NITROGEN FIXATION IN DEEP-SEA SEDIMENTS (Invited Presentation)

Nitrogen fixation, the biological conversion of N₂ to NH₃, is the largest natural source of nitrogen to the biosphere, and can regulate productivity in nitrogen-limited habitats. Although intensely studied in the marine photic zone, little is known about N₂ fixation in the deep sea. If occurring in deep-sea sediments, which cover about two-thirds of the planet, it could be an important factor in both benthic productivity and ocean-scale elemental cycling. To determine the rates, spatial distribution, and biological mediators of deep-sea benthic nitrogen fixation, we collected diverse sediments from 100 to 4,500 meters water depth, including methane seeps, whale falls, and representative background sediments. We combined molecular analyses of the 16S rRNA and nifH genes with ¹⁵N₂ incubations followed by both bulk and single-cell isotope analyses (isotope ratio mass spectrometry and nanoscale secondary ion mass spectrometry, respectively). We found that nitrogen fixation was widespread, and exceeded previous rate measurements in deep sediments by over an order of magnitude in some locations. At methane seeps, nitrogen fixation is mediated primarily by methanotrophic archaea (ANME), but outside of methane seeps, N₂ fixation appears to be mediated by a diversity of anaerobic microorganisms. Our observations suggest a bigger role for N₂ fixation on the seafloor – and potentially the deep-biosphere – than previously realized.