Is the CH4, H2 and CO venting from submarine hydrothermal systems produced by thermophilic bacteria?

Abstract

Submarine hydrothermal vents are a major source of methane to the oceans1,2. The methane, as well as H2 and CO, are generally believed to result from degassing of the mantle or from abiogenic water–rock reactions1, a conclusion supported by direct correlations between 3He and CH4, and generally between CH4, H2 and CO and dissolved silicate in hydrothermal waters2,3. An alternative source for these gases might be microbiological. This would imply that active bacterial communities exist in deep-sea hot water environments, some of which have temperatures exceeding 100 °C; this inference is without precedent. We have now found that the super-heated waters emanating from sulphide chimneys at 21 °N along the East Pacific Rise and samples from the sulphide chimneys themselves harbour complex communities of bacteria capable of growing with generation times of 37–65 min, producing CH4, CO, H2 and traces of N2O in media containing S2O2−3, Mn2+ and Fe2+ as energy sources, and oxidizing CH4, at 100 ± 2 °C at 1 atm. These microbial communities consist of three to five morphologically distinct types and include both oxidative and anaerobic species. These mixed cultures will not grow at temperatures below 70–75 °C. Even though some of the communities originated from water of temperatures >300 °C, it is not known if they can grow and produce CH4, CO and H2 in super-heated waters kept liquid due to hydrostatic pressure. The discovery of these obligately thermophilic, gas-producing and consuming bacterial communities associated with submarine volcanic environments has interesting and important implications for prokaryotic evolution, marine geochemistry, industrial microbiology and exobiology.