Methane production by microbial mats under low sulphate concentrations

Cyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulphate concentrations ([SO(4)(2-)]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm palaeoclimates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulphate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [SO(4)(2-)] (c. 70 m M) and 'Archean' [SO(4)(2-)] (< 0.2 mM). At low [SO(4)(2-)], primary production in the mats was essentially unaffected, while rates of sulphate reduction decreased by a factor of three, and methane fluxes increased by up to 10-fold. However, remineralization by methanogenesis still amounted to less than 0.4% of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats were probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low environmental levels oxygen and sulphate.