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

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Science  16 Oct 2009:
Vol. 326, Issue 5951, pp. 422-426
DOI: 10.1126/science.1178223

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Balancing the Nitrogen Budget

Setting the global budget for elements presents difficult challenges, such as accounting for possibly unknown sources or sinks. An unresolved imbalance in the oceanic nitrogen budget suggests that there may be additional sources of biological nitrogen fixation in the deep sea. Using high-resolution imaging techniques, Dekas et al. (p. 422; see the Perspective by Fulweiler) observed direct assimilation of isotopically labeled N2 by anaerobic methane-oxidizing archaea from deep marine sediment and the subsequent transfer of nitrogen to their sulfate-reducing bacterial symbionts. This slow and energetically costly conversion by archaea is dependent upon methane and requires physical contact with the associated bacterial partner. Such syntrophic consortia represent a potential source of nitrogen in the oceans and may help to balance the global nitrogen budget.


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 15N incorporation, we showed that deep-sea anaerobic methane-oxidizing archaea fix N2, 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 N2 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 N2 fixation and reveals a link between the global carbon, nitrogen, and sulfur cycles.

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