Niche construction by non-diazotrophs for N2 fixers in the eastern tropical North Atlantic Ocean

Diazotrophic dinitrogen (N2) fixation contributes ~76% to “new” nitrogen inputs to the sunlit open ocean, but environmental factors determining N2 fixation rates are not well constrained. Excess phosphate (phosphate–nitrate/16 > 0) and iron availability control N2 fixation rates in the eastern tropical North Atlantic (ETNA), but it remains an open question how excess phosphate is generated within or supplied to the phosphate-depleted sunlit layer. Our observations in the ETNA region (8°N–15°N, 19°W–23°W) suggest that Prochlorococcus and Synechococcus, the two ubiquitous non-diazotrophic cyanobacteria with cellular N:P ratios higher than the Redfield ratio, create an environment of excess phosphate, which cannot be explained by diapycnal mixing, atmospheric, and riverine inputs. Thus, our results unveil a new biogeochemical niche construction mechanism by non-diazotrophic cyanobacteria for their diazotrophic phylum group members (N2 fixers). Our observations may help to understand the prevalence of diazotrophy in low-phosphate, oligotrophic regions. Plain Language Summary Diazotrophic cyanobacteria, only phytoplankton group capable to utilize dinitrogen as N source, flourish in warm oceans when phosphate and iron nutrients are available. However, some of the phosphate-depleted warm waters of the Atlantic Ocean are mysteriously known for their existence of diazotrophs. Upwelling fluxes, atmospheric deposition, and river inputs are depleted in phosphate compared to nitrate in this region. Hence, the reasons behind the source of excess phosphate supply for diazotrophy in this region are not known. Here we have discovered that non-diazotrophic cyanobacteria create an environment of excess phosphate through a virtual biological membrane for their diazotrophic phylum group members. In an environment of sustained phosphate supply in the oligotrophic waters, iron inputs seem to determine the magnitude of nitrogen fixing rates.