© The Author(s), 2016. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 198 (2017): 131-150, doi:10.1016/j.gca.2016.10.029.
The role of nitrogen cycling in submarine hydrothermal systems is far less studied than
that of other biologically reactive elements such as sulfur and iron. In order to address
this knowledge gap, we investigated nitrogen redox processes at Loihi Seamount,
Hawaii, using a combination of biogeochemical and isotopic measurements,
bioenergetic calculations and analysis of the prokaryotic community composition in
venting fluids sampled during four cruises in 2006, 2008, 2009 and 2013.
Concentrations of NH4+ were positively correlated to dissolved Si and negatively
correlated to NO3-+NO2-, while NO2- was not correlated to NO3-+NO2-, dissolved Si or
NH4+. This is indicative of hydrothermal input of NH4+ and biological mediation
influencing NO2- concentrations. The stable isotope ratios of NO3- (d15N and d18O) was
elevated with respect to background seawater, with d18O values exhibiting larger
changes than corresponding d15N values, reflecting the occurrence of both production
and reduction of NO3- by an active microbial community. d15N-NH4+ values ranged from
0‰ to +16.7‰, suggesting fractionation during consumption and potentially N-fixation
as well. Bioenergetic calculations reveal that several catabolic strategies involving the
reduction of NO3- and NO2- coupled to sulfide and iron oxidation could provide energy to
microbes in Loihi fluids, while 16S rRNA gene sequencing of Archaea and Bacteria in
the fluids reveals groups known to participate in denitrification and N-fixation. Taken
together, our data support the hypothesis that microbes are mediating N-based redox
processes in venting hydrothermal fluids at Loihi Seamount.
was supported by the NSF Microbial Observatories program (MCB 0653265), the
Gordon and Betty Moore Foundation (GBMF1609), NSF-OCE 0648287, the Center for
Dark Energy Biosphere Investigations (C-DEBI) and the NASA Astrobiology Institute — Life Underground (NAI-LU). Sequence data was generated as part of the Alfred P.
Sloan Foundation's ICoMM field project and the W. M. Keck Foundation.
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