Anaerobic methane oxidation associated with marine gas hydrates:: superlight C-isotopes from saturated and unsaturated C20 and C25 irregular isoprenoids

The activity of anaerobic methane-oxidizing microbes is revealed by extremely light C-isotope values of saturated and unsaturated C-20 and C-25 isoprenoids (e.g.. crocetane. 2,6,10,15,19-pentamethyleicosane) in reducing sediments of the Hydrate Ridge of the Cascadia continental margin. The C-13 values were up to -123.8% vs. Pee Dee Belemnite; they represent the isotopically lightest carbon so far isolated from the marine environment. At Hydrate Ridge destabilization of gas hydrate generates an enormous methane plume in the lower water column and saturates the sediments with methane. Based on isotope evidence of the biomarkers we conclude that methanogenic archaebacteria in a syntrophic consortium with sulfate reducers may be able to switch their metabolism from one of methane formation to another, favoring methane consumption. Methane hydrate destabilization at Hydrate Ridge and globally at cold seeps at all convergent margins supplies sufficiently high amounts of free methane to favor net methane oxidation under anaerobic conditions.