MOLECULAR ISOTOPIC EVIDENCE FOR CARBON-CYCLING IN ANAEROBIC METHANE OXIDIZING MICROBIAL COMMUNITIES IN EASTERN MEDITERRANEAN MUD VOLCANOS

Fluxes of methane to and from the atmosphere are influential in moderating earth's climate, potentially over both long (e.g. millions of years) and very short (e.g. instantaneous) time scales due to the strength of methane as a greenhouse gas. Methane seeps have been identified in mud volcano fields in the eastern Mediterranean Sea, which could potentially contribute significantly to the flux of methane to the atmosphere. However, these cold methane seeps stimulate and support a significant biological community in surficial sediments, including microbes (archaea and bacteria) as well as macrofauna such as clams, mussels, and tube worms, which apparently completely scavenges the evolving methane as it exits the sediments, consequently this methane never reaches the atmosphere. Previous studies have implicated anaerobic methane oxidation as the primary process scavenging methane from cold seeps and incorporating it into organic matter, however, the specific mechanisms and pathways of methane assimilation in these biological communities remain poorly understood. The present study investigates possible pathways of methane incorporation into microbial biomass. A number of extremely carbon isotopically depleted biomarkers have been identified which suggest that the process of anaerobic methane oxidation is being mediated primarily by a consortium of methane oxidizing archaea and sulfate reducing bacteria. Archaeal biomarkers identified include archaeol, hydroxyarchaeol, glycerol dialkyl glycerol tetraethers, and isoprenoids. Bacterial biomarkers identified include hopanoids (such as diploptene) and non-isoprenoid dialkyl glycerol diethers. Other biomarkers examined include those of terrestrial higher plants (n-alkanes, isoprenoids) and bactivorous ciliates (tetrahymanol). The distributions and carbon isotopic compositions of these biomarkers vary significantly within a few centimeters depth in a single core, and suggest that reverse methanogenesis (methane oxidation) is occurring in the sediments surrounding these methane cold seeps, possibly via the reverse aceticlastic methanogenesis pathway.