Investigating Biological Nitrogen Cycling in Marine Methane Seeps via FISH-SIMS (Geobiology graduate students Anne Dekas and Abbie Green)
The biological anaerobic oxidation of methane (AOM) fuels thriving communities of micro- and macro-fauna at marine methane seeps throughout the world. AOM also provides a significant sink for naturally produced methane that would otherwise have a greenhouse warming effect in the atmosphere. Currently little is known about nutrient cycling in seep communities, and therefore what may limit growth and ultimately methane consumption. In particular, the sources of bioavailable nitrogen to these seep communities have not been fully elucidated. Variable measured nitrate concentrations, as well as low δ15N values in biomass, suggest that seep ecosystems may be nitrogen limited and are at least partially dependent on nitrogen fixation, the process by which atmospheric nitrogen is reduced to ammonia, to obtain bioavailable nitrogen. Although nif genes – the genes required for nitrogen fixation – have been recovered from seep sediments, the identity and activity of diazotrophic microorganisms within methane seeps is unknown. Recent metagenomic data collected from seep samples enriched in methane-oxidizing archaea and their sulfate-reducing bacterial symbionts suggest that these organisms are the source of the observed nif genes. This would imply that these remarkable consortia of archaea and bacteria are not only the main source of carbon to these diverse communities, but also of fixed nitrogen.
To investigate nitrogen cycling in marine methane seeps, bulk sediment from the Eel River Basin, CA, is incubated in vitro with methane (CH4) and one of several isotopically labeled nitrogen species, including dinitrogen (15N2), nitrate (15NO3), ammonia (15NH3) and cyanide (CN-) for up to six months. Florescence in situ hybridization is coupled with secondary ion mass spectroscopy (FISH-SIMS) to demonstrate the incorporation of 15N from these nitrogen species into seep organisms over time. The Caltech Nano-SIMS is employed to collect high resolution elemental and isotopic images to investigate the spatial distribution of 15N incorporation within individual aggregates of methane oxidizing archaea and the sulfate-reducing bacterial symbionts. More traditional molecular approaches, such as the characterization of nifH mRNA and DNA extracted from seep sediment are also employed. Preliminary results show 15N incorporation in aggregates incubated with 15N2, demonstrating that microorganisms at methane seeps can fix nitrogen and may be a source of new nitrogen for these high-productivity seep communities
Dekas, A.D., R. S. Poretsky, and V.J. Orphan (2009) Deep-sea archaea fix and share nitrogen in methane-consuming microbial consortia. Science. 326: 422-426
NSF MO/MIP "Collaborative Research: Examination of diverse anaerobic methane oxidizing Archaea and associated syntrophic relationships using high resolution molecular and isotopic methods" In collaboration with Christopher House, Penn State.