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

Diazotrophic dinitrogen (N-2) fixation contributes similar to 76% to new nitrogen inputs to the sunlit open ocean, but environmental factors determining N-2 fixation rates are not well constrained. Excess phosphate (phosphate-nitrate/16 > 0) and iron availability control N-2 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 degrees N-15 degrees N, 19 degrees W-23 degrees 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 (N-2 fixers). Our observations may help to understand the prevalence of diazotrophy in low-phosphate, oligotrophic regions.