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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.

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