The impact of irradiance on optimal and cellular nitrogen to phosphorus ratios in phytoplankton

Elemental and macromolecular plasticity of Chlamydomonas reinhardtii (Chlorophyta) in response to resource limitation and growth rate

With the ongoing differential disruption of the biogeochemical cycles of major elements that are essential for all life (carbon, nitrogen, and phosphorus), organisms are increasingly faced with a heterogenous supply of these elements in nature. Given that photosynthetic primary producers form the base of aquatic food webs, impacts of changed elemental supply on these organisms are particularly important. One way that phytoplankton cope with the differential availability of nutrients is through physiological changes, resulting in plasticity in macromolecular and elemental biomass composition. Here, we assessed how the green alga Chlamydomonas reinhardtii adjusts its macromolecular (e.g., carbohydrates, lipids, and proteins) and elemental (C, N, and P) biomass pools in response to changes in growth rate and the modification of resources (nutrients and light). We observed that Chlamydomonas exhibits considerable plasticity in elemental composition (e.g., molar ratios ranging from 124 to 971 for C:P, 4.5 to 25.9 for C:N, and 15.1 to 61.2 for N:P) under all tested conditions, pointing to the adaptive potential of Chlamydomonas in a changing environment. Exposure to low light modified the elemental and macromolecular composition of cells differently than limitation by nutrients. These observed differences, with potential consequences for higher trophic levels, included smaller cells, shifts in C:N and C:P ratios (due to proportionally greater N and P contents), and differential allocation of C among macromolecular pools (proportionally more lipids than carbohydrates) with different energetic value. However, substantial pools of N and P remained unaccounted for, especially at fast growth, indicating accumulation of N and P in forms we did not measure.

Assessment of eutrophication from Xiaoqing River estuary to Laizhou Bay: Further warning of ecosystem degradation in typically polluted estuary

The coast of Laizhou Bay is plagued by a number of environmental issues, such as eutrophication, which are likely to worsen over the next few decades as a result of trends toward industrialization and urbanization. High nutrient levels in the Xiaoqing River are believed to be the main cause of Laizhou Bay becoming eutrophicated. Therefore, we conducted two cruises from the Xiaoqing River estuary to Laizhou Bay in August 2022 and December 2022, respectively, in the wet and dry periods to assess the potential impact of status of eutrophication due to human activities. The results showed that the concentration of DIN was higher than the quality standard for water (f_i > 1) in both the wet season (August 2022) and the dry season (December 2022). DIN has major environmental impacts in Laizhou Bay. The eutrophication level index and organic pollution index have obvious spatial and temporal characteristics. In terms of time, the dry season is higher than the wet season. In space, Xiaoqing estuary is higher than Laizhou Bay. In the two surveys, DIN and DIP concentrations were significantly positively correlated, indicating that N and P pollution in the region had a common source and destination, and the spatial distribution was also similar. In addition, the current environmental conditions in the region are not ideal, reaching moderate and severe eutrophication levels, which proves that the ecosystem has the risk of aggravating degradation. As the Xiaoqing River is about to resume full navigation, human-related nutrient input (especially DIN) will continue to increase, and it is expected that the eutrophication risk in this area will increase in the next few years due to the increase in nutrient load.

The crucial influence of trophic status on the relative requirement of nitrogen to phosphorus for phytoplankton growth

Clarifying the pattern of relative nitrogen (N)-to-phosphorus (P) requirements for phytoplankton growth is of great significance for eutrophication mitigation and aquatic system management. The relative N-to-P requirement for phytoplankton growth is considered an essential trait determining species dominance within ecosystems and explaining phytoplankton response to nutrient availability. These requirements vary with environmental trophic status, though this variation remains unclear. Here, we evaluated the relative N-to-P requirements under different absolute nutrient levels using previous and current experimental data on eight phytoplankton species (three studied by us and five extrapolated from previous studies). Results showed that relative N-to-P requirements for phytoplankton growth decreased as absolute nutrient levels increased. Thus, N may be crucial for enhancing phytoplankton growth under low nutrient conditions, whereas P may be the primary limiting factor of phytoplankton growth under sufficient nutrient conditions. This result applies to single species as well as species assemblages, which are independent of species shifts occurring along water N:P gradients. The response observed in our large trophic status gradient may help elucidate the relative importance of N and P reductions in mitigating the impact of eutrophication on ecosystems.

Influence of abiotic conditions on the biofouling formation of flagellated microalgae culture

This work analyses the adhesion of flagellated microalgae to seven surfaces that have different water adhesion tension characteristics. Chlamydomonas reinhardtii and Isochrysis galbana, were cultivated in batch and fed-batch mode at four nitrogen/phosphorus (N/P) ratios (from 1.29 to 70) and subjected to four irradiance levels (50, 100, 200 and 400 µE·s^-1·m^-2) at 23 °C. Cell adhesion was greater in C. reinhardtii and a higher biomass concentration was obtained for this strain, reaching 2 g·L^-1 compared to 1 g·L^-1 for I. galbana. The adhesion of cells and exopolymeric substances was measured upon the batch and the first fed-batch reaching the stationary growth phase, observing a direct correlation between them and inversely to biomass generation in the cultures. The protein adhesion data for the different materials are comparable to those for cell adhesion coinciding with minimums of Baier's theory and Vogler. It is observed displacements in the curves as a function of the irradiance level.

Nanocosm: a well plate photobioreactor for environmental and biotechnological studies

Phytoplankton are key primary producers at the bottom of the aquatic food chain. They are a highly diverse group of organisms essential for the functioning of our ecosystems and because of their characteristics, their biomass is considered for various commercial applications. A full appreciation of their abundance, diversity and potential is only feasible by using systems that enable simultaneous testing of strains and/or variables in a fast and easy way. A major bottleneck is the lack of a cost-effective method with the capacity for complex experimental set-ups that enable fast and reproducible screening and analysis. In this study, we present nanocosm, a versatile LED-based micro-scale photobioreactor (PBR) that allows simultaneous testing of multiple variables such as temperature and light within the same plate. Every well can be independently controlled for intensity, temporal variation and light type (RGB, white, UV). We show that our systems guarantee homogeneous conditions because of controlled temperature and evaporation and adjustments for light crosstalk. By ensuring controlled environmental conditions the nanocosm is suitable for running factorial experimental designs where each well can be used as an independent micro-PBR. To validate culture performances, we assess well-to-well reproducibility and our results show minimal well-to-well variability for all the conditions tested. Possible modes of operation and application are discussed together with future development of the system.

Water Browning Controls Adaptation and Associated Trade-Offs in Phytoplankton Stressed by Chemical Pollution

The acquisition of tolerance to an environmental stressor can result in organisms displaying slower growth after stress release. While well-grounded in the theory, empirical evidence of the trade-off between stress tolerance and organism fitness is scarce and blurred by the interaction with different environmental factors. Here, we report the effects of water browning on the responses, tolerance acquisition, and associated trade-offs in a population of microalgae exposed to sublethal concentrations of organic micropollutants over multiple generations. Our results show that dissolved organic matter (DOM) reduces toxic responses and modulates tolerance acquisition by the algae, possibly by complexing micropollutants. Microalgae that acquire tolerance allocate resources to fitness at the cost of reduced cell size. They yield higher productivity than nonadapted ones when grown in the presence of micropollutants but lower in their absence. The net trade-off was positive, indicating that adaptation can result in a higher productivity and fitness in tolerant species in recurrently stressed environments.

Shifting of phytoplankton assemblages in a regulated Chinese river basin after streamflow and water quality changes

Phytoplankton is critical to river ecosystems. These organisms are sensitive to streamflow and water quality changes and, therefore, used to determine stability of river ecosystems, especially in regulated rivers. However, exactly how such disturbances alter spatial distribution of phytoplankton remains unclear, particularly during different seasons. A thorough understanding of these mechanisms is required to better analyze impact of environmental factors on regulated rivers. Given this, phytoplankton communities, streamflow, and water quality factors were assessed in areas sampled four times from 2015 to 2016 in upper and middle Huai River Basin. Biodiversity indices, as well as cluster and rank analyses, were used to (1) determine phytoplankton composition and distribution and (2) clarify impacts of key streamflow and water quality factors on such communities. It was found phytoplankton composition deteriorated over time, with phyla number decreasing from six to three. Moreover, proportion of Bacillariophyta increased from 51.83% to 68.13%. Phytoplankton in three regions, upstream region (Shannon-Wiener index 1.39-2.95), midstream region (0.70-4.55), and downstream region (0.22 to 2.97), were spatially clustered. The most impact factors impacting variation in composition and distribution were water quality factors and then hydrological factors. Of these, the most important factors in wet seasons were total nitrogen and maximum runoff, while ammonia nitrogen and low flow discharge were the most important factors during dry seasons. Streamflow and water quality contributed the most in midstream region, which was significantly affected by numbers of high and low flow. Contributions of these factors to downstream region were the strongest during dry seasons, which were significantly affected by numbers of low flow. Collectively, these results reveal significant impact of streamflow and water quality factors on phytoplankton deterioration in upper and middle Huai River Basin. Critically, this study provides scientific and technological support for increased biomonitoring and ecohydrological studies in regulated river basins.

From molecular manipulation of domesticated Chlamydomonas reinhardtii to survival in nature

In the mid-20th century, the unicellular and genetically tractable green alga Chlamydomonas reinhardtii was first developed as a model organism to elucidate fundamental cellular processes such as photosynthesis, light perception and the structure, function and biogenesis of cilia. Various studies of C. reinhardtii have profoundly advanced plant and cell biology, and have also impacted algal biotechnology and our understanding of human disease. However, the 'real' life of C. reinhardtii in the natural environment has largely been neglected. To extend our understanding of the biology of C. reinhardtii, it will be rewarding to explore its behavior in its natural habitats, learning more about its abundance and life cycle, its genetic and physiological diversity, and its biotic and abiotic interactions.

Global change-driven effects on dissolved organic matter composition: Implications for food webs of northern lakes

Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial-aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)-the messenger between terrestrial and lake ecosystems-with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change-driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice-out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.

A Platform for High-Throughput Assessments of Environmental Multistressors

A platform compatible with microtiter plates to parallelize environmental treatments to test the complex impacts of multiple stressors, including parameters relevant to climate change and point source pollutants is developed. This platform leverages (1) the high rate of purely diffusive gas transport in aerogels to produce well-defined centimeter-scale gas concentration gradients, (2) spatial light control, and (3) established automated liquid handling. The parallel gaseous, aqueous, and light control provided by the platform is compatible with multiparameter experiments across the life sciences. The platform is applied to measure biological effects in over 700 treatments in a five-parameter full factorial study with the microalgae Chlamydomonas reinhardtii. Further, the CO2 response of multicellular organisms, Lemna gibba and Artemia salina under surfactant and nanomaterial stress are tested with the platform.

Ecological Stoichiometry of Ocean Plankton

Marine plankton elemental stoichiometric ratios can deviate from the Redfield ratio (106C:16N:1P); here, we examine physiological and biogeochemical mechanisms that lead to the observed variation across lineages, regions, and seasons. Many models of ecological stoichiometry blend together acclimative and adaptive responses to environmental conditions. These two pathways can have unique molecular mechanisms and stoichiometric outcomes, and we attempt to disentangle the two processes. We find that interactions between environmental conditions and cellular growth are key to understanding stoichiometric regulation, but the growth rates of most marine plankton populations are poorly constrained. We propose that specific physiological mechanisms have a strong impact on plankton and community stoichiometry in nutrient-rich environments, whereas biogeochemical interactions are important for the stoichiometry of the oligotrophic gyres. Finally, we outline key areas with missing information that is needed to advance understanding of the present and future ecological stoichiometry of ocean plankton.

Cylindrospermopsin induced changes in growth, toxin production and antioxidant response of Acutodesmus acuminatus and Microcystis aeruginosa under differing light and nitrogen conditions

Growing evidence suggests that some bioactive metabolites (e.g. cyanotoxins) produced by cyanobacteria have allelopathic potential, due to their inhibitory or stimulatory effects on competing species. Although a number of studies have shown that the cyanotoxin cylindrospermopsin (CYN) has variable effects on phytoplankton species, the impact of changing physicochemical conditions on its allelopathic potential is yet to be investigated. We investigated the physiological response of Microcystis aeruginosa (Cyanobacteria) and Acutodesmus acuminatus (Chlorophyta) to CYN under varying nitrogen and light conditions. At 24h, higher microcystins content of M. aeruginosa was recorded under limited light in the presence of CYN, while at 120h the lower levels of the toxins were observed in the presence of CYN under optimum light. Total MCs concentration was significantly (p<0.05) lowered by CYN after 120h of exposure under limited and optimum nitrogen conditions. On the other hand, there were no significant (p>0.05) changes in total MCs concentrations after exposure to CYN under high nitrogen conditions. As expected, limited light and limited nitrogen conditions resulted in lower cell density of both species, while CYN only significantly (p<0.05) inhibited the growth of M. aeruginosa. Regardless of the light or nitrogen condition, the presence of CYN increased internal H₂O₂ content of both species, which resulted in significant (p<0.05) changes in antioxidant enzyme (catalase, peroxidase, superoxide dismutase and glutathione S-transferase) activities. The oxidative stress caused by CYN was higher under limited light and limited nitrogen. These results showed that M. aeruginosa and A. acuminatus have variable response to CYN under changing light and nitrogen conditions, and demonstrate that need to consider changes in physicochemical conditions during ecotoxicological and ecophysiological investigations.

Changes in Stoichiometry, Cellular RNA, and Alkaline Phosphatase Activity of Chlamydomonas in Response to Temperature and Nutrients

Phytoplankton may respond both to elevated temperatures and reduced nutrients by changing their cellular stoichiometry and cell sizes. Since increased temperatures often cause increased thermal stratification and reduced vertical flux of nutrients into the mixed zone, it is difficult to disentangle these drivers in nature. In this study, we used a factorial design with high and low levels of phosphorus (P) and high and low temperature to assess responses in cellular stoichiometry, levels of RNA, and alkaline phosphatase activity (APA) in the chlorophyte Chlamydomonas reinhardtii. Growth rate, C:P, C:N, N:P, RNA, and APA all responded primarily to P treatment, but except for N:P and APA, also temperature contributed significantly. For RNA, the contribution from temperature was particularly strong with higher cellular levels of RNA at low temperatures, suggesting a compensatory allocation to ribosomes to maintain protein synthesis and growth. These experiments suggest that although P-limitation is the major determinant of growth rate and cellular stoichiometry, there are pronounced effects of temperature also via interaction with P. At the ecosystem level, nutrients and temperature will thus interact, but temperatures would likely exert a stronger impact on these phytoplankton traits indirectly via its force on stratification regimes and vertical nutrient fluxes.