Deep-Sea Archaea Fix and Share Nitrogen in Methane-Consuming Microbial Consortia

Nitrogen-fixing (diazotrophic) microorganisms regulate productivity in diverse ecosystems; however, the identities of diazotrophs are unknown in many oceanic environments. Using single-cell–resolution nanometer secondary ion mass spectrometry images of ^(15)N incorporation, we showed that deep-sea anaerobic methane-oxidizing archaea fix N_2, as well as structurally similar CN^–, and share the products with sulfate-reducing bacterial symbionts. These archaeal/bacterial consortia are already recognized as the major sink of methane in benthic ecosystems, and we now identify them as a source of bioavailable nitrogen as well. The archaea maintain their methane oxidation rates while fixing N_2 but reduce their growth, probably in compensation for the energetic burden of diazotrophy. This finding extends the demonstrated lower limits of respiratory energy capable of fueling N_2 fixation and reveals a link between the global carbon, nitrogen, and sulfur cycles.