FORMATION OF DIMETHYL SULFIDE AND METHANETHIOL IN ANOXIC FRESH-WATER SEDIMENTS

Concentrations of volatile organic sulfur compounds (VOSC) were measur ed in water and sediment columns of ditches in a minerotrophic peatlan d in The Netherlands. VOSC, with methanethiol (4 to 40 nM) as the majo r compound, appeared td be mainly of sediment origin. Both VOSC and hy drogen sulfide concentrations decreased dramatically towards the water surface. High methanethiol and high dimethyl sulfide concentrations i n the sediment and just above the sediment surface coincided with high concentrations of hydrogen sulfide (correlation factors, r = 0.91 and r = 0.81, respectively). Production and degradation of VOSC were stud ied in 32 sediment slurries collected from various freshwater systems in The Netherlands. Maximal endogenous methanethiol production rates o f the sediments tested (up to 1.44 mu mol per liter of sediment slurry .day(-1)) were determined after inhibition of methanogenic and sulfate -reducing populations in order to stop VOSC degradation. These experim ents showed that the production and degradation of VOSC in sediments a re well balanced. Statistical analysis revealed multiple relationships of methanethiol production rates with the combination of methane prod uction rates (indicative of total anaerobic mineralization) and hydrog en sulfide concentrations (r = 0.90) or with the combination of methan e production rates and the sulfate/iron ratios in the sediment (r = 0. 82). These findings and the observed stimulation of methanethiol forma tion in sediment slurry incubations in which the hydrogen sulfide conc entrations were artificially increased provided strong evidence that t he anaerobic methylation of hydrogen sulfide is the main mechanism for VOSC formation in most freshwater systems. Methoxylated aromatic comp ounds are likely a major source of methyl groups for this methylation of hydrogen sulfide, since they are important degradation products of the abundant biopolymer lignin. Increased sulfate concentrations in se veral fresh,vater ecosystems caused by the inflow of water from the ri ver Rhine into these systems result in higher hydrogen sulfide concent rations. As a consequence, higher flues of VOSC towards the atmosphere are conceivable.