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One of the most serious problems facing the whole world today is global warming. Their lease of greenhouse gases is exacerbating the effects of global warming. Reduced greenhouse gas emissions and searching for alternative energy sources are becoming increasingly crucial. This study aims to review that one of the effective methods for lowering greenhouse gas emissions is the creation of biogas from agricultural waste using anaerobic digester plants. A lab-scale 5-liter batch fermenter was incubated at room temperature, specifically mesophilic (35°C). Biogas is a clean, reasonably priced, and sustainable energy source produced by the anaerobic fermentation of waste and organic waste. Mixing sludge and waste to react with each other during biogas production is essential. Factors affecting biogas, such as loading rate, retention time, operating ambient temperature, pH, mixing, etc., are also discussed. Asia is a region where the generation of this form of renewable energy is widespread, particularly in nations like China and India. The generation of biogas never has any adverse environmental effects, but it also yields environmentally safe byproducts. Agricultural waste is a large and anthropogenic source of methane in the atmosphere. It can be converted into nutrient-rich fertilizer. Agricultural waste, food waste, animal or human manure, and other organic waste are all converted into energy (in the form of biogas or electricity) using anaerobic digesters. Another advantage of biological fermentation is that it leaves behind a high-quality organic fertilizer [ABSTRACT FROM AUTHOR]

Today, the world is becoming more dependent on fossil fuels. The major drawbacks of these non-renewable energy resources include an extreme environmental pollution and an extinction threat. Several technologies including microalgal biodiesel production, biomass gasification, and bioethanol production have been explored for the generation of renewable energy especially, biofuels. One such promising research has been carried out in the generation of biohythane which has the potential to become an alternative fuel to the existing non-renewable ones. It has been reported that biohydrogen can be produced from organic wastes or agricultural feedstocks with the help of acidogens. Dark fermentation can be carried out by acidogens to produce biohydrogen under anaerobic conditions by utilizing lignocellulosic biomass or sugarcane feedstocks in the absence of light. The spent medium contains volatile short-chain fatty acids like acetate, butyrate, and propionate that can serve as substrates for acetogenesis followed by methane biosynthesis by methanogens. Therefore, the sequential two-stage anaerobic digestion (AD) involves a production of biohydrogen followed by the biosynthesis of methane. This combined process is termed as a single eponym "Biohythane" (hydrogen + methane). Several studies have demonstrated about the effectiveness of biofuel, and it is believed to have a greater energy recovery, environmental friendliness, and shorter fermentation time. Biohythane can serve as an alternative future green biofuel and solve the present energy crisis in India as well as the entire world. [ABSTRACT FROM AUTHOR]

One of the most serious problems facing the whole world today is global warming. Their lease of greenhouse gases is exacerbating the effects of global warming. Reduced greenhouse gas emissions and searching for alternative energy sources are becoming increasingly crucial. This study aims to review that one of the effective methods for lowering greenhouse gas emissions is the creation of biogas from agricultural waste using anaerobic digester plants. A lab-scale 5-liter batch fermenter was incubated at room temperature, specifically mesophilic (35°C). Biogas is a clean, reasonably priced, and sustainable energy source produced by the anaerobic fermentation of waste and organic waste. Mixing sludge and waste to react with each other during biogas production is essential. Factors affecting biogas, such as loading rate, retention time, operating ambient temperature, pH, mixing, etc., are also discussed. Asia is a region where the generation of this form of renewable energy is widespread, particularly in nations like China and India. The generation of biogas never has any adverse environmental effects, but it also yields environmentally safe byproducts. Agricultural waste is a large and anthropogenic source of methane in the atmosphere. It can be converted into nutrient-rich fertilizer. Agricultural waste, food waste, animal or human manure, and other organic waste are all converted into energy (in the form of biogas or electricity) using anaerobic digesters. Another advantage of biological fermentation is that it leaves behind a high-quality organic fertilizer

Subsurface hydrogen storage would be one of the main possible solutions for a successful energy transition in the future of world energy. However, hydrogen gas is an electron donor which may lead to microbial activity when storing hydrogen in the subsurface. Methanogenesis, acetogenesis, and sulfate reduction are the primary biological reactions that can lead to hydrogen loss through biochemical processes. This paper presents a novel bio-geochemical modelling approach which considers the kinetics of microbial reactions as well as the equilibrated geochemical reactions in the context of underground hydrogen storage. The findings demonstrate that the type of mineralogy and formation water composition may greatly influence biochemical reactions and thus potentially the storage/recovery performance of hydrogen. This study provides a detailed comparison and analysis of the behaviour occurring in calcite, dolomite, and quartz systems assuming the North Sea seawater composition as the in-situ brine. It is found that dolomite is the least favourable mineralogy in terms of hydrogen loss and hydrogen sulfide production, while calcite and quartz are better suited in a seawater-rich environment. • The interplay between biochemical and geochemical reactions during UHS is studied. • There is a strong interplay between biochemical and geochemical reactions. • Small concentrations of sulfate may lead to dangerous level of gas toxicity. [ABSTRACT FROM AUTHOR]

Simple Summary: The lower termite Coptotermes gestroi is widely distributed in Northern Vietnam and is an important urban pest that destroys wooden constructions such as pagodas, temples, and furniture. Although the metagenomic DNA of prokaryotes freely living in the termite gut has been sequenced and analyzed, the overall picture of the prokaryotic diversity, including archaea and their function, has not been investigated. The present study, for the first time, revealed the differences in the structure and function of free-living prokaryotes in the termite gut. The bacterial community was formed and adapted to aid the hosts' survival and development even in the presence of pesticides. Beyond the potential function of the bacteria towards lignocellulose digestion, free-living bacteria and archaea also harbor diverse genes coding for the enzymes/proteins involved in reductive acetogenesis, methanogenesis, methane and sulfur metabolism, and nitrogen fixation and recycling to supply energy for the host; the synthesis of antibiotics for host defense; and the detoxification of aromatic compounds. The present study provides a valuable scientific basis for the mining of novel bacterial genes and the isolation of bacteria from the termite gut for agricultural, environmental, pharmaceutical, and medical applications. Termites' digestive systems, particularly in lower termites with the presence of protozoa, are unique ecological niches that shelter a diverse microbiota with a variety of functions for the host and the environment. In 2012, the metagenomic DNA (5.4 Gb) of the prokaryotes that freely live in the gut of the lower termite Coptotermes gestroi were sequenced. A total of 125,431 genes were predicted and analyzed in order to mine lignocellulolytic genes. however, the overall picture of the structure, diversity, and function of the prokaryotic gut microbiota was not investigated. In the present study, these 125,431 genes were taxonomically classified by MEGAN and functionally annotated by the Kyoto Encyclopedia of Genes and Genomes (KEGG) and by the Carbohydrate-Active enZYmes (CAZy) and HMMER databases. As a result, 95,751 bacterial genes were classified into 35 phyla. The structure of the bacteria, typified by a high ratio of Firmicutes to Bacterioidetes, was distinct from the structure of the entirety of the bacteria in the lower or higher termites' guts. The archaea (533 genes) were distributed into 4 phyla, 10 classes, 15 orders, 21 families, 47 genera, and 61 species. Although freely living in the guts, the prokaryotic community was formed, developed, and adapted to exhibit unique interactions in order to perform mutual roles of benefit to their hosts. Methanobacteriales, accounting for 61% of the archaea symbionts, seem to play an important role in methanogenesis. Concomitantly, bacterial methanotrophs in the gut utilize methane and combine with other bacterial groups, including potential lignocellulolytic degraders, acetogens, sulfur bacteria, and nitrogen-recycling bacteria, to efficiently convert wood with little nitrogen into acetates via certain pathway modules specified by prokaryotes that freely live in the gut. This forms an important energy source for the termites. Furthermore, bacteria carry 2223 genes involved in the biosynthesis of 17 antibiotic groups. The gut bacteria also possess genes for the degradation of 18 toxic aromatic compounds, of which four are commercial pesticides against termites commonly used for the preservation of wooden constructions. Eight of the eighteen pathways were the first to be reported from the termite gut. Overall, this study sheds light on the roles of the freely living bacteria and archaea in the C. gestroi gut, providing evidence that the gut microbiome acts as the second host genome, contributing both nutrients and immunity to support the host's existence, growth, and development. [ABSTRACT FROM AUTHOR]

Anaerobic fermentation technology enables the production of medium chain carboxylates and alcohols through microbial chain elongation. This involves steering reactor microbiomes to yield desired products, with CO2 supply playing a crucial role in controlling ethanol-based chain elongation and facilitating various bioprocesses simultaneously. In the absence of CO2 supply (Phase I), chain elongation predominantly led to n-caproate with a high selectivity of 96 Cmol%, albeit leaving approximately 80% of ethanol unconverted. During this phase, C. kluyveri and Proteiniphilum-related species dominated the reactors. In Phase II, with low CO2 input (2.0 NmL L−1 min−1), formation of n-butyrate, butanol, and hexanol was stimulated. Increasing CO2 doses in Phase III (6 NmL L−1 min−1) led to CO2 utilization via homoacetogenesis, coinciding with the enrichment of Clostridium luticellarii, a bacterium that can use CO2 as an electron acceptor. Lowering CO2 dose to 0.5 NmL L−1 min−1 led to a shift in microbiome composition, diminishing the dominance of C. luticellarii while increasing C. kluyveri abundance. Additionally, other Clostridia, Proteiniphilum, and Lactobacillus sakei-related species became prevalent. This decrease in CO2 load from 6 to 0.5 NmL L−1 min−1 minimized excessive ethanol oxidation from 30%–50% to 0%–3%, restoring a microbiome favoring net n-butyrate consumption and n-caproate production. The decreased ethanol oxidation coincided with the resurgence of hydrogen formation at partial pressures above 1%. High concentrations of butyrate, caproate, and ethanol in the reactor, along with low acetate concentration, promoted the formation of butanol and hexanol. It is evident that CO2 supply is indispensable for controlling chain elongation in an open culture and it can be harnessed to stimulate higher alcohol formation or induce CO2 utilization as an electron acceptor.

Abstract Background A large proportion of prokaryotic microbes in marine sediments remains uncultured, hindering our understanding of their ecological functions and metabolic features. Recent environmental metagenomic studies suggested that many of these uncultured microbes contribute to the degradation of organic matter, accompanied by acetogenesis, but the supporting experimental evidence is limited. Results Estuarine sediments were incubated with different types of organic matters under anaerobic conditions, and the increase of uncultured bacterial populations was monitored. We found that (1) lignin stimulated the increase of uncultured bacteria within the class Dehalococcoidia. Their ability to metabolize lignin was further supported by the presence of genes associated with a nearly complete degradation pathway of phenolic monomers in the Dehalococcoidia metagenome-assembled genomes (MAGs). (2) The addition of cellulose stimulated the increase of bacteria in the phylum Ca. Fermentibacterota and family Fibrobacterales, a high copy number of genes encoding extracellular endoglucanase or/and 1,4-beta-cellobiosidase for cellulose decomposition and multiple sugar transporters were present in their MAGs. (3) Uncultured lineages in the order Bacteroidales and the family Leptospiraceae were enriched by the addition of casein and oleic acid, respectively, a high copy number of genes encoding extracellular peptidases, and the complete β-oxidation pathway were found in those MAGs of Bacteroidales and Leptospiraceae, respectively. (4) The growth of unclassified bacteria of the order Clostridiales was found after the addition of both casein and cellulose. Their MAGs contained multiple copies of genes for extracellular peptidases and endoglucanase. Additionally, 13C-labeled acetate was produced in the incubations when 13C-labeled dissolved inorganic carbon was provided. Conclusions Our results provide new insights into the roles of microorganisms during organic carbon degradation in anaerobic estuarine sediments and suggest that these macro and single molecular organic carbons support the persistence and increase of uncultivated bacteria. Acetogenesis is an additional important microbial process alongside organic carbon degradation. Video Abstract

One of the important current issues of bioenergetics is the establishment of the thermodynamic limits of life. There is still no final understanding of what is the minimum value of the energy yield of a reaction that is sufficient to be used by an organism (the so-called "biological quantum of energy"). A reasonable model for determination of the minimal energy yield would be microorganisms capable of living on low-energy substrates, such as acetogenic prokaryotes. The most prominent metabolic feature of acetogens is autotrophic growth with molecular hydrogen and carbon dioxide as the substrates, which is hardly competitive in environments. Most probably, that is why only facultative autotrophic acetogens have been known so far. Here, we describe the first obligately autotrophic acetogenic bacterium Aceticella autotrophica gen. nov., sp. nov., strain 3443-3AcT. Phylogenetically, the new genus falls into a monophyletic group of heterotrophic bacteria of the genera Thermoanaerobacterium, Thermoanaerobacter, and Caldanaerobacter (hereinafter referred to as TTC group), where the sole acetogenic representative has so far been the facultatively autotrophic Thermoanaerobacter kivui. A. autotrophica and T. kivui both are acetogens employing energy-converting hydrogenase (Ech-acetogens) that are likely to have inherited the acetogenesis capacity vertically from common ancestor. However, their acetogenic machineries have undergone different adjustments by gene replacements due to horizontal gene transfers from different donors. Obligate autotrophy of A. autotrophica is associated with the lack of many sugar transport systems and carbohydrate catabolism enzymes that are present in other TTC group representatives, including T. kivui. [ABSTRACT FROM AUTHOR]

The depletion of fossil fuels coupled with stringent environmental laws has encouraged us to develop sustainable renewable energy. Due to its numerous benefits, anaerobic digestion (AD) has emerged as an environment-friendly technology. Biogas generated during AD is primarily a mixture of CH4 (65–70%) and CO2 (20–25%) and a potent energy source that can combat the energy crisis in today's world. Here, an attempt has been made to provide a broad understanding of AD and delineate the effect of various operational parameters influencing AD. The characteristics of fruit and vegetable waste (FVW) and its feasibility as a potent substrate for AD have been studied. This review also covers traditional challenges in managing FVW via AD, the implementation of various bioreactor systems to manage large amounts of organic waste and their operational boundaries, microbial consortia involved in each phase of digestion, and various strategies to increase biogas production. [ABSTRACT FROM AUTHOR]

An anaerobic granular sludge was enriched to utilize H2/CO2 in a continuous gas‐fed up‐flow anaerobic sludge reactor by applying operating conditions expected to produce acetic acid, butyric acid, and ethanol. Three stages of fermentation were found: Stage I with acetic acid accumulation with the highest concentration of 35 mM along with a pH decrease from initial 6 to 4.5. In Stage II, H2/CO2 was replaced by 100% H2 to induce solventogenesis, whereas butyric acid was produced with the highest concentration of 2.5 mM. At stage III with 10 µM tungsten (W) addition, iso‐valeric acid, valeric acid, and caproic acid were produced at pH 4.5–5.0. In the batch tests inoculated with the enriched sludge taken from the bioreactor (day 70), however, methane production occurred at pH 6. Exogenous 15 mM acetate addition enhanced both the H2 and CO2 consumption rate compared to exogenous 10, 30, and 45 mM acetate by the enriched sludge. Exogenous acetate was failed to be converted to ethanol using H2 as electron donor by the enriched acetogens. [ABSTRACT FROM AUTHOR]

One of the important current issues of bioenergetics is the establishment of the thermodynamic limits of life. There is still no final understanding of what is the minimum value of the energy yield of a reaction that is sufficient to be used by an organism (the so-called “biological quantum of energy”). A reasonable model for determination of the minimal energy yield would be microorganisms capable of living on low-energy substrates, such as acetogenic prokaryotes. The most prominent metabolic feature of acetogens is autotrophic growth with molecular hydrogen and carbon dioxide as the substrates, which is hardly competitive in environments. Most probably, that is why only facultative autotrophic acetogens have been known so far. Here, we describe the first obligately autotrophic acetogenic bacterium Aceticella autotrophica gen. nov., sp. nov., strain 3443-3AcT. Phylogenetically, the new genus falls into a monophyletic group of heterotrophic bacteria of the genera Thermoanaerobacterium, Thermoanaerobacter, and Caldanaerobacter (hereinafter referred to as TTC group), where the sole acetogenic representative has so far been the facultatively autotrophic Thermoanaerobacter kivui. A. autotrophica and T. kivui both are acetogens employing energy-converting hydrogenase (Ech-acetogens) that are likely to have inherited the acetogenesis capacity vertically from common ancestor. However, their acetogenic machineries have undergone different adjustments by gene replacements due to horizontal gene transfers from different donors. Obligate autotrophy of A. autotrophica is associated with the lack of many sugar transport systems and carbohydrate catabolism enzymes that are present in other TTC group representatives, including T. kivui.

Abstract An anaerobic granular sludge was enriched to utilize H2/CO2 in a continuous gas‐fed up‐flow anaerobic sludge reactor by applying operating conditions expected to produce acetic acid, butyric acid, and ethanol. Three stages of fermentation were found: Stage I with acetic acid accumulation with the highest concentration of 35 mM along with a pH decrease from initial 6 to 4.5. In Stage II, H2/CO2 was replaced by 100% H2 to induce solventogenesis, whereas butyric acid was produced with the highest concentration of 2.5 mM. At stage III with 10 µM tungsten (W) addition, iso‐valeric acid, valeric acid, and caproic acid were produced at pH 4.5–5.0. In the batch tests inoculated with the enriched sludge taken from the bioreactor (day 70), however, methane production occurred at pH 6. Exogenous 15 mM acetate addition enhanced both the H2 and CO2 consumption rate compared to exogenous 10, 30, and 45 mM acetate by the enriched sludge. Exogenous acetate was failed to be converted to ethanol using H2 as electron donor by the enriched acetogens.

Introduction. The volume of accumulated organ-containing non-recyclable waste products in Russia is increasing. Existing technologies make it possible to recycle these waste products through using anaerobic digestion to produce biogas and fertilizers. The organic portions of waste consist mostly of hard-to-degrade lignin, cellulose, and extracellular polymers, which make anaerobic digestion difficult. Pretreating is necessary to make the raw materials available to microorganisms for bioavailability and deeper degradation. There are many different types of pretreating with their own specific effects on the raw materials. To improve understanding the process of pretreating organic raw materials prior to anaerobic digestion, and to improve the quality of the decisions made, pretreatment methods need to be identified and systematized. Materials and Methods. The subject of the study is the relationship between the effects of methods, techniques and specific operations of pretreatment for anaerobic digestion. The main method of this study is the analysis of data on the application of different methods to increase the bioavailability and degree of decomposition of organic matter of waste products during anaerobic digestion. Results. The scheme of preparatory operations and methods for pre-treatment of organic waste products before anaerobic digestion is developed. It is revealed that each method (mechanical, thermal, electromagnetic, chemical, biological and combined) contains many practices, which are divided into particular techniques. The mechanisms of influence that improve further processing are described. Discussion and Conclusion. The choice methods of pretreatment depends on the purpose and tasks performed, the physical and chemical composition of raw materials, access to energy, biological and chemical resources. Advantages and disadvantages of various methods should be studied in more detail and ranked into private methods according to energy costs and the effect they produce. The comparison of the research results reveals disadvantages and difficulties.

Termites’ digestive systems, particularly in lower termites with the presence of protozoa, are unique ecological niches that shelter a diverse microbiota with a variety of functions for the host and the environment. In 2012, the metagenomic DNA (5.4 Gb) of the prokaryotes that freely live in the gut of the lower termite Coptotermes gestroi were sequenced. A total of 125,431 genes were predicted and analyzed in order to mine lignocellulolytic genes. however, the overall picture of the structure, diversity, and function of the prokaryotic gut microbiota was not investigated. In the present study, these 125,431 genes were taxonomically classified by MEGAN and functionally annotated by the Kyoto Encyclopedia of Genes and Genomes (KEGG) and by the Carbohydrate-Active enZYmes (CAZy) and HMMER databases. As a result, 95,751 bacterial genes were classified into 35 phyla. The structure of the bacteria, typified by a high ratio of Firmicutes to Bacterioidetes, was distinct from the structure of the entirety of the bacteria in the lower or higher termites’ guts. The archaea (533 genes) were distributed into 4 phyla, 10 classes, 15 orders, 21 families, 47 genera, and 61 species. Although freely living in the guts, the prokaryotic community was formed, developed, and adapted to exhibit unique interactions in order to perform mutual roles of benefit to their hosts. Methanobacteriales, accounting for 61% of the archaea symbionts, seem to play an important role in methanogenesis. Concomitantly, bacterial methanotrophs in the gut utilize methane and combine with other bacterial groups, including potential lignocellulolytic degraders, acetogens, sulfur bacteria, and nitrogen-recycling bacteria, to efficiently convert wood with little nitrogen into acetates via certain pathway modules specified by prokaryotes that freely live in the gut. This forms an important energy source for the termites. Furthermore, bacteria carry 2223 genes involved in the biosynthesis of 17 antibiotic groups. The gut bacteria also possess genes for the degradation of 18 toxic aromatic compounds, of which four are commercial pesticides against termites commonly used for the preservation of wooden constructions. Eight of the eighteen pathways were the first to be reported from the termite gut. Overall, this study sheds light on the roles of the freely living bacteria and archaea in the C. gestroi gut, providing evidence that the gut microbiome acts as the second host genome, contributing both nutrients and immunity to support the host’s existence, growth, and development.

Abstract Agriculture is responsible for a great share of the anthropogenic sources of greenhouse gases that, by warming the earth, threaten its biodiversity. Among greenhouse gas emissions, enteric CH4 from livestock is an important target to slow down climate changes. The CH4 is originated from rumen fermentation and its concentration is affected by several factors, including genetics and nutrition. Ruminants have an extraordinary symbiosis with microorganisms (bacteria, fungi, and protozoa) that ferment otherwise indigestible carbohydrates, from which they obtain energy to grow and continue actively producing, among other products, volatile fatty acids, CO2 and H2. Detrimental ruminal accumulation of H2 is avoided by methanogenesis carried out by Archaea methanogens. Importantly, methanogenesis is not the only H2 sink pathway. In fact, other bacteria can reduce substrates using metabolic hydrogen formed during carbohydrate fermentation, namely propionate production and reductive acetogenesis, thus lowering the CH4 produced. Although the complexity of rumen poses challenges to mitigate CH4 production, the emergence of sequencing techniques that allow the study of microbial communities, gene expression, and metabolome are largely contributing to unravel pathways and key players in the rumen. Indeed, it is now recognized that in vivo emissions of CH4 are correlated to microbial communities, and particularly with the abundance of methanogens, several bacterial groups, and their genes. The goal of CH4 mitigation is to work in favor of the natural processes, without compromising rumen function, animal health, and productivity. Notwithstanding, the major challenge continues to be the feasibility and affordability of the proposed solutions.

Microbial electrosynthesis (MES) enables the bioproduction of multicarbon compounds from CO2 using electricity as the driver. Although high salinity can improve the energetic performance of bioelectrochemical systems, acetogenic processes under elevated salinity are poorly known. Here MES under 35–60 g L−1 salinity was evaluated. Acetate production in two-chamber MES systems at 35 g L−1 salinity (seawater composition) gradually decreased within 60 days, both under −1.2 V cathode potential (vs. Ag/AgCl) and −1.56 A m−2 reductive current. Carbonate precipitation on cathodes (mostly CaCO3) likely declined the production through inhibiting CO2 supply, the direct electrode contact for acetogens and H2 production. Upon decreasing Ca2+ and Mg2+ levels in three-chamber reactors, acetate was stably produced over 137 days along with a low cathode apparent resistance at 1.9 ± 0.6 mΩ m2 and an average production rate at 3.80 ± 0.21 g m−2 d−1. Increasing the salinity step-wise from 35 to 60 g L−1 gave the most efficient acetate production at 40 g L−1 salinity with average rates of acetate production and CO2 consumption at 4.56 ± 3.09 and 7.02 ± 4.75 g m−2 d−1, respectively. The instantaneous coulombic efficiency for VFA averaged 55.1 ± 31.4%. Acetate production dropped at higher salinity likely due to the inhibited CO2 dissolution and acetogenic metabolism. Acetobacterium up to 78% was enriched on cathodes as the main acetogen at 35 g L−1. Under high-salinity selection, 96.5% Acetobacterium dominated on the cathode along with 34.0% Sphaerochaeta in catholyte. This research provides a first proof of concept that MES starting from CO2 reduction can be achieved at elevated salinity.

Little is known of acetogens in contemporary serpentinizing systems, despite widely supported theories that serpentinite-hosted environments supported the first life on Earth via acetogenesis. To address this knowledge gap, genome-resolved metagenomics was applied to subsurface fracture water communities from an area of active serpentinization in the Samail Ophiolite, Sultanate of Oman. Two deeply branching putative bacterial acetogen types were identified in the communities belonging to the Acetothermia (hereafter, types I and II) that exhibited distinct distributions among waters with lower and higher water–rock reaction (i.e., serpentinization influence), respectively. Metabolic reconstructions revealed contrasting core metabolic pathways of type I and II Acetothermia, including in acetogenic pathway components (e.g., bacterial- vs. archaeal-like carbon monoxide dehydrogenases [CODH], respectively), hydrogen use to drive acetogenesis, and chemiosmotic potential generation via respiratory (type I) or canonical acetogen ferredoxin-based complexes (type II). Notably, type II Acetothermia metabolic pathways allow for use of serpentinization-derived substrates and implicate them as key primary producers in contemporary hyperalkaline serpentinite environments. Phylogenomic analyses indicate that 1) archaeal-like CODH of the type II genomes and those of other serpentinite-associated Bacteria derive from a deeply rooted horizontal transfer or origin among archaeal methanogens and 2) Acetothermia are among the earliest evolving bacterial lineages. The discovery of dominant and early-branching acetogens in subsurface waters of the largest near-surface serpentinite formation provides insight into the physiological traits that likely facilitated rock-supported life to flourish on a primitive Earth and possibly on other rocky planets undergoing serpentinization [ABSTRACT FROM AUTHOR]

Alkaline fluids venting from chimneys of the Lost City hydrothermal field flow from a potentially vast microbial habitat within the seafloor where energy and organic molecules are released by chemical reactions within rocks uplifted from Earth’s mantle. In this study, we investigated hydrothermal fluids venting from Lost City chimneys as windows into subseafloor environments where the products of geochemical reactions, such as molecular hydrogen (H2), formate, and methane, may be the only available sources of energy for biological activity. Our deep sequencing of metagenomes and metatranscriptomes from these hydrothermal fluids revealed a few key species of archaea and bacteria that are likely to play critical roles in the subseafloor microbial ecosystem. We identified a population of Thermodesulfovibrionales (belonging to phylum Nitrospirota) as a prevalent sulfate-reducing bacterium that may be responsible for much of the consumption of H2 and sulfate in Lost City fluids. Metagenome-assembled genomes (MAGs) classified as Methanosarcinaceae and Candidatus Bipolaricaulota were also recovered from venting fluids and represent potential methanogenic and acetogenic members of the subseafloor ecosystem. These genomes share novel hydrogenases and formate dehydrogenase-like sequences that may be unique to hydrothermal environments where H2 and formate are much more abundant than carbon dioxide. The results of this study include multiple examples of metabolic strategies that appear to be advantageous in hydrothermal and subsurface alkaline environments where energy and carbon are provided by geochemical reactions. [ABSTRACT FROM AUTHOR]

The importance of acetogens for H2 turnover and overall anaerobic degradation in peatlands remains elusive. In the well-studied minerotrophic peatland fen Schlöppnerbrunnen, H2-consuming acetogens are conceptualized to be largely outcompeted by iron reducers, sulfate reducers, and hydrogenotrophic methanogens in bulk peat soil. However, in root zones of graminoids, fermenters thriving on rhizodeposits and root litter might temporarily provide sufficient H2 for acetogens. In the present study, root-free peat soils from around the roots of Molinia caerulea and Carex rostrata (i.e., two graminoids common in fen Schlöpnnerbrunnen) were anoxically incubated with or without supplemental H2 to simulate conditions of high and low H2 availability in the fen. In unsupplemented soil treatments, H2 concentrations were largely below the detection limit (~10 ppmV) and possibly too low for acetogens and methanogens, an assumption supported by the finding that neither acetate nor methane substantially accumulated. In the presence of supplemental H2, acetate accumulation exceeded CH4 accumulation in Molinia soil whereas acetate and methane accumulated equally in Carex soil. However, reductant recoveries indicated that initially, additional unknown processes were involved either in H2 consumption or the consumption of acetate produced by H2-consuming acetogens. 16S rRNA and 16S rRNA gene analyses revealed that potential acetogens (Clostridium, Holophagaceae), methanogens (Methanocellales, Methanobacterium), iron reducers (Geobacter), and physiologically uncharacterized phylotypes (Acidobacteria, Actinobacteria, Bacteroidetes) were stimulated by supplemental H2 in soil treatments. Phylotypes closely related to clostridial acetogens were also active in soil-free Molinia and Carex root treatments with or without supplemental H2. Due to pronounced fermentation activities, H2 consumption was less obvious in root treatments, and acetogens likely thrived on root organic carbon and fermentation products (e.g., ethanol) in addition to H2. Collectively, the data highlighted that in fen Schlöppnerbrunnen, acetogens are associated to graminoid roots and inhabit the peat soil around the roots, where they have to compete for H2 with methanogens and iron reducers. Furthermore, the study underscored that the metabolically flexible acetogens do not rely on H2, potentially a key advantage over other H2 consumers under the highly dynamic conditions characteristic for the root-zones of graminoids in peatlands. [ABSTRACT FROM AUTHOR]

The adaption of the phylum Chloroflexota to various geochemical conditions is thought to have originated in primitive microbial ecosystems, involving hydrogenotrophic energy conservation under ferruginous anoxia. Oligotrophic deep waters displaying anoxic ferruginous conditions, such as those of Lake Towuti, and their sediments may thus constitute a preferential ecological niche for investigating metabolic versatility in modern Chloroflexota. Combining pore water geochemistry, cell counts, sulfate reduction rates, 16S rRNA genes with in-depth analysis of metagenome-assembled genomes, we show that Chloroflexota benefit from cross-feeding on metabolites derived from canonical respiration chains and fermentation. Detailing their genetic contents, we provide molecular evidence that Anaerolineae have metabolic potential to use unconventional electron acceptors, different cytochromes and multiple redox metalloproteins to cope with oxygen fluctuations, and thereby effectively colonizing the ferruginous sediment-water interface. In sediments, Dehalococcoidia evolved to be acetogens, scavenging fatty acids, haloacids and aromatic acids, apparently bypassing specific steps in carbon assimilation pathways to perform energy-conserving secondary fermentations combined with CO2 fixation via the Wood-Ljungdahl pathway. Our study highlights the partitioning of Chloroflexota populations according to alternative electron acceptors and donors available at the sediment-water interface and below. Chloroflexota would have developed analogous primeval features due to oxygen fluctuations in ancient ferruginous ecosystems., (© The Author(s) 2024. Published by Oxford University Press on behalf of FEMS.)

The importance of acetogens for H2 turnover and overall anaerobic degradation in peatlands remains elusive. In the well-studied minerotrophic peatland fen Schlöppnerbrunnen, H2-consuming acetogens are conceptualized to be largely outcompeted by iron reducers, sulfate reducers, and hydrogenotrophic methanogens in bulk peat soil. However, in root zones of graminoids, fermenters thriving on rhizodeposits and root litter might temporarily provide sufficient H2 for acetogens. In the present study, root-free peat soils from around the roots of Molinia caerulea and Carex rostrata (i.e., two graminoids common in fen Schlöpnnerbrunnen) were anoxically incubated with or without supplemental H2 to simulate conditions of high and low H2 availability in the fen. In unsupplemented soil treatments, H2 concentrations were largely below the detection limit (∼10 ppmV) and possibly too low for acetogens and methanogens, an assumption supported by the finding that neither acetate nor methane substantially accumulated. In the presence of supplemental H2, acetate accumulation exceeded CH4 accumulation in Molinia soil whereas acetate and methane accumulated equally in Carex soil. However, reductant recoveries indicated that initially, additional unknown processes were involved either in H2 consumption or the consumption of acetate produced by H2-consuming acetogens. 16S rRNA and 16S rRNA gene analyses revealed that potential acetogens (Clostridium, Holophagaceae), methanogens (Methanocellales, Methanobacterium), iron reducers (Geobacter), and physiologically uncharacterized phylotypes (Acidobacteria, Actinobacteria, Bacteroidetes) were stimulated by supplemental H2 in soil treatments. Phylotypes closely related to clostridial acetogens were also active in soil-free Molinia and Carex root treatments with or without supplemental H2. Due to pronounced fermentation activities, H2 consumption was less obvious in root treatments, and acetogens likely thrived on root organic carbon and fermentation products (e.g., ethanol) in addition to H2. Collectively, the data highlighted that in fen Schlöppnerbrunnen, acetogens are associated to graminoid roots and inhabit the peat soil around the roots, where they have to compete for H2 with methanogens and iron reducers. Furthermore, the study underscored that the metabolically flexible acetogens do not rely on H2, potentially a key advantage over other H2 consumers under the highly dynamic conditions characteristic for the root-zones of graminoids in peatlands.

ABSTRACT Acetogenic bacteria are a group of strictly anaerobic bacteria that make a living from acetate formation from two molecules of CO2 via the Wood-Ljungdahl pathway (WLP). The free energy change of this reaction is very small and allows the synthesis of only a fraction of an ATP. How this pathway is coupled to energy conservation has been an enigma since its discovery ~90 years ago. Here, we describe an electron transport chain in the cytochrome- and quinone-containing acetogen Sporomusa ovata that leads from molecular hydrogen as an electron donor to an intermediate of the WLP, methylenetetrahydrofolate (methylene-tetrahydrofolate [THF]), as an electron acceptor. The catalytic site of the hydrogenase is periplasmic and likely linked cytochrome b to the membrane. We provide evidence that the MetVF-type methylenetetrahydrofolate reductase is linked proteins MvhD and HdrCBA to the cytoplasmic membrane. Membrane preparations catalyzed the H2-dependent reduction of methylene-THF to methyl-THF. In our model, a transmembrane electrochemical H+ gradient is established by both scalar and vectorial protons that leads to the synthesis of 0.5 mol ATP/mol methylene-THF by a H+-F1Fo ATP synthase. This H2- and methylene-THF-dependent electron transport chain may be present in other cytochrome-containing acetogens as well and represents a third way of chemiosmotic energy conservation in acetogens, but only in addition to the well-established respiratory enzymes Rnf and Ech. IMPORTANCE Acetogenic bacteria grow by making acetate from CO2 and are considered the first life forms on Earth since they couple CO2 reduction to the conservation of energy. How this is achieved has been an enigma ever since. Recently, two respiratory enzymes, a ferredoxin:NAD+ oxidoreductase (Rnf) and a ferredoxin:H+ oxidoreductase (Ech), have been found in cytochrome-free acetogenic model bacteria. However, some acetogens contain cytochromes in addition, and there has been a long-standing assumption of a cytochrome-containing electron transport chain in those acetogens. Here, we provide evidence for a respiratory chain in Sporomusa ovata that has a cytochrome-containing hydrogenase as the electron donor and a methylenetetrahydrofolate reductase as the terminal electron acceptor. This is the third way of chemiosmotic energy conservation found in acetogens.

Tylosin eliminated in animal waste, during therapeutic treatment, can be efficiently removed in anaerobic systems. The present study investigated the influence of tylosin concentration and assessed its degradation kinetics and the microorganisms involved in each stage of its anaerobic digestion (hydrolysis/acidogenesis; acetogenesis; methanogenesis). The results showed a stimulating effect on methane production with increasing tylosin concentration in the poultry litter up to 80 mg kg−1 tylosin (232.9 NL CH4 kg SV−1). As for tylosin degradation, greater removal of antibiotics was observed in the methanogenic phase (88%), followed by acetogenic (84%) and hydrolytic/acidogenic (76%) phases. The higher rate of tylosin degradation obtained in the methanogenic step, is mainly related to the co-metabolic effect exerted by the presence of acetate and its degradation by acetoclastic methanogens. Indeed, metagenomic analyses suggested a syntrophic action between archaea of ​​the genus Methanobacterium, and bacteria such as Clostridium and Flexilinea, which seemed decisive for tylosin degradation. [ABSTRACT FROM AUTHOR]

Abstract Background During the acetogenic step of anaerobic digestion, the products of acidogenesis are oxidized to substrates for methanogenesis: hydrogen, carbon dioxide and acetate. Acetogenesis and methanogenesis are highly interconnected processes due to the syntrophic associations between acetogenic bacteria and hydrogenotrophic methanogens, allowing the whole process to become thermodynamically favorable. The aim of this study is to determine the influence of the dominant acidic products on the metabolic pathways of methane formation and to find a core microbiome and substrate-specific species in a mixed biogas-producing system. Results Four methane-producing microbial communities were fed with artificial media having one dominant component, respectively, lactate, butyrate, propionate and acetate, for 896 days in 3.5-L Up-flow Anaerobic Sludge Blanket (UASB) bioreactors. All the microbial communities showed moderately different methane production and utilization of the substrates. Analyses of stable carbon isotope composition of the fermentation gas and the substrates showed differences in average values of δ13C(CH4) and δ13C(CO2) revealing that acetate and lactate strongly favored the acetotrophic pathway, while butyrate and propionate favored the hydrogenotrophic pathway of methane formation. Genome-centric metagenomic analysis recovered 234 Metagenome Assembled Genomes (MAGs), including 31 archaeal and 203 bacterial species, mostly unknown and uncultivable. MAGs accounted for 54%–67% of the entire microbial community (depending on the bioreactor) and evidenced that the microbiome is extremely complex in terms of the number of species. The core microbiome was composed of Methanothrix soehngenii (the most abundant), Methanoculleus sp., unknown Bacteroidales and Spirochaetaceae. Relative abundance analysis of all the samples revealed microbes having substrate preferences. Substrate-specific species were mostly unknown and not predominant in the microbial communities. Conclusions In this experimental system, the dominant fermentation products subjected to methanogenesis moderately modified the final effect of bioreactor performance. At the molecular level, a different contribution of acetotrophic and hydrogenotrophic pathways for methane production, a very high level of new species recovered, and a moderate variability in microbial composition depending on substrate availability were evidenced. Propionate was not a factor ceasing methane production. All these findings are relevant because lactate, acetate, propionate and butyrate are the universal products of acidogenesis, regardless of feedstock.

[Display omitted] • SP-EP enhanced the production of ethanol, acetate and methane. • SP-EP enriched methanogens such as Methanothrix and Methanoculleus. • EP promoted the conversion of ethanol to acetaldehyde. • SP promoted the production of CO 2 and H 2 from pyruvate. • SP-EP enhanced acetyl-CoA production and hydrogenotrophic methanogenic pathway. In anaerobic digestion (AD) of food waste (FW), the acetogenesis stage is thermodynamically limited and often leads to acid inhibition. This study investigated the impact of optimizing ethanol pre-fermentation (EP) of FW through saccharification pretreatment (SP) on AD by regulating the acidification pathway of substrates. The results showed that the ethanol concentration of FW after SP-EP reached 50.81 g/L, which was 2.11-fold of that in EP. SP-EP enriched hydrolytic and acidogenic bacteria such as Alkaliphilus , Caldicoprobacter , Sphaerochaeta , and Fermentimonas. The analysis showed that the abundance of enzyme-encoding genes catalyzing the conversion of ethanol to acetaldehyde in SP-EP was 1.17-fold higher than control, which further indicated that SP-EP facilitated the conversion of substrate to glucose → ethanol → acetaldehyde → acetate. This process provided abundant acetate for AD, and enriched the acetoclastic methanogens Methanothrix. Consequently, the cumulative methane yield of SP-EP (557 mL/g vs) was 7.49 %–17.79 % higher than the other three treatments. Hydrogenotrophic methanogens such as Methanoculleus were also enriched in SP-EP due to the fact that acetogenesis was accompanied by hydrogen production, which enhanced the hydrogenotrophic methanogenic pathway. This study provides a potential strategy for balancing the processes of acetogenesis and methanogenesis, providing a practical basis for the industrial application of methane production from FW. [ABSTRACT FROM AUTHOR]

The ancient origins of metabolism may be rooted deep in oceanic crust, and these early metabolisms may have persisted in the habitable thermal anoxic aquifer where conditions remain similar to those when they first appeared. The Wood–Ljungdahl pathway for acetogenesis is a key early biosynthetic pathway with the potential to influence ocean chemistry and productivity, but its contemporary role in oceanic crust is not well established. Here, we describe the genome of a novel acetogen from a thermal suboceanic aquifer olivine biofilm in the basaltic crust of the Juan de Fuca Ridge (JdFR) whose genome suggests it may utilize an ancient chemosynthetic lifestyle. This organism encodes the genes for the complete canonical Wood–Ljungdahl pathway, but is potentially unable to use sulfate and certain organic carbon sources such as lipids and carbohydrates to supplement its energy requirements, unlike other known acetogens. Instead, this organism may use peptides and amino acids for energy or as organic carbon sources. Additionally, genes involved in surface adhesion, the import of metallic cations found in Fe-bearing minerals, and use of molecular hydrogen, a product of serpentinization reactions between water and olivine, are prevalent within the genome. These adaptations are likely a reflection of local environmental micro-niches, where cells are adapted to life in biofilms using ancient chemosynthetic metabolisms dependent on H2 and iron minerals. Since this organism is phylogenetically distinct from a related acetogenic group of Clostridiales, we propose it as a new species, Candidatus Acetocimmeria pyornia. [ABSTRACT FROM AUTHOR]

Fen Schlöppnerbrunnen is a moderately acidic methane-emitting peatland overgrown by Molinia caerulea and other wetland graminoids (e.g. Carex rostrata). Recently, the accumulation of H2, an indicator for fermentation, was observed with anoxically incubated C. rostrata roots but not with root-free fen soil. Based on this finding, we hypothesized that root-derived organic carbon has a higher capacity to promote fermentation processes than peat organic carbon from root-free fen soil. To address this hypothesis, C. rostrata and M. caerulea roots were anoxically incubated with or without fen soil and the product profiles of root treatments were compared with those of root-free soil treatments. Ethanol, acetate, propionate, butyrate, H2 and CO2 accumulated in root treatments and collective amounts of carbon in accumulating products were 20–200 times higher than those in root-free soil treatments, in which mainly CO2 accumulated. Analyses of 16S rRNA and 16S rRNA gene sequences revealed that Clostridium , Propionispira and Rahnella , representatives of butyrate, propionate and mixed acid fermenters, respectively, were relatively enriched in root treatments. In contrast, differences of the microbial community before and after incubation were marginal in root-free soil treatments. Collectively, these findings supported the assumed stimulatory effect of root-derived organic carbon on fen fermenters. [ABSTRACT FROM AUTHOR]

The ancient origins of metabolism may be rooted deep in oceanic crust, and these early metabolisms may have persisted in the habitable thermal anoxic aquifer where conditions remain similar to those when they first appeared. The Wood–Ljungdahl pathway for acetogenesis is a key early biosynthetic pathway with the potential to influence ocean chemistry and productivity, but its contemporary role in oceanic crust is not well established. Here, we describe the genome of a novel acetogen from a thermal suboceanic aquifer olivine biofilm in the basaltic crust of the Juan de Fuca Ridge (JdFR) whose genome suggests it may utilize an ancient chemosynthetic lifestyle. This organism encodes the genes for the complete canonical Wood–Ljungdahl pathway, but is potentially unable to use sulfate and certain organic carbon sources such as lipids and carbohydrates to supplement its energy requirements, unlike other known acetogens. Instead, this organism may use peptides and amino acids for energy or as organic carbon sources. Additionally, genes involved in surface adhesion, the import of metallic cations found in Fe-bearing minerals, and use of molecular hydrogen, a product of serpentinization reactions between water and olivine, are prevalent within the genome. These adaptations are likely a reflection of local environmental micro-niches, where cells are adapted to life in biofilms using ancient chemosynthetic metabolisms dependent on H2 and iron minerals. Since this organism is phylogenetically distinct from a related acetogenic group of Clostridiales, we propose it as a new species, Candidatus Acetocimmeria pyornia.

ABSTRACT Acetogens synthesize acetyl-CoA via the CO2-fixing Wood-Ljungdahl pathway. Despite their ecological and biotechnological importance, their translational regulation of carbon and energy metabolisms remains unclear. Here, we report how carbon and energy metabolisms in the model acetogen Acetobacterium woodii are translationally controlled under different growth conditions. Data integration of genome-scale transcriptomic and translatomic analyses revealed that the acetogenesis genes, including those of the Wood-Ljungdahl pathway and energy metabolism, showed changes in translational efficiency under autotrophic growth conditions. In particular, genes encoding the Wood-Ljungdahl pathway are translated at similar levels to achieve efficient acetogenesis activity under autotrophic growth conditions, whereas genes encoding the carbonyl branch present increased translation levels in comparison to those for the methyl branch under heterotrophic growth conditions. The translation efficiency of genes in the pathways is differentially regulated by 5′ untranslated regions and ribosome-binding sequences under different growth conditions. Our findings provide potential strategies to optimize the metabolism of syngas-fermenting acetogenic bacteria for better productivity. IMPORTANCE Acetogens are capable of reducing CO2 to multicarbon compounds (e.g., ethanol or 2,3-butanediol) via the Wood-Ljungdahl pathway. Given that protein synthesis in bacteria is highly energy consuming, acetogens living at the thermodynamic limit of life are inevitably under translation control. Here, we dissect the translational regulation of carbon and energy metabolisms in the model acetogen Acetobacterium woodii under heterotrophic and autotrophic growth conditions. The latter may be experienced when acetogen is used as a cell factory that synthesizes products from CO2 during the gas fermentation process. We found that the methyl and carbonyl branches of the Wood-Ljungdahl pathway are activated at similar translation levels during autotrophic growth. Translation is mainly regulated by the 5′-untranslated-region structure and ribosome-binding-site sequence. This work reveals novel translational regulation for coping with autotrophic growth conditions and provides the systematic data set, including the transcriptome, translatome, and promoter/5′-untranslated-region bioparts.

The utilization of foul condensate (FC) collected from a Kraft pulp mill for the anaerobic production of volatile fatty acids (VFA) was tested in upflow anaerobic sludge blanket (UASB) reactors operated at 22, 37 and 55°C at a hydraulic retention time (HRT) of ∼75 h. The FC consisted mainly of 11370, 500 and 592 mg/L methanol, ethanol and acetone, respectively. 42–46% of the organic carbon (methanol, ethanol and acetone) was utilized in the UASB reactors operated at an organic loading of ∼8.6 gCOD/L.d and 52–70% of the utilized organic carbon was converted into VFA. Along with acetate, also propionate, isobutyrate, butyrate, isovalerate and valerate were produced from the FC. Prior to acetogenesis of FC, enrichment of the acetogenic biomass was carried out in the UASB reactors for 113 d by applying operational parameters that inhibit methanogenesis and induce acetogenesis. Activity tests after 158 d of reactor operation showed that the biomass from the 55°C UASB reactor exhibited the highest activity after the FC feed compared to the biomass from the reactors at 22 and 37°C. Activity tests at 37°C to compare FC utilization for CH4 versus VFA production showed that an organic carbon utilization >98% for CH4 production occurred in batch bottles, whereas the VFA production batch bottles showed 51% organic carbon utilization. Furthermore, higher concentrations of C3–C5 VFA were produced when FC was the substrate compared to synthetic methanol rich wastewater. [ABSTRACT FROM AUTHOR]

Background: During the acetogenic step of anaerobic digestion, the products of acidogenesis are oxidized to substrates for methanogenesis: hydrogen, carbon dioxide and acetate. Acetogenesis and methanogenesis are highly interconnected processes due to the syntrophic associations between acetogenic bacteria and hydrogenotrophic methanogens, allowing the whole process to become thermodynamically favorable. The aim of this study is to determine the influence of the dominant acidic products on the metabolic pathways of methane formation and to find a core microbiome and substrate-specific species in a mixed biogas-producing system. Results: Four methane-producing microbial communities were fed with artificial media having one dominant component, respectively, lactate, butyrate, propionate and acetate, for 896 days in 3.5-L Up-flow Anaerobic Sludge Blanket (UASB) bioreactors. All the microbial communities showed moderately different methane production and utilization of the substrates. Analyses of stable carbon isotope composition of the fermentation gas and the substrates showed differences in average values of δ13C(CH4) and δ13C(CO2) revealing that acetate and lactate strongly favored the acetotrophic pathway, while butyrate and propionate favored the hydrogenotrophic pathway of methane formation. Genome-centric metagenomic analysis recovered 234 Metagenome Assembled Genomes (MAGs), including 31 archaeal and 203 bacterial species, mostly unknown and uncultivable. MAGs accounted for 54%–67% of the entire microbial community (depending on the bioreactor) and evidenced that the microbiome is extremely complex in terms of the number of species. The core microbiome was composed of Methanothrix soehngenii (the most abundant), Methanoculleus sp., unknown Bacteroidales and Spirochaetaceae. Relative abundance analysis of all the samples revealed microbes having substrate preferences. Substrate-specific species were mostly unknown and not predominant in the microbial communities. Conclusions: In this experimental system, the dominant fermentation products subjected to methanogenesis moderately modified the final effect of bioreactor performance. At the molecular level, a different contribution of acetotrophic and hydrogenotrophic pathways for methane production, a very high level of new species recovered, and a moderate variability in microbial composition depending on substrate availability were evidenced. Propionate was not a factor ceasing methane production. All these findings are relevant because lactate, acetate, propionate and butyrate are the universal products of acidogenesis, regardless of feedstock. [ABSTRACT FROM AUTHOR]

The functional pathways of volatile fatty acids production at different biological hydrolysis temperatures and times were investigated in terms of microbial profile, network analysis, and phylogenetic investigation of communities by reconstruction of unobserved states. The principal component analysis result showed that biological hydrolysis at 35 °C and 42 °C were good for propionate production, and biological hydrolysis at 55 °C facilitated acetate production within 4.5 days of biological hydrolysis time but increased butyrate production with the biological hydrolysis time beyond 4.5 days. Network analysis revealed that the increased abundance of Chitinophagales and Flavobacteriales reduced the amount of propionate, butyrate, and iso-butyrate but increased the amount of acetate at biological hydrolysis 35 °C, 42 °C, and 55 °C. Moreover, compared to Clostridiales , Bacteroidales is more specific for other bacteria in the biological hydrolysis of waste activated sludge. Thirty-three enzymes were identified in the primary volatile fatty acid metabolism by phylogenetic investigation of communities by reconstruction of unobserved states. Acetate was produced dominantly through acetyl-CoA pathway and regulated by the functional genes atoA and frhG , and propionate was regulated by the functional gene sucD and acs via succinyl-CoA pathway; Butyrate was generated by enriching the functional genes ptb , buk , and atoA. These genes were regulated by biological hydrolysis temperatures, resulting in variations in the distribution of acetate, propionate, and butyrate under different temperature conditions. This study is significant for the optimization of biological hydrolysis for the pretreatment of waste activated sludge. [Display omitted] • Acetic acid was the dominant VFA produced in BH of WAS. • Propionic acid production decreased with BH temperature. • Chitinophagales and Flavobacteriales were associated with acetogenesis in BH. • Acetic acid production was regulated by genes atoA and frhG. • Propionic acid production was regulated by sucD , oadA , and acs. [ABSTRACT FROM AUTHOR]

Anaerobic fermentation technology enables the production of medium chain carboxylates and alcohols through microbial chain elongation. This involves steering reactor microbiomes to yield desired products, with CO 2 supply playing a crucial role in controlling ethanol-based chain elongation and facilitating various bioprocesses simultaneously. In the absence of CO 2 supply (Phase I), chain elongation predominantly led to n-caproate with a high selectivity of 96 Cmol%, albeit leaving approximately 80% of ethanol unconverted. During this phase, C. kluyveri and Proteiniphilum -related species dominated the reactors. In Phase II, with low CO 2 input (2.0 NmL L -1  min -1 ), formation of n-butyrate, butanol, and hexanol was stimulated. Increasing CO 2 doses in Phase III (6 NmL L -1  min -1 ) led to CO 2 utilization via homoacetogenesis, coinciding with the enrichment of Clostridium luticellarii , a bacterium that can use CO 2 as an electron acceptor. Lowering CO 2 dose to 0.5 NmL L -1  min -1 led to a shift in microbiome composition, diminishing the dominance of C. luticellarii while increasing C. kluyveri abundance. Additionally, other Clostridia , Proteiniphilum , and Lactobacillus sakei -related species became prevalent. This decrease in CO 2 load from 6 to 0.5 NmL L -1  min -1 minimized excessive ethanol oxidation from 30%-50% to 0%-3%, restoring a microbiome favoring net n-butyrate consumption and n-caproate production. The decreased ethanol oxidation coincided with the resurgence of hydrogen formation at partial pressures above 1%. High concentrations of butyrate, caproate, and ethanol in the reactor, along with low acetate concentration, promoted the formation of butanol and hexanol. It is evident that CO 2 supply is indispensable for controlling chain elongation in an open culture and it can be harnessed to stimulate higher alcohol formation or induce CO 2 utilization as an electron acceptor., Competing Interests: Author DL was employed by the ChainCraft B.V. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors declare that this study received funding from ChainCraft B.V. The funder had the following involvement in the study: study design, collection, analysis, interpretation of data and the writing of this article., (Copyright © 2024 de Leeuw, van Willigen, Vrauwdeunt and Strik.)

ABSTRACT A longstanding conundrum in Treponema pallidum biology concerns how the spirochete generates sufficient energy to fulfill its complex pathogenesis processes during human syphilitic infection. For decades, it has been assumed that the bacterium relies solely on glucose catabolism (via glycolysis) for generation of its ATP. However, the organism’s robust motility, believed to be essential for human tissue invasion and dissemination, would require abundant ATP likely not provided by the parsimony of glycolysis. As such, additional ATP generation, either via a chemiosmotic gradient, substrate-level phosphorylation, or both, likely exists in T. pallidum. Along these lines, we have hypothesized that T. pallidum exploits an acetogenic energy conservation pathway that relies on the redox chemistry of flavins. Central to this hypothesis is the apparent existence in T. pallidum of an acetogenic pathway for the conversion of d-lactate to acetate. Herein we have characterized the structural, biophysical, and biochemical properties of the first enzyme (d-lactate dehydrogenase [d-LDH]; TP0037) predicted in this pathway. Binding and enzymatic studies showed that recombinant TP0037 consumed d-lactate and NAD+ to produce pyruvate and NADH. The crystal structure of TP0037 revealed a fold similar to that of other d-acid dehydrogenases; residues in the cofactor-binding and active sites were homologous to those of other known d-LDHs. The crystal structure and solution biophysical experiments revealed the protein’s propensity to dimerize, akin to other d-LDHs. This study is the first to elucidate the enzymatic properties of T. pallidum’s d-LDH, thereby providing new compelling evidence for a flavin-dependent acetogenic energy conservation (ATP-generating) pathway in T. pallidum. IMPORTANCE Because T. pallidum lacks a Krebs cycle and the capability for oxidative phosphorylation, historically it has been difficult to reconcile how the syphilis spirochete generates sufficient ATP to fulfill its energy needs, particularly for its robust motility, solely from glycolysis. We have postulated the existence in T. pallidum of a flavin-dependent acetogenic energy conservation pathway that would generate additional ATP for T. pallidum bioenergetics. In the proposed acetogenic pathway, first d-lactate would be converted to pyruvate. Pyruvate would then be metabolized to acetate in three additional steps, with ATP being generated via substrate-level phosphorylation. This study provides structural, biochemical, and biophysical evidence for the first T. pallidum enzyme in the pathway (TP0037; d-lactate dehydrogenase) requisite for the conversion of d-lactate to pyruvate. The findings represent the first experimental evidence to support a role for an acetogenic energy conservation pathway that would contribute to nonglycolytic ATP production in T. pallidum.

ABSTRACT How microbial metabolism is translated into cellular reproduction under energy-limited settings below the seafloor over long timescales is poorly understood. Here, we show that microbial abundance increases an order of magnitude over a 5 million-year-long sequence in anoxic subseafloor clay of the abyssal North Atlantic Ocean. This increase in biomass correlated with an increased number of transcribed protein-encoding genes that included those involved in cytokinesis, demonstrating that active microbial reproduction outpaces cell death in these ancient sediments. Metagenomes, metatranscriptomes, and 16S rRNA gene sequencing all show that the actively reproducing community was dominated by the candidate phylum “Candidatus Atribacteria,” which exhibited patterns of gene expression consistent with fermentative, and potentially acetogenic, metabolism. “Ca. Atribacteria” dominated throughout the 8 million-year-old cored sequence, despite the detection limit for gene expression being reached in 5 million-year-old sediments. The subseafloor reproducing “Ca. Atribacteria” also expressed genes encoding a bacterial microcompartment that has potential to assist in secondary fermentation by recycling aldehydes and, thereby, harness additional power to reduce ferredoxin and NAD+. Expression of genes encoding the Rnf complex for generation of chemiosmotic ATP synthesis were also detected from the subseafloor “Ca. Atribacteria,” as well as the Wood-Ljungdahl pathway that could potentially have an anabolic or catabolic function. The correlation of this metabolism with cytokinesis gene expression and a net increase in biomass over the million-year-old sampled interval indicates that the “Ca. Atribacteria” can perform the necessary catabolic and anabolic functions necessary for cellular reproduction, even under energy limitation in millions-of-years-old anoxic sediments. IMPORTANCE The deep subseafloor sedimentary biosphere is one of the largest ecosystems on Earth, where microbes subsist under energy-limited conditions over long timescales. It remains poorly understood how mechanisms of microbial metabolism promote increased fitness in these settings. We discovered that the candidate bacterial phylum “Candidatus Atribacteria” dominated a deep-sea subseafloor ecosystem, where it exhibited increased transcription of genes associated with acetogenic fermentation and reproduction in million-year-old sediment. We attribute its improved fitness after burial in the seabed to its capabilities to derive energy from increasingly oxidized metabolites via a bacterial microcompartment and utilize a potentially reversible Wood-Ljungdahl pathway to help meet anabolic and catabolic requirements for growth. Our findings show that “Ca. Atribacteria” can perform all the necessary catabolic and anabolic functions necessary for cellular reproduction, even under energy limitation in anoxic sediments that are millions of years old.

Electroautotrophy is a novel and fascinating microbial metabolism, with tremendous potential for CO2 storage and valorization into chemicals and materials made thereof. Research attention has been devoted toward the characterization of acetogenic and methanogenic electroautotrophs. In contrast, here we characterize the electrophysiology of a sulfate-reducing bacterium, Desulfosporosinus orientis, harboring the Wood-Ljungdahl pathway and, thus, capable of fixing CO2 into acetyl-CoA. For most electroautotrophs the mode of electron uptake is still not fully clarified. Our electrochemical experiments at different polarization conditions and Fe0 corrosion tests point to a H2- mediated electron uptake ability of this strain. This observation is in line with the lack of outer membrane and periplasmic multi-heme c-type cytochromes in this bacterium. Maximum planktonic biomass production and a maximum sulfate reduction rate of 2 ± 0.4 mM day–1 were obtained with an applied cathode potential of −900 mV vs. Ag/AgCl, resulting in an electron recovery in sulfate reduction of 37 ± 1.4%. Anaerobic sulfate respiration is more thermodynamically favorable than acetogenesis. Nevertheless, D. orientis strains adapted to sulfate-limiting conditions, could be tuned to electrosynthetic production of up to 8 mM of acetate, which compares well with other electroacetogens. The yield per biomass was very similar to H2/CO2 based acetogenesis. Acetate bioelectrosynthesis was confirmed through stable isotope labeling experiments with Na-H13CO3. Our results highlight a great influence of the CO2 feeding strategy and start-up H2 level in the catholyte on planktonic biomass growth and acetate production. In serum bottles experiments, D. orientis also generated butyrate, which makes D. orientis even more attractive for bioelectrosynthesis application. A further optimization of these physiological pathways is needed to obtain electrosynthetic butyrate production in D. orientis biocathodes. This study expands the diversity of facultative autotrophs able to perform H2-mediated extracellular electron uptake in Bioelectrochemical Systems (BES). We characterized a sulfate-reducing and acetogenic bacterium, D. orientis, able to naturally produce acetate and butyrate from CO2 and H2. For any future bioprocess, the exploitation of planktonic growing electroautotrophs with H2-mediated electron uptake would allow for a better use of the entire liquid volume of the cathodic reactor and, thus, higher productivities and product yields from CO2-rich waste gas streams.

Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H2 dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H2 evolution reaction on the cathode or Fe(0) surface. Interestingly, low H2 partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H2 partial pressures by microbial H2 consumption, which thermodynamically favors the H2 evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H2 partial pressure affects the H2 evolution onset potential and the H2 evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H2 consumption characteristics, i.e., H2 threshold (thermodynamic limit for H2 consumption) and H2 consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H2 consumption characteristics. Different acetogenic strains are thus expected to maintain different H2 partial pressures on a cathode or Fe(0) surface, while those that maintain lower H2 partial pressures (lower H2 threshold, higher H2 affinity) more strongly increase the H2 evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H2 consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.

Acetogens have the ability to fixate carbon during fermentation by employing the Wood-Ljungdahl pathway (WLP), which is highly conserved across Bacteria and Archaea. In a previous study, product stoichometries in galacturonate-limited, anaerobic enrichment cultures of “Candidatus Galacturonibacter soehngenii,” from a novel genus within the Lachnospiraceae, suggested the simultaneous operation of a modified Entner-Doudoroff pathway for galacturonate fermentation and a WLP for acetogenesis. However, a draft metagenome-assembled genome (MAG) based on short reads did not reveal homologs of genes encoding a canonical WLP carbon-monoxide-dehydrogenase/acetyl-Coenzyme A synthase (CODH/ACS) complex. In this study, NaH13CO3 fed to chemostat-grown, galacturonate-limited enrichment cultures of “Ca. G. soehngenii” was shown to be incorporated into acetate. Preferential labeling of the carboxyl group of acetate was consistent with acetogenesis via a WLP in which the methyl group of acetate was predominately derived from formate. This interpretation was further supported by high transcript levels of a putative pyruvate-formate lyase gene and very low transcript levels of a candidate gene for formate dehydrogenase. Reassembly of the “Ca. G. soehngenii” MAG with support from long-read nanopore sequencing data produced a single-scaffold MAG, which confirmed the absence of canonical CODH/ACS-complex genes homologs. However, high CO-dehydrogenase activities were measured in cell extracts of “Ca. G. soehngenii” enrichment cultures, contradicting the absence of corresponding homologs in the MAG. Based on the highly conserved amino-acid motif associated with anaerobic Ni-CO dehydrogenase proteins, a novel candidate was identified which could be responsible for the observed activities. These results demonstrate operation of an acetogenic pathway, most probably as a yet unresolved variant of the Wood-Ljungdahl pathway, in anaerobic, galacturonate-limited cultures of “Ca. G. soehngenii.”

The thermophilic acetogen Thermacetogenium phaeum uses the Wood-Ljungdahl pathway (WLP) in both directions, either for the production of acetate from various compounds or for the oxidation of acetate in syntrophic cooperation with methanogens. In this study, energy–conserving enzyme systems in T. phaeum were investigated in both metabolic directions. A gene cluster containing a membrane-bound periplasmically oriented formate dehydrogenase directly adjacent to putative menaquinone synthesis genes was identified in the genome. The protein products of these genes were identified by total proteome analysis, and menaquinone MK-7 had been found earlier as the dominant quinone in the membrane. Enzyme assays with membrane preparations and anthraquinone-2,6-disulfonate as electron acceptor verified the presence of a quinone-dependent formate dehydrogenase. A quinone-dependent methylene–THF reductase is active in the soluble fraction and in the membrane fraction. From these results we conclude a reversed electron transport system from methyl-THF oxidation to CO2 reduction yielding formate as reduced product which is transferred to the methanogenic partner. The redox potential difference between methyl-THF (Eo’ = −200 mV) and formate (Eo’ = −432 mV) does not allow electron transfer through syntrophic formate removal alone. We postulate that part of the ATP conserved by substrate-level phosphorylation has to be invested into the generation of a transmembrane proton gradient by ATPase. This proton gradient could drive the endergonic oxidation of methyl-THF in an enzyme reaction similar to the membrane-bound reversed electron transport system previously observed in the syntrophically butyrate-oxidizing bacterium Syntrophomonas wolfei. To balance the overall ATP budget in acetate oxidation, we postulate that acetate is activated through an ATP-independent path via aldehyde:ferredoxin oxidoreductase (AOR) and subsequent oxidation of acetaldehyde to acetyl-CoA.

Methane production usually increases from the acidic sphagnum-dominated ombrotrophic peatlands to minerotrophic ones with more neutral pH and higher coverage of vascular plants. Along this ombrotrophic–minerotrophic gradient, pH, microbial communities, and properties of dissolved organic matter in porewater all vary greatly. The hydrographic change resulted from permafrost thaw and projected global warming can potentially connect the minerotrophic and ombrotrophic sites via porewater and turn acidic bogs to minerotrophic fens. It is thus very important to investigate how the anaerobic carbon degradation processes respond to changes in fundamental factors like pH, temperature, properties of dissolved organic matter, and microbial communities resulted from such hydrographic change. In this study, one ombrotrophic (pH = 3.9) and one minerotrophic peatland site were sampled in Fairbanks, Alaska in Sep 2017 and a 42-day-period anaerobic laboratory incubation was conducted to study the changes in anaerobic carbon degradation processes including primary and secondary fermentation, methanogenesis, and acetogenesis when pH, temperature, and porewater were manipulated individually and a combination of two or three of these factors. The results suggested lowering pH would inhibit many anaerobic carbon degradation processes in the minerotrophic peatland except primary fermentation. Elevating pH in the ombrotrophic site did not stimulate its methanogen community, but primary fermentation responded better with increasing pH than with increasing temperature alone. Replacing the porewater in the minerotrophic site with that from the ombrotrophic site with high aromaticity did not inhibit methanogenesis but potentially brought in highly efficient primary fermenters. Acetoclastic methanogenesis, acetogenesis, and syntrophy only exist in the minerotrophic site but not at the ombrotrophic one. Porewater from the minerotrophic site could potentially introduce acetoclastic methanogens and syntrophs to the ombrotrophic site but would not make them active unless both pH and temperature were increased. When ground water connects ombrotrophic and minerotrophic peatlands due to thawing of permafrost, secondary fermenters and acetoclastic methanogens could be introduced to acidic bogs and cooperate efficiently to degrade the stored carbon in ombrotrophic peatlands especially under elevated temperature conditions. [ABSTRACT FROM AUTHOR]

Acetogens have the ability to fixate carbon during fermentation by employing the Wood-Ljungdahl pathway (WLP), which is highly conserved across Bacteria and Archaea. In a previous study, product stoichometries in galacturonate-limited, anaerobic enrichment cultures of " Candidatus Galacturonibacter soehngenii," from a novel genus within the Lachnospiraceae , suggested the simultaneous operation of a modified Entner-Doudoroff pathway for galacturonate fermentation and a WLP for acetogenesis. However, a draft metagenome-assembled genome (MAG) based on short reads did not reveal homologs of genes encoding a canonical WLP carbon-monoxide-dehydrogenase/acetyl-Coenzyme A synthase (CODH/ACS) complex. In this study, NaH13CO3 fed to chemostat-grown, galacturonate-limited enrichment cultures of " Ca. G. soehngenii" was shown to be incorporated into acetate. Preferential labeling of the carboxyl group of acetate was consistent with acetogenesis via a WLP in which the methyl group of acetate was predominately derived from formate. This interpretation was further supported by high transcript levels of a putative pyruvate-formate lyase gene and very low transcript levels of a candidate gene for formate dehydrogenase. Reassembly of the " Ca. G. soehngenii" MAG with support from long-read nanopore sequencing data produced a single-scaffold MAG, which confirmed the absence of canonical CODH/ACS-complex genes homologs. However, high CO-dehydrogenase activities were measured in cell extracts of " Ca. G. soehngenii" enrichment cultures, contradicting the absence of corresponding homologs in the MAG. Based on the highly conserved amino-acid motif associated with anaerobic Ni-CO dehydrogenase proteins, a novel candidate was identified which could be responsible for the observed activities. These results demonstrate operation of an acetogenic pathway, most probably as a yet unresolved variant of the Wood-Ljungdahl pathway, in anaerobic, galacturonate-limited cultures of " Ca. G. soehngenii." [ABSTRACT FROM AUTHOR]

Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H2 dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H2 evolution reaction on the cathode or Fe(0) surface. Interestingly, low H2 partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H2 partial pressures by microbial H2 consumption, which thermodynamically favors the H2 evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H2 partial pressure affects the H2 evolution onset potential and the H2 evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H2 consumption characteristics, i.e., H2 threshold (thermodynamic limit for H2 consumption) and H2 consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H2 consumption characteristics. Different acetogenic strains are thus expected to maintain different H2 partial pressures on a cathode or Fe(0) surface, while those that maintain lower H2 partial pressures (lower H2 threshold, higher H2 affinity) more strongly increase the H2 evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H2 consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms. [ABSTRACT FROM AUTHOR]

The thermophilic acetogen Thermacetogenium phaeum uses the Wood-Ljungdahl pathway (WLP) in both directions, either for the production of acetate from various compounds or for the oxidation of acetate in syntrophic cooperation with methanogens. In this study, energy–conserving enzyme systems in T. phaeum were investigated in both metabolic directions. A gene cluster containing a membrane-bound periplasmically oriented formate dehydrogenase directly adjacent to putative menaquinone synthesis genes was identified in the genome. The protein products of these genes were identified by total proteome analysis, and menaquinone MK-7 had been found earlier as the dominant quinone in the membrane. Enzyme assays with membrane preparations and anthraquinone-2,6-disulfonate as electron acceptor verified the presence of a quinone-dependent formate dehydrogenase. A quinone-dependent methylene–THF reductase is active in the soluble fraction and in the membrane fraction. From these results we conclude a reversed electron transport system from methyl-THF oxidation to CO2 reduction yielding formate as reduced product which is transferred to the methanogenic partner. The redox potential difference between methyl-THF (Eo' = −200 mV) and formate (Eo' = −432 mV) does not allow electron transfer through syntrophic formate removal alone. We postulate that part of the ATP conserved by substrate-level phosphorylation has to be invested into the generation of a transmembrane proton gradient by ATPase. This proton gradient could drive the endergonic oxidation of methyl-THF in an enzyme reaction similar to the membrane-bound reversed electron transport system previously observed in the syntrophically butyrate-oxidizing bacterium Syntrophomonas wolfei. To balance the overall ATP budget in acetate oxidation, we postulate that acetate is activated through an ATP-independent path via aldehyde:ferredoxin oxidoreductase (AOR) and subsequent oxidation of acetaldehyde to acetyl-CoA. [ABSTRACT FROM AUTHOR]

Syngas fermentation has been successfully implemented in commercial-scale plants and can enable the biochemical conversion of the driest fractions of biomass through synthesis gas (H2, CO2, and CO). The process relies on optimized acetogenic strains able to reach and maintain high productivity of ethanol and acetate. In parallel, microbial communities have shown to be the best choice for the production of valuable medium-chain carboxylates through anaerobic fermentation of biomass, demanding low technical complexity and being able to realize simultaneous hydrolysis of the substrate. Each of the two technologies benefits from different strong points and has different challenges to overcome. This review discusses the rationales for merging these two seemingly disparate technologies by analyzing previous studies and drawing opinions based on the lessons learned from such studies. For keeping the technical demands of the resulting process low, a case is built for using microbial communities instead of pure strains. For that to occur, a shift from conventional syngas-based to "syngas-aided" anaerobic fermentation is suggested. Strategies for tackling the intricacies of working simultaneously with communities and syngas, such as competing pathways, and thermodynamic aspects are discussed as well as the stoichiometry and economic feasibility of the concept. Overall, syngas-aided anaerobic fermentation seems to be a promising concept for the biorefinery of the future. However, the effects of process parameters on microbial interactions have to be understood in greater detail, in order to achieve and sustain feasible medium-chain carboxylate and alcohol productivity. [ABSTRACT FROM AUTHOR]

The essential oils in citrus waste poses challenges for the anaerobic digestion application to energy recovery from these wastes due to the potential to inhibit the bioprocess. The two-stage system is a practical alternative to overcome these limitations of anaerobic co-digestion of citrus peel waste and wastewater. This study proposes to overcome drawbacks associated with the use of these substrates in anaerobic digestion by separating the phases, performing parameters optimization of the methanogenic stage. To obtain optimized conditions for the methanogenic stage, the Rotational Central Composite Design (RCCD) and Response Surface Methodology (RSM) were applied, evaluating the effects of diluting the Fermentation Reactor Content (FRC) of the first stage (35.7%–100%) and alkalinization (up to 0.60 g sodium bicarbonate.gCOD−1), in methane production and organic matter removal. Values from 2100.4 to 6320.6 mLCH 4.L−1 were obtained in the experimental design assays, with no inhibition identified by the high substrate concentration, nor by the limonene content (1.5–5.0 mg.L−1). Lower dilution of the fermentation reactor content (90%–100%) resulted in higher accumulated methane production, while the variation in the alkalinization did not imply significant effects on this response. The optimized condition for maximum production potential was obtained with 99.3% of fermentation reactor content and 0.58 gNaCHO 3.gCOD−1, reaching a production of 6549.7 mLCH 4.L−1, with organic matter removal (rCOD) of 87.1%, and 58.6% of reduction in solid waste content. For this condition, acetogenic metabolism predominance was observed, in the conversion of butyric acid and propionic acid into acetic acid. In the analysis of the microbial community of the optimized condition assay, a high relative abundance of hydrolytic and acidogenic bacteria (AUTHM297, Lachnospiraceae, and Hungateiclostridiaceae) was observed, as well as methanogenic archaea related to acetoclastic, hydrogenotrophic, and methylotrophic methanogenesis (Methanoregula, Methanolinea, Methanomassiliicoccus, Candidatus Methanomethylicus, and Methanofolis). [Display omitted] • Low dilution of the fermentation reactor content resulted in higher P. • No inhibitory effects on methanogenesis related to higher substrate concentrations. • The impact of limonene on methanogenesis was attenuated by its removal in the first stage. • Methanolinea and Methanoregula were identified in high abundance. • The energy potential obtained can supply the entire electricity demand of an industry. [ABSTRACT FROM AUTHOR]

Large emissions of atmospheric carbon dioxide (CO 2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO 2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO 2 to high–value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO 2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO 2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO 2 reduction. [Display omitted] • Influencing factors & regulation methods of microbial electrosynthesis are reviewed • Acetogenic bacteria play key roles in carbon dioxide reduction • Reactor configurations and gas supply are crucial for microbial electrosynthesis • Applications and future perspectives of microbial electrosynthesis are presented [ABSTRACT FROM AUTHOR]

Syngas (CO, CO 2 , H 2) was effectively bioconverted into lipids in a two-stage process. In the first stage, C 1 -gases were bioconverted into acetic acid by the acetogenic species Clostridium aceticum through the Wood-Ljungdahl metabolic pathway in a stirred tank bioreactor, reaching a maximum acetic acid concentration of 11.5 g/L, with a production rate of 0.05 g/L·h. Throughout this experiment, samples were extracted at different periods, i.e., different concentrations, to be used in the second stage, aiming at the production of lipids from acetic acid. The yeast Rhodosporidium toruloides , inoculated in the acetogenic medium, was able to efficiently accumulate lipids from acetic acid generated in the first stage. The best results, in terms of lipid content, dry biomass, biomass yield (Y (X/S)) and lipid yield (Y (L/S)) were 39.5% g/g dry cell weight, 3 g/L, 0.35 and 0.107, respectively. In terms of abundance, the lipid profile followed the order: C18:1 > C16:0 > C18:2 > C18:0 > Others. Experiments were also performed to determine the toxicity exerted by high concentrations of acetic acid on R. toruloides, resulting in inhibition at initial acid concentrations around 18 g/L leading to a higher lag phase and being lethal to the yeast at initial acetic acid concentrations around 22 g/L and above. This research paves the way for a novel method of growing oleaginous yeasts to produce sustainable biofuels from syngas or C 1 -pollutant gases. [Display omitted] • Efficient syngas valorization and lipid production by oleaginous yeasts is feasible. • A process allowing to yield sustainable biofuels from C 1 gases and syngas is shown. • Initial acetic acid concentrations above 18 g/L are inhibitory for R. toruloides. • 39.5% g/g lipid content was achieved at an acetic acid concentration of 11 g/L. • The lipid profile followed the order: C18:1 > C16:0 > C18:2 > C18:0 > Others. [ABSTRACT FROM AUTHOR]