Dinitrogen Gas Production From Epi- and Endolithic Basalt Communities at Loihi Seamount: A New Challenge for the Marine Nitrogen Budget

Current knowledge of the nitrogen cycle in the oceans indicates a net loss of biologically available nitrogen from the oceans. One marine habitat that is conspicuously absent from current nitrogen budget estimations is seafloor-exposed lavas, such as occur at young ridge flanks around mid-ocean ridge systems and seamounts. Basalt from two locations on Loihi Seamount, an actively venting volcano southeast of the island of Hawai'i, were incubated shipboard with 15N labelled nitrate and ammonium additions to determine the rate of nitrogen loss mediated by epi- and endolithic microbial communities. In all incubations a net production of dinitrogen gas occurred with concurrent consumption of oxidized nitrogen species (nitrate plus nitrite) and ammonium. The availability of reduced iron and manganese from the basalts could serve as potential electron donors for microbial metabolisms, including denitrification and anaerobic ammonia oxidizing (anammox) bacteria. The presence of 16S rRNA gene sequences that align with the anammox clade of Planctomycetes and ongoing anammox enrichment cultures inoculated with Loihi basalt suggest a role for anammox bacteria as well. This initial study indicates an active and significant role for basalt associated microbial communities in the nitrogen budget of the oceans.