Even though the relative contribution to the global warming potential from anthropogenically produced CO2 and CH4 is estimated at 49 and 17 percent, respectively, the generation of CH4 is of primary concern because it is not only increasing at a rate of 1.1 percent per year but it is also 32 times more effective in absorbing energy from the sun. The primary source of methane is suspected to be biological in origin and, according to the 1990 report of the Japanese Intergovernmental Panel for

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... ate Change (IPCC), about 45 percent of the total methane emissions are produced not only in lake, marsh and coastal zone ecosystems but also rice paddy fields and the digestive systems of ruminant animals. The objective of the present study was to elucidate the mechanisms of interaction between sulfate-reducing and methane-producing bacteria as sources of methane emission to the atmosphere. The experiments, carried out in cylindrical microcosms containing lagoon sediments covered by seawater of varying salinity, showed that the sulfate-reducing reaction was dominant in suppressing the methane fermentation reaction. Next, these competitive bacterial interactions where investigated as a function of both salinity and the organic and sulfur concentrations in the sediments. This was accomplished by covering the respective sediment samples with seawater, seawater diluted with deionized water and deionized water, each with and without the addition of glucose solution, (used to simulate organic loadings to brackish marsh ecosystems from wastewater effluents). The data showed that the amount of sulfide produced in the sediments followed a decreasing order: 100%, 50%, 25% and 0% seawater, respectively. In addition, it was established that this decreasing order, as a function of the dilution effect, also occurred for cations such as K+ and Na+. The addition of glucose significantly enhanced the sulfide formation. Conversely, the decreasing sulfide production as a function of decreasing salinity coincided with an increase in the production of methane. These results suggested that, in brackish marsh ecosystems, the sulfate-reduction reaction is favored and appears to retard the methane fermentation reaction.