Biogeographic drivers of diazotrophs in the western Pacific Ocean

The global budget of marine nitrogen (N) is not balanced, with N removal largely exceeding N fixation. One of the major causes of this imbalance is our inadequate understanding of the diversity and distribution of marine N-2 fixers (diazotrophs) as well as their contribution to N-2 fixation. Here, we performed a large-scale cross-system study spanning the South China Sea, Luzon Strait, Philippine Sea, and western tropical Pacific Ocean to compare the biogeography of seven major diazotrophic groups and N-2 fixation rates in these ecosystems. Distinct spatial niche differentiation was observed. Trichodesmium was dominant in the South China Sea and western equatorial Pacific, whereas the unicellular cyanobacterium UCYN-B dominated in the Philippine Sea. Furthermore, contrasting diel patterns of Trichodesmium nifH genes and UCYN-B nifH gene transcript activity were observed. The heterotrophic diazotroph Gamma A phylotype was widespread throughout the western Pacific Ocean and occupied an ecological niche that overlapped with that of UCYN-B. Moreover, Gamma A (or other possible unknown/undetected diazotrophs) rather than Trichodesmium and UCYN-B may have been responsible for the high N-2 fixation rates in some samples. Regional biogeochemistry analyses revealed cross-system variations in N-2-fixing community composition and activity constrained by sea surface temperature, aerosol optical thickness, current velocity, mixed-layer depth, and chlorophyll a concentration. These factors except for temperature essentially control/reflected iron supply/bioavailability and thus drive diazotroph biogeography. This study highlights biogeographical controls on marine N-2 fixers and increases our understanding of global diazotroph biogeography.