Symbiotic Methane Oxidizers

Symbiotic methane-oxidizing bacteria are found at hydrothermal vents and cold seeps in the deep sea and in terrestrial wetlands. These very different habitats share a common feature: They provide access to both oxic, electron acceptor-rich fluids, and anoxic, methane-rich fluids. By teaming up with bacteria that can gain energy from the oxidation of CH4 with O2, the plant or animal host is indirectly able to live from an energy source that is otherwise only available to methanotrophic microorganisms. Methane is both an energy and carbon source for the bacteria, which contribute organic compounds to their host as a source of nutrition. The host, in turn, provides a stable environment for the bacteria, and provides access to both electron donors and acceptors. As no symbiotic methane oxidizer is available in pure culture, all evidence to date for these symbiotic associations comes from ultrastructural, enzymatic, physiological, stable isotope, and molecular biological studies of the symbiotic host tissues. We present an overview of the range of hosts and habitats in which symbiotic methane oxidizers are found, summarize the ways in which methanotrophic metabolism in the symbiotic bacteria has been confirmed in the past, and identify directions for future research and the methodological developments that will help us understand how symbiotic methane oxidizers interact with their hosts, and how both are influenced by their environment.