Anaerobic versus aerobic degradation of dimethyl sulfide and methanethiol in anoxic freshwater sediments

Degradation of dimethyl sulfide and methanethiol in slurries prepared from sediments of minerotrophic peatland ditches were studied under various cond itions, Maximal aerobic dimethyl sulfide-degrading capacities (4.95 nmol pe r mi of sediment slurry . h(-1)), measured in bottles shaken under an air a tmosphere, were 10-fold higher than the maximal anaerobic degrading capacit ies determined from bottles shaken under N-2 or H-2 atmosphere (0.37 and 0. 32 nmol per mi of sediment slurry h(-1), respectively). Incubations under e xperimental conditions which mimic the in situ conditions (i.e., not shaken and with an air headspace), however, revealed that aerobic degradation of dimethyl sulfide and methanethiol in freshwater sediments is low due to oxy gen limitation. Inhibition studies with bromoethanesulfonic acid and sodium tungstate demonstrated that the degradation of dimethyl sulfide and methan ethiol in these incubations originated mainly from methanogenic activity. p rolonged incubation under a H-2 atmosphere resulted in lower dimethyl sulfi de degradation rates. Kinetic analysis of the data resulted in apparent K-m values (6 to 8 mu M) for aerobic dimethyl sulfide degradation which are co mparable to those reported for Thiobacillus spp., Hhyphomicrobium spp., and other methylotrophs. Apparent K-m values determined for anaerobic degradat ion of dimethyl sulfide (3 to 8 mu M) were of the same order of magnitude. The low apparent K-m values obtained explain the low dimethyl sulfide and m ethanethiol concentrations in freshwater sediments that we reported previou sly. Our observations point to methanogenesis as the major mechanism of dim ethyl sulfide and methanethiol consumption in freshwater sediments.