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

Marine Ecology Progress Series

Maren Voss¹  Deniz Bombar¹  Joachim W. Dippner¹  Jonathan P. Zehr¹  Birte Karfeld¹  Rachel A. Foster¹  Pia H. Moisander¹ 

关键词: Nitrogen fixation;    Phytoplankton ecology;    Trichodesmium;    Diatoms;    Symbiosis;    South China Sea;    River discharge;   

DOI  :  10.3354/meps08976

学科分类：海洋学与技术

来源: Inter-Research

【 摘 要 】

ABSTRACT: N₂ fixation by marine pelagic prokaryotes plays a critical role in supplying new N to the ocean, and there is growing evidence that oceanic N₂ fixation is generally enhanced in tropical river plumes, where N₂ 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 N₂ 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 N₂ at high rates.

【 授权许可】

Unknown