Effects of Temperature and Nutrient Supply on Resource Allocation, Photosynthetic Strategy, and Metabolic Rates of Synechococcus sp

Seasonal Variations of Community Structure and Functional Genes of Synechococcus in the Subtropical Coastal Waters: Insights from FACS and High-Throughput Sequencing

Synechococcus plays a pivotal role in the marine biogeochemical cycle. Advances in isolation techniques and high-throughput sequencing have expanded our understanding of the diversity of the Synechococcus community. However, their genomic diversity, functional dynamics and seasonal variations in the coastal waters are still not well known. Here, seawater samples were collected seasonally (March, June, August, December) from three stations in the coastal waters of Xiamen. Using fluorescence-activated cell sorting (FACS), we isolated 1000 Synechococcus cells per sample and performed ITS amplicon sequencing and metagenomic sequencing to analyze the seasonal variations in community structure and functional genes of Synechococcus. Firstly, we conducted a comparative analysis of in situ data and FACS data from three sampling sites in August. FACS samples revealed low-abundance Synechococcus strains underdetected by in situ samples. In addition, 24 clades representing Synechococcus subclusters S5.1, S5.2, and S5.3 were detected from three in situ samples and twelve FACS samples, suggesting the high diversity of Synechococcus in the coastal waters of Xiamen. Furthermore, the Synechococcus community displayed pronounced seasonal variations, and temperature significantly influenced the variations in Synechococcus community composition. Additionally, Synechococcus populations exhibit seasonal functional dynamics, with enhanced metabolic activity in summer characterized by higher numbers of functional genes associated with metabolic pathways compared to winter samples. Altogether, this study underscored the significance of FACS and high-throughput sequencing to reveal the diversity and functional dynamics of Synechococcus.

Effect of temperature, nutrients and growth rate on picophytoplankton cell size across the Atlantic Ocean

The cell size of picophytoplankton populations affects their ecology and biogeochemical role, but how different environmental drivers control its variability is still not well understood. To gain insight into the role of temperature and nutrient availability as determinants of picophytoplankton population mean cell size, we carried out five microcosm experiments across the Atlantic Ocean (45°N-27°S) in which surface plankton assemblages were incubated under all combinations of three temperatures (in situ, 3 °C cooling and 3 °C warming) and two nutrient levels (unamended and addition of nitrogen and phosphorus). The overall range of variability in cell volume was 5-fold for Prochlorococcus, 8-fold for Synechococcus and 6-fold for the picoeukaryotes. We observed, in all the treatments and in the control, a consistent trend toward larger mean cell sizes over time for both Prochlorococcus and Synechococcus, which was likely the result of sample confinement. Changes in temperature and nutrient status alone did not cause clear changes in cell size, relative to the control, but the combination of warming and nutrient addition resulted in an increase in Prochlorococcus and Synechococcus cell size. The largest increases in cell volume were associated with slow or negative population net growth rates. Our results emphasize the importance of considering changes in biovolume to obtain accurate estimates of picophytoplankton biomass and suggest that the inverse relationship between growth rate and population mean cell size may be a general pattern in marine phytoplankton.

Present and future potential role of toxin-producing Synechococcus in the tropical region

As anthropogenic induced temperature rises and nutrient loadings increase in fresh and brackish environments, the ecological function of the phytoplankton community is expected to favour the picocyanobacteria, of the genus Synechococcus. Synechococcus is already a ubiquitous cyanobacterium found in both freshwater and marine environments, notwithstanding that the toxigenic species still remains unexplored in many freshwaters. Their fast growth rate and their ability to produce toxins make Synechococcus a potential dominant player in harmful algal blooms under climate change scenarios. This study examines the responses of a novel toxin-producing Synechococcus (i.e., one belonging to a freshwater clade; the other belonging to a brackish clade) to environmental changes that reflect climate change effects. We conducted a series of controlled experiments under present and predicted future temperatures, as well as under various N and P nutrients loadings. Our findings highlight how Synechococcus can be altered by the differing reactions to increasing temperature and nutrients, which resulted in considerable variations in cell abundance, growth rate, death rate, cellular stoichiometry and toxin production. Synechococcus had the highest growth observed at 28 °C, and further increases in temperature resulted in a decline for both fresh and brackish waters. Cellular stoichiometry was also altered, where more nitrogen (N) per cell was required, and the plasticity of N:P was more severe for the brackish clade. However, Synechococcus become more toxic under future scenario. Anatoxin-a (ATX) saw the greatest spike when temperature was at 34 °C especially under P-enrichment conditions. In contrast, Cylindrospermopsin (CYN) was promoted at the lowest tested temperature (25 °C) and under N-limitation. Overall, both temperature and external nutrients are the dominant control over Synechococcus toxins production. A model was also created to assess Synechococcus toxicity to zooplankton grazing. Zooplankton grazing was reduced by two folds under nutrient limitation, but temperature accounted for very insignificant change.

Phytoplankton responses to changing temperature and nutrient availability are consistent across the tropical and subtropical Atlantic

Temperature and nutrient supply interactively control phytoplankton growth and productivity, yet the role of these drivers together still has not been determined experimentally over large spatial scales in the oligotrophic ocean. We conducted four microcosm experiments in the tropical and subtropical Atlantic (29°N-27°S) in which surface plankton assemblages were exposed to all combinations of three temperatures (in situ, 3 °C warming and 3 °C cooling) and two nutrient treatments (unamended and enrichment with nitrogen and phosphorus). We found that chlorophyll a concentration and the biomass of picophytoplankton consistently increase in response to nutrient addition, whereas changes in temperature have a smaller and more variable effect. Nutrient enrichment leads to increased picoeukaryote abundance, depressed Prochlorococcus abundance, and increased contribution of small nanophytoplankton to total biomass. Warming and nutrient addition synergistically stimulate light-harvesting capacity, and accordingly the largest biomass response is observed in the warmed, nutrient-enriched treatment at the warmest and least oligotrophic location (12.7°N). While moderate nutrient increases have a much larger impact than varying temperature upon the growth and community structure of tropical phytoplankton, ocean warming may increase their ability to exploit events of enhanced nutrient availability.

Microcosm experiments in the tropical and subtropical Atlantic reveal consistent responses of phytoplankton to changing temperature and nutrient availability, with implications for the impacts of ocean warming in oligotrophic ecosystems.

Picocyanobacterial Synechococcus in marine ecosystem: Insights from genetic diversity, global distribution, and potential function

Marine Synechococcus, a main group of picocyanobacteria, has been ubiquitously observed across the global oceans. Synechococcus exhibits high phylogenetical and phenotypical diversity, and horizontal gene transfer makes its genetic evolution much more intricate. With the development of measurement technologies and analysis methods, the genomic information and niche partition of each Synechococcus lineage tend to be precisely described, but the global analysis is still lacking. Therefore, it is necessary to summarize existing studies and integrate published data to gain a comprehensive understanding of Synechococcus on genetic variation, niche division, and potential functions. In this review, the maximum likelihood trees are constructed based on existing sequence data, including both phylogenetic and pigmentary gene markers. The global distribution characteristics of abundance, lineages, and pigment types are concluded through pooled analysis of more than 700 samples obtained from approximately 50 scientific research cruises. The potential functions of Synechococcus are explored in element cycles and biological interactions. Future work on Synechococcus is suggested to focus on not only elucidating the nature of Synechococcus biodiversity but also demonstrating its interactions with the ecosystem by combining bioinformatics and macroscopic isotope-labeled environmental parameters.

Modeled temperature dependencies of macromolecular allocation and elemental stoichiometry in phytoplankton

Warming oceans may affect how phytoplankton allocate nutrients to essential cellular processes. Despite the potential impact of such processes on future biogeochemical cycles, questions remain about how temperature affects macromolecular allocation and elemental stoichiometry within phytoplankton cells. Here, we present a macromolecular model of phytoplankton and the effect of increasing temperature on the intracellular allocation of nutrients at a constant growth rate. When temperature increases under nitrogen (N) and phosphorus (P) co-limitation, the model shows less investment in phosphorus-rich RNA molecules relative to nitrogen-rich proteins, leading to a more severe decrease in cellular P:C than N:C causing increased cellular N:P values. Under P limitation, the model shows a similar pattern, but when excess P is available under N limitation, we predict lowered N:P due to the effect of luxury uptake of P. We reflected our model result on the surface ocean showing similar latitudinal patterns in N:P and P:C to observation and other model predictions, suggesting a considerable impact of temperature on constraining the elemental stoichiometry in the ocean.

Carboxysome Mispositioning Alters Growth, Morphology, and Rubisco Level of the Cyanobacterium Synechococcus elongatus PCC 7942

Cyanobacteria are the prokaryotic group of phytoplankton responsible for a significant fraction of global CO2 fixation. Like plants, cyanobacteria use the enzyme ribulose 1,5-bisphosphate carboxylase/oxidase (Rubisco) to fix CO2 into organic carbon molecules via the Calvin-Benson-Bassham cycle. Unlike plants, cyanobacteria evolved a carbon-concentrating organelle called the carboxysome-a proteinaceous compartment that encapsulates and concentrates Rubisco along with its CO2 substrate. In the rod-shaped cyanobacterium Synechococcus elongatus PCC 7942, we recently identified the McdAB system responsible for uniformly distributing carboxysomes along the cell length. It remains unknown what role carboxysome positioning plays with respect to cellular physiology. Here, we show that a failure to distribute carboxysomes leads to slower cell growth, cell elongation, asymmetric cell division, and elevated levels of cellular Rubisco. Unexpectedly, we also report that even wild-type S. elongatus undergoes cell elongation and asymmetric cell division when grown at the cool, but environmentally relevant, growth temperature of 20°C or when switched from a high- to ambient-CO2 environment. The findings suggest that carboxysome positioning by the McdAB system functions to maintain the carbon fixation efficiency of Rubisco by preventing carboxysome aggregation, which is particularly important under growth conditions where rod-shaped cyanobacteria adopt a filamentous morphology. IMPORTANCE Photosynthetic cyanobacteria are responsible for almost half of global CO2 fixation. Due to eutrophication, rising temperatures, and increasing atmospheric CO2 concentrations, cyanobacteria have gained notoriety for their ability to form massive blooms in both freshwater and marine ecosystems across the globe. Like plants, cyanobacteria use the most abundant enzyme on Earth, Rubisco, to provide the sole source of organic carbon required for its photosynthetic growth. Unlike plants, cyanobacteria have evolved a carbon-concentrating organelle called the carboxysome that encapsulates and concentrates Rubisco with its CO2 substrate to significantly increase carbon fixation efficiency and cell growth. We recently identified the positioning system that distributes carboxysomes in cyanobacteria. However, the physiological consequence of carboxysome mispositioning in the absence of this distribution system remains unknown. Here, we find that carboxysome mispositioning triggers changes in cell growth and morphology as well as elevated levels of cellular Rubisco.

Development of photosynthetic carbon fixation model using multi-excitation wavelength fast repetition rate fluorometry in Lake Biwa

Direct measurements of gross primary productivity (GPP) in the water column are essential, but can be spatially and temporally restrictive. Fast repetition rate fluorometry (FRRf) is a bio-optical technique based on chlorophyll a (Chl-a) fluorescence that can estimate the electron transport rate (ETR_PSII) at photosystem II (PSII) of phytoplankton in real time. However, the derivation of phytoplankton GPP in carbon units from ETR_PSII remains challenging because the electron requirement for carbon fixation (Ф_e,C), which is mechanistically 4 mol e⁻ mol C⁻¹ or above, can vary depending on multiple factors. In addition, FRRf studies are limited in freshwater lakes where phosphorus limitation and cyanobacterial blooms are common. The goal of the present study is to construct a robust Ф_e,C model for freshwater ecosystems using simultaneous measurements of ETR_PSII by FRRf with multi-excitation wavelengths coupled with a traditional carbon fixation rate by the ¹³C method. The study was conducted in oligotrophic and mesotrophic parts of Lake Biwa from July 2018 to May 2019. The combination of excitation light at 444, 512 and 633 nm correctly estimated ETR_PSII of cyanobacteria. The apparent range of Ф_e,C in the phytoplankton community was 1.1–31.0 mol e⁻ mol C⁻¹ during the study period. A generalised linear model showed that the best fit including 12 physicochemical and biological factors explained 67% of the variance in Ф_e,C. Among all factors, water temperature was the most significant, while photosynthetically active radiation intensity was not. This study quantifies the in situ FRRf method in a freshwater ecosystem, discusses core issues in the methodology to calculate Ф_e,C, and assesses the applicability of the method for lake GPP prediction.