Contrasted geographical distribution of N2 fixation rates and nifH phylotypes in the Coral and Solomon Seas (southwestern Pacific) during austral winter conditions

Abstract

Biological dinitrogen (N2) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (southwestern Pacific) during austral winter conditions. N2 fixation rates were measurable at every station, but integrated (0-150 m) rates were an order of magnitude higher in the Solomon Sea (30 to 5449 μmol N m-2 d-1) compared to those measured in the Coral Sea (2 to 109 μmol N m-2 d-1). Rates measured in the Solomon Sea were in the upper range (100-1000 μmol N m-2 d-1) or higher than rates compiled in the global MARine Ecosystem biomass DATa database, indicating that this region has some of the highest N2 fixation rates reported in the global ocean. While unicellular diazotrophic cyanobacteria from group A (UCYN-A1 and UCYN-A2) and the proteobacteria γ-24774A11 dominated in the Coral Sea and were correlated with N2 fixation rates (p < 0.05), Trichodesmium and UCYN-B dominated in the Solomon Sea and were correlated (p < 0.05) with N2 fixation rates. UCYN-A were totally absent in the Solomon Sea. The biogeographical distribution of diazotrophs is discussed within the context of patterns in measured environmental parameters.