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 oceans^1,2. 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 waters^2,3. 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.