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Biological dinitrogen (N2) fixation is a key metabolic process exclusively performed by prokaryotes, some of which are symbiotic with eukaryotes. Species of the marine haptophyte algae Braarudosphaera bigelowii harbor the N2-fixing endosymbiotic cyanobacteria UCYN-A, which might be evolving organelle-like characteristics. We found that the size ratio between UCYN-A and their hosts is strikingly conserved across sublineages/species, which is consistent with the size relationships of organelles in this symbiosis and other species. Metabolic modeling showed that this size relationship maximizes the coordinated growth rate based on trade-offs between resource acquisition and exchange. Our findings show that the size relationships of N2-fixing endosymbionts and organelles in unicellular eukaryotes are constrained by predictable metabolic underpinnings and that UCYN-A is, in many regards, functioning like a hypothetical N2-fixing organelle (or nitroplast)

AbstractThe proportion of major elements in marine organic matter links cellular processes to global nutrient, oxygen and carbon cycles. Differences in the C:N:P ratios of organic matter have been observed between ocean biomes, but these patterns have yet to be quantified from the underlying small-scale physiological and ecological processes. Here we use an ecosystem model that includes adaptive resource allocation within and between ecologically distinct plankton size classes to attribute the causes of global patterns in the C:N:P ratios. We find that patterns of N:C variation are largely driven by common physiological adjustment strategies across all phytoplankton, while patterns of N:P are driven by ecological selection for taxonomic groups with different phosphorus storage capacities. Although N:C varies widely due to cellular adjustment to light and nutrients, its latitudinal gradient is modest because of depth-dependent trade-offs between nutrient and light availability. Strong latitudinal variation in N:P reflects an ecological balance favouring small plankton with lower P storage capacity in the subtropics, and larger eukaryotes with a higher cellular P storage capacity in nutrient-rich high latitudes. A weaker N:P difference between southern and northern hemispheres, and between the Atlantic and Pacific oceans, reflects differences in phosphate available for cellular storage. Despite simulating only two phytoplankton size classes, the emergent global variability of elemental ratios resembles that of all measured species, suggesting that the range of growth conditions and ecological selection sustain the observed diversity of stoichiometry among phytoplankton.

Biological dinitrogen (N2)-fixation is an important source of N in the environment and is catalyzed exclusively by prokaryotes, some of which are symbionts with multicellular or unicellular eukaryotes. The marine cyanobacteria UCYN-A are unusual N2-fixing endosymbionts of unicellular haptophyte algae closely related to Braarudosphaera bigelowii. They depend on each other’s carbon (C) and N2 fixation, establishing symbiotic pairs between different UCYN-A lineages and host species that can vary in cell volume up to 1000-fold. However, whether this natural size variation might inform about the metabolic interdependency of such symbiotic interactions remains unknown. We found that the size ratio between UCYN-A and their algal hosts is strikingly conserved across different lineages/species and metabolic modeling shows that this size relationship maximizes coordinated growth rate based on tradeoffs between nutrient acquisition and exchange. The data also show that size relationships in the UCYN-A symbiosis are similar to those between organelle and cell-size in the eukaryotic plankton, suggesting similar metabolic and growth constraints. These findings suggest that the UCYN-A symbiont is functioning similarly to an organelle, with implications for the evolution of organelles as well as the potential for bioengineering of N2-fixing plant cells

Diatoms are a major phytoplankton group responsible for approximately 20% of carbon fixation on Earth. They perform photosynthesis using light-harvesting chlorophylls located in plastids, an organelle obtained through eukaryote-eukaryote endosymbiosis. Microbial rhodopsin, a photoreceptor distinct from chlorophyll-based photosystems, was recently identified in some diatoms. However, the physiological function of diatom rhodopsin remains unclear. Heterologous expression techniques were herein used to investigate the protein function and subcellular localization of diatom rhodopsin. We demonstrated that diatom rhodopsin acts as a light-driven proton pump and localizes primarily to the outermost membrane of four membrane-bound complex plastids. Using model simulations, we also examined the effects of pH changes inside the plastid due to rhodopsin-mediated proton transport on photosynthesis. The results obtained suggested the involvement of rhodopsin-mediated local pH changes in a photosynthetic CO2-concentrating mechanism in rhodopsin-possessing diatoms.

Prochlorococcus and Synechococcus are the two dominant picocyanobacteria in the low-nutrient surface waters of the subtropical ocean, but the basis for their coexistence has not been quantitatively demonstrated. Here, we combine in situ microcosm experiments and an ecological model to show that this coexistence can be sustained by specialization in the uptake of distinct nitrogen (N) substrates at low-level concentrations that prevail in subtropical environments. In field incubations, the response of both Prochlorococcus and Synechococcus to nanomolar N amendments demonstrates N limitation of growth in both populations. However, Prochlorococcus showed a higher affinity to ammonium, whereas Synechococcus was more adapted to nitrate uptake. A simple ecological model demonstrates that the differential nutrient preference inferred from field experiments with these genera may sustain their coexistence. It also predicts that as the supply of NO3 2 decreases, as expected under climate warming, the dominant genera should undergo a nonlinear shift from Synechococcus to Prochlorococcus, a pattern that is supported by subtropical field observations. Our study suggests that the evolution of differential nutrient affinities is an important mechanism for sustaining the coexistence of genera and that climate change is likely to shift the relative abundance of the dominant plankton genera in the largest biomes in the ocean. IMPORTANCE Our manuscript addresses the following fundamental question in microbial ecology: how do different plankton using the same essential nutrients coexist? Prochlorococcus and Synechococcus are the two dominant picocyanobacteria in the low-nutrient surface waters of the subtropical ocean, which support a significant amount of marine primary production. The geographical distributions of these two organisms are largely overlapping, but the basis for their coexistence in these biomes remains unclear. In this study, we combined in situ microcosm experiments a

The saturating relationship between phytoplankton growth rate and environmental nutrient concentration has been widely observed, yet the mechanisms behind the relationship remain elusive. Here, we use a mechanistic model of phytoplankton and show that the saturating relationship between growth rate and phosphorous concentration can be interpreted by intracellular macromolecular allocation. At low nutrient levels, the diffusive nutrient transport linearly increases with the phosphorous concentration, while the internal phosphorous requirement increases with the growth rate, leading to a non-linear increase in the growth rate with phosphorous. This increased phosphorous requirement is due to the increased allocation to biosynthetic and photosynthetic molecules. The allocation to these molecules reaches a maximum at high-phosphorous concentration, and the growth rate no longer increases despite the rise in phosphorous concentration. The produced growth rate and phosphorous relationships are consistent with the data of phytoplankton across taxa. Our study suggests that the key control of phytoplankton growth is internal, and nutrient uptake is only a single step in the overall process.

Biological nitrogen fixation is a key metabolic process exclusively performed by prokaryotes, some of which are in symbiosis with eukaryotes. Symbiotic interactions led to the evolution of organelles for photosynthesis and respiration but a true nitrogen-fixing organelle has never been reported. We found that the size ratio between the marine symbiotic nitrogen-fixing cyanobacteria UCYN-A and their algal hosts is strikingly conserved across different lineages, and consistent with size relationships of organelles in this symbiosis and other species. Metabolic modeling showed that this size relationship maximizes the coordinated growth rate based on tradeoffs between nutrient acquisition and exchange. This study suggests that size relationships of endosymbionts and organelles in unicellular organisms are constrained by predictable metabolic underpinnings, and that UCYN-A is functioning as a nitrogen-fixing organelle, or ‘nitroplast’. Keywords: Microbial symbiosis, nitrogen fixation, organellogenesis, UCYN-A

There is considerable debate about the benefits and trade-offs for colony formation in a major marine nitrogen fixer, Trichodesmium. To quantitatively analyze the trade-offs, we developed a metabolic model based on carbon fluxes to compare the performance of Trichodesmium colonies and free trichomes under different scenarios. Despite reported reductions in carbon fixation and nitrogen fixation rates for colonies relative to free trichomes, we found that model colonies can outperform individual cells in several cases. The formation of colonies can be advantageous when respiration rates account for a high proportion of the carbon fixation rate. Negative external influence on vital rates, such as mortality due to predation or micronutrient limitations, can also create a net benefit for colony formation relative to individual cells. In contrast, free trichomes also outcompete colonies in many scenarios, such as when respiration rates are equal for both colonies and individual cells or when there is a net positive external influence on rate processes (i.e., optimal environmental conditions regarding light and temperature or high nutrient availability). For both colonies and free trichomes, an increase in carbon fixation relative to nitrogen fixation rates would increase their relative competitiveness. These findings suggest that the formation of colonies in Trichodesmium might be linked to specific environmental and ecological circumstances. Our results provide a road map for empirical studies and models to evaluate the conditions under which colony formation in marine phytoplankton can be sustained in the natural environment.

Crocosphaera watsonii (hereafter referred to as Crocosphaera) is a key nitrogen (N) fixer in the ocean, but its ability to consume combined-N sources is still unclear. Using in situ microcosm incubations with an ecological model, we show that Crocosphaera has high competitive capability both under low and moderately high combined-N concentrations. In field incubations, Crocosphaera accounted for the highest consumption of ammonium and nitrate, followed by picoeukaryotes. The model analysis shows that cells have a high ammonium uptake rate (;7 mol N [mol N]21 d21 at the maximum), which allows them to compete against picoeukaryotes and nondiazotrophic cyanobacteria when combined N is sufficiently available. Even when combined N is depleted, their capability of nitrogen fixation allows higher growth rates compared to potential competitors. These results suggest the high fitness of Crocosphaera in combined-N limiting, oligotrophic oceans heightening its potential significance in its ecosystem and in biogeochemical cycling.

Rising temperatures are associated with reduced body size in many marine species, but the biological cause and generality of the phenomenon is debated. We derive a predictive model for body size responses to temperature and oxygen (O2) changes based on thermal and geometric constraints on organismal O2 supply and demand across the size spectrum. The model reproduces three key aspects of the observed patterns of intergenerational size reductions measured in laboratory warming experiments of diverse aquatic ectotherms (i.e., the “temperature-size rule” [TSR]). First, the interspecific mean and variability of the TSR is predicted from species' temperature sensitivities of hypoxia tolerance, whose nonlinearity with temperature also explains the second TSR pattern-its amplification as temperatures rise. Third, as body size increases across the tree of life, the impact of growth on O2 demand declines while its benefit to O2 supply rises, decreasing the size dependence of hypoxia tolerance and requiring larger animals to contract by a larger fraction to compensate for a thermally driven rise in metabolism. Together our results support O2 limitation as the mechanism underlying the TSR, and they provide a physiological basis for projecting ectotherm body size responses to climate change from microbes to macrofauna. For small species unable to rapidly migrate or evolve greater hypoxia tolerance, ocean warming and O2 loss in this century are projected to induce >20% reductions in body mass. Size reductions at higher trophic levels could be even stronger and more variable, compounding the direct impact of human harvesting on size-structured ocean food webs.

The dominant marine filamentous N2 fixer, Trichodesmium, conducts photosynthesis and N2 fixation during the daytime. Because N2 fixation is sensitive to O2, some previous studies suggested that spatial segregation of N2 fixation and photosynthesis is essential in Trichodesmium. However, this hypothesis conflicts with some observations where all the cells contain both photosystems and the N2-fixing enzyme nitrogenase. Here, we construct a systematic model simulating Trichodesmium metabolism, showing that the hypothetical spatial segregation is probably useless in increasing the Trichodesmium growth and N2 fixation, unless substances can efficiently transfer among cells with low loss to the environment. The model suggests that Trichodesmium accumulates fixed carbon in the morning and uses that in respiratory protection to reduce intracellular O2 during the mid-daytime, when photosynthesis is downregulated, allowing the occurrence of N2 fixation. A cell membrane barrier against O2 and alternative non-O2 evolving electron transfer also contribute to maintaining low intracellular O2. Our study provides a mechanism enabling N2 fixation despite the presence of photosynthesis across Trichodesmium.

Diatom–diazotroph associations (DDAs) are one of the most important symbiotic dinitrogen (N2) fixing groups in the oligotrophic ocean. Despite their capability to fix N2, ammonium (NH4+) remains a key nitrogen (N) source for DDAs, and the effect of NH4+ on their metabolism remains elusive. Here, we developed a coarse-grained, cellular model of the DDA with NH4+ uptake and quantified how the level of extracellular NH4+ influences metabolism and nutrient exchange within the symbiosis. The model shows that, under a fixed growth rate, an increased NH4+ concentration may lower the required level of N2 fixation and photosynthesis, and decrease carbon (C) and N exchange. A low-NH4+ environment leads to more C and N in nutrient exchange and more fixed N2 to support a higher growth rate. With higher growth rates, nutrient exchange and metabolism increased. Our study shows a strong effect of NH4+ on metabolic processes within DDAs, and thus highlights the importance of in situ measurement of NH4+ concentrations.

The publisher regrets that the published version of some references contained errors that did not exist in the original manuscript. Correctly, references [26–28, 30, 40] should read as follows: [26] Inomura K, Bragg J, Riemann L, Follows MJ. A quantitative model of nitrogen fixation in the presence of ammonium. PLoS One 2018;13(11):e0208282. [27] Inomura K, Wilson ST, Deutsch C. Mechanistic model for the coexistence of nitrogen fixation and photosynthesis in marine Trichodesmium. mSystems 2019;4(4):e00210-19. [28] Inomura K, Follett CL, Masuda T, Eichner M, Prášil O, Deutsch C. Carbon transfer from the host diatom enables fast growth and high rate of N2 fixation by symbiotic heterocystous cyanobacteria. Plants 2020;9(2):192. [30] Inomura K, Deutsch C, Wilson ST, Masuda T, Lawrenz E, Bučinská L, et al. Quantifying oxygen management and temperature and light dependencies of nitrogen fixation by Crocosphaera watsonii. mSphere 2019;4(6):e00531-19. [40] Mulderij G, Mooij WM, Smolders AJP, Van Donk E. Allelopathic inhibition of phytoplankton by exudates from Stratiotes aloides. Aquat Bot 2005;82(4):284–96. The publisher would like to apologise for any inconvenience caused.

Phytoplankton account for about a half of photosynthesis in the world, making them a key player in the ecological and biogeochemical systems. One of the key traits of phytoplankton is their growth rate because it indicates their productivity and affects their competitive capability. The saturating relationship between phytoplankton growth rate and environmental nutrient concentration has been widely observed yet the mechanisms behind the relationship remain elusive. Here we use a mechanistic model and metadata of phytoplankton to show that the saturating relationship between growth rate and nitrate concentration can be interpreted by intracellular macromolecular allocation. At low nitrate levels, the diffusive nitrate transport linearly increases with the nitrate concentration, while the internal nitrogen requirement increases with the growth rate, leading to a non-linear increase in the growth rate with nitrate. This increased nitrogen requirement is due to the increased allocation to biosynthetic and photosynthetic molecules. The allocation to these molecules reaches a maximum at high nitrate concentration and the growth rate ceases to increase despite high nitrate availability due to carbon limitation. The produced growth rate and nitrate relationships are consistent with the data of phytoplankton across taxa. Our study provides a macromolecular interpretation of the widely observed growth-nutrient relationship and highlights that the key control of the phytoplankton growth exists within the cell.

Diatoms are important microorganisms involved in global primary production, nutrient cycling, and carbon sequestration. A unique feature of diatoms is their silica frustules, which impact sinking speed, defense against predators and viruses, and growth cycling. Thus, frustules are inherently linked to their role in ecosystems and biogeochemical cycles. However, constraints on cellular silicon levels remain unclear and few existing models resolve diatom elemental stoichiometry to specifically include variable silicon levels. Here, we use a coarse-grained model of the diatom, Thalassiosira pseudonana, compared with laboratory results to illustrate the relationship of silicon uptake with elemental stoichiometry of other nutrients. The model-data comparison suggests the balance between growth rate and silicon uptake constrains the amount of cellular silicon. Additionally, it expresses relationships between silicon, nitrogen, phosphorus, and carbon to changing growth rates in nitrogen-limited and phosphorus-limited regimes. First, our model-data comparison suggests Si uptake hits a maximum cellular quota at low growth rates and below this maximum there is independent Si uptake. In each nutrient regime, Si:N, Si:P, and Si:C decrease exponentially with growth rate when Si is below the maximum limit. This is explained by independent Si uptake and increased loss of Si to new cells. These results provide predictions of diatom stoichiometry and allocation, which can be used in ecosystem models to differentiate phytoplankton types to better represent diatoms’ contribution to global biogeochemical cycles and ecosystems.