Contrasted geographical distribution of N-2 fixation rates and nifH phylotypes in the Coral and Solomon Seas (southwestern Pacific) during austral winter conditions

Biological dinitrogen (N-2) 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. N-2 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 mu mol N m(-2) d(-1)) compared to those measured in the Coral Sea (2 to 10(9) mu mol N m(-2) d(-1)). Rates measured in the Solomon Sea were in the upper range (100-1000 mu mol 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 N-2 fixation rates reported in the global ocean. While unicellular diazotrophic cyanobacteria from group A (UCYN-A1 and UCYN-A2) and the proteobacteria gamma-24774A11 dominated in the Coral Sea and were correlated with N-2 fixation rates (p < 0.05), Trichodesmium and UCYN-B dominated in the Solomon Sea and were correlated (p < 0.05) with N-2 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