Mixotrophic Algae Research Articles

Toxic effects of the emerging Alexandrium pseudogonyaulax (Dinophyceae) on multiple trophic levels of the pelagic food web

The dinoflagellate Alexandrium pseudogonyaulax, a harmful algal bloom species, is currently appearing in increasing frequency and abundance across Northern European waters, displacing other Alexandrium species. This mixotrophic alga produces goniodomins (GDs) and bioactive extracellular substances (BECs) that may pose a threat to coastal ecosystems and other marine resources. This study demonstrated the adverse effects of A. pseudogonyaulax on four marine trophic levels, including microalgae (Rhodomonas salina), microzooplankton (Polykrikos kofoidii) and mesozooplankton (Acartia tonsa), as well as fish gill cells (RTgill-W1, Oncorhynchus mykiss), ultimately leading to enhanced mortality and cell lysis. Furthermore, cell-free supernatants collected from A. pseudogonyaulax cultures caused complete loss of metabolic activity in the RTgill-W1 cell line, indicating ichthyotoxic properties, while all tested GDs were much less toxic. In addition, cell-free supernatants of A. pseudogonyaulax led to cell lysis of R. salina, while all tested GDs were non-lytic. Finally, reduced egg hatching rates of A. tonsa eggs exposed to cell-free supernatants of A. pseudogonyaulax and impaired mobility of P. kofoidii and A. tonsa exposed to A. pseudogonyaulax were also observed. Altogether, bioassay results suggest that the toxicity of A. pseudogonyaulax is mainly driven by BECs and not by GDs, although further research into factors modulating the lytic activity of Alexandrium spp. are needed.

Investigating the role of hydrological connectivity on the processing of organic carbon in tropical aquatic ecosystems

Inland waters are highways of carbon and nutrient flows between the land and ocean. Aquatic environments integrate multiple sources and processes over space and time that influence ecosystem functionality. The complexity of these systems and their multiple interactions with the surrounding environment are conceptualised, but often lack empirical scrutiny that allows further understanding of how inland waters mobilise, transport, and utilise carbon and nutrients. This is particularly evident in tropical waters. Here, we apply advanced geochemical analyses of dissolved organic matter (DOM) composition in conjunction with algal pigment biomarkers, to determine the seasonal variability of organic matter production, processing and export for a tropical, floodpulse wetland, Tasik Chini (Malaysia). We identify two phases in the hydrological cycle: Phase 1 signifying a transition from the wet season with high suspended sediment and dissolved organic carbon concentrations. DOM is composed of humic substances, building blocks and lower molecular weight compounds. Towards the end this phase then are periods of increased water clarity and algal productivity. This is followed by Phase 2, which has a greater contribution of autochthonous DOM, composed of proteinaceous material, concomitant with lower dissolved nutrient concentrations, increased mixotrophic algae and emergent vegetation. Based on this framework, we highlight the role of such tropical wetland lakes as hydrological “bottlenecks,” through a lentic/lotic switch that shifts aquatic transport of carbon and nutrients from lateral river continuum supply to flood pulses. We highlight the need to consider inherent biases of spatial and temporal scaling when examining freshwater ecosystems along the land-ocean aquatic continuum.

Mixotrophic culture of Chaetoceros sp. and the synergistic carbon and energy metabolism

This research studied the metabolic mechanism of the mixotrophic Chaetoceros sp. The results showed this alga had the highest cell density and growth rate of 47.72 × 105 cells mL−1 and 0.41 d-1, respectively, with a maximum dry weight of 2.90 g/L, when compared to photoautotrophic and photoheterotrophic modes. Compared to photoheterotrophy, transcriptomics results showed the Rubisco, PGK, and GAPDH related genes were separately up-regulated by 1.03, 2.36, and 1.36 times in CBB cycle in mixotrophic mode, suggesting intermediate metabolites of EMP and PPP can enter the chloroplast via transporter proteins, or membrane permeation, and feedback inhibition regulates the reduction of multiple reactions in CBB cycle. Chaetoceros sp. achieves high biomass by utilizing ATP and carbon structures from EMP and PPP pathways, and the addition of NaHCO3 leads to an up-regulation of CBB cycle for the mixotrophic alga, resulting in higher biomass compared to the photoheterotrophic mode.

The second life of terrestrial and plastic carbon as nutritionally valuable food for aquatic consumers.

Primary production is the basis for energy and biomolecule flow in food webs. Nutritional importance of terrestrial and plastic carbon via mixotrophic algae to upper trophic level is poorly studied. We explored this question by analysing the contribution of osmo- and phagomixotrophic species in boreal lakes and used 13 C-labelled materials and compound-specific isotopes to determine biochemical fate of carbon backbone of leaves, lignin-hemicellulose and polystyrene at four-trophic level experiment. Microbes prepared similar amounts of amino acids from leaves and lignin, but four times more membrane lipids from lignin than leaves, and much less from polystyrene. Mixotrophic algae (Cryptomonas sp.) upgraded simple fatty acids to essential omega-3 and omega-6 polyunsaturated fatty acids. Labelled amino and fatty acids became integral parts of cell membranes of zooplankton (Daphnia magna) and fish (Danio rerio). These results show that terrestrial and plastic carbon can provide backbones for essential biomolecules of mixotrophic algae and consumers at higher trophic levels.

Experimental additions of allochthonous dissolved organic matter reveal multiple trophic pathways to stimulate planktonic food webs

Abstract There is still considerable debate as to whether the allochthonous (terrestrial) dissolved organic matter (DOM) mobilised during large flow events plays an important role in supporting secondary production in riverine food webs. Understanding how food webs respond to large pulses of terrestrial DOM (tDOM) is important for conceptualising the relative importance of energy sources supporting food webs. A mesocosm experiment (1,000 L) using three concentrations of leachate (1, 4, 8 mg C/L and a control) made from floodplain DOM was run for 34 days in a dam filled with water from Gunbower Creek in Northern Victoria, Australia. Nutrients, phytoplankton, zooplankton and stable isotope (δ13C signatures) data were collected to examine how floodplain nutrients affected growth and community structure within the lower food web typical of an Australian lowland river. All leachate additions led to very high concentrations of zooplankton and mixotrophic algae compared to the control. Mixotrophs dominated the algal biovolume of all leachate additions until Day (D)20 and appeared to drive changes in δ13C signatures of particulate organic matter (POM) which were significantly related to changes in zooplankton δ13C signatures. tDOM additions did not significantly suppress obligate autotroph growth which also appeared important as a food source, as reflected in the δ13C signatures of zooplankton and POM after D10. These results show the ability of phytoplankton and zooplankton communities in lowland rivers to respond quickly to changes in resource availability and quality. Mixotrophs appeared to provide an important trophic link between allochthonous carbon and primary consumers, and increased complimentarily to autotrophic production. This resulted in large net increases to phytoplankton biovolume and potentially played a significant role in driving changes in zooplankton growth. We suggest that allochthonous DOM may be highly bioavailable and support production through several different trophic pathways, offering a large boost to production via both autotrophy and heterotrophy. Furthermore, we contend that mixotrophy may be an important pathway for allochthonous organic matter to enter riverine food webs and support secondary production.

Metabolic plasticity of mixotrophic algae is key for their persistence in browning environments

Light availability is the main regulator of primary production, shaping photosynthetic communities and their production of ecologically important biomolecules. In freshwater ecosystems, increasing dissolved organic carbon (DOC) concentrations, commonly known as browning, leads to lower light availability and the proliferation of mixotrophic phytoplankton. Here, a mixotrophic algal species (Cryptomonas sp.) was grown under five increasing DOC concentrations to uncover the plastic responses behind the success of mixotrophs in browning environments and their effect in the availability of nutritionally important biomolecules. In addition to the browning treatments, phototrophic, heterotrophic and mixotrophic growth conditions were used as controls. Despite reduced light availability, browning did not impair algal growth compared to phototrophic conditions. Comparative transcriptomics showed that genes related to photosynthesis were down‐regulated, whereas phagotrophy gene categories (phagosome, lysosome and endocytosis) were up‐regulated along the browning gradient. Stable isotope analysis of phospholipid fractions validated these results, highlighting that the studied mixotroph increases its reliance on heterotrophic processes with browning. Metabolic pathway reconstruction using transcriptomic data suggests that organic carbon is acquired through phagotrophy and used to provide energy in conjunction with photosynthesis. Although metabolic responses to browning were observed, essential fatty acid content was similar between treatments while sterol content was slightly higher upon browning. Together, our results provide a mechanistic model of how a mixotrophic alga responds to browning and how such responses affect the availability of nutritionally essential biomolecules for higher trophic levels.

Biocrude Oil Production by Integrating Microalgae Polyculture and Wastewater Treatment: Novel Proposal on the Use of Deep Water-Depth Polyculture of Mixotrophic Microalgae

Microalgae have attracted significant attention worldwide as one of the most promising feedstock fossil fuel alternatives. However, there are a few challenges for algal fuels to compete with fossil fuels that need to be addressed. Therefore, this study reviews the R&D status of microalgae-based polyculture and biocrude oil production, along with wastewater treatment. Mixotrophic algae are free to some extent from light restrictions using organic matter and have the ability to grow well even in deep water-depth cultivation. It is proposed that integrating the mixotrophic microalgae polyculture and wastewater treatment process is the most promising and harmonizing means to simultaneously increase capacities of microalgae biomass production and wastewater treatment with a low land footprint and high robustness to perturbations. A large amount of mixotrophic algae biomass is harvested, concentrated, and dewatered by combining highly efficient sedimentation through flocculation and energy efficient filtration, which reduce the carbon footprint for algae fuel production and coincide with the subsequent hydrothermal liquefaction (HTL) conversion. HTL products are obtained with a relatively low carbon footprint and separated into biocrude oil, solid, aqueous, and gas fractions. Algae biomass feedstock-based HTL conversion has a high biocrude oil yield and quality available for existing oil refineries; it also has a bioavailability of the recycled nitrogen and phosphorus from the aqueous phase of algae community HTL. The HTL biocrude oil represents higher sustainability than conventional liquid fuels and other biofuels for the combination of greenhouse gas (GHG) and energy return on investment (EROI). Deep water-depth polyculture of mixotrophic microalgae using sewage has a high potential to produce sustainable biocrude oil within the land area of existing sewage treatment plants in Japan to fulfill imported crude oil.

From adverse to beneficial: Contrasting dietary effects of freshwater mixotrophs on zooplankton

Abstract The importance of mixotrophic algae as key bacterivores in microbial food webs is increasingly acknowledged, but their effects on the next trophic level remain poorly understood. Their high stoichiometric food quality is contrasted by anti‐grazing strategies. We tested the quality of freshwater mixotrophs as prey for zooplankton, using four non‐colonial chrysophyte species and a cryptophyte as a high quality reference food. We (1) analyzed the stoichiometric and biochemical (fatty acid) composition of the mixotrophs, and (2) quantified their dietary effects on Daphnia longispina survival. Survival of D. longispina significantly depended on the identity of species provided as food, ranging from higher to lower as compared to starvation. This was not reflected in differences in cellular stoichiometry or fatty acid profiles of the mixotrophs. We suggest that toxicity may be the driver for the observed differences. Generalization of the dietary effects of mixotrophic chrysophytes does not appear straightforward. Besides fundamental species‐specific differences, potential toxic effects may vary depending on environmental cues or physiological strategies. Notably in our study, Ochromonas tuberculata, a species previously reported to be deleterious, turned out to be a beneficial food source in terms of enabling high survival of D. longispina. We challenge the generality of the assumption that chrysophytes are of low value as food for zooplankton. We recommend that future studies test how environmental conditions and physiological strategies shape the quality of mixotrophs as food for consumers at higher trophic levels, specifically focusing on effects of dietary toxicity.

An under-ice bloom of mixotrophic haptophytes in low nutrient and freshwater-influenced Arctic waters

The pelagic spring bloom is essential for Arctic marine food webs, and a crucial driver of carbon transport to the ocean depths. A critical challenge is understanding its timing and magnitude, to predict its changes in coming decades. Spring bloom onset is typically light-limited, beginning when irradiance increases or during ice breakup. Here we report an acute 9-day under-ice algal bloom in nutrient-poor, freshwater-influenced water under 1-m thick sea ice. It was dominated by mixotrophic brackish water haptophytes (Chrysochromulina/ Prymnesium) that produced 5.7 g C m−2 new production. This estimate represents about half the annual pelagic production, occurring below sea ice with a large contribution from the mixotrophic algae bloom. The freshwater-influenced, nutrient-dilute and low light environment combined with mixotrophic community dominance implies that phagotrophy played a critical role in the under-ice bloom. We argue that such blooms dominated by potentially toxic mixotrophic algae might become more common and widespread in the future Arctic Ocean.

Mixotrophic cultivation of Thalassiosira pseudonana with pure and crude glycerol: Impact on lipid profile

The mixotrophic cultivation of microalgae often couples high biomass production with lipid accumulation, and thus it is considered a useful approach to obtain high quantities of added value microalgal biomass. In this study, the marine diatom Thalassiosira pseudonana was supplied with glycerol to determine the potential of the mixotrophic alga for the accumulation of lipids. In preliminarily experiments, T. pseudonana was cultivated in the presence of increasing concentrations of glucose, acetate and glycerol in order to select the best organic carbon source for growth promotion, and glycerol was selected for further experiments. The best glycerol dose for both biomass productivity and lipid production was 2.5 g L−1. To overcome the costs of the pure glycerol supply, the crude glycerol derived from a biodiesel production plant was also tested, as a low-cost alternative. Beside growth and photosynthetic parameters useful to monitor the algal conditions, lipid amount and profile were compared between autotrophic and pure/crude glycerol-treated algae. Interestingly, an almost doubled lipid content was accumulated in mixotrophic algae compared to the autotrophic ones. Moreover, an evident increase in a relatively short-chain fatty acid (C14:0, myristic acid), potentially useful for bio-jet fuels, and in a long-chain omega3-polyunsaturated fatty acid (C18:4; stearidonic acid), interesting for nutritional purposes, was highlighted in mixotrophic samples, especially when cultivated with crude glycerol. Differently, the fatty acid profile obtained from the autotrophic cultures suggested a preferable employment of the algal biomass for biodiesel production, owing to the prevalence of palmitic and palmitoleic acids. This study highlights that the mixotrophic cultivation of T. pseudonana drives the algal metabolism towards the production of different lipids, suitable for diversified applications. Moreover, the use of biodiesel-derived crude glycerol could make mixotrophy economically advantageous, in particular considering a bio-refinery approach.

Evaluation of Galdieria sulphuraria for nitrogen removal and biomass production from raw landfill leachate

This study evaluates the potential of thermophilic mixotrophic alga, Galdieria sulphuraria, to efficiently perform on-site treatment of landfill leachate (LL). Nutrient removal and algae growth experiments using various dilutions (20%, 40%, 60%, 80%, 100%) of actual municipal LL were carried out in a 14-day lab scale set-up. The effect of N/P mass ratio (255:1; 100:1; 50:1; 25:1; 10:1; 4.55:1) was also tested. Based on the results, it was evident that the growth reactors with 20% LL outperformed the other growth reactors. The final algal biomass density in 20% LL was 2.16 ± 0.22 g L−1 (twice more than compare to standard media) and the observed growth rate was 159 mg L−1d−1. Fourteen-day removal efficiencies were 99.9% for ammoniacal-nitrogen and 34% for phosphate, with removal rate of 22.78 mg L−1d−1 and 2.91 mg L−1d−1, respectively. Addition of P increased NH4-N removal rate from 16.3 to 23.6 mg L−1 day−1 when N/P mass ratio was changed from 255:1 to 4.55:1. Furthermore, range of N/P mass ratio from 100:1 to 25: 1 has been found to achieve almost 100% removal of both NH4-N and PO4-P from 20% LL in 14 days. Overall, this study showed a promising algal-based treatment system for LL nutrient removal and producing algal-biomass for subsequent value-added product generation.

Grazing resistance and poor food quality of a widespread mixotroph impair zooplankton secondary production

Growing evidence suggests that global climate change promotes the dominance of mixotrophic algae especially in oligotrophic aquatic ecosystems. While theory predicts that mixotrophy increases trophic transfer efficiency in aquatic food webs, deleterious effects of some mixotrophs on consumers have also been reported. Here, using a widespread mixotrophic algal genus Dinobryon, we aimed to quantify how colonial taxa contribute to secondary production in lakes. We, therefore, studied the dietary effects of Dinobryon divergens on Cladocera (Daphnia longispina) and Copepoda (Eudiaptomus gracilis), representing two main taxonomic and functional groups of zooplankton. In feeding experiments, we showed that Dinobryon was largely grazing resistant and even inhibited the uptake of the high-quality reference food in Daphnia. Eudiaptomus could to some extent compensate with selective feeding, but a negative long-term food quality effect was also evident. Besides, Eudiaptomus was more sensitive to the pure diet of Dinobryon than Daphnia. Low lipid content and high C:P elemental ratio further supported the low nutritional value of the mixotroph. In a stable isotope approach analysing a natural plankton community, we found further evidence that carbon of Dinobryon was not conveyed efficiently to zooplankton. Our results show that the increasing dominance of colonial mixotrophs can result in reduced dietary energy transfer to consumers at higher trophic levels. In a wider perspective, global climate change favours the dominance of some detrimental mixotrophic algae which may constrain pelagic trophic transfer efficiency in oligotrophic systems, similarly to cyanobacteria in eutrophic lakes.

Effects of light, dissolved nutrients and prey on ingestion and growth of a newly identified mixotrophic alga, Chrysolepidomonas dendrolepidota (Chrysophyceae)

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.

Detection of Phagotrophy in the Marine Phytoplankton Group of the Coccolithophores (Calcihaptophycidae, Haptophyta) During Nutrient-replete and Phosphate-limited Growth.

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 2days), 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.

Small Lakes Ecosystems under the Impact of Non-Ferrous Metallurgy (Russia, Murmansk Region)

This paper presents integrated research on ecosystems of small lakes experiencing the direct impact of a copper-nickel ore processing plant, the “Kolskaya GMK” (MS KGMK), near the town of Monchegorsk Kola Peninsula, Russia. An integrated research method with the analysis of both abiotic and biotic components of aquatic ecosystems was used. It was found that the water ecosystems developed under the conditions of extreme pollution depleted the species composition of the hydrobionts and the number indices. Much of the pollution resulted in a transformation in the phytoplankton community structure: the share of mixotrophic algae and Cyanobacteria increased. Anthropogenic eutrophication resulted in a decrease in toxic impact. Despite high anthropogenic load, Salmonid and Coregonid species were found in a number of water bodies. The size and weight indices and the heavy metal accumulation intensity had a distinct gradient nature. The macrozoobenthos in the water bodies studied was characterized by depleted taxonomic composition and extremely low numbers. The basis of the zoobenthos was formed by chironomids Psectrocladius, Procladius, Cricotopus, and Orthocladius, spread widely in water bodies polluted with heavy metals.

Interacting Temperature, Nutrients and Zooplankton Grazing Control Phytoplankton Size-Abundance Relationships in Eight Swiss Lakes.

Biomass distribution among size classes follows a power law where the Log-abundance of taxa scales to Log-size with a slope that responds to environmental abiotic and biotic conditions. The interactions between ecological mechanisms controlling the slope of locally realized size-abundance relationships (SAR) are however not well understood. Here we tested how warming, nutrient levels, and grazing affect the slope of phytoplankton community SARs in decadal time-series from eight Swiss lakes of the peri-alpine region, which underwent environmental forcing due to climate change and oligotrophication. We expected rising temperature to have a negative effect on slope (favoring small phytoplankton), and increasing nutrient levels and grazing pressure to have a positive effect (benefiting large phytoplankton). Using a random forest approach to extract robust patterns from the noisy data, we found that the effects of temperature (direct and indirect through water column stability), nutrient availability (phosphorus and total biomass), and large herbivore (copepods and daphnids) grazing and selectivity on slope were non-linear and interactive. Increasing water temperature or total grazing pressure, and decreasing phosphorus levels, had a positive effect on slope (favoring large phytoplankton, which are predominantly mixotrophic in the lake dataset). Our results therefore showed patterns that were opposite to the expected long-term effects of temperature and nutrient levels, and support a paradigm in which (i) small phototrophic phytoplankton appear to be favored under high nutrients levels, low temperature and low grazing, and (ii) large mixotrophic algae are favored under oligotrophic conditions when temperature and grazing pressure are high. The effects of temperature were stronger under nutrient limitation, and the effects of nutrients and grazing were stronger at high temperature. Our study shows that the phytoplankton local SARs in lakes respond to both the independent and the interactive effects of resources, grazing and water temperature in a complex, unexpected way, and observations from long-term studies can deviate significantly from general theoretical expectations.

Tracing the fate of microplastic carbon in the aquatic food web by compound-specific isotope analysis

Increasing abundance of microplastics (MP) in marine and freshwaters is currently one of the greatest environmental concerns. Since plastics are fairly resistant to chemical decomposition, breakdown and reutilization of MP carbon complexes requires microbial activity. Currently, only a few microbial isolates have been shown to degrade MPs, and direct measurements of the fate of the MP carbon are still lacking. We used compound-specific isotope analysis to track the fate of fully labelled 13C-polyethylene (PE) MP carbon across the aquatic microbial-animal interface. Isotopic values of respired CO2 and membrane lipids showed that MP carbon was partly mineralized and partly used for cell growth. Microbial mineralization and assimilation of PE-MP carbon was most active when inoculated microbes were obtained from highly humic waters, which contain recalcitrant substrate sources. Mixotrophic algae (Cryptomonas sp.) and herbivorous zooplankton (Daphnia magna) used microbial mediated PE-MP carbon in their cell membrane fatty acids. Moreover, heteronanoflagellates and mixotrophic algae sequestered MP carbon for synthesizing essential ω-6 and ω-3 polyunsaturated fatty acids. Thus, this study demonstrates that aquatic micro-organisms can produce, biochemically upgrade, and trophically transfer nutritionally important biomolecules from PE-MP.

Investigating the modulation of metabolites under high light in mixotrophic alga Asteracys sp. using a metabolomic approach

Metabolomic fluctuations in microalgal cells under mixotrophic growth regimes are scarcely understood. Our study explores these changes in an oleaginous microalga, Asteracys sp. when grown mixotrophically under high irradiances. Biomass production under autotrophic and mixotrophic regimes were studied at two light intensities (LL; 100 μmol photons m−2 s−1 and HL; 900 μmol photons m−2 s−1) for Asteracys sp. Studies on supplementation of external carbon such as glucose (mixotrophic mode) under different light intensities has not been demonstrated in context with enhanced biomass production. In the present study, we have provided external source of carbon as glucose in LL and HL regimes and observed a marked influence of glucose assimilation on the overall machinery of the cell. Our qualitative metabolomics reveals a stress-like phenomenon in Asteracys sp. under HL with enhanced biomass. In overview, our data analysis highlights few metabolites such as trehalose, proline and α-tocopherol that showed significant changes in response to light as well as in growth regime. Our analysis predicts an interrelation cross talk between maltose and sucrose pathways within cells owing to possible alteration in starch degradation under the influence of light. However, mixotrophy with additional carbon supplementation in presence of light did not alter or enhance lipid biosynthesis. In conclusion, our hypothesis indicates modulation of metabolites in presence of glucose under high light would rather facilitate rearrangement in metabolic pathways for enhanced biomass production.

Predicting removal kinetics of biochemical oxygen demand (BOD) and nutrients in a pilot scale fed-batch algal wastewater treatment system

Our previous reports have presented single-step treatment of primary-settled urban wastewater by a mixotrophic alga, Galdieria sulphuraria. Here, we present an approach to predict the operational cycle time, t*, required to meet the discharge standards for ammoniacal-nitrogen (NH3-N), phosphate (PO4) and biochemical oxygen demand (BOD) in fed-batch mode, under varying influent concentrations. First order removal rates of NH3-N, PO4 and BOD, determined using data from 10 fed-batch cycles in a pilot scale system (700 L), were used to predict their temporal fate and t* in 30 other cycles. Predicted concentrations of NH3-N, PO4, and BOD and t* over a wide range of influent concentrations agreed well with the measured ones, with root mean square errors, respectively, of 2.06 mg/L, 0.23 mg/L, 2.88 mg/L and 0.30 days. The coefficient of determination between the measured and the predicted values were as follows: for NH3-N: r2 = 0.82, n = 90; for PO4: r2 = 0.87, n = 90; for BOD: r2 = 0.70, n = 30; and for t*: r2 = 0.61, n = 30. The validity of the model in predicting temporal concentration profiles of NH3-N, PO4 and BOD reflected by Janus quotient (respectively of 0.85, 0.87 and 0.61) is comparable to or better than those reported in the literature. Sensitivity analysis procedures identified the influent concentration of PO4 as the most sensitive parameter in predicting t*.

Nutritional stress by means of high C:N ratios in the diet and starvation affects nitrogen isotope ratios and trophic fractionation of omnivorous copepods.

Nutritional stress, from feeding on low-quality diets or starvation, may cause changes in consumers' nitrogen isotope ratios (δ15N = 15N/14N) and trophic fractionation (∆15N = δ15Nconsumer - δ15Nfood source), however, research has shown mixed results in the magnitude and the direction of the change. This is potentially more complex for omnivores whose diets span a wide range of food resources. We conducted seasonal field samplings in Patagonian lakes and analyzed the relationship between seston (SES) quality parameters and the δ15N and ∆15N of an omnivorous copepod, Boeckella gracilipes (Bg). We also performed a 7-day laboratory starvation experiment, an extreme form of nutritional stress, to investigate if lack of food led to changes in δ15NBg values. Our field results showed that increasing values of the seston carbon to nitrogen ratio (C:NSES), chlorophyll a (Chl a), and δ15NSES were related to higher δ15NBg values. C:NSES and Chl a were also positively related to ∆15N; yet, C:NSES alone explained 70% of the variation. C:NSES values correlated with the presence of mixotrophic algae and ciliates that are key food resources for B. gracilipes. In our laboratory starvation experiment, the δ15NBg values increased significantly, pointing to use of internal N sources; yet, the change associated with starvation was less pronounced than that related to C:NSES changes in the field, suggesting depletion of the substrate pool in the former. We found that ∆15N values of omnivorous species consuming a low-quality diet would be higher than that from a conspecific with a high-quality diet; though fasting animals would show intermediate values.