Measurement of in situ Methane Oxidation Rate in Hydrothermal Vent Area, Suiyo Seamount in the Izu-Bonin Arc

Methane is one of the important carbon sources for the hydrothermal vent ecosystem. Extensive studies about dissolved methane concentration and methane carbon isotope composition at hydrothermal vent area have been investigated, much remains to be learned about methane production and oxidation by microorganisms at the ecosystem. The quantitative estimation of the organic carbon supply to the hydrothermal vent ecosystem with methane oxidation by the microorganisms is an important study in order to grasp carbon flow of the ecosystem. In the present study, we measured in situ oxidation rate of dissolved methane at hydrothermal vent area in the Suiyo Seamount, Izu-Bonin (Ogasawara) Arc. The oxidation measuring equipment which was combined Niskin water sampler with Time-series water sampler having 12 acrylic cylinders was attached to the Japanese manned submersible SHINKAI 2000. At the sea floor, Niskin sampler was hold up just above the low temperature vent and sea water in the sampler was replaced in the hydrothermal fluid. Then the Niskin sampler closed just before starting time-series sampling and incubation experiment started on site. Hydrothermal fluid in Niskin sampler was sampled in the 30 min or 1 hour interval by Time-series water sampler and methane oxidation was stopped immediately by mercuric chloride solution added in each cylinder previously. Time-series samples were immediately transferred to the 30-ml glass bottles on board. Bottles were slowly overflowed with sample water and sealed with no headspace. After sealing, the samples were stored in freezer until methane concentration analysis at the laboratory. The dissolved methane concentration was measured with an automatic system consisting of a purge and trap apparatus and a gas chromatograph with a flame ionization detector. The in situ methane oxidation rate at the hydrothermal vent area was 0.016/hr, and methane consumption rate, which is obtained from multiplying the methane oxidation rate by the methane concentration, was 13.6 nM CH₄/hr. It suggests that dissolved methane oxidation by microorganisms should be an important organic carbon source for the hydrothermal vent ecosystem at the Suiyo Seamount.