Coexistence of Dominant Marine Phytoplankton Sustained by Nutrient Specialization

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- 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 and an ecosystem model to show that the coexistence of these two organisms can arise from specialization in the uptake of distinct nitrogen substrates; Prochlorococcus prefers ammonium, whereas Synechococcus prefers nitrate when these nutrients exist at low concentrations. Our framework can be used for simulating and predicting the coexistence in the future ocean and may provide hints toward understanding other similar types of coexistence.

Evaluation of origin-depended nitrogen input through atmospheric deposition and its effect on primary production in coastal areas of western Kyusyu, Japan

Long term monitoring of atmospheric wet and dry depositions and associated nutrients fluxes was conducted on the coast of Japan facing the East China Sea continuously for 1 year and 2 months, with the origin of air mass investigated based on isotope analyses (Sr, Nd, and NO₃). During the same period, intensive observations of ocean conditions and the chemical composition of sinking particles collected using sediment traps were conducted to investigate the effects of atmospheric deposition-derived nutrients on phytoplankton blooms. Dry-deposition-derived nutrient inputs to the surface ocean were larger during autumn to spring than in summer due to the effect of continental air mass occasionally carrying Asian dust (yellow sand). However, these nutrients fluxes were limited (1.1-1.5 mg-N m^-2 day^-1 on average) and didn't appear to cause phytoplankton blooms through the year. Although average dissolved inorganic nitrogen (DIN) concentrations in rainwater were lower in oceanic air masses compared to continental air masses, wet-deposition-derived nutrient inputs to the surface ocean on rainy days during the summer (26.0 mg-N m^-2 day^-1 on average) were large due to higher precipitation from oceanic air masses. Wet-deposition-derived nutrients significantly increased nutrient concentrations in the surface ocean and seemed to cause phytoplankton blooms in the warm rainy season when nutrients in the surface were depleted due to increased stratification. The increase in phytoplankton biomass was reflected in increased particle sinking into the bottom layer, as well as changing chemical characteristics. The supply of flesh phytoplankton-derived labile organic matter into the bottom layer could be expected to promote rapid bacterial decomposition and contribute to the formation of hypoxic water masses in early summer when the ocean was strongly stratified. Atmospheric deposition-derived nutrients in East Asia will have important impacts on not only the oligotrophic outer ocean but also surrounding coastal areas in the warm rainy season.

Nitrogen burden from atmospheric deposition in East Asian oceans in 2010 based on high-resolution regional numerical modeling

East Asian oceans are possibly affected by a high nitrogen (N) burden because of the intense anthropogenic emissions in this region. Based on high-resolution regional chemical transport modeling with horizontal grid scales of 36 and 12 km, we investigated the N burden into East Asian oceans via atmospheric deposition in 2010. We found a high N burden of 2-9 kg N ha^-1 yr^-1 over the Yellow Sea, East China Sea (ECS), and Sea of Japan. Emissions over East Asia were dominated by ammonia (NH₃) over land and nitrogen oxides (NO_x) over oceans, and N deposition was dominated by reduced N over most land and open ocean, whereas it was dominated by oxidized N over marginal seas and desert areas. The verified numerical modeling identified that the following processes were quantitatively important over East Asian oceans: the dry deposition of nitric acid (HNO₃), NH₃, and coarse-mode (aerodynamic diameter greater than 2.5 μm) NO₃^-, and wet deposition of fine-mode (aerodynamic diameter less than 2.5 μm) NO₃^- and NH₄⁺. The relative importance of the dry deposition of coarse-mode NO₃^- was higher over open ocean. The estimated N deposition to the whole ECS was 390 Gg N yr^-1; this is comparable to the discharge from the Yangtze River to the ECS, indicating the significant contribution of atmospheric deposition. Based on the high-resolution modeling over the ECS, a tendency of high deposition in the western ECS and low deposition in the eastern ECS was found, and a variety of deposition processes were estimated. The dry deposition of coarse-mode NO₃^- and wet deposition of fine-mode NH₄⁺ were the main factors, and the wet deposition of fine-mode NO₃^- over the northeastern ECS and wet deposition of coarse-mode NO₃^- over the southeastern ECS were also found to be significant processes determining N deposition over the ECS.

Active nitrogen fixation by Crocosphaera expands their niche despite the presence of ammonium - A case study

Unicellular nitrogen fixer Crocosphaera contributes substantially to nitrogen fixation in oligotrophic subtropical gyres. They fix nitrogen even when significant amounts of ammonium are available. This has been puzzling since fixing nitrogen is energetically inefficient compared with using available ammonium. Here we show that by fixing nitrogen, Crocosphaera can increase their population and expand their niche despite the presence of ammonium. We have developed a simple but mechanistic model of Crocosphaera based on their growth in steady state culture. The model shows that the growth of Crocosphaera can become nitrogen limited despite their capability to fix nitrogen. When they fix nitrogen, the population increases by up to 78% relative to the case without nitrogen fixation. When we simulate a simple ecological situation where Crocosphaera exists with non-nitrogen-fixing phytoplankton, the relative abundance of Crocosphaera increases with nitrogen fixation, while the population of non-nitrogen-fixing phytoplankton decreases since a larger fraction of fixed nitrogen is consumed by Crocosphaera. Our study quantitatively supports the benefit of nitrogen fixation despite the high electron/energy costs, even when an energetically efficient alternative is available. It demonstrates a competitive aspect of Crocosphaera, permitting them to be regionally significant nitrogen fixers.

Biogeochemical controls of surface ocean phosphate

Surface ocean phosphate is commonly below the standard analytical detection limits, leading to an incomplete picture of the global variation and biogeochemical role of phosphate. A global compilation of phosphate measured using high-sensitivity methods revealed several previously unrecognized low-phosphate areas and clear regional differences. Both observational climatologies and Earth system models (ESMs) systematically overestimated surface phosphate. Furthermore, ESMs misrepresented the relationships between phosphate, phytoplankton biomass, and primary productivity. Atmospheric iron input and nitrogen fixation are known important controls on surface phosphate, but model simulations showed that differences in the iron-to-macronutrient ratio in the vertical nutrient supply and surface lateral transport are additional drivers of phosphate concentrations. Our study demonstrates the importance of accurately quantifying nutrients for understanding the regulation of ocean ecosystems and biogeochemistry now and under future climate conditions.