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

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 CH 4 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. CH 4 emission rates did not increase under anaerobic conditions in the dark. The rate of CH 4 emission also did not increase in the presence of potential inhibitors of CH 4 oxidation, i.e. acetylene (≤ 8%), methyl fluoride (1.5%), or dimethyl ether (≤ 10%) indicating that the CH 4 flux was not affected by CH 4 oxidation. However, addition of 20% acetylene inhibited CH 4 production and resulted in zero emission. Aerobic incubation of mat pieces in the presence of 0.1 to 10% CH 4 did not result in uptake of CH 4. Attempts to obtain enrichment cultures of methanotrophic bacteria from the microbial mats at 9% salinity failed. Measurement of O 2 microprofiles using a poiarographic O 2 electrode showed that O 2 was available in darkness to at least 0.5 mm depth. in the light, O 2 was produced by oxygenic photosynthesis, reached supersaturation at about 1.5 mm depth, and penetrated to at least 2.5 mm depth. Measurement of CH 4 microprofiles using a gas diffusion probe showed that CH 4 concentrations increased linearly from the surface down to > 20 mm depth. The CH 4 flux calculated from the CH 4 gradient was the same as the flux that was actually measured. All these experiments indicate that hypersaiine microbial mats contain no aerobic CH 4 oxidation activity, and probably no methanotrophic bacteria, although both O 2 and CH 4 are available.