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The uptake of dissolved organic compounds, that is, osmotrophy, has been shown to be an efficient nutritional strategy for algae. However, this mode of nutrition may affect the biochemical composition, for example, the fatty acid (FA) contents, of algal cells. This study focused on the osmotrophic assimilation of glucose and leucine by selected seven algal strains belonging to chlorophytes, chrysophytes, cryptophytes, dinoflagellates and euglenoids. Our laboratory experiments with stable isotope labeling showed that osmotrophy occurred in four of the selected seven strains. However, only three of these produced long chain omega-3 FAs eicosapentaenoic acid (EPA; 20:5ω3) and docosahexaenoic acid (DHA; 22:6ω3). High glucose content (5 mg L−1) affected negatively on the total FAs of Mallomonas kalinae and the total omega-3 FAs of Cryptomonas sp. Further, glucose assimilation explained 35% (negative effect) and leucine assimilation 48% (positive effect) of the variation of EPA, DHA and the FAs related to their synthesis in Cryptomonas sp. Moderate glucose concentration (2 mg L−1) was found to enhance the growth of Cryptomonas ozolinii, whereas low leucine (20 µg L−1) enhanced the growth of M. kalinae. However, no systematic effect of osmotrophy on growth rates was detected. Our study shows that osmotrophic assimilation of algae is species and compound specific, and that the effects of the assimilated compounds on algal metabolism also varies depending on the species.

Abstract In this study, Tisochrysis lutea was cultivated in mixotrophic and heterotrophic cultures with glycerol as a carbon source and with glucose and acetate for comparison; autotrophic cultivation was the control group without a carbon source. It was found that T. lutea used glycerol and did not use glucose and acetate under mixotrophy. Mixotrophy slightly elevated the docosahexaenoic acid (DHA) and total fatty acids (TFA) content in the dry-weight and enhanced the DHA and TFA production in medium (41.3 and 31.9% respectively) at the end of a 16-day cultivation, while heterotrophy reduced the DHA content and TFA production. Under the mixotrophy, the glycerol contribution to the DHA production (16.19 mg/L) and the TFA production (97.8 mg/L) was not very high and the DHA yield [2.63% chemical oxygen demand (COD)] and TFA yield (13.1% COD) were also very low. Furthermore, T. lutea using glycerol had a period of adaptation, indicating that T. lutea was not an ideal microalga for organic carbon utilization.

Current mixotrophic culture systems for Dunaliella salina have technical limitations to achieve high growth and productivity. The purpose of this study was to optimize the mixotrophic conditions imposed by glycerol, light, and salinity that lead to the highest biomass and β-carotene yields in D. salina. The combination of 12.5 mM glycerol, 3.0 M salinity, and 50 μmol photons m−2 s−1 light intensity enabled significant assimilation of glycerol by D. salina and consequently enhanced growth (2.1 × 106 cell mL−1) and β-carotene accumulation (4.43 pg cell−1). The saline and light shock induced the assimilation of glycerol by this microalga. At last stage of growth, the increase in light intensity (300 μmol photons m−2 s−1) caused the β-carotene to reach values higher than 30 pg cell−1 and tripled the β-carotene values obtained from photoautotrophic cultures using the same light intensity. Increasing the salt concentration from 1.5 to 3.0 M NaCl (non-isosmotic salinity) produced higher growth and microalgal β-carotene than the isosmotic salinity 3.0 M NaCl. The mixotrophic strategy developed in this work is evidenced in the metabolic capability of D. salina to use both photosynthesis and organic carbon, viz., glycerol that leads to higher biomass and β-carotene productivity than that of an either phototrophic or heterotrophic process alone. The findings provide insights into the key role of exogenous glycerol with a strategic combination of salinity and light, which evidenced unknown roles of this polyol other than that in osmoregulation, mainly on the growth, pigment accumulation, and carotenogenesis of D. salina.

We utilized the trait-based approach in a novel way to examine how specific phytoplankton traits are related to physical features connected to global change, water quality features connected to catchment change, and nutrient availability connected to nutrient loading. For the analyses, we used summertime monitoring data originating from the coastal northern Baltic Sea and generalized additive mixed modeling (GAMM). Of the physical features connected to global climate change, temperature was the most important affecting several studied traits. Nitrogen-fixing, buoyant, non-motile, and autotrophic phytoplankton, as well as harmful cyanobacteria, benefited from a higher temperature. Salinity and stratification did not have clear effects on the traits. Water transparency, which in the Baltic Sea is connected to catchment change, had a mostly negative relation to the studied traits. Harmfulness was negatively correlated with transparency, while the share of non-harmful and large-sized phytoplankton was positively related to it. We used nutrient loading source type and total phosphorus (TP) as proxies for nutrient availability connected to anthropogenic eutrophication. The nutrient loading source type did not relate to any of the traits. Our result showing that N-fixing was not related to TP is discussed. The regionality analysis demonstrated that traits should be calculated in both absolute terms (biomass) and proportions (share of total biomass) to get a better view of community changes and to potentially supplement the environmental status assessments.

In this work, white wine lees (WWL), cheese whey (CW), and glycerol (GLY) were used as carbon (C) sources to mixotrophically support the production of the microalga Nannochloropsis salina, replacing CO2 supply. In doing so, the alga was allowed to grow on C sources dosed at 2 g L−1, 3 g L−1, and 4 g L−1 of C, in the presence and absence of CO2 supply. WWL and CW were not able to support the algal growth due to a fungal contamination that was genomically identified, while GLY gave interesting results in particular with 3 g L−1 of C. GLY-C was able to replace CO2-C completely when the latter was omitted, showing an algal biomass production similar to those obtained in autotrophy. If CO2-C was provided jointly with GLY-C, biomass production and lipid contents increased more than 30% and 23%, respectively, compared to autotrophy. [ABSTRACT FROM AUTHOR]

Chlamydomonas reinhardtii P.A. Dangeard is a unicellular green alga capable to assimilate acetate. C. reinhardtii growth and metabolism distinctly depend on trophic conditions. Its influence on batch culture interferes with changes in the medium composition and metabolism of microalgae occurring during culture growth. The aim of this work was to estimate the effect of acetate on changes in the expression of 32 genes encoding enzymes of central metabolism and plastid transporters during growth of batch cultures of C. reinhardtii cc-124. In autotrophic conditions, transcription profiles considerably differed from the profiles in the presence of acetate. The strongest influence of trophic conditions was observed in the log phase of growth. In the presence of acetate, a more intense expression of gene ACS2 encoding plastid acetyl-CoA synthase and of the genes encoding subunits of enzymes directing acetyl groups to the synthesis of fatty acids was recorded. Elevated expression of genes PCK1 and PPT1 under mixotrophic conditions may be corresponded to the entry of acetate carbon to gluconeogenesis. In the presence of acetate, a high expression level of the starch metabolism genes was observed. Autotrophic conditions were notable for an elevated accumulation of transcripts of the genes encoding subunits of citrate lyase, which may be responsible for an outflow of acetyl groups from the Krebs cycle. Moreover, a higher level of expression was shown for genes of plastid transporters participating in the export of sugars from plastids, which is associated with the redistribution of reducing power in the cell. It was concluded that, on the level of transcription, similarity between mixotrophic and autotrophic cultures becomes more pronounced in the course of development. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Suetsugu et al. (2020), 227: 1519–1529. [ABSTRACT FROM AUTHOR]

Summary: Mycoheterotrophic plants depend entirely on fungal associations for organic nutrients. While most mycoheterotrophic plants are associated with the mycorrhizal partners of surrounding green plants, some mycoheterotrophs are believed to obtain carbon from decaying litter or dead wood by parasitising saprotrophic fungi, based on culture experiments and 13C and 15N isotopic signatures.The carbon age (the time since carbon was fixed from atmospheric CO2 by photosynthesis) can be estimated by measuring the concentration of 14C arising from the bomb tests of the 1950s and 1960s. Given that mycorrhizal fungi obtain photosynthate from their plant partners, and saprotrophic wood‐decaying fungi obtain carbon from older sources, radiocarbon could represent a new and powerful tool to investigate carbon sources of mycoheterotrophic plants.We showed that the Δ14C values of mycoheterotrophs exploiting ectomycorrhizal fungi were close to 0‰, similar to those of autotrophic plants. By contrast, the Δ14C values of mycoheterotrophs exploiting saprotrophic fungi ranged from 110.7‰ to 324.8‰, due to the 14C‐enriched bomb carbon from dead wood via saprotrophic fungi.Our study provides evidence supporting that some mycoheterotrophic orchids depend on forest woody debris. Our study also indicates that radiocarbon could be used to predict the trophic strategies of mycoheterotroph‐associated fungal symbionts. See also the Commentary on this article by Hatté et al., 227: 1284–1288. [ABSTRACT FROM AUTHOR]

Gymnodinium smaydae is a newly described mixotrophic dinoflagellate that feeds on only Heterocapsa spp. and Scrippsiella acuminata among 19 tested algal prey. It is one of the fastest growing dinoflagellates when feeding, but does not grow well without prey. To investigate its spatial-temporal distributions in Korean waters, we quantified its abundance in water samples that were seasonally collected from 28 stations along the Korean Peninsula from April 2015 to October 2018, using quantitative real-time polymerase chain reactions. This dinoflagellate had a wide distribution, as reflected by the detection of G. smaydae cells at 23 of the sampling stations. However, this distribution had a strong seasonality; it was detected at 21 stations in the summer and only one station in winter. The abundance of G. smaydae was significantly and positively correlated with chlorophyll a concentration as well as with water temperature. However, there were no significant correlations between the abundance of G. smaydae and salinity, concentrations of nutrients, or dissolved oxygen concentration. During the study period, G. smaydae was present when water temperatures were 7.6- 28.0°C, salinities were 9.6-34.1, concentrations of NO3 were not detectable-106.0 μM, and concentrations of PO4 were not detectable-3.4 μM. The highest abundance of G. smaydae was 18.5 cells mL-1 in the coastal waters of Jinhae in July 2017 when the chlorophyll a concentration was 127 mg m-3 and water temperature was 23.8°C. Therefore, the spatialtemporal distribution of G. smaydae in Korean coastal waters may be affected by chlorophyll a concentration and water temperature. [ABSTRACT FROM AUTHOR]

There is a growing demand for exploiting cost-effective carbon sources for mixotrophic cultivation of microalgae. Fucoidan and alginate were extracted from the brown seaweed Cystoseira trinodis and were subjected to hydrolysis using H 2 O, HCl, or NaOH for the mixotrophic cultivation of Dunaliella salina. The algal biomass was enhanced to 0.46 g L−1 and biomass productivity reached 41.83 mg L−1 day−1 using alkali hydrolyzed alginate, which was ∼1.6-fold higher than the values under the photoautotrophic conditions. Lipid content was increased to 21.81% w/w using alkali hydrolyzed fucoidan which was 1.78-times higher than the control. Several biodiesel quality parameters were calculated and indicated that mixotrophic cultivation enhanced the biodiesel properties. The biochemical composition showed that amino acid and insoluble and soluble protein productivities were promoted 2-3-folds in relation to the control. Similarly, glycerol and soluble carbohydrate (starch) productivities were increased ∼ 1.4-times, while insoluble carbohydrate productivity was 3.3-times higher than the control. Principal component analysis demonstrated that the starch content of D. salina was reduced in compensatory of lipid accumulation. In addition, the algal growth behavior was statistically fitted using modified logistic model which indicated that lag-phase and maximum growth rate were 0.75–3.89 day and 0.019–0.066 day−1, respectively. Image 1 • Fucoidan and alginate oligosaccharide enhanced the D. salina growth. • Alkali hydrolyzed alginate promoted the biodiesel characteristics. • NaOH-hydrolyzed alginate induced high biomass productivity (41.83 mg L−1 day−1). • Starch content was reduced in compensation of lipid accumulation. • High lipid production (7.39 mg L−1 day−1) was in H 2 O-hydrolyzed alginate medium. [ABSTRACT FROM AUTHOR]

Despite the benefits of phototrophy, many algae have lost photosynthesis and have converted back to heterotrophy. Parasitism is a heterotrophic strategy, with apicomplexans being among the most devastating parasites for humans. The presence of a nonphotosynthetic plastid in apicomplexan parasites suggests their phototrophic ancestry. The discovery of related phototrophic chromerids has unlocked the possibility to study the transition between phototrophy and parasitism in the Apicomplexa. The chromerid Chromera velia can live as an intracellular parasite in coral larvae as well as a free-living phototroph, combining phototrophy and parasitism in what I call photoparasitism. Since early-branching apicomplexans live extracellularly, their evolution from an intracellular symbiont is unlikely. In this opinion article I discuss possible evolutionary trajectories from an extracellular photoparasite to an obligatory apicomplexan parasite. Apicomplexan parasites evolved from a phototrophic ancestor. Parasitism evolved multiple times in the Apicomplexa and in Apicomplexa-like protists. Chromerid algae are the closest known phototrophic relatives of parasitic Apicomplexa. The chromerid Chromera velia is a free-living alga that can infect coral larvae and live like a parasite. [ABSTRACT FROM AUTHOR]

Mixotrophic algae that combine photoautotrophy with phagotrophy in a single cell are prevalent in marine ecosystems. Here, we assessed the ability of food ingestion in coccolithophores, an important group of calcifying haptophytes inhabiting the oceans. We tested four species from different coccolithophore lineages (Emiliania huxleyi, Calcidiscus leptoporus, Coccolithus braarudii, and Calyptrosphaera sphaeroidea). For both E. huxleyi and C. leptoporus we included different life phases (haploid and diploid). For C. braarudii we only tested diploid heterococcolithophore cells, while for C. sphaeroidea we only tested haploid holococcolithophore cells. Phagotrophy was assessed using fluorescently labeled bacteria (FLB) as model prey item, under nutrient‐replete and phosphate‐limited conditions. We detected by microscopy ingestion of FLB by all species, except the diploid C. braarudii strain. However, a previous study detected ingestion by haploid cells of C. braarudii. These overall results indicate that mixotrophy and the ability to ingest prey is widespread in coccolithophores. Yet, in all tested species the ingestion of FLB was low (<1% of the population contained prey at all time points over 2 days), namely for E. huxleyi and the diploid cells from C. leptoporus where detection of ingestion was sporadic. Moreover, no clear differences were detected between life phases in E. huxleyi and C. leptoporus under equal circumstances, or between replete and limited growth conditions. [ABSTRACT FROM AUTHOR]

Chrysolepidomonas dendrolepidota was described recently and little is known of its distribution, physiology or ecology. Although many photosynthetic chrysophytes have been identified as mixotrophic, combining photosynthesis and phagotrophy, this study provides the first evidence of mixotrophic nutrition in C. dendrolepidota. In a 2 × 2 factorial experiment with relatively high and low levels of light and nutrients, the ingestion rate was highest in the high-light, low-nutrient treatment. Growth rates and abundance increased in high- versus low-light conditions. In additional experiments with nitrogen and phosphorus modified separately, ingestion rate was different from the control only in the treatment with both macronutrients reduced. Supplementing low-nutrient treatments with additional bacteria led to an increase in algal abundance relative to controls without added bacteria. These results indicate that acquisition of major nutrients is a likely driver of mixotrophic nutrition in C. dendrolepidota. Although the distribution of C. dendrolepidota is unknown, mixotrophic chrysophytes can seasonally dominate planktonic algal abundance in some aquatic systems, and nutrient limitation is known to increase bacterivory in some of those species. Identifying and understanding how different mixotrophic species respond to changing environmental cues is crucial to understanding their roles in aquatic food webs. [ABSTRACT FROM AUTHOR]

Chloroflexales (Chloroflexi) are typical members of the anoxygenic photosynthesizing component of microbial mats and have mostly been characterized from communities associated to hot springs. Here, we report the assembly of five metagenome-assembled genomes (MAGs) of a novel lineage of Chloroflexales found in mesophilic lithifying microbial mats (microbialites) in Lake Alchichica (Mexico). Genomic and phylogenetic analyses revealed that the bins shared 92% of their genes, and these genes were nearly identical despite being assembled from samples collected along a depth gradient (1–15 m depth). We tentatively name this lineage Candidatus Lithoflexus mexicanus. Metabolic predictions based on the MAGs suggest that these chlorosome-lacking mixotrophs share features in central carbon metabolism, electron transport, and adaptations to life under oxic and anoxic conditions, with members of two related lineages, Chloroflexineae and Roseiflexineae. Contrasting with the other diverse microbialite community members, which display much lower genomic conservation along the depth gradient, Ca. L. mexicanus MAGs exhibit remarkable similarity. This might reflect a particular flexibility to acclimate to varying light conditions with depth or the capacity to occupy a very specific spatial ecological niche in microbialites from different depths. Alternatively, Ca. L. mexicanus may also have the ability to modulate its gene expression as a function of the local environmental conditions during diel cycles in microbialites along the depth gradient. [ABSTRACT FROM AUTHOR]

All orchids and pyroloids are mycoheterotrophic at least in the early stage. Many species are predisposed to mycoheterotrophic nutrition even in the adult stage, due to the initial mycoheterotrophy during germination. Although other green plants, such as gentian species, also produce numerous minute seeds, whose germination may depend on fungal associations to meet C demands, physiological evidence for partial mycoheterotrophy in the adult stage is lacking for most candidate taxa. Here, we compared the natural abundances of 13C and 15N isotopes in the AM-associated gentian species Pterygocalyx volubilis growing in high-light-intensity habitats with those of co-occurring autotrophic C3 and C4 plants and AM fungal spores. We found that P. volubilis was significantly enriched in 13C compared with the surrounding C3 plants, which suggests the transfer of some C from the surrounding autotrophic plants through shared AM networks. In addition, the intermediate δ15N values of P. volubilis, between those of autotrophic plants and AM fungal spores, provide further evidence for partial mycoheterotrophy in P. volubilis. Although it is often considered that light deficiency selects partial mycoheterotrophy, we show that partial mycoheterotrophy in AM-forming plants can evolve even under light-saturated conditions. The fact that there have been relatively few descriptions of partial mycoheterotrophy in AM plants may not necessarily reflect the rarity of such associations. In conclusion, partial mycoheterotrophy in AM plants may be more common than hitherto believed. [ABSTRACT FROM AUTHOR]

Trophic strategy determines stoichiometry of plankton. In general, heterotrophic zooplankton have lower and more stable C∶N and C∶P ratios than photoautotrophic phytoplankton, whereas mixotrophic protists, which consume prey and photosynthesize, have stoichiometry between zooplankton and phytoplankton. As trophic strategies change with cell size, body size may be a key trait influencing eukaryotic plankton stoichiometry. However, the relationship between body size and stoichiometry remains unclear. Here we measured plankton size-fractionated C∶N ratios under different intensities of light and nutrient supply in subtropical freshwater and marine systems. We found a unimodal body size–C∶N ratio pattern, with a maximum C∶N ratio at ∼50 μm diameter in marine and freshwater systems. Moreover, the variation in C∶N ratios is explained mainly by body size, followed by light intensity and nutrient concentration. To investigate the mechanisms behind this unimodal pattern, we constructed a size-based plankton food web model in which the trophic strategy and C∶N ratio are an emerging result. Our model simulations reproduce the unimodal pattern with a C∶N ratio of photoautotrophs ≤50 μm increasing with body size due to increase of photosynthetic carbon, whereas C∶N ratios of organisms >50 μm decrease with size due to decreasing photoautotrophic but increasing heterotrophic uptake. Based on our field observations and simulation, we extend the classic "light-nutrient" theory that determines plankton C∶N ratio to include body size and trophic strategy dependency. We conclude that body size and size-dependent uptake of resources (light, nutrients, and prey) determine plankton stoichiometry at various light and nutrient supplies. [ABSTRACT FROM AUTHOR]

Premise: Difficulties in comparing extremely divergent features in fully mycoheterotrophic plants with those in closely related chlorophyllous plants have complicated attempts to reveal the evolutionary patterns and processes of fully mycoheterotrophic plants. Albino mutants of partially mycoheterotrophic plants, generally observed in Orchidaceae, have provided an ideal model for investigating the evolution of mycoheterotrophy within similar genetic backgrounds. In 2018, we found a putative albino population of Pyrola (Ericaceae). Here we aimed to reveal the identity of the albino pyroloid and confirm its fully mycoheterotrophic status. Methods: To reveal the putative albino pyroloid's identity, we examined its morphology and sequenced its chloroplast DNA. In addition, we assessed the trophic status of the putative albino pyroloid by analyzing chlorophyll fluorescence, chlorophyll concentration, and natural 13C and 15N abundances. Results: We identified albino individuals as P. japonica—otherwise a partially mycoheterotrophic species. We confirmed their albino status by their considerably lower chlorophyll fluorescence and concentrations than those of sympatrically occurring chlorophyllous plants. 13C abundance in the albino individuals was significantly higher than in the green individuals of P. japonica. Conclusions: This first report of albino mutants from partially mycoheterotrophic species in angiosperms other than orchids will play a valuable role in further studies focused on mycoheterotrophy. For instance, their δ13C and δ15N values represent a reference for fully mycoheterotrophic plants in Pyrola. Our findings also indicate the strong dependence of some leafy Pyrola species on fungal C during their entire life cycle. [ABSTRACT FROM AUTHOR]

To investigate the spatio-temporal distributions of the mixotrophic dinoflagellate Yihiella yeosuensis in Korean coastal waters and its grazing impact on prey populations, water samples were seasonally collected from 28 stations in the East, West, and South Seas of Korea and Jeju Island from April 2015 to October 2018. The abundances of Y. yeosuensis in the water samples were quantified using quantitative real-time polymerase chain reaction (qPCR). Simultaneously, the physical and chemical properties of water from all sampled stations were determined, and the abundances of the optimal prey species of Y. yeosuensis, the prasinophyte Pyramimonas sp. and the cryptophyte Teleaulax amphioxeia, were quantified using qPCR. Y. yeosuensis has a wide distribution, as is reflected by the detection of Y. yeosuensis cells at 23 sampling stations; however, this distribution has a strong seasonality, which is indicated by its detection at 22 stations in summer but only one station in winter. The abundance of Y. yeosuensis was significantly and positively correlated with those of Pyramimonas sp. and T. amphioxeia, as well as with water temperature. The highest abundance of Y. yeosuensis was 48.5 cells mL-1 in Buan in July 2017, when the abundances of Pyramimonas sp. and T. amphioxeia were 917.6 and 210.4 cells mL-1, respectively. The growth rate of Y. yeosuensis on Pyramimonas sp., calculated by interpolating the growth rates at the same abundance, was 0.49 d-1, which is 37% of the maximum growth rate of Y. yeosuensis on Pyramimonas sp. obtained in the laboratory. Therefore, the field abundance of Pyramimonas sp. obtained in the present study can support a moderate positive growth of Y. yeosuensis. The maximum grazing coefficient for Y. yeosuensis on the co-occurring Pyramimonas sp. was 0.42 d-1, indicating that 35% of the Pyramimonas sp. population were consumed in 1 d. Therefore, the spatio-temporal distribution of Y. yeosuensis in Korean coastal waters may be affected by those of the optimal prey species and water temperature. Moreover, Y. yeosuensis may potentially have considerable grazing impacts on populations of Pyramimonas sp. [ABSTRACT FROM AUTHOR]

• The biomass and sequestration of Chlorella can be enhanced using new mixtrophic cultivation. • The cell quality under mixotrophic condition is better than those of photoautotrophic and photoheterotrophic cultures. • The metabolism of carbohydrates and amino acids were significantly enhanced under mixotrophic conditions. • This result of new mixtrophic culture was confirmed in a pilot open pond. CO 2 sequestration using microalgae has been deemed as a promising way, which can fix CO 2 and simultaneously produce valuable bio-products. However, large scale open-pond production of Chlorella vulgaris is normally cultured using acetic acid as a carbon source, which is costly and shows a low efficiency in CO 2 fixation. Here, a new mixotrophic culture strategy using both CO 2 and acetic acid was developed and evaluated for CO 2 sequestration by a C. vulgaris culture in an open pond. Our results show that the growth rate of C. vulgaris under this new mixotrophic condition reaches 0.24 g/L/d, which is higher than the 0.15 and 0.21 g/L/d of photoheterotrophic culture with acetic acids and photoautotrophic culture with CO 2 , respectively. The averaged CO 2 fixation rate was determined as 0.29 g/L/d, which is much higher than heterotrophic method but slightly lower than photoautotrophic method. This result was further confirmed in a 125 m2 open raceway pond. Physiological and biochemical characterization showed that the cell quality of C. vulgaris under mixotrophic conditions is better than those of photoautotrophic method and photoheterotrophic cultures. The enzyme activity assay and transcriptome sequencing analysis revealed that the metabolism of carbohydrates and amino acids was significantly enhanced under mixotrophic condition compared with other groups, which may attribute to the increased biomass and CO 2 sequestration of C. vulgaris. Our results suggest that this mixotrophic strategy can be applied in large-scale cultivation of C. vulgaris for biomass production and CO 2 sequestration. [ABSTRACT FROM AUTHOR]

Many marine microbial eukaryotes combine photosynthetic with phagotrophic nutrition, but incomplete understanding of such mixotrophic protists, their functional diversity, and underlying physiological mechanisms limits the assessment and modeling of their roles in present and future ocean ecosystems. We developed an experimental system to study responses of mixotrophic protists to availability of living prey and light, and used it to characterize contrasting physiological strategies in two stramenopiles in the genus Ochromonas. We show that oceanic isolate CCMP1393 is an obligate mixotroph, requiring both light and prey as complementary resources. Interdependence of photosynthesis and heterotrophy in CCMP1393 comprises a significant role of mitochondrial respiration in photosynthetic electron transport. In contrast, coastal isolate CCMP2951 is a facultative mixotroph that can substitute photosynthesis by phagotrophy and hence grow purely heterotrophically in darkness. In contrast to CCMP1393, CCMP2951 also exhibits a marked photoprotection response that integrates non‐photochemical quenching and mitochondrial respiration as electron sink for photosynthetically produced reducing equivalents. Facultative mixotrophs similar to CCMP2951 might be well adapted to variable environments, while obligate mixotrophs similar to CCMP1393 appear capable of resource efficient growth in oligotrophic ocean environments. Thus, the responses of these phylogenetically close protists to the availability of different resources reveals niche differentiation that influences impacts in food webs and leads to opposing carbon cycle roles. [ABSTRACT FROM AUTHOR]

Microalgae with high lipid productivity offer great promise as a potential source of biodiesel, an alternative to fossil diesel. Since a target of both high biomass yield and lipid content is difficult to attain with the classical photoautotrophic mode of nutrition, mixotrophy has evolved as a preferred mode for enhancement of growth in microalgae. In the present study, this strategy was explored extensively with six different exogenous carbon sources in eight different concentrations to investigate the effect of these carbon sources on the biomass and lipid production potential of the green alga Tetradesmus obliquus (SAG 276-3a). To further augment the effect of mixotrophy on the growth and lipid accumulation of the test microalga, selected carbon sources with their specific concentrations showing enhancement in both biomass yield and lipid accumulation in the first phase of the experiment were interacted with each other. Significant rise in biomass yield by twofold was seen in the case of 0.16% citrate + 0.08% bicarbonate interaction, whereas for lipid accumulation, maximum rise in lipid content (% dry cell wt.) by 2.3-fold and lipid yield (g L-1) by 4.5-fold was observed in the case of 0.16% acetate + 0.16% citrate-supplemented cultures. The biodiesel samples as analyzed using GC–MS were predominated with saturated and monounsaturated fatty acid methyl esters. Fuel property analysis of the biodiesel samples found them to be well suited to the Indian, American (ASTM), and European (EN) biodiesel standards. [ABSTRACT FROM AUTHOR]

The extremophilic red alga Galdieria partita is a facultative heterotroph that occupies mostly low-light microhabitats. However, the exceptional detection of abundant populations of G. partita in sunlight-exposed soil raises the possibility that exogenous organic carbon sources protect cells from photo-oxidative damage. The present study aimed to identify the photoprotective process activated by exogenous glucose under photo-oxidative stress. We demonstrated that exogenous glucose mitigated the photo-oxidative damage of cells exposed to 300 μmol photons m–2 s–1 photosynthetic active radiation. Photosynthesis carbon assimilation scarcely contributed to the cell growth in the presence of glucose, but the photosynthetic apparatus was nevertheless maintained and protected by glucose in a concentration-dependent manner. Supplementation of glucose increased expression of the L-gulonolactone oxidase gene essential for ascorbic acid biosynthesis, whereas no enhanced expression of the genes involved in carotenoid or tocopherol biosynthesis was observed. Under the photo-oxidative stress condition, the ascorbic acid content was strongly enhanced by exogenous glucose. We propose that the biosynthesis of ascorbic acid is one of the major photoprotective processes induced by exogenous glucose. The elucidation of how ascorbic acid is involved in scavenging reactive oxygen species provides key insights into the photoprotective mechanism in red algae. [ABSTRACT FROM AUTHOR]

The uptake of dissolved organic compounds, that is, osmotrophy, has been shown to be an efficient nutritional strategy for algae. However, this mode of nutrition may affect the biochemical composition, for example, the fatty acid (FA) contents, of algal cells. This study focused on the osmotrophic assimilation of glucose and leucine by selected seven algal strains belonging to chlorophytes, chrysophytes, cryptophytes, dinoflagellates and euglenoids. Our laboratory experiments with stable isotope labeling showed that osmotrophy occurred in four of the selected seven strains. However, only three of these produced long chain omega-3 FAs eicosapentaenoic acid (EPA; 20:5ω3) and docosahexaenoic acid (DHA; 22:6ω3). High glucose content (5 mg L-1) affected negatively on the total FAs of Mallomonas kalinae and the total omega-3 FAs of Cryptomonas sp. Further, glucose assimilation explained 35% (negative effect) and leucine assimilation 48% (positive effect) of the variation of EPA, DHA and the FAs related to their synthesis in Cryptomonas sp. Moderate glucose concentration (2 mg L-1) was found to enhance the growth of Cryptomonas ozolinii, whereas low leucine (20 μg L-1) enhanced the growth of M. kalinae. However, no systematic effect of osmotrophy on growth rates was detected. Our study shows that osmotrophic assimilation of algae is species and compound specific, and that the effects of the assimilated compounds on algal metabolism also varies depending on the species. [ABSTRACT FROM AUTHOR]

Clostridium formicoaceticum , a Gram-negative mixotrophic homoacetogen, produces acetic acid as the sole metabolic product from various carbon sources, including fructose, glycerol, formate, and CO 2. Its genome of 4.59-Mbp contains a highly conserved Wood-Ljungdahl pathway gene cluster with the same layout as that in other mixotrophic acetogens, including Clostridium aceticum , Clostridium carboxidivorans , and Clostridium ljungdahlii. For energy conservation, C. formicoaceticum does not have all the genes required for the synthesis of cytochrome or quinone used for generating proton gradient in H+-dependent acetogens such as Moorella thermoacetica ; instead, it has the Rnf system and a Na+-translocating ATPase similar to the one in Acetobacterium woodii. Its growth in both heterotrophic and autotrophic media were dependent on the sodium concentration. C. formicoaceticum has genes encoding acetaldehyde dehydrogenases, alcohol dehydrogenases, and aldehyde oxidoreductases, which could convert acetyl-CoA and acetate to ethanol and butyrate to butanol under excessive reducing equivalent conditions. • Complete genomic sequencing and analysis of mixotrophic acetogen C. formicoaceticum • Can use both organic and inorganic carbon sources for growth with acetate as the sole product • Has a highly conserved Wood-Ljungdahl pathway gene cluster for autotrophic growth • Uses a Na+-translocating ATPase and Rnf system for energy conversion • Can guide future metabolic engineering of this bacterium for biofuels and chemicals production [ABSTRACT FROM AUTHOR]

Mixotrophy in marine organisms is an important aspect of ecology and evolution. The discovery of mixotrophic abilities in phototrophic dinoflagellates alters our understanding of the dynamics of red tides. In the phototrophic dinoflagellate genus Alexandrium, some species are mixotrophic, but others are exclusively autotrophic. There are differences in the ecological roles of autotrophic and mixotrophic Alexandrium in marine food webs. However, of the 34 known Alexandrium species, the mixotrophic ability of >20 species has yet to be explored. In this study, the mixotrophic capabilities of Alexandrium insuetum CCMP2082, Alexandrium mediterraneum CCMP3433, Alexandrium pacificum CCMP3434, Alexandrium tamutum ATSH1609, and Alexandrium margalefii CAWD10 were investigated by providing each species with 22 diverse prey items including bacterium-sized microbeads (1 µm), the cyanobacterium Synechococcus sp., algal prey species, and the ciliate Mesodinium rubrum. None of the 5 Alexandrium species fed on any of the prey items. These results increase the number of Alexandrium species lacking mixotrophic abilities to 9, compared to the 7 known mixotrophic Alexandrium species. Furthermore, the Alexandrium phylogenetic tree based on the large subunit ribosomal DNA contained 3 large clades, each of which had species with and without mixotrophic abilities. Thus, the acquisition or loss of mixotrophic abilities in Alexandrium might readily occur. [ABSTRACT FROM AUTHOR]

Photosynthesis in eukaryotes first arose through phagocytotic processes wherein an engulfed cyanobacterium was not digested, but instead became a permanent organelle. Other photosynthetic lineages then arose when eukaryotic cells engulfed other already photosynthetic eukaryotic cells. Some of the resulting lineages subsequently lost their ability for phagocytosis, while many others maintained the ability to do both processes. These mixotrophic taxa have more complicated ecological roles, in that they are both primary producers and consumers that can shift more towards producing the organic matter that forms the base of aquatic food chains, or towards respiring and releasing CO2. We still have much to learn about which taxa are predatory mixotrophs as well as about the physiological consequences of this lifestyle, in part, because much of the diversity of unicellular eukaryotes in aquatic ecosystems remains uncultured. Here, we discuss existing methods for studying predatory mixotrophs, their individual biases, and how single-cell approaches can enhance knowledge of these important taxa. The question remains what the gold standard should be for assigning a mixotrophic status to ill-characterized or uncultured taxa—a status that dictates how organisms are incorporated into carbon cycle models and how their ecosystem roles may shift in future lakes and oceans. This article is part of a discussion meeting issue 'Single cell ecology'. [ABSTRACT FROM AUTHOR]

Reef‐building corals are mixotrophic organisms that can obtain nutrition from endosymbiotic microalgae (autotrophy) and particle capture (heterotrophy). Heterotrophic nutrition is highly beneficial to many corals, particularly in times of stress. Yet, the extent to which different coral species rely on heterotrophic nutrition remains largely unknown because it is challenging to quantify.We developed a quantitative approach to investigate coral nutrition using carbon isotope (δ13C) analysis of six essential amino acids (AAESS) in a common Indo‐Pacific coral (Pocillopora meandrina) from the fore reef habitat of Palmyra Atoll. We sampled particulate organic matter (POM) and zooplankton as the dominant heterotrophic food sources in addition to the coral host and endosymbionts. We also measured bulk tissue carbon (δ13C) and nitrogen (δ15N) isotope values of each sample type.Patterns among δ13C values of individual AAESS provided complete separation between the autotrophic (endosymbionts) and heterotrophic nutritional sources. In contrast, bulk tissue δ13C and δ15N values were highly variable across the putative food sources and among the coral and endosymbiont fractions, preventing accurate estimates of coral nutrition on Palmyra.We used linear discriminant analysis to quantify differences among patterns of AAESS δ13C values, or 'fingerprints', of the food resources available to corals. This allowed for the development of a quantitative continuum of coral nutrition that can identify the relative contribution of autotrophic and heterotopic nutrition to individual colonies. Our approach revealed exceptional variation in conspecific colonies at scales of metres to kilometres. On average, 41% of AAESS in P. meandrina on Palmyra are acquired via heterotrophy, but some colonies appear capable of obtaining the majority of AAESS from one source or the other.The use of AAESS δ13C fingerprinting analysis offers a significant improvement on the current methods for quantitatively assessing coral trophic ecology. We anticipate that this approach will facilitate studies of coral nutrition in the field, which are essential for comparing coral trophic ecology across taxa and multiple spatial scales. Such information will be critical for understanding the role of heterotrophic nutrition in coral resistance and/or resilience to ongoing environmental change. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

In this study, Arthrospira platensis was grown in the presence of different glycerol concentrations (0.5–9 g/L) under three light intensities (5, 10 and 15 Klux) in semi-continuous mode and under non-axenic conditions. The aim of this study was to investigate the growth performance, the biomass biochemical composition and any interactions between A. platensis and bacteria that would potentially grow as well on glycerol. The results here show that glycerol did not have any positive effect on biomass production of A. platensis. In contrast, it was observed that by increasing glycerol concentration the growth performance of A. platensis was restricted, while a gradual increase of bacteria population was observed, which apparently outcompeted and repressed A. platensis growth. Chlorophyll fluorescence measurements (Quantum Yields) revealed that glycerol was not an inhibiting factor per se of photosynthesis. On the other hand, cyanobacterial biomass grown on glycerol displayed a higher content in proteins and lipids. Especially, protein productivity was enhanced around 15–35% with the addition of glycerol compared to the control. In distinction, carbohydrate and photosynthetic pigments (phycocyanin and chlorophyll-α) content decreased with the increase of glycerol concentration. The results here suggest that A. platensis did not utilize glycerol for biomass production but most probably as metabolic energy carrier towards synthesis of proteins and lipids, which are more energy consuming metabolites compared to carbohydrates. The study revealed that the addition of glycerol at amounts of 0.5–1.5 g/L could be a strategy to improve protein productivity by A. platensis. [ABSTRACT FROM AUTHOR]

Microalgae are promising resources for the sustainable production of biofuels, feed, and high-value chemicals. Several strains can grow heterotrophically or mixotrophically on multiple organic substrates even if the high cost associated to their use can hinder scalability and economical sustainability of the overall process. The use of agri-food waste biomass hydrolysates might make the cultivation procedure more sustainable, while at the same time valorising underutilized by-products. In this study, Chlorella vulgaris biomass production and sugar utilization was investigated during mixotrophic cultivation on hydrolysates of two inexpensive and widely available recalcitrant agri-food waste biomasses: barley straw (BS) and citrus processing waste (CPW). CPW hydrolysate supported enhanced biomass production, compared to BS digestate, likely because of the presence, besides glucose, of significant amounts of galactose, which is rapidly metabolized by the algae. Notably, when pure monosaccharides were provided as sole organic carbon, growth stopped before complete sugar consumption. Arrested growth in presence of pure monosaccharides correlated with a drastic drop in extracellular pH, which appears to depend on both carbon and nitrogen sources. Our results show that mixotrophic cultivation of C. vulgaris on BS or CPW hydrolysates results in more efficient conversion of organic carbon into biomass, compared to growth on pure sugars, indicating that these agri-food by-products can be utilized as valid feedstocks for sustainable algal biomass production. • Agri-food waste digestates support C. vulgaris growth and biomass accumulation. • C. vulgaris uptakes biomass derived monosaccharides more efficiently than pure sugars. • C. vulgaris sugar metabolism is deeply influenced by the available nitrogen source. [ABSTRACT FROM AUTHOR]