Dominance of unicellular N2 fixation in the western tropical North Pacific Ocean (WTNP) during 2012 and 2013

Our understanding of a major exogenous new nitrogen sources to the global ocean, N2 fixation, in the rapidly changing western tropical North Pacific Ocean (WTNP) is yet limited. We conducted measurements of N2 fixation rate (157 N isotope tracer assay) in 2012 (July-September) and the diazotroph community structure (using the molecular method) in 2013 (May-June), which first spanned a large spatial range (13–20°N, 120–160°E) in the WTNP. Our independent evidences from both cruises show the dominance of unicellular N2 fixation, either for in situ N2 fixation activity or diazotroph community structure, over majority of the WTNP surface waters during the sampling periods. Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria Crocosphaera watsonii-dominated to Trichodesmium spp. dominated toward the western boundary current (Kuroshio) was also observed in 2013. We propose that nutrient availability may have played a major role in regulating the biogeography of N2 fixation. The role of N2 fixation in biogeochemical cycling and ecosystem function in this climate change-vulnerable region calls for further investigations.