Reductive dechlorination of the model compound 2,4-dichlorophenol in Chesapeake Bay sediments: Effects of sulfur biogeochemistry

Sulfate inhibition of microbial reductive dechlorination of chlorinated aryl compounds has been widely observed in many environments. Environmental factors that promoted the onset of dechlorination of the model compound, 2,4-dichlorophenol (DCP), were investigated in sulfate-reducing sediments from three sites along the salinity gradient of the Chesapeake Bay.; Sediments from a seasonally sulfate-limited mid Bay (MB) site were repeatedly assayed for dechlorination activity. In fresh MB sediments, ortho dechlorination of DCP to 4-chlorophenol (4CP) was initiated rapidly. Rates of ortho dechlorination were biphasic in surface sediments: slow and steady in the presence of sulfate and rapid upon depletion of sulfate with concomitant increases in hydrogen levels. 4CP persisted at levels nearly stoichiometric to the amount of DCP degraded in the absence of sulfate, but was degraded under sulfate-reducing conditions. Rates of dechlorination decreased and inhibition of sulfate reduction (SR) by high doses of DCP increased with sediment depth. Sulfate partially inhibited dechlorination rates, but inhibition was most evident when SR rates were also limited by carbon or electron donor levels. In both the presence and absence of sulfate, the greatest controlling variable on reductive dechlorination rates was the rate of electron donor production.; Reductive dechlorination was severely inhibited by molybdate, an inhibitor of sulfate reducing bacteria (SRB), in both the presence and absence of sulfate. SR was active in low sulfate, methanogenic MB incubations, and apparently maintained by anoxic sulfur cycling. SRB or other bacteria participating in sufur cycling were strongly implicated as the dechlorinating bacteria.; A rapid onset of ortho reductive dechlorination of DCP was also observed in sediments from the upper and lower Bay (LB) sites. Initiation of dechlorination did not appear to be linked with variations in salinity, total organic matter, redox potential or sulfate levels among the three sites. However, relative rates of dechlorination with depth mirrored relative rates of SR, which followed labile carbon distributions. DCP was degraded under either nitrogen or air atmospheres in LB sediment incubations. Sulfate did not inhibit anoxic ortho dechlorination at LB and was required for degradation of the metabolites, 4CP and phenol; involvement of SRB in these reactions was indicated.