Anaerobic methane oxidation associated with marine gas hydrates: superlight C-isotopes from saturated and unsaturated C-20 and C-25 irregular isoprenoids

The activity of anaerobic methane-oxidizing microbes is revealed by extreme ly light C-isotope values of saturated and unsaturated C-20 and C-25 isopre noids (e.g.. crocetane. 2,6,10,15,19-pentamethyleicosane) in reducing sedim ents of the Hydrate Ridge of the Cascadia continental margin. The C-13 valu es were up to -123.8% vs. Pee Dee Belemnite; they represent the isotopicall y lightest carbon so far isolated from the marine environment. At Hydrate R idge destabilization of gas hydrate generates an enormous methane plume in the lower water column and saturates the sediments with methane. Based on i sotope evidence of the biomarkers we conclude that methanogenic archaebacte ria 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.