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Photoautotrophic algal cultivation is challenging due to the shadowing effect produced by an increase in the number of cells; under such circumstances, mixotrophic growth might be an efficient alternative. We grew cultures of the autochthonous acidotolerant microalga Elliptochloris sp. on crude technical glycerin or glucose bubbled with either only air or air containing 2.5 % (v/v) CO2. We found that CO2 strongly influenced the production of Elliptochloris sp., as higher growth occurred in mixotrophy with CO2-enriched air compared to that with only air. Mixotrophy with CO2-enriched air allowed to reach higher biomass productivities and facilitated an increase in the relative abundance of saturated fatty acids. Nevertheless, mixotrophy with only air resulted in an increase of both saturated and unsaturated fatty acids, but biomass productivities were lower. These results, along with the limited biological contamination facilitated by low pH, suggest that this microalga might be attractive for large-scale production within the circular economy model. [ABSTRACT FROM AUTHOR]

Liquid byproducts and organic wastes generated from dairy processing units contribute as the largest source of industrial food wastewater. Though bacteria-mediated treatment strategies are largely implemented, a more effective and innovative management system is needed of the hour. Thus, the current study involves the cultivation of centric diatoms, Chaetoceros gracilis, and Thalassiosira weissflogii in simulated dairy wastewater (SDWW) formulated using varying amounts of milk powder with artificial seawater f/2 media (ASW). The results revealed that cell density and biomass productivity were highest in the 2.5% SDWW treatment cultures of both the strains, the maximum being in C. gracilis (7.5 × 106 cells mL − 1; 21.1 mg L−1 day−1). Conversely, the total carotenoid, chrysolaminarin, and phenol content were negatively impacted by SDWW. However, a considerable enhancement in the total lipid content was reported in the 2.5% SDWW culture of both species. Furthermore, the fatty acid profiling revealed that though the total polyunsaturated fatty acid (PUFA) content was highest in the control setups, the total mono polyunsaturated fatty acid (MUFA) content was higher in the 5% SDWW setups (30.66% in C. gracilis and 33.21% in T. weissflogii). In addition to it, in the cultures utilizing energy from external carbon sources provided by SDWW, the biodiesel produced was also enhanced owing to the heightened cetane number. Thus, the current study evidently highlights the organic carbon acquisition potential of marine diatoms with the scope of providing sustainable biorefinery. [ABSTRACT FROM AUTHOR]

Leptothrix ochracea creates distinctive iron-mineralized mats that carpet streams and wetlands. Easily recognized by its iron-mineralized sheaths, L. ochracea was one of the first microorganisms described in the 1800s. Yet it has never been isolated and does not have a complete genome sequence available, so key questions about its physiology remain unresolved. It is debated whether iron oxidation can be used for energy or growth and if L. ochracea is an autotroph, heterotroph, or mixotroph. To address these issues, we sampled L. ochracea-rich mats from three of its typical environments (a stream, wetlands, and a drainage channel) and reconstructed nine high-quality genomes of L. ochracea from metagenomes. These genomes contain iron oxidase genes cyc2 and mtoA, showing that L. ochracea has the potential to conserve energy from iron oxidation. Sox genes confer potential to oxidize sulfur for energy. There are genes for both carbon fixation (RuBisCO) and utilization of sugars and organic acids (acetate, lactate, and formate). In silico stoichiometric metabolic models further demonstrated the potential for growth using sugars and organic acids. Metatranscriptomes showed a high expression of genes for iron oxidation; aerobic respiration; and utilization of lactate, acetate, and sugars, as well as RuBisCO, supporting mixotrophic growth in the environment. In summary, our results suggest that L. ochracea has substantial metabolic flexibility. It is adapted to iron-rich, organic carbon-containing wetland niches, where it can thrive as a mixotrophic iron oxidizer by utilizing both iron oxidation and organics for energy generation and both inorganic and organic carbon for cell and sheath production. [ABSTRACT FROM AUTHOR]

Ciliates are a well‐defined, species‐rich, versatile group of heterotrophic and mixotrophic protists. Their response to increased temperature during global warming is critical for the structure and functioning of freshwater food webs.I conducted a meta‐analysis of the literature from field studies and experimental evidence to assess the parameters characterising the thermal response of freshwater ciliates. The shape of the thermal performance curve predicts the ciliates' survival at supraoptimal temperatures (i.e., the width of the thermal safety margin).The ciliates' typical thermal safety margin is c. 5°C. More than two‐thirds of the freshwater ciliates dwelling permanently or occasionally in the pelagial cannot survive at temperatures exceeding 30°C. Likewise, cold‐stenothermic species, which represent a significant fraction of euplanktonic ciliates, cannot survive by evolutionary adaptation to rapidly warming environments.The statistical analysis revealed that the ciliates' thermal performance is affected by their planktonic lifestyle (euplanktonic vs. tychoplanktonic), ability to form cysts and nutritional ecology. Bactivorous ciliates have the widest temperature niche, and algivorous ciliates have the narrowest temperature niche.Phenotypic plasticity and genetic variation, favouring the selection of pre‐adapted species in a new environment, are widespread among freshwater ciliates. However, the lack of evidence for the temperature optima and imprecisely defined tolerance limits of most species hampers the present analysis. The extent of acclimation and adaptation requires further research with more ciliate species than the few chosen thus far.Recent eco‐evolutionary experimental work and modelling approaches have demonstrated that the ciliates' thermal responses follow general trends predicted by the metabolic theory of ecology and mechanistic functions inherent in enzyme kinetics. The present analysis identified current knowledge gaps and avenues for future research that may serve as a model study for other biota. Thermal adaptation may conflict with adaptation to other stressors (predators, food availability, pH), making general predictions on the future role of freshwater ciliates in a warmer environment difficult, if not impossible, at present. [ABSTRACT FROM AUTHOR]

A novel Gram-stain-negative, facultatively anaerobic, and mixotrophic bacterium, designated as strain LZ166T, was isolated from the bathypelagic seawater in the western Pacific Ocean. The cells were short rod-shaped, oxidase- and catalase-positive, and motile by means of lateral flagella. The growth of strain LZ166T was observed at 10–45 °C (optimum 34–37 °C), at pH 5–10 (optimum 6–8), and in the presence of 0–5% NaCl (optimum 1–3%). A phylogenetic analysis based on the 16S rRNA gene showed that strain LZ166T shared the highest similarity (98.58%) with Aquibium oceanicum B7T and formed a distinct branch within the Aquibium genus. The genomic characterization, including average nucleotide identity (ANI, 90.73–76.79%), average amino identity (AAI, 88.50–79.03%), and digital DNA–DNA hybridization (dDDH, 36.1–22.2%) values between LZ166T and other species within the Aquibium genus, further substantiated its novelty. The genome of strain LZ166T was 6,119,659 bp in size with a 64.7 mol% DNA G+C content. The predominant fatty acid was summed feature 8 (C18:1ω7c and/or C18:1ω6c). The major polar lipids identified were diphosphatidylglycerol (DPG), phosphatidylethanolamine (PE), glycolipid (GL), and phosphatidylglycerol (PG), with ubiquinone-10 (Q-10) as the predominant respiratory quinone. The genomic annotation indicated the presence of genes for a diverse metabolic profile, including pathways for carbon fixation via the Calvin–Benson–Bassham cycle and inorganic sulfur oxidation. Based on the polyphasic taxonomic results, strain LZ166T represented a novel species of the genus Aquibium, for which the name Aquibium pacificus sp. nov. is proposed, with the type strain LZ166T (=MCCC M28807T = KACC 23148T = KCTC 82889T). [ABSTRACT FROM AUTHOR]

In this study, an Artificial Neural Network-Genetic Algorithm (ANN-GA) approach was successfully applied to optimise the physicochemical factors influencing the synthesis of unsaturated fatty acids (UFAs) in the microalgae P. kessleri UCM 001. The optimized model recommended specific cultivation conditions, including glucose at 29 g/L, NaNO3 at 2.4 g/L, K2HPO4 at 0.4 g/L, red LED light, an intensity of 1000 lx, and an 8:16-h light-dark cycle. Through ANN-GA optimisation, a remarkable 66.79% increase in UFAs production in P. kessleri UCM 001 was achieved, compared to previous studies. This underscores the potential of this technology for enhancing valuable lipid production. Sequential variations in the application of physicochemical factors during microalgae culture under mixotrophic conditions, as optimized by ANN-GA, induced alterations in UFAs production and composition in P. kessleri UCM 001. This suggests the feasibility of tailoring the lipid profile of microalgae to obtain specific lipids for diverse industrial applications. The microalgae were isolated from a high-mountain lake in Colombia, highlighting their adaptation to extreme conditions. This underscores their potential for sustainable lipid and biomaterial production. This study demonstrates the effectiveness of using ANN-GA technology to optimise UFAs production in microalgae, offering a promising avenue for obtaining valuable lipids. The microalgae's unique origin in a high-mountain environment in Colombia emphasises the importance of exploring and harnessing microbial resources in distinctive geographical regions for biotechnological applications.

In this work, microalgae cultivation trials were carried out in a membrane bioreactor to investigate fouling when the cultures of Chlorellavulgaris were grown under mixotrophic, heterotrophic, and phototrophic cultivation regimes. The Chlorella cultures were cultivated in wastewater as a source of nutrients that contained a high concentration of ammonium. In mixotrophic cultivation trials, the results showed that the elevated contents of carbohydrates in the soluble microbial product and proteins in extracellular polymeric substances probably initiated membrane fouling. In this case, the highest protein content was also found in extracellular polymeric substances due to the high nitrogen removal rate. Consequently, transmembrane pressure significantly increased compared to the phototrophic and heterotrophic regimes. The data indicated that cake resistance was the main cause of fouling in all cultivations. Higher protein content in the cake layer made the membrane surface more hydrophobic, while carbohydrates had the opposite effect. Compared to a mixotrophic culture, a phototrophic culture had a larger cell size and higher hydrophobicity, leading to less membrane fouling. Based on our previous data, the highest ammonia removal rate was reached in the mixotrophic cultures; nevertheless, membrane fouling appeared to be the fundamental problem. HIGHLIGHTS An algal membrane bioreactor was used for ammonium-rich wastewater treatment.; The highest fouling was in a mixotrophic culture and independent of nitrogen sources.; High nitrogen removal leads to an increase in extracellular substances and fouling.; Increased fouling rate in mixotrophic culture due to increased carbohydrates.; The inverse relationship between the particle size and the membrane fouling rate.;

Summary: Some cyanobacteria can grow photoautotrophically or photomixotrophically by using simultaneously CO2 and glucose. The switch between these trophic modes and the role of glycogen, their main carbon storage macromolecule, was investigated.We analysed the effect of glucose addition on the physiology, metabolic and photosynthetic state of Synechocystis sp. PCC 6803 and mutants lacking phosphoglucomutase and ADP‐glucose pyrophosphorylase, with limitations in glycogen synthesis.Glycogen acted as a metabolic buffer: glucose addition increased growth and glycogen reserves in the wild‐type (WT), but arrested growth in the glycogen synthesis mutants. Already 30 min after glucose addition, metabolites from the Calvin–Benson–Bassham cycle and the oxidative pentose phosphate shunt increased threefold more in the glycogen synthesis mutants than the WT. These alterations substantially affected the photosynthetic performance of the glycogen synthesis mutants, as O2 evolution and CO2 uptake were both impaired.We conclude that glycogen synthesis is essential during transitions to photomixotrophy to avoid metabolic imbalance that induces inhibition of electron transfer from PSII and subsequently accumulation of reactive oxygen species, loss of PSII core proteins, and cell death. Our study lays foundations for optimising photomixotrophy‐based biotechnologies through understanding the coordination of the crosstalk between photosynthetic electron transport and metabolism. [ABSTRACT FROM AUTHOR]

The outcome of species competition strongly depends on the traits of the competitors and associated trade‐offs, as well as on environmental variability. Here, we investigate the relevance of consumer trait variation for species coexistence in a ciliate consumer–microalgal prey system under fluctuating regimes of resource supply. We focus on consumer competition and feeding traits, and specifically on the consumer's ability to overcome periods of resource limitation by mixotrophy, that is, the ability of photosynthetic carbon fixation via algal symbionts in addition to phagotrophy. In a 48‐day chemostat experiment, we investigated competitive interactions of different heterotrophic and mixotrophic ciliates of the genera Euplotes and Coleps under different resource regimes, providing prey either continuously or in pulses under constant or fluctuating light, entailing periods of resource depletion in fluctuating environments, but overall providing the same amount of prey and light. Although ultimate competition results remained unaffected, population dynamics of mixotrophic and heterotrophic ciliates were significantly altered by resource supply mode. However, the effects differed among species combinations and changed over time. Whether mixotrophs or heterotrophs dominated in competition strongly depended on the genera of the competing species and thus, species‐specific differences in the minimum resource requirements that are associated with feeding on shared prey, nutrient uptake, light harvesting, and access to additional resources such as bacteria. Potential differences in the curvature of the species' resource‐dependent growth functions may have further mediated the species‐specific responses to the different resource supply modes. Overall, our study demonstrates that genus‐ or species‐specific traits other than that related to nutritional mode may override the relevance of acquired phototrophy by heterotrophs in competitive interactions, and that the potential advantage of photosynthetic carbon fixation of symbiont‐bearing mixotrophs in competition with pure heterotrophs may differ greatly among different mixotrophs, playing out under different environmental conditions and depending on the specific requirements of the species. Complex trophic interactions determine the outcome of competition, which can only be understood by taking on a multidimensional trait perspective. [ABSTRACT FROM AUTHOR]

Protist plankton can be divided into three main groups: phytoplankton, zooplankton, and mixoplankton. In situ methods for studying phytoplankton and zooplankton are relatively straightforward since they generally target chlorophyll/photosynthesis or grazing activity, while the integration of both processes within a single cell makes mixoplankton inherently challenging to study. As a result, we understand less about mixoplankton physiology and their role in food webs, biogeochemical cycling, and ecosystems compared to phytoplankton and zooplankton. In this paper, we posit that by merging conventional techniques, such as microscopy and physiological data, with innovative methods like in situ single-cell sorting and omics datasets, in conjunction with a diverse array of modeling approaches ranging from singlecell modeling to comprehensive Earth system models, we can propel mixoplankton research into the forefront of aquatic ecology. We present eight crucial research questions pertaining to mixoplankton and mixotrophy, and briefly outline a combination of existing methods and models that can be used to address each question. Our intent is to encourage more interdisciplinary research on mixoplankton, thereby expanding the scope of data acquisition and knowledge accumulation for this understudied yet critical component of aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Cryptophytes (class Cryptophyceae) are bi‐flagellated eukaryotic protists with mixed nutritional modes and cosmopolitan distribution in aquatic environments. Despite their ubiquitous presence, their molecular diversity is understudied in coastal waters. Weekly 18S rRNA gene amplicon sequencing at the Scripps Institution of Oceanography pier (La Jolla, California) in 2016 revealed 16 unique cryptophyte amplicon sequence variants (ASVs), with two dominant "clade 4" ASVs. The diversity of cryptophytes was lower than what is often seen in other phytoplankton taxa. One ASV represented a known Synechococcus grazer, while the other one appeared not to have cultured representatives and an unknown potential for mixotrophy. These two dominant ASVs were negatively correlated, suggesting possible niche differentiation. The cryptophyte population in nearby San Diego Bay was surveyed in 2019 and showed the increasing dominance of a different clade 4 ASV toward the back of the bay where conditions are warmer, saltier, and shallower relative to other areas in the bay. An ASV representing a potentially chromatically acclimating cryptophyte species also suggested that San Diego Bay exerts differing ecological selection pressures than nearby coastal waters. Cryptophyte and Synechococcus cell abundance at the SIO Pier from 2011 to 2017 showed that cryptophytes were consistently present and had a significant correlation with Synechococcus abundance, but no detectable seasonality. The demonstrated mixotrophy of some cryptophytes suggests that grazing on these and perhaps other bacteria is important for their ecological success. Using several assumptions, we calculated that cryptophytes could consume up to 44% (average 6%) of the Synechococcus population per day. This implies that cryptophytes could significantly influence Synechococcus abundance. [ABSTRACT FROM AUTHOR]

In this study, changes in the growth and biochemical parameters of Chlorella sorokiniana under mixotrophic culture conditions with 50 mM acetate and acetate supplemented with 0.25, 0.50 and 1 mM of urea were investigated. Acetate increased fresh (FW) and dry (DW) weights by 54 and 40%, respectively, compared with the control. Acetate treatment also increased the amount of chlorophyll a and carotenoids by 2.93 and 3.32 times, respectively, compared with the control, but decreased the amount of chlorophyll b, protein, and soluble sugars. There was a significant increase in growth and in the amount of metabolites analyzed when urea was used together with acetate. The most suitable concentration of urea for increasing FW and DW, chl a and b, and protein was 1 mM, for increasing carotenoid 0.25 mM and for increasing soluble sugars 0.50 mM. The best results in the simultaneous treatment with acetate and urea for each of the parameters analyzed were as follows: carotenoid 5 times, FW 2.1 times, DW 2.5 times, chl a 3.35 times, chl b 2 times, protein 1.4 times and soluble sugar 1.3 times increase compared with the control. These results showed that the simultaneous use of a carbon source (acetate) and a nitrogen source (urea) was more effective in increasing growth and the amount of metabolites. [ABSTRACT FROM AUTHOR]

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Purpose: Herein, we examined C. necator for its production of bioplastic under mixotrophic fermentation. The mixotrophic process utilized dual carbon sources of mixed CO2 and glucose for the production of PHB. Methods: C. necator was optimized through adaptive laboratory evolution under a mixed carbon sources of CO2 and glucose. The isolated mutant was then studied for its ability to co-utilize glucose and CO2 carbon sources for growth and for PHB production. Experimental design based on central composition design was implemented to optimize PHB production under mixotrophic fermentation. Parameters effecting PHB accumulation including CO2 and glucose substrate ratio, cell dosage and aeration were studied. Results: Under optimized mixotrophic batch process, 0.22 g/L of PHB and 28% PHB content was reached from mixed carbon sources. Further CO2 and glucose co-feeding strategy optimization in mixotrophic fed-batch, PHB titer was elevated to 0.41 g/L. Conclusion: Overall, this study offered a promising alternative for CO2 valorization through the mixotrophic conversion of CO2 and glucose to PHB by C. necator, which could provide basis in future Bio-CCU technology development for climate change mitigation. [ABSTRACT FROM AUTHOR]

Although the absence of normal leaves is often considered a sign of full heterotrophy, some plants remain at least partially autotrophic despite their leafless habit. Leafless orchids with green stems and capsules probably represent a late evolutionary stage toward full mycoheterotrophy and serve as valuable models for understanding the pathways leading to this nutritional strategy. In this study, based on molecular barcoding and isotopic analysis, we explored the physiological ecology of the leafless orchid Eulophia zollingeri, which displays green coloration, particularly during its fruiting phase. Although previous studies had shown that E. zollingeri, in its adult stage, is associated with Psathyrellaceae fungi and exhibits high 13C isotope signatures similar to fully mycoheterotrophic orchids, it remained uncertain whether this symbiotic relationship is consistent throughout the orchid's entire life cycle and whether the orchid relies exclusively on mycoheterotrophy for its nutrition during the fruiting season. Our study has demonstrated that E. zollingeri maintains a specialized symbiotic relationship with Psathyrellaceae fungi throughout all life stages. However, isotopic analysis and chlorophyll data have shown that the orchid also engages in photosynthesis to meet its carbon needs, particularly during the fruiting stage. This research constitutes the first discovery of partial mycoheterotrophy in leafless orchids associated with saprotrophic non-rhizoctonia fungi. [ABSTRACT FROM AUTHOR]

Phytoplankton are usually considered autotrophs, but an increasing number of studies show that many taxa are able also to use organic carbon. Acquiring nutrients and energy from different sources might enable an efficient uptake of required substances and provide a strategy to deal with varying resource availability, especially in highly dynamic ecosystems such as estuaries. In our study, we investigated the effects of 31 organic carbon sources on the growth (proxied by differences in cell counts after 24 h exposure) of 17 phytoplankton strains from the Elbe estuary spanning four functional groups. All of our strains were able to make use of at least 1 and up to 26 organic compounds for growth. Pico-sized green algae such as Mychonastes, as well as the nano-sized green alga Monoraphidium in particular were positively affected by a high variety of substances. Reduced light availability, typically appearing in turbid estuaries and similar habitats, resulted in an overall poorer ability to use organic substances for growth, indicating that organic carbon acquisition was not primarily a strategy to deal with darkness. Our results give further evidence for mixotrophy being a ubiquitous ability of phytoplankton and highlight the importance to consider this trophic strategy in research. [ABSTRACT FROM AUTHOR]

The treatment and utilization of cheese whey (W) represents one of the major challenges facing the dairy industry. In this work the production of Chlorella vulgaris and its potential use in recovering the main nutrients (N, P and C) from cheese whey was evaluated in batch and continuous mode. Four different treatments were evaluated: Fertilizers-control (100% freshwater plus chemical fertilizers); Fertilizers-2.5%W (v/v), Fertilizers-5%W (v/v) and Fertilizers-10%W (v/v). The biomass productivity was affected by the percentage of cheese whey added, resulting in a 2.8-fold increase when adding 10% cheese whey to the culture medium. In addition, the cheese whey concentration in the culture medium also affected the overall macromolecular composition of the biomass along with the appearance and growth of aerobic mesophilic bacteria. Despite the high concentration of organic matter in the cheese whey (COD, 60.000 mg L-1), C.vulgaris was able to recover more than 80% of COD in continuous mode, along with 94 % and 60% of TN and TP, respectively. These promising results can be explained by the mixotrophic activity of C.vulgaris, which was demonstrated by the decrease in the chlorophyll content when cheese whey was added to the culture medium and by photo-respirometric assays. The use of cheese whey for microalgal biomass production represents a sustainable alternative to revaluate this by-product stream and contribute to the circular economy. [ABSTRACT FROM AUTHOR]

BACKGROUND: Eicosapentaenoic acid (EPA, 20:5) is an essential fatty acid of the omega‐3 family and its importance for prevention and treatment of several disorders has been documented. Phaeodactylum tricornutum is a prominent candidate as an EPA producer. In several microalgal species the mixotrophic growth allows improvement of cell growth and EPA production, although P. tricornutum does not have glucose transporter proteins, limiting glucose assimilation. In the present study, the growth and EPA production of P. tricornutum UTEX 646 and its transformant T‐HUP*(8), in which the glucose transporter hup1 gene was expressed, was evaluated under mixotrophic cultivation using glucose as carbon source. RESULTS: Insertion of the gene hup1 in the T‐HUP*(8) strain allowed an increase in the glucose uptake of 4.4‐fold compared to the glucose consumption observed in the wild type strain. In T‐HUP*(8) the lipid content and productivity were 42% and 63% higher than in the wild type; these differences were reflected in the EPA concentration, which was enhanced up to twofold in cultivation of T‐HUP*(8) strain. The improvement observed in lipid and EPA production in the strain T‐HUP*(8) can be explained by increases of 2.97‐, 5.71‐, and 9.44‐fold in the relative expression of the genes acc1, me1, and ptd5, respectively. CONCLUSION: Increase in glucose consumption in cultivation of the P. tricornutum T‐HUP*(8) strain caused metabolic changes that increased the lipid content and EPA specific concentration, making this strain a good candidate for improved EPA production. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI). [ABSTRACT FROM AUTHOR]

Abstract The outcome of species competition strongly depends on the traits of the competitors and associated trade‐offs, as well as on environmental variability. Here, we investigate the relevance of consumer trait variation for species coexistence in a ciliate consumer–microalgal prey system under fluctuating regimes of resource supply. We focus on consumer competition and feeding traits, and specifically on the consumer's ability to overcome periods of resource limitation by mixotrophy, that is, the ability of photosynthetic carbon fixation via algal symbionts in addition to phagotrophy. In a 48‐day chemostat experiment, we investigated competitive interactions of different heterotrophic and mixotrophic ciliates of the genera Euplotes and Coleps under different resource regimes, providing prey either continuously or in pulses under constant or fluctuating light, entailing periods of resource depletion in fluctuating environments, but overall providing the same amount of prey and light. Although ultimate competition results remained unaffected, population dynamics of mixotrophic and heterotrophic ciliates were significantly altered by resource supply mode. However, the effects differed among species combinations and changed over time. Whether mixotrophs or heterotrophs dominated in competition strongly depended on the genera of the competing species and thus, species‐specific differences in the minimum resource requirements that are associated with feeding on shared prey, nutrient uptake, light harvesting, and access to additional resources such as bacteria. Potential differences in the curvature of the species' resource‐dependent growth functions may have further mediated the species‐specific responses to the different resource supply modes. Overall, our study demonstrates that genus‐ or species‐specific traits other than that related to nutritional mode may override the relevance of acquired phototrophy by heterotrophs in competitive interactions, and that the potential advantage of photosynthetic carbon fixation of symbiont‐bearing mixotrophs in competition with pure heterotrophs may differ greatly among different mixotrophs, playing out under different environmental conditions and depending on the specific requirements of the species. Complex trophic interactions determine the outcome of competition, which can only be understood by taking on a multidimensional trait perspective.

ABSTRACT Mixotrophy is an important trophic strategy for bacterial survival in the ocean. However, the global relevance and identity of the major mixotrophic taxa remain largely elusive. Here, we combined phylogenetic, metagenomic, and metatranscriptomic analyses to characterize ubiquitous Arcobacteraceae based on our deep-sea in situ incubations and the global data. The phylogenomic tree of Arcobacteraceae is divided into three large clades, among which members of clades A and B are almost all from terrestrial environments, while those of clade C are widely distributed in various marine habitats in addition to some terrestrial origins. All clades harbor genes putatively involved in chitin degradation, sulfide oxidation, hydrogen oxidation, thiosulfate oxidation, denitrification, dissimilatory nitrate reduction to ammonium, microaerophilic respiration, and metal (iron/manganese) reduction. Additionally, in clade C, more unique pathways were retrieved, including thiosulfate disproportionation, ethanol fermentation, methane oxidation, fatty acid oxidation, cobalamin synthesis, and dissimilatory reductions of sulfate, perchlorate, and arsenate. Within this clade, two mixotrophic Candidatus genera represented by UBA6211 and CAIJNA01 harbor genes putatively involved in the reverse tricarboxylic acid pathway for carbon fixation. Moreover, the metatranscriptomic data in deep-sea in situ incubations indicated that the latter genus is a mixotroph that conducts carbon fixation by coupling sulfur oxidation and denitrification and metabolizing organic matter. Furthermore, global metatranscriptomic data confirmed the ubiquitous distribution and global relevance of Arcobacteraceae in the expression of those corresponding genes across all oceanic regions and depths. Overall, these results highlight the contribution of previously unrecognized Arcobacteraceae to carbon, nitrogen, and sulfur cycling in global oceans.IMPORTANCEMarine microorganisms exert a profound influence on global carbon cycling and ecological relationships. Mixotrophy, characterized by the simultaneous utilization of both autotrophic and heterotrophic nutrition, has a significant impact on the global carbon cycling. This report characterizes a group of uncultivated bacteria Arcobacteraceae that thrived on the “hot time” of bulky particulate organic matter and exhibited mixotrophic strategy during the in situ organic mineralization. Compared with clades A and B, more unique metabolic pathways were retrieved in clade C, including the reverse tricarboxylic acid pathway for carbon fixation, thiosulfate disproportionation, methane oxidation, and fatty acid oxidation. Global metatranscriptomic data from the Tara Oceans expeditions confirmed the ubiquitous distribution and extensive transcriptional activity of Arcobacteraceae with the expression of genes putatively involved in carbon fixation, methane oxidation, multiple sulfur compound oxidation, and denitrification across all oceanic regions and depths.

Protist plankton can be divided into three main groups: phytoplankton, zooplankton, and mixoplankton. In situ methods for studying phytoplankton and zooplankton are relatively straightforward since they generally target chlorophyll/photosynthesis or grazing activity, while the integration of both processes within a single cell makes mixoplankton inherently challenging to study. As a result, we understand less about mixoplankton physiology and their role in food webs, biogeochemical cycling, and ecosystems compared to phytoplankton and zooplankton. In this paper, we posit that by merging conventional techniques, such as microscopy and physiological data, with innovative methods like in situ single-cell sorting and omics datasets, in conjunction with a diverse array of modeling approaches ranging from single-cell modeling to comprehensive Earth system models, we can propel mixoplankton research into the forefront of aquatic ecology. We present eight crucial research questions pertaining to mixoplankton and mixotrophy, and briefly outline a combination of existing methods and models that can be used to address each question. Our intent is to encourage more interdisciplinary research on mixoplankton, thereby expanding the scope of data acquisition and knowledge accumulation for this understudied yet critical component of aquatic ecosystems.

A novel Gram-stain-negative, facultatively anaerobic, and mixotrophic bacterium, designated as strain LZ166T, was isolated from the bathypelagic seawater in the western Pacific Ocean. The cells were short rod-shaped, oxidase- and catalase-positive, and motile by means of lateral flagella. The growth of strain LZ166T was observed at 10–45 °C (optimum 34–37 °C), at pH 5–10 (optimum 6–8), and in the presence of 0–5% NaCl (optimum 1–3%). A phylogenetic analysis based on the 16S rRNA gene showed that strain LZ166T shared the highest similarity (98.58%) with Aquibium oceanicum B7T and formed a distinct branch within the Aquibium genus. The genomic characterization, including average nucleotide identity (ANI, 90.73–76.79%), average amino identity (AAI, 88.50–79.03%), and digital DNA–DNA hybridization (dDDH, 36.1–22.2%) values between LZ166T and other species within the Aquibium genus, further substantiated its novelty. The genome of strain LZ166T was 6,119,659 bp in size with a 64.7 mol% DNA G+C content. The predominant fatty acid was summed feature 8 (C18:1ω7c and/or C18:1ω6c). The major polar lipids identified were diphosphatidylglycerol (DPG), phosphatidylethanolamine (PE), glycolipid (GL), and phosphatidylglycerol (PG), with ubiquinone-10 (Q-10) as the predominant respiratory quinone. The genomic annotation indicated the presence of genes for a diverse metabolic profile, including pathways for carbon fixation via the Calvin–Benson–Bassham cycle and inorganic sulfur oxidation. Based on the polyphasic taxonomic results, strain LZ166T represented a novel species of the genus Aquibium, for which the name Aquibium pacificus sp. nov. is proposed, with the type strain LZ166T (=MCCC M28807T = KACC 23148T = KCTC 82889T).

Ciliates have been recognized as one of the major components of the microbial food web, especially in ultra-oligotrophic waters, such as the Eastern Mediterranean Sea, where nutrients are scarce and the microbial community is dominated by pico- and nano-sized organisms. For this reason, ciliates play an important role in these ecosystems since they are the main planktonic grazers. Regardless the importance of these organisms, little is known about the community structure of heterotrophic and mixotrophic ciliates and how they are associated to their potential prey. In this study, we used 18S V4 rRNA gene metabarcoding to analyze ciliate community dynamics and how the relationship with potential prey changes according to different seasons and depths. Samples were collected seasonally at two stations of the Eastern Mediterranean Sea (HCB: coastal, M3A: offshore) from the surface and deep chlorophyll maximum (DCM) layers. The ciliate community structure varied across depths in HCB and across seasons in M3A, and the network analysis showed that in both stations, mixotrophic oligotrichs were positively associated with diatoms and showed few negative associations with ASVs annotated as marine Stramenopiles (MAST). On the other hand, heterotrophic tintinnids showed negative relationships in both HCB and M3A stations, mostly with Ochrophyta and Chlorophyta. These results showed, in first place that, although the two stations are close to each other, the ciliate dynamics differed between them. Moreover, mixotrophic and heterotrophic ciliates may have different ecological niches since mixotrophic ciliates may be more selective compared to heterotrophic species regarding their prey. These findings are the first glimpse into an understanding of the dynamics between heterotrophic and mixotrophic ciliates and their role inmicrobial assemblages and dynamics of ultra-oligotrophic environments. [ABSTRACT FROM AUTHOR]

The growth of the polyextremophile red microalga Galdieria sulphuraria ACUF 064 was evaluated under mixotrophy and heterotrophy in a 13-L lab-scale stirred photobioreactor, using buttermilk as a carbon source. Beforehand, G. sulphuraria ACUF 064 growth on glucose, galactose and lactose was evaluated. No significant differences were observed in terms of growth when lactose, glucose or galactose was used as a carbon source. Overall, the biomass yield on carbon was 70% higher in mixotrophy than heterotrophy for galactose (2.0 gx gC−1) and lactose (2.0 gx gC−1), while it was similar for glucose (1.5 gx gC−1). At the same time, the yield of biomass on nitrogen for cultures grown in lactose was the lowest in comparison to all the other tested substrates. This was also supported by a higher C-phycocyanin content, with 5.9% wC-PC/wx as the highest value in mixotrophy. A preliminary experiment in flasks under mixotrophic conditions with different buttermilk dilutions revealed that a dilution ratio of 40% v/v of buttermilk (corresponding to 2.0 gC L−1) was optimal for biomass production. Finally, G. sulphuraria ACUF 064 was grown in the optimal buttermilk dilution ratio in a 13 L photobioreactor. The highest biomass productivity was also obtained in mixotrophy (0.55 gx L−1 d−1), corresponding to a carbon removal of 61%. Overall, lactose-containing substrates such as buttermilk hold promise as a substrate for the growth of G. sulphuraria while revalorizing an industrial effluent. [ABSTRACT FROM AUTHOR]

Symbiotic microalgal–bacterial biofilms can be very attractive for potato wastewater treatment. Microalgae remove nitrogen and phosphorus and simultaneously produce the oxygen that is required for the aerobic, heterotrophic degradation of organic pollutants. In this study, symbiotic microalgal–bacterial biofilms were grown in flow cells with ammonium and phosphate, and with acetate as a simulated biodegradable organic pollutant. The symbiotic biofilms removed acetate without an external oxygen or carbon dioxide supply, but ammonium and phosphate could not be completely removed. The biofilm was shown to have a considerable heterotrophic denitrification capacity. The symbiotic relationship between microalgae and aerobic heterotrophs was proven by subsequently removing light and acetate. In both cases, this resulted in the cessation of the symbiosis and in increasing effluent concentrations of both acetate and the nutrients ammonium and phosphate. [ABSTRACT FROM AUTHOR]

ABSTRACTCyanobacteria fix carbon dioxide and release carbon-containing compounds into the wider ecosystem, yet they are sensitive to small metabolites that may impact their growth and physiology. Several cyanobacteria can grow mixotrophically, but we currently lack a molecular understanding of how specific nutrients may alter the compounds they release, limiting our knowledge of how environmental factors might impact primary producers and the ecosystems they support. In this study, we develop a high-throughput phytoplankton culturing platform and identify how the model cyanobacterium Synechocystis sp. PCC 6803 responds to nutrient supplementation. We assess growth responses to 32 nutrients at two concentrations, identifying 15 that are utilized mixotrophically. Seven nutrient sources significantly enhance growth, while 19 elicit negative growth responses at one or both concentrations. High-throughput exometabolomics indicates that oxidative stress limits Synechocystis’ growth but may be alleviated by antioxidant metabolites. Furthermore, glucose and valine induce strong changes in metabolite exudation in a possible effort to correct pathway imbalances or maintain intracellular elemental ratios. This study sheds light on the flexibility and limits of cyanobacterial physiology and metabolism, as well as how primary production and trophic food webs may be modulated by exogenous nutrients.IMPORTANCECyanobacteria capture and release carbon compounds to fuel microbial food webs, yet we lack a comprehensive understanding of how external nutrients modify their behavior and what they produce. We developed a high throughput culturing platform to evaluate how the model cyanobacterium Synechocystis sp. PCC 6803 responds to a broad panel of externally supplied nutrients. We found that growth may be enhanced by metabolites that protect against oxidative stress, and growth and exudate profiles are altered by metabolites that interfere with central carbon metabolism and elemental ratios. This work contributes a holistic perspective of the versatile response of Synechocystis to externally supplied nutrients, which may alter carbon flux into the wider ecosystem.

Symbiotic microalgal–bacterial biofilms can be very attractive for potato wastewater treatment. Microalgae remove nitrogen and phosphorus and simultaneously produce the oxygen that is required for the aerobic, heterotrophic degradation of organic pollutants. In this study, symbiotic microalgal–bacterial biofilms were grown in flow cells with ammonium and phosphate, and with acetate as a simulated biodegradable organic pollutant. The symbiotic biofilms removed acetate without an external oxygen or carbon dioxide supply, but ammonium and phosphate could not be completely removed. The biofilm was shown to have a considerable heterotrophic denitrification capacity. The symbiotic relationship between microalgae and aerobic heterotrophs was proven by subsequently removing light and acetate. In both cases, this resulted in the cessation of the symbiosis and in increasing effluent concentrations of both acetate and the nutrients ammonium and phosphate.

Abstract Background Diatoms have been viewed as ideal cell factories for production of some high-value bioactive metabolites, such as fucoxanthin, but their applications are restrained by limited biomass yield. Mixotrophy, by using both CO2 and organic carbon source, is believed effective to crack the bottleneck of biomass accumulation and achieve a sustainable bioproduct supply. Results Glycerol, among tested carbon sources, was proved as the sole that could significantly promote growth of Cylindrotheca sp. with illumination, a so-called growth pattern, mixotrophy. Biomass and fucoxanthin yields of Cylindrotheca sp., grown in medium with glycerol (2 g L−1), was increased by 52% and 29%, respectively, as compared to the autotrophic culture (control) without compromise in photosynthetic performance. As Cylindrotheca sp. was unable to use glycerol without light, a time-series transcriptomic analysis was carried out to elucidate the light regulation on glycerol utilization. Among the genes participating in glycerol utilization, GPDH1, TIM1 and GAPDH1, showed the highest dependence on light. Their expressions decreased dramatically when the alga was transferred from light into darkness. Despite the reduced glycerol uptake in the dark, expressions of genes associating with pyrimidine metabolism and DNA replication were upregulated when Cylindrotheca sp. was cultured mixotrophically. Comparative transcriptomic and metabolomic analyses revealed amino acids and aminoacyl-tRNA metabolisms were enhanced at different timepoints of diurnal cycles in mixotrophic Cylindrotheca sp., as compared to the control. Conclusions Conclusively, this study not only provides an alternative for large-scale cultivation of Cylindrotheca, but also pinpoints the limiting enzymes subject to further metabolic manipulation. Most importantly, the novel insights in this study should aid to understand the mechanism of biomass promotion in mixotrophic Cylindrotheca sp.

The effect of sugarcane molasses on lipid content and fatty acids methyl ester (FAME) profile by Chromochloris zofingiensis is investigated in this work. For this purpose, the strain has been cultivated under mixotrophic conditions in a medium amended with specific concentrations of molasses (0.5 g/L, 1 g/L, and 2 g/L) able to sustain microalgae growth. Better biomass concentration, lipid content, and lipid productivity (1.6 g/L, 38% wt, and 286 mg/L/day) than the control (1.32 g/L, 16% wt, and 139 mg/L/day) were obtained with 2 g/L of molasses. The highest value of the total lipid content (42% wt) was reached with 1 g/L of molasses. FAME profile revealed a 97% composition in C16-C18 with no statistically meaningful differences among the three concentrations of molasses tested. The most represented fatty acids were C18:1 oleic (> 38% wt), C18:2 linoleic (> 21% wt), and C16:0 palmitic (> 14% wt). When using 2 g/L of molasses unsaturated fatty acids reached the highest portion (77%) than the control (61%). Specifically, all FAMEs have been quantitatively and qualitatively analyzed in order to improve biodiesel properties. The main characteristics of biodiesel obtainable from the algal lipids appeared in compliance with ASTM standards for unblended biodiesel and fulfilled European regulations (EN 14,214 and EN 590) for the quality of biodiesel. Owing the compliance of the corresponding biodiesel characteristics with international standards, a profitable biodiesel can be obtained under mixotrophic conditions triggered by the use of a food industry waste. Highlights: Sugarcane molasses can be used by Chromochloris zofingiensis to produce lipids. Molasses enhanced better cell growth than control medium. Molasses triggered biomass and lipid productivity better than control medium. Internal cell metabolism was modified by molasses to improve lipid production. Saturation and unsaturation levels in FAMEs were directly influenced by molasses. [ABSTRACT FROM AUTHOR]

Context: Giving up photosynthetic function is a bold evolutionary step for a plant, yet the evolutionary transition from autotrophy to mycoheterotrophy has occurred dozens of times. Comparing plastome sequences of mycoheterotrophs and autotrophs has identified recurring patterns of gene loss throughout a range of land plants, though more comparisons are required to see how broadly these patterns apply across the diversity of terrestrial plants. Mycoheterotrophy is especially common in Orchidaceae, with more than 40 transitions from autotrophy to mycoheterotrophy in the family. Aims: We sought to test generalised patterns of plastome degradation seen in other mycoheterotrophs by comparing two species in the genus Dipodium (Orchidaceae); one species is photosynthetic and the other appears to be a facultative mycoheterotroph species. Methods: We sequenced and assembled the plastomes of two Dipodium species and compared the two genomes to identify gene degradation or loss. Results: The two plastomes were nearly identical, with no degradation of photosynthesis genes in the putative mycoheterotroph, and both species have undergone loss or pseudogenisation of all plastid ndh (nicotinamide adenine dinucleotide + hydrogen specific dehydrogenase) genes. Conclusions: These results contrast with most other comparisons between photosynthetic and likely mycoheterotrophic relatives, where rapid degradation in mycoheterotroph plastome genes is common, and may suggest the leafless Dipodium species are capable of photosynthesis and may be in the early stages of transitioning to a fully heterotrophic lifestyle. Implications: Further investigation of trophic transitions in Dipodium , including sequencing more plastomes and measuring photosynthetic capability of the putative heterotrophs, will yield insights into the evolution of plant lineages that lose the ability to photosynthesise. Some plant species rely on fungal symbioses for energy, rather than the sun; such plants typically follow a general pattern of gene loss as they transition away from being photosynthetic. We found that an Australian orchid species typically regarded as being fully reliant on the fungal symbiont, retains the genes required for photosynthesis. This species may be able to utilise both energy sources, or may rather be only in the early stages of transitioning to fungal reliance. [ABSTRACT FROM AUTHOR]

This study aimed to examine the impact of crude glycerol as the main carbon source on the growth, cell morphology, and production of high-value-added metabolites of two microalgal species, namely Chlorella vulgaris and Scenedesmus quadricauda, under heterotrophic and mixotrophic conditions, using monochromatic illumination from light-emitting diodes (LEDs) emitting blue, red, yellow, and white (control) light. The findings indicated that both microalgae strains exhibited higher biomass yield on the mixotrophic growth system when compared to the heterotrophic one, while S. quadricauda generally performed better than C. vulgaris. In mixotrophic mode, the use of different monochromatic illumination affected biomass production differently on both strains. In S. quadricauda, growth rate was higher under red light (μmax = 0.89 d−1), while the highest biomass concentration and yield per gram of consumed glycerol were achieved under yellow light, reaching 1.86 g/L and Yx/s = 0.18, respectively. On the other hand, C. vulgaris demonstrated a higher growth rate on blue light (μmax = 0.45 d−1) and a higher biomass production on white (control) lighting (1.34 g/L). Regarding the production of metabolites, higher yields were achieved during mixotrophic mode in both strains. In C. vulgaris, the highest lipid (26.5% of dry cell weight), protein (63%), and carbohydrate (20.3%) contents were obtained under blue, red, and yellow light, respectively, thus indicating that different light wavelengths probably activate different metabolic pathways. Similar results were obtained for S. quadricauda with red light leading to higher lipid content, while white lighting caused higher production of proteins and carbohydrates. Overall, the study demonstrated the potential of utilizing crude glycerol as a carbon source for the growth and metabolite production of microalgae and, furthermore, revealed that the strains' behavior varied depending on lighting conditions. [ABSTRACT FROM AUTHOR]

Ciliates have been recognized as one of the major components of the microbial food web, especially in ultra-oligotrophic waters, such as the Eastern Mediterranean Sea, where nutrients are scarce and the microbial community is dominated by pico- and nano-sized organisms. For this reason, ciliates play an important role in these ecosystems since they are the main planktonic grazers. Regardless the importance of these organisms, little is known about the community structure of heterotrophic and mixotrophic ciliates and how they are associated to their potential prey. In this study, we used 18S V4 rRNA gene metabarcoding to analyze ciliate community dynamics and how the relationship with potential prey changes according to different seasons and depths. Samples were collected seasonally at two stations of the Eastern Mediterranean Sea (HCB: coastal, M3A: offshore) from the surface and deep chlorophyll maximum (DCM) layers. The ciliate community structure varied across depths in HCB and across seasons in M3A, and the network analysis showed that in both stations, mixotrophic oligotrichs were positively associated with diatoms and showed few negative associations with ASVs annotated as marine Stramenopiles (MAST). On the other hand, heterotrophic tintinnids showed negative relationships in both HCB and M3A stations, mostly with Ochrophyta and Chlorophyta. These results showed, in first place that, although the two stations are close to each other, the ciliate dynamics differed between them. Moreover, mixotrophic and heterotrophic ciliates may have different ecological niches since mixotrophic ciliates may be more selective compared to heterotrophic species regarding their prey. These findings are the first glimpse into an understanding of the dynamics between heterotrophic and mixotrophic ciliates and their role in microbial assemblages and dynamics of ultra-oligotrophic environments. [ABSTRACT FROM AUTHOR]

Many phytoplankton taxa function on multiple trophic levels by combining photosynthesis and ingestion of bacteria, termed mixotrophy. Despite the recognition of mixotrophy as a universal functional trait, we have yet to fully resolve how environmental conditions influence community grazing rates in situ. A microcosm study was used to assess bacterivory by mixotrophic nanoflagellates following nutrient enrichment and light attenuation in a temperate lake. We found contrasting results based on assessment of mixotroph abundance or bacterivory. Despite an interactive effect of nutrient enrichment and light attenuation on mixotroph abundance, significant differences within light treatments were observed only after enrichment with P or N + P. The greatest abundance of mixotrophs across treatments occurred under co‐nutrient enrichment with full exposure to irradiance. However, bacterivory by mixotrophic nanoflagellates was greatest under shaded conditions after either N or P enrichment. We suggest that PAR availability dampened the stimulatory effect of nutrient limitation, and bacterivory supplemented a suboptimal photosynthetic environment. In a saturating light regime, the mixotrophic community was less driven to ingest bacteria because photosynthesis was able to satisfy energetic demands. These findings quantify community bacterivory in response to environmental drivers that may characterize future ecosystem conditions and highlight the importance of considering grazing rates in conjunction with abundance of mixotrophic protists. [ABSTRACT FROM AUTHOR]

This study investigated the mechanism of lactose assimilation in Nannochloropsis oceanica for dairy-wastewater bioremediation and co-production of valuable feed/food ingredients in a circular dairy system (β-galactosidase and omega-3 polyunsaturated fatty acids). Mixotrophic cultivation was found to be mandatory for lactose assimilation in N. oceanica, with biomass production in mixotrophic cultures reaching a fourfold increase over that under heterotrophic conditions. Under mixotrophic conditions, the microalgae were able to produce β-galactosidase enzyme to hydrolyse lactose, with maximum extracellular secretion recorded on day 8 of growth cycle at 41.47 ± 0.33 U gbiomass−1. No increase in the concentration of glucose or galactose was observed in the medium, confirming the ability of microalgae to indiscriminately absorb the resultant monosaccharides derived from lactose breakdown. Population analysis revealed that microalgae cells were able to maintain dominance in the mixotrophic culture, with bacteria accounting for < 12% of biomass. On the other hand, under heterotrophic conditions, native bacteria took over the culture (occupying over 95% of total biomass). The bacteria, however, were also unable to effectively assimilate lactose, resulting in limited biomass increase and negligible production of extracellular β-galactosidase. Results from the study indicate that N. oceanica can be effectively applied for onsite dairy wastewater treatment under strict mixotrophic conditions. This is commercially disadvantageous as it rules out the possibility of deploying heterotrophic fermentation with low-cost bioreactors and smaller areal footprint. [ABSTRACT FROM AUTHOR]

Microalgae cultivation using wastewater has been conducted to treat wastewater and lower carbon footprints. Wastewater can contribute to microalgae culture by supplying nutrients such as nitrogen, phosphorus, and organic carbon sources. Meanwhile, greenhouse gas emissions from fossil fuel consumption are also increasing rapidly, and global warming is intensifying. To overcome this phenomenon, Carbon Capture, Utilization, and Storage technology, a highly efficient greenhouse gas treatment technology that can capture and separate CO2, has been developed and comprehensively discussed in this review. As a technology for capturing carbon, microalgae are cultured by supplying high concentrations of CO2 using industrial flue gas. In addition, microalgae are used for removing organic substances, phosphorus, and nitrogen in wastewater. Through such biological treatment processes, it is possible to simultaneously reduce CO2, remove biocontaminants, and produce high-value-added substances. Recently, research has been conducted on promoting microalgae growth by supplying wastewater and flue gas by enabling mixotrophic cultivation of microalgae. The review finally provides that sewage and industrial flue gas have the potential to improve the productivity of microalgae, reduce cost, and encourage environmental sustainability. [ABSTRACT FROM AUTHOR]

The quality of water has a great impact on the life of aquatic species. The frequency of algal proliferation has increased all over the world due to changes in the water quality factors of the water systems. This study determined the correlation between Uroglena americana (U. americana) and water quality factors. U. americana mainly proliferated in late April at the surface level of the source water when the water temperature was in the range of 15.8–17.9 °C. Water temperature was negatively correlated with U. americana throughout the sampling period. In spring, the dissolved nitrogen (DN), dissolved organic carbon (DOC), fluorescence substance (peaks 1, 2, 3, 4, and 5), and electrical conductivity (EC) were positively correlated with the U. americana whereas the pH, dissolved oxygen (DO), oxidative reduction potential (ORP), turbidity, color, and dissolved phosphorus (DP) were negatively correlated. The total bacteria (16S rDNA) of source water showed a strong positive correlation (r = 0.66 at p = 0.05) with U. americana. The proliferation of U. americana also could be influenced by the associated microalgae species of source water. The Peridinium and Asterionella species were negatively correlated with U. americana, whereas the Euglena, Chlamydomonas, and Nitzchia species were positively correlated; among them, Euglena sp showed a significant positive correlation (r = 0.90, at p = 0.01) with U. americana. The obtained results of this study could be applied to the source water quality management and the optimization of the operating conditions of drinking water treatment plants to ensure the quality of drinking water. [ABSTRACT FROM AUTHOR]

Abstract Background Chlorella sorokiniana FZU60 is a promising lutein producing microalga. A mixotrophy/photoautotrophy two-stage strategy can achieve high biomass concentration at stage 1 and high lutein content at stage 2, leading to excellent lutein production efficiency in C. sorokiniana FZU60. However, the underlying molecular mechanisms are still unclear, restraining the further improvement of lutein production. Results In this study, physiological and biochemical analysis revealed that photochemical parameters (Fv/Fm and NPQ) and photosynthetic pigments contents increased during the shift from mixotrophy to photoautotrophy, indicating that photosynthesis and photoprotection enhanced. Furthermore, transcriptomic analysis revealed that the glyoxylate cycle and TCA cycle were suppressed after the shift to photoautotrophy, leading to a decreased cell growth rate. However, the gene expression levels of photosynthesis, CO2 fixation, autophagy, and lutein biosynthesis were upregulated at the photoautotrophy stage, demonstrating that microalgal cells could obtain more precursor to synthesize lutein for enhancing photosynthesis and reducing reactive oxygen species. Conclusions The findings help to elucidate the molecular mechanisms for high lutein production efficiency of C. sorokiniana FZU60 under the mixotrophy/photoautotrophy strategy, identify key functional genes responsible for lutein biosynthesis, and shed light on further improvement of lutein production by genetic or metabolic engineering in future studies.

Abstract Background Production of monocarboxylates using microbial communities is highly dependent on local and degradable biomass feedstocks. Syngas or different mixtures of H2, CO, and CO2 can be sourced from biomass gasification, excess renewable electricity, industrial off-gases, and carbon capture plants and co-fed to a fermenter to alleviate dependence on local biomass. To understand the effects of adding these gases during anaerobic fermentation of plant biomass, a series of batch experiments was carried out with different syngas compositions and corn silage (pH 6.0, 32 °C). Results Co-fermentation of syngas with corn silage increased the overall carboxylate yield per gram of volatile solids (VS) by up to 29% (0.47 ± 0.07 g gVS −1; in comparison to 0.37 ± 0.02 g gVS −1 with a N2/CO2 headspace), despite slowing down biomass degradation. Ethylene and CO exerted a synergistic effect in preventing methanogenesis, leading to net carbon fixation. Less than 12% of the electrons were misrouted to CH4 when either 15 kPa CO or 5 kPa CO + 1.5 kPa ethylene was used. CO increased the selectivity to acetate and propionate, which accounted for 85% (electron equivalents) of all products at 49 kPa CO, by favoring lactic acid bacteria and actinobacteria over n-butyrate and n-caproate producers. Inhibition of n-butyrate and n-caproate production by CO happened even when an inoculum preacclimatized to syngas and lactate was used. Intriguingly, the effect of CO on n-butyrate and n-caproate production was reversed when formate was present in the broth. Conclusions The concept of co-fermenting syngas and plant biomass shows promise in three aspects: by making anaerobic fermentation a carbon-fixing process, by increasing the yields of short-chain carboxylates (propionate and acetate), and by minimizing electron losses to CH4. Moreover, a model was proposed for how formate can alleviate CO inhibition in certain acidogenic bacteria. Testing the fermentation of syngas and plant biomass in a continuous process could potentially improve selectivity to n-butyrate and n-caproate by enriching chain-elongating bacteria adapted to CO and complex biomass.

Mountain lakes provide a variety of ecosystem services (ES) such as water, habitat, recreation and aesthetic values. However, global change pressures increasingly affect lake ecosystems, necessitating suitable indicators to monitor changes in ES so that targeted protection measures can be developed. Limnological data in ES assessments are mostly limited to transparency for evaluating recreational and aesthetic values, but to date, limnological parameters related to plankton community composition have rarely been associated with ES. This study therefore aims to link limnological parameters to potential ES provision using widely applied standard parameters as well as metabarcoding based on 16S/18S rDNA high-throughput sequencing (HTS) to obtain information on phytoplankton communities. We collected 10 limnological parameters of 26 mountain lakes in the European Alps (1,489 to 2,922 m a.s.L.) and related them to six ES identified by local stakeholders. Our results generally indicate low to moderate concentrations of total phosphorus and chlorophyll a and high Secchi depths. Therefore, the trophic state of the majority of lakes was classified as ultra- (n = 6) or oligotrophic (n = 18), while two lakes were assigned to oligo-mesotrophic conditions. The phytoplankton functional groups were generally in agreement with the trophic state, i.e., we observed a decreasing relative abundance of mixotrophic vs. phototrophic phytoplankton with increasing trophic state. With regard to the ES, our results indicate that most lakes had high potential in providing multiple ES. Only six of the 26 lakes were inhabited by larger cladocerans, indicating reduced food availability for newts and thus less potential for the maintenance of populations and habitats. The high transparency of most lakes indicated a high potential for aesthetic and recreational values, whereas a few lakes with a greater proportion (>30%) of phototrophic phytoplankton manifested reduced water clarity, probably due to eutrophication. Our findings suggest that a deterioration of ecological conditions in mountain lakes may negatively affect future ES provision. Therefore, it will become increasingly important to protect pristine lakes, with data on plankton community composition potentially providing useful guidance for deciding which lakes should receive priority for protection measures.

Orchid mycorrhiza forms unique symbiotic associations between members of the Orchidaceae and multiple ecological guilds of fungi. Because orchids associate with a wide variety of fungi with different ecological functions, they represent an ideal study system to address fundamental questions about the evolution and ecophysiology of mycorrhizal symbiosis. Although it is well established that shifts in mycorrhizal associations are linked to transitions in plant trophic mode, it remains unclear what ecological drivers promote these evolutionary changes.Here, we investigated mycorrhizal communities and isotope signatures across six populations of the terrestrial orchid Neottia ovata growing under contrasting light conditions in temperate Europe. We hypothesized that plants growing in forests would associate with different mycorrhizal fungi than plants occurring in grasslands and that the limited light availability in forests leads to a higher contribution of fungi to the carbon budget of orchids.Our results showed that N. ovata predominantly associated with rhizoctonia fungi of the family Serendipitaceae in both habitats, but plants in forests also recruited ectomycorrhizal fungi. Root communities highly resembled soil communities and variation in root communities was significantly related to habitat type and edaphic factors. In contrast, isotope signatures (13C, 15N, 2H and 18O) and N concentration showed no significant relationship with habitat type. In addition, both 13C and 2H were not significantly correlated to habitat's light availability.Although it has been suggested that the presence of a wide variety of ectomycorrhizal fungi in root communities of orchids can serve as a precursor for evolutionary shifts to partial mycoheterotrophy (mixotrophy), the presence or absence of ectomycorrhizal fungi did not substantially influence the isotope signatures of N. ovata. These results indicate that rhizoctonia fungi played the major functional role in C and nutrient supply and that ectomycorrhizal fungi did not substantially contribute to the carbon budget of the plants. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Mixotrophs are ubiquitous and integral to microbial food webs, but their impacts on the dynamics and functioning of broader ecosystems are largely unresolved.Here, we show that mixotrophy produces a unique type of food web module that exhibits unusual ecological dynamics, with surprising consequences for carbon flux under warming. We develop a generalizable model of a mixotrophic food web module that incorporates dynamic switching between phototrophy and phagotrophy to assess ecological dynamics and total system CO2 flux.We find that warming switches mixotrophic systems between alternative stable carbon states—including a phototrophy‐dominant carbon sink state, a phagotrophy‐dominant carbon source state and cycling between these two. Moreover, warming always shifts this mixotrophic system from a carbon sink state to a carbon source state, but a coordinated increase in nutrients can erase early warning signals of this transition and expand hysteresis.This suggests that mixotrophs can generate critical carbon tipping points under warming that will be more abrupt and less reversible when combined with increased nutrient levels, having widespread implications for ecosystem functioning in the face of rapid global change. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Phago-mixotrophy, the combination of photoautotrophy and phagotrophy in mixoplankton, organisms that can combine both trophic strategies, have gained increasing attention over the past decade. It is now recognized that a substantial number of protistan plankton species engage in phago-mixotrophy to obtain nutrients for growth and reproduction under a range of environmental conditions. Unfortunately, our current understanding of mixoplankton in aquatic systems significantly lags behind our understanding of zooplankton and phytoplankton, limiting our ability to fully comprehend the role of mixoplankton (and phago-mixotrophy) in the plankton food web and biogeochemical cycling. Here, we put forward five research directions that we believe will lead to major advancement in the field: (i) evolution: understanding mixotrophy in the context of the evolutionary transition from phagotrophy to photoautotrophy; (ii) traits and trade-offs: identifying the key traits and trade-offs constraining mixotrophic metabolisms; (iii) biogeography: large-scale patterns of mixoplankton distribution; (iv) biogeochemistry and trophic transfer: understanding mixoplankton as conduits of nutrients and energy; and (v) in situ methods: improving the identification of in situ mixoplankton and their phago-mixotrophic activity. [ABSTRACT FROM AUTHOR]