Subseafloor nitrogen redox processes at Loihi Seamount, Hawai

Abstract:

Loihi seamount is a mid-plate hotspot volcano located 30 kilometers off the southeast corner of the Big Island of Hawai'i. We measured temperature, concentrations of nitrate + nitrite (NO_x), ammonium (NH₄⁺), nitrite (NO₂^-) and dissolved silica (dSi), as well as the stable N and O isotopic composition of nitrate (δ¹⁵N and δ¹⁸O) in end-member hydrothermal fluids and microbial mats during three cruises in 2008, 2009 and 2013. We also sequenced 16S rRNA from archaea and bacteria from the same sites during 2006. NH₄⁺ ranged ~0.71-7.5 μM, was positively correlated to dSi and negatively correlated to NO_x. NO₂^- ranged from below detection to 0.49 μM and was not correlated to NO_x, dSi or NH₄⁺. In the microbial mats found at the Ula Nui site at 5000 m, NO₂^-, dSi and NH₄⁺ all increase with depth in the mat, creating a vertical zonation of niches within these mounds. Measured δ¹⁵N and δ¹⁸O of nitrate suggest biological production and reduction of nitrate are both occurring. Analysis of δ¹⁵N-NH₄⁺ is underway and will help constrain the relative magnitude of nitrification to NO₂^‑ and the potential for N-fixation. Using the measured concentrations of NO_x, NO₂^- and NH₄⁺ and other published data from Loihi, Gibbs energy calculations reveal that several catabolic strategies involving the reduction of NO₃^- and NO₂^- could provide energy to microbes in Loihi fluids. Thermodynamically feasible electron donors including HS^-, CH₄, Fe²⁺ and NH₄⁺, and the amount of energy available, in units of Joules per kg H₂O, varies by orders of magnitude from one sample site to the next. Pyrosequencing of the V6 region of bacterial and archaeal 16S rRNA from four sites at Loihi detected groups known to participate in denitrification, N-fixation and NO₂^- oxidation. Among these, the most abundant putative N-reducing microbes include Caldithrix, Epsilonproteobacteria, Thiohalophilus and members of the SAR324 clade. Putative N-fixers detected include members of the bacterial order Chlorobiales and archaeal methanogens in the genera Methanococcus and Methanothermococcus. Members of the NO₂^- oxidizing phylum Nitrispirae are present in all four samples, and are very abundant in two of them. All this data together reveals that N redox processes are significant sources of energy in subsurface Loihi fluids, and possibly at diffuse flow hydrothermal sites elsewhere