Stoichiometry of Prochlorococcus, Synechococcus , and small eukaryotic populations in the western North Atlantic Ocean

Latitudinal and meridional patterns of picophytoplankton variability are contrastingly associated with Ekman pumping and the warm pool in the tropical western Pacific

Marine picophytoplankton plays a major role in marine cycling and energy conversion, and its effects on the carbon cycle and global climate change have been well documented. In this study, we investigated the response of picophytoplankton across a broad range of physicochemical conditions in two distinct regions of the tropical western Pacific. Our analysis considered the abundance, carbon biomass, size fraction, distribution, and regulatory factors of the picophytoplankton community, which included the cyanobacteria Prochlorococcus and Synechococcus, and small eukaryotic phytoplankton (picoeukaryotes). The first region was a latitudinal transect along the equator (142-163° E, 0° N), characterized by stratified oligotrophic conditions. The second region was a meridional transect (143° E, 0-22° N) known for its high-nutrient and low-chlorophyll (HNLC) conditions. Results showed that picophytoplankton contributed >80% of the chlorophyll a (Chl a), and was mainly distributed above 100 m. Prochlorococcus was the dominant organism in terms of cell abundance and estimated carbon biomass in both latitudinal and meridional transects, followed by Synechococcus and picoeukaryotes. In the warm pool, Prochlorococcus was primarily distributed below the isothermal layer, with the maximum subsurface abundance forming below it. The maximum Synechococcus abundance was restricted to the west-warm pool, due to the high temperature, and the second-highest Synechococcus abundance was associated with frontal interaction between the east-warm pool and the westward advance of Middle East Pacific water. In contrast, picoeukaryotes formed a maximum subsurface abundance corresponding to the subsurface Chl a maximum. In the mixed HNLC waters, the cell abundance and biomass of the three picophytoplankton groups were slightly lower than those in the warm pool. Due to a cyclonic eddy, the contours of the maximum subsurface Prochlorococcus abundance were uplifted, evidently with a lower value than the surrounding water. Synechococcus abundance varied greatly in patches, forming a weakly high subsurface peak when the isothermal layer rose to the near-surface (<50 m). The subsurface maximum picoeukaryote abundance was also highly consistent with that of the subsurface Chl a maximum. Correlation analysis and generalized additive models of environmental factors showed that nutrient availability had a two-faceted role in regulating the spatial patterns of picophytoplankton in diverse latitudinal and meridional environments. We concluded through regression that temperature and light irradiance were the key determinants of picophytoplankton variability in the tropical western Pacific. This study provides insights into the changing picophytoplankton community structure with potential future changing hydroclimatic force.

Disentangling environmental effects on picophytoplankton communities in the Eastern Indian Ocean

Photosynthesis by picophytoplankton provides energy for higher organisms and is essential in the food chain and global carbon cycle. In 2020 and 2021, we investigated the spatial distribution and vertical changes of picophytoplankton in the euphotic layer of the Eastern Indian Ocean (EIO) and estimated their carbon biomass contributions through two cruise surveys. The abundance of picophytoplankton was composed of Prochlorococcus (69.94%), Synechococcus (22.21%) and picoeukaryotes (7.85%). Synechococcus was mainly found in the surface layer, while Prochlorococcus and picoeukaryotes had high abundances in the subsurface layer. The surface picophytoplankton community was greatly affected by fluorescence, the middle layer was significantly regulated by temperature and dissolved oxygen concentration, and the lower layer was dominated by nutrients and apparent oxygen utilization (AOU). Aggregated boosted tree (ABT) and Generalized Additive models (GAM) indicated that temperature, salinity, AOU, and fluorescence were strong influencing factors of picophytoplankton communities in EIO. The mean carbon biomass contribution of picophytoplankton in the surveyed area was 0.565 μg C/L, which was contributed by Prochlorococcus (39.32%), Synechococcus (38.88%) and picoeukaryotes (21.80%). These findings contribute to our understanding of the effects of different environmental factors on picophytoplankton communities and the influence of picophytoplankton contributions to the carbon pools of the oligotrophic ocean.

Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN)

Concentrations and elemental stoichiometry of suspended particulate organic carbon, nitrogen, phosphorus, and oxygen demand for respiration (C:N:P:−O2) play a vital role in characterizing and quantifying marine elemental cycles. Here, we present Version 2 of the Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN) dataset. Version 1 is a previously published dataset of particulate organic matter from 70 different studies between 1971 and 2010, while Version 2 is comprised of data collected from recent cruises between 2011 and 2020. The combined GO-POPCORN dataset contains 2673 paired surface POC/N/P measurements from 70°S to 73°N across all major ocean basins at high spatial resolution. Version 2 also includes 965 measurements of oxygen demand for organic carbon respiration. This new dataset can help validate and calibrate the next generation of global ocean biogeochemical models with flexible elemental stoichiometry. We expect that incorporating variable C:N:P:-O2 into models will help improve our estimates of key ocean biogeochemical fluxes such as carbon export, nitrogen fixation, and organic matter remineralization.

Adaptive carbon export response to warming in the Sargasso Sea

Ocean ecosystem models predict that warming and increased surface ocean stratification will trigger a series of ecosystem events, reducing the biological export of particulate carbon to the ocean interior. We present a nearly three-decade time series from the open ocean that documents a biological response to ocean warming and nutrient reductions wherein particulate carbon export is maintained, counter to expectations. Carbon export is maintained through a combination of phytoplankton community change to favor cyanobacteria with high cellular carbon-to-phosphorus ratios and enhanced shallow phosphorus recycling leading to increased nutrient use efficiency. These results suggest that surface ocean ecosystems may be more responsive and adapt more rapidly to changes in the hydrographic system than is currently envisioned in earth ecosystem models, with positive consequences for ocean carbon uptake.

The ability of the ocean’s biota to sequester carbon is thought to be negatively affected by climate change. Here the authors use time-series data in the Sargasso Sea to show that biotic processes can buffer against these negative impacts.

Environmental control of marine phytoplankton stoichiometry in the North Atlantic Ocean

The stoichiometric coupling of carbon to limiting nutrients in marine phytoplankton regulates the magnitude of biological carbon sequestration in the ocean. While clear links between plankton C:N ratios and environmental drivers have been identified, the nature and direction of these links, as well as their underlying physiological and ecological controls, remain uncertain. We show, with a well-constrained mechanistic model of plankton ecophysiology, that while nitrogen availability and temperature emerge as the main drivers of phytoplankton C:N stoichiometry in the North Atlantic, the biological mechanisms involved vary depending on the spatiotemporal scale and region considered. We find that phytoplankton C:N stoichiometry is overall controlled by nitrogen availability below 40° N, predominantly driven by ecoevolutionary shifts in the functional composition of the phytoplankton communities, while phytoplankton stoichiometric plasticity in response to dropping temperatures and increased grazing pressure dominates at higher latitudes. Our findings highlight the potential of "organisms-to-ecosystems" modeling approaches based on mechanistic models of plankton biology accounting for physiology, ecology, and trait evolution to explore and explain complex observational data and ultimately improve the predictions of global ocean models.

Nanomolar phosphate supply and its recycling drive net community production in the subtropical North Pacific

Seasonal drawdown of dissolved inorganic carbon (DIC) in the subtropical upper ocean makes a significant contribution to net community production (NCP) globally. Although NCP requires macronutrient supply, surface macronutrients are chronically depleted, and their supply has been unable to balance the NCP demand. Here, we report nanomolar increases in surface nitrate plus nitrite (N+N, ~20 nM) and phosphate (PO₄, ~15 nM) from summer to winter in the western subtropical North Pacific. Molar ratios of upward fluxes of DIC:N+N:PO₄ to the euphotic zone (< 100 m) were in near-stoichiometric balance with microbial C:N:P ratios (107~243:16~35:1). Comparison of these upward influxes with other atmospheric and marine sources demonstrated that total supply is largely driven by the other sources for C and N (93~96%), but not for P (10%), suggesting that nanomolar upward supply of P and its preferential recycling play a vital role in sustaining the NCP.

Phosphorus Release and Regeneration Following Laboratory Lysis of Bacterial Cells

The availability of phosphorus limits primary production in large regions of the oceans, and marine microbes use a variety of strategies to overcome this limitation. One strategy is the production of alkaline phosphatase (APase), which allows hydrolysis of larger dissolved organic phosphorus (DOP) compounds in the periplasm or at the cell surface for transport of orthophosphate into the cell. Cell lysis, driven by grazing and viral infection, releases phosphorus-containing cell components, along with active enzymes that could persist after lysis. The importance of this continued enzymatic activity for orthophosphate regeneration is unknown. We used three model bacteria – Escherichia coli K-12 MG1655, Synechococcus sp. WH7803, and Prochlorococcus sp. MED4 – to assess the impact of continued APase activity after cell lysis, via lysozyme treatment, on orthophosphate regeneration. Direct release of orthophosphate scaled with cell size and was reduced under phosphate-starved conditions where APase activity continued for days after lysis. All lysate incubations showed post-lysis orthophosphate generation suggesting phosphatases other than APase maintain activity. Rates of DOP hydrolysis and orthophosphate remineralization varied post-lysis among strains. Escherichia coli K-12 MG1655 rates of remineralization were 0.6 and 1.2 amol cell^–1hr^–1 under deplete and replete conditions; Synechococcus WH7803 lysates ranged from 0.04 up to 0.3 amol cell^–1hr^–1 during phosphorus deplete and replete conditions, respectively, while in Prochlorococcus MED4 lysates, rates were stable at 0.001 amol cell^–1hr^–1 in both conditions. The range of rates of hydrolysis and regeneration underscores the taxonomic and biochemical variability in the process of nutrient regeneration and further highlights the complexity of quantitatively resolving the major fluxes within the microbial loop.

Linking regional shifts in microbial genome adaptation with surface ocean biogeochemistry

Linking 'omics measurements with biogeochemical cycles is a widespread challenge in microbial community ecology. Here, we propose applying genomic adaptation as 'biosensors' for microbial investments to overcome nutrient stress. We then integrate this genomic information with a trait-based model to predict regional shifts in the elemental composition of marine plankton communities. We evaluated this approach using metagenomic and particulate organic matter samples from the Atlantic, Indian and Pacific Oceans. We find that our genome-based trait model significantly improves our prediction of particulate C : P (carbon : phosphorus) across ocean regions. Furthermore, we detect previously unrecognized ocean areas of iron, nitrogen and phosphorus stress. In many ecosystems, it can be very challenging to quantify microbial stress. Thus, a carefully calibrated genomic approach could become a widespread tool for understanding microbial responses to environmental changes and the biogeochemical outcomes. This article is part of the theme issue 'Conceptual challenges in microbial community ecology'.

Vertical migration by bulk phytoplankton sustains biodiversity and nutrient input to the surface ocean

Phytoplankton subsumes the great variety of unicellular photoautotrophs that perform roughly half of Earth's primary production. They achieve this despite their challenging oceanic habitat, with opposing vertical gradients of nutrients (which often limit their growth near the surface) and light (which becomes limiting with increasing depth). Most phytoplankton species are commonly assumed to be incapable of moving actively between the zones of light and nutrient availability, which are separated vertically by from 30-120 m. Here we propose that a considerable fraction of phytoplankton vertically traverse these gradients over time scales from hours to weeks, employing variations of a common migration strategy to acquire multiple resources. We present a mechanistic Lagrangian model resolving phytoplankton growth linked to optimal migration behaviour and demonstrate unprecedented agreement of its calculated vertical CHL-a distributions with 773 profiles observed at five prominent marine time-series stations. Our simulations reveal that vertically cycling phytoplankton can pump up enough nutrient to sustain as much as half of oceanic Net Primary Production (NPP). Active locomotion is therefore a plausible mechanism enabling relatively high NPP in the oligotrophic surface ocean. Our simulations also predict similar fitness for a variety of very different migration strategies, which helps to explain the puzzling diversity of phytoplankton observed in the ocean.

Dynamic responses of picophytoplankton to physicochemical variation in the eastern Indian Ocean

Picophytoplankton were investigated during spring 2015 and 2016 extending from near-shore coastal waters to oligotrophic open waters in the eastern Indian Ocean (EIO). They were typically composed of Prochlorococcus (Pro), Synechococcus (Syn), and picoeukaryotes (PEuks). Pro dominated most regions of the entire EIO and were approximately 1-2 orders of magnitude more abundant than Syn and PEuks. Under the influence of physicochemical conditions induced by annual variations of circulations and water masses, no coherent abundance and horizontal distributions of picophytoplankton were observed between spring 2015 and 2016. Although previous studies reported the limited effects of nutrients and heavy metals around coastal waters or upwelling zones could constrain Pro growth, Pro abundance showed strong positive correlation with nutrients, indicating the increase in nutrient availability particularly in the oligotrophic EIO could appreciably elevate their abundance. The exceptional appearance of picophytoplankton with high abundance along the equator appeared to be associated with the advection processes supported by the Wyrtki jets. For vertical patterns of picophytoplankton, a simple conceptual model was built based upon physicochemical parameters. However, Pro and PEuks simultaneously formed a subsurface maximum, while Syn generally restricted to the upper waters, significantly correlating with the combined effects of temperature, light, and nutrient availability. The average chlorophyll a concentrations (Chl a) of picophytoplankton accounted for above 49.6% and 44.9% of the total Chl a during both years, respectively, suggesting that picophytoplankton contributed a significant proportion of the phytoplankton community in the whole EIO.

NanoSIMS single cell analyses reveal the contrasting nitrogen sources for small phytoplankton

Nitrogen (N) is a limiting nutrient in vast regions of the world's oceans, yet the sources of N available to various phytoplankton groups remain poorly understood. In this study, we investigated inorganic carbon (C) fixation rates and nitrate (NO₃^-), ammonium (NH₄⁺) and urea uptake rates at the single cell level in photosynthetic pico-eukaryotes (PPE) and the cyanobacteria Prochlorococcus and Synechococcus. To that end, we used dual ¹⁵N and ¹³C-labeled incubation assays coupled to flow cytometry cell sorting and nanoSIMS analysis on samples collected in the North Pacific Subtropical Gyre (NPSG) and in the California Current System (CCS). Based on these analyses, we found that photosynthetic growth rates (based on C fixation) of PPE were higher in the CCS than in the NSPG, while the opposite was observed for Prochlorococcus. Reduced forms of N (NH₄⁺ and urea) accounted for the majority of N acquisition for all the groups studied. NO₃^- represented a reduced fraction of total N uptake in all groups but was higher in PPE (17.4 ± 11.2% on average) than in Prochlorococcus and Synechococcus (4.5 ± 6.5 and 2.9 ± 2.1% on average, respectively). This may in part explain the contrasting biogeography of these picoplankton groups. Moreover, single cell analyses reveal that cell-to-cell heterogeneity within picoplankton groups was significantly greater for NO₃^- uptake than for C fixation and NH₄⁺ uptake. We hypothesize that cellular heterogeneity in NO₃^- uptake within groups facilitates adaptation to the fluctuating availability of NO₃^- in the environment.

Mixotrophic metabolism by natural communities of unicellular cyanobacteria in the western tropical South Pacific Ocean

Cyanobacteria are major contributors to ocean biogeochemical cycling. However, mixotrophic metabolism and the relative importance of inorganic and organic carbon assimilation within the most abundant cyanobacteria are still poorly understood. We explore the ability of Prochlorococcus and Synechococcus to assimilate organic molecules with variable C:N:P composition and its modulation by light availability and photosynthetic impairment. We used a combination of radiolabelled molecules incubations with flow cytometry cell sorting to separate picoplankton groups from the western tropical South Pacific Ocean. Prochlorococcus and Synechococcus assimilated glucose, leucine and ATP at all stations, but cell-specific assimilation rates of N and P containing molecules were significantly higher than glucose. Incubations in the dark or with an inhibitor of photosystem II resulted in reduced assimilation rates. Light-enhanced cell-specific glucose uptake was generally higher for cyanobacteria (∼50%) than for the low nucleic acid fraction of bacterioplankton (LNA, ∼35%). Our results confirm previous findings, based mainly on cultures and genomic potentials, showing that Prochlorococcus and Synechococcus have a flexible mixotrophic metabolism, but demonstrate that natural populations remain primarily photoautotrophs. Our findings indicate that mixotrophy by marine cyanobacteria is more likely to be an adaptation to low inorganic nutrient availability rather than a facultative pathway for carbon acquisition.

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.