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Deciphering the diversity of diazotrophs taking up dissolved organic matter in the north pacific ocean
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Edité par CCSD -
International audience. Dinitrogen (N2) fixation by diazotrophs sustains biological productivity in the ocean. Diazotroph groups include photoautotrophic cyanobacteria and non-cyanobacterial diazotrophs (NCDs). The metabolism and ecology of NCDs are almost unknown despite being prevalent in nifH gene surveys. NCDs are thought to be primarily heterotrophs, therefore must acquire carbon and energy from external sources of dissolved organic matter (DOM) to meet their physiological needs, still their interaction with organic matter remains unclear. Controversially, diazotrophic cyanobacteria thought to be mainly photoautotrophic have been shown to take up DOM to cope with nutritional and energetic stress. Traditional DOM amendment incubations do not allow for discerning which diazotrophs benefit directly from DOM uptake. Here we amended surface microbial communities from the North Pacific Ocean with 13C-labeled DOM derived from phytoplankton cultures and used a DNA stable isotope probing and nifH gene amplicon sequencing to identify diazotrophs capable of using DOM. We further used a metabolomics approach to characterize the 13C-labeled DOM composition. Bulk N2 fixation and DOC uptake rates in response to DOM additions were also measured. We observe that both cyanobacterial diazotrophs from the order Chroococcales and NCDs, mainly gamma- and alphaproteobacteria, uptake DOM, with a significant spatial variability driven by the background biogeochemical environment. Our results provide valuable insights into the use of DOM by diazotrophs as an alternative resource and their role in organic matter cycling in the ocean.
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