Distribution of diazotrophic microorganisms and nifH gene expression in the Mekong River plume during intermonsoon

N ₂ fixation by marine pelagic prokaryotes plays a critical role in supplying new N to the ocean, and there is growing evidence that oceanic N ₂ fixation is generally enhanced in tropical river plumes, where N ₂ fixers (diazotrophs) benefit from riverine phosphorus and/or iron. Here we used nifH gene quantitative polymerase chain reaction (QPCR) and reverse transcription (RT) QPCR to investigate the horizontal distribution and activity of 9 diazotroph phylotypes in the Mekong River plume, South China Sea (SCS), in April 2007 (intermonsoon, lowest annual discharge). The nifH gene diversity was investigated by cloning and sequencing. Hydrodynamic modeling of the surface water advection revealed that the same water masses were sampled during the entire study, and helped to elucidate the physical forcing on diazotroph abundances and distributions. According to our estimates of nifH abundances and expression, the high N ₂ fixation rates detected in the Mekong plume can primarily be attributed to Trichodesmium spp. and the symbiotic diatom-diazotroph associations (DDAs) Rhizosolenia-Richelia and Hemiaulus -Richelia. NifH phylotypes from unicellular cyanobacterial groups B and C and a gammaproteobacterial group (24774A11) were exclusively detected and expressed at oceanic stations. Clone libraries of nifH were dominated by Trichodesmium spp. but also included proteobacterial- and cluster III-like nifH sequences, including new lineages, possibly representing other active diazotrophs in the Mekong plume and SCS offshore waters. This study extends the known distribution of quantitatively important oceanic diazotrophs, and shows that the Mekong River plume hosts a diverse diazotroph community, of which particularly Trichodesmium spp. and DDAs fix N ₂ at high rates.