RADIOISOTOPE AND CHEMICAL INHIBITOR MEASUREMENTS OF DIMETHYL SULFIDE CONSUMPTION RATES AND KINETICS IN ESTUARINE WATERS

Microbial consumption rates of 1 to 20 nM dimethyl sulfide (DMS) in estuarine whole-water samples from coastal Georgia (USA) were measured by 2 methods: production of radiolabeled CO2 and particulates > 0.2 mum from ((CH3)2S)-C-14, and specific inhibition by addition of 500 muM chloroform. The combination and comparison of these 2 methods helped overcome some inherent limitations of each one, and both methods gave short turnover times (< 1 d) at low DMS concentrations (1 to 3 nM). DMS production was often as rapid as its consumption, suggesting very tight cycling. However, when exogenous DMS was added to increase concentrations, consumption rates saturated below 20 nM, suggesting high affinities for DMS. Kinetics and rates appeared similar over a several week period in samples taken from the Duplin River, a coastal tidal river where DMS concentrations were consistently in the range of 1 to 3 nM. In contrast, samples of water which had recently flooded the marsh had much higher DMS concentrations, but consumption rates were similar to those observed in the creek waters before they flooded the marsh. The turnover times calculated for the waters over the marsh were on the order of 3 to 7 d. The radioisotope method showed that DMS assimilation into cell material was approximately equal to that respired as CO2, and confirmed 500 muM CHCl3 as an excellent inhibitor of DMS consumption. However, it appears that chloroform may in some cases increase dissolved DMSP concentrations and hence DMS production, leading to overestimates of DMS consumption rates, Two other inhibitors of DMS consumption, dimethyl disulfide (100 nM) and dimethyl ether (30 muM), gave similar but lower consumption rates compared to chloroform, and show promise as specific inhibitors for future studies.