Methane emission from hypersaline microbial mats: Lack of aerobic methane oxidation activity

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Abstract
Methane emission was measured in intact cores of microbial mats taken from hypersaline Solar Lake (Sinai) and from salterns of the city of Eilat at salinities of 9% and 13%, respectively. The CH4 emission rates were 0.4–2.2 nmol cm−2 h−1 irrespectively of the incubation conditions, i.e. incubation in the light versus dark, with air versus argon headspace. CH4 emission rates did not increase under anaerobic conditions in the dark. The rate of CH4 emission also did not increase in the presence of potential inhibitors of CH4 oxidation, i.e. acetylene (≤ 8%), methyl fluoride (1.5%), or dimethyl ether (≤ 10%) indicating that the CH4 flux was not affected by CH4 oxidation. However, addition of 20% acetylene inhibited CH4 production and resulted in zero emission. Aerobic incubation of mat pieces in the presence of 0.1 to 10% CH4 did not result in uptake of CH4. Attempts to obtain enrichment cultures of methanotrophic bacteria from the microbial mats at 9% salinity failed. Measurement of O2 microprofiles using a poiarographic O2 electrode showed that O2 was available in darkness to at least 0.5 mm depth. in the light, O2 was produced by oxygenic photosynthesis, reached supersaturation at about 1.5 mm depth, and penetrated to at least 2.5 mm depth. Measurement of CH4 microprofiles using a gas diffusion probe showed that CH4 concentrations increased linearly from the surface down to > 20 mm depth. The CH4 flux calculated from the CH4 gradient was the same as the flux that was actually measured. All these experiments indicate that hypersaiine microbial mats contain no aerobic CH4 oxidation activity, and probably no methanotrophic bacteria, although both O2 and CH4 are available.

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