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Diversity and activity of nitrogen‐fixing communities across ocean basins
Author(s) -
Gradoville Mary R.,
Bombar Deniz,
Crump Byron C.,
Letelier Ricardo M.,
Zehr Jonathan P.,
White Angelicque E.
Publication year - 2017
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.1002/lno.10542
Subject(s) - diazotroph , mesopelagic zone , biology , upwelling , ocean gyre , cyanobacteria , nitrogenase , pelagic zone , nitrogen fixation , ecology , oceanography , subtropics , geology , bacteria , genetics
Abstract Recent observations of N 2 fixation rates (NFR) and the presence of nitrogenase ( nifH ) genes from heterotrophic N 2 ‐fixing (diazotrophic) prokaryotes in unusual habitats challenge the paradigm that pelagic marine N 2 fixation is constrained to cyanobacteria in warm, oligotrophic, surface waters. Here, we compare NFR and diazotrophic diversity (assessed via high‐throughput nifH sequencing) from a region known to be dominated by cyanobacterial diazotrophs (the North Pacific Subtropical Gyre, NPSG) to two regions dominated by heterotrophic diazotrophs: the Eastern South Pacific (ESP, from the Chilean upwelling system to the subtropical gyre) and the Pacific Northwest coastal upwelling system (PNW). We observed distinct biogeographical patterns among the three regions. Diazotrophic community structure differed strongly between the NPSG, dominated by cyanobacterium UCYN‐A, and the ESP, dominated by heterotrophic nifH group 1J/1K, yet surface NFR were similar in magnitude (up to 5.1 nmol N L −1 d −1 ). However, while diverse, predominantly heterotrophic nifH genes were recovered from the PNW and the mesopelagic of the NPSG, NFR were undetectable in both of these environments (although glucose amendments stimulated low rates in the deep NPSG). Our work suggests that while diazotrophs may be nearly omnipresent in marine waters, the activity of this functional group is regionally restricted. Further, we show that the detection limits of the 15 N 2 fixation assay suggest that many of the low NFR reported for the mesopelagic (often < 0.1 nmol N L −1 d −1 in the literature) are not indicative of active diazotrophy, highlighting the challenges of assessing the ecosystem significance of heterotrophic diazotrophs.