Microbial consumption rates of 1 to 20 nM dimethyl sulfide (DMS) in estuanne wholewater samples from coastal Georgia (USA) were measured by 2 methods production of radiolabeled CO2 and particulates >O 2 pm from ('4CH,)2S, and specific inhibition by addition of 500 FM chloroform The combination and companson 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 Kinehcs and rates appeared s m d a r over a several week penod in samples taken from the Duplin River, a coastal tidal nver where DMS concentrations were consistently in the range of 1 to 3 nM In contrast, samples of water whlch had recently flooded the marsh had much higher DMS concentrations, but consumption rates were simllar to those observed in the creek waters before they flooded the marsh The turnover tlmes calculated for the waters over the marsh were on the order of 3 to 7 d The radioisotope method showed that DMS assinillation into cell matenal was approximately equal to that respired as CO2, and confirmed 500 pM CHC13 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 PM), gave similar but lower consumption rates compared to chloroform, and show promise as specif~c inhibitors for future studies

ABSTRACT. Dimethylsulfon~opropionate (DMSP) is an organic sulfur compound which is produced by many marine phytoplankton and which is ubiquitous in the euphotlc zone of the ocean. DMSP is degraded through complex interactions within the food web and studies of its dynamics may lead to greater understanding of microbial ecology and food web interactions. In this study we examined the degradation of dissolved DMSP [DMSP(d)] in coastal water samples and tested glycine betaine (GBT), a structural analog of DMSP, as a potential inhibitor of this important biogeochemical reaction. The addition of 1 to 50 pM GBT to water samples from the northern Gulf of Mexico strongly inhibited the consumption of 50 nM added DMSP(d). The production of dimethyl sulfide (DMS) from DMSP(d) was also inhibited by GBT, but was slightly less sens~tive than overall DMSP degradation. The inhibitory effects of GBT were short-lived, lasting only 5 to 6 h, after which time net DMSP(d) consumption resumed. Several analogs of GBT were also found to be inhibitory to DMSP(d) degradation but unrelated compounds had no effects. Consistent with the inhibitory effects of GBT, we found that endogenous DMSP(d) concentrations increased at steady rates in response to GBT additions. These GBT-induced accumulation rates ranged from 4 to 28 nM d-' in water samples collected over the course of a year and may represent the natural turnover rates of DMSP(d). We found no significant effects of GBT on particulate DMSP concentrations in natural water samples or in an axenic culture of the prasinophyte Tetraselmis subcordiformis. However, addition of 50 pM GBT to the phytoplankton culture caused an accumulation of DMSP(d) (equivalent to 2 % of the particulate DMSP in the culture) for a period of 1 h with no change thereafter. GBT may be a useful inhibitor of DMSP(d) degradation (and DMS production) under some circumstances. However, the short-lived inhibitory effects of GBT and the potential for it to cause some direct release from the particulate DMSP pool may limit its application.