Effects of Metals on Methanogenesis, Sulfate Reduction, Carbon Dioxide Evolution, and Microbial Biomass in Anoxic Salt Marsh Sediments

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Abstract
The effects of several metals on microbial CH4, CO2 and production and microbial ATP were examined in sediments from Spartina alterniflora communities. Anaerobically homogenized sediments were amended with 1000 ppm (ratio of wt of metal to dry wt of sediment) of various metals. Time courses in controls were similar for CH4, H2S and CO2, with short initial lags (0-4 h) followed by periods of constant gas production (1-2 days) and declining rates thereafter. Comparisons were made between control and experimental assays with respect to initial rates of production (after lag) and overall production. CH4 evolution was inhibited both initially and overall by CH3HgCl, HgS and NaAsO2. A period of initial inhibition was followed by a period of overall stimulation with Hg, Pb, Ni, Cd and Cu, all as chlorides, and with ZnSO4, K2CrO4 and K2Cr2O7. Production of CO2 was generally less affected by the addition of metals. Inhibition was noted with NaAsO2, CH3HgCl and Na2MoO4. Minor stimulation of CO2 production occurred over the long term with chlorides of Hg, Pb and Fe. Sulfate reduction was inhibited in the short term by all metals tested and over the long term by all but FeCl2 and NiCl2. Microbial biomass was decreased by FeCl2, K2Cr2O7, ZnSO4, CdCl2 and CuCl2 but remained generally unaffected by PbCl2, HgCl2, and NiCl2. Although the majority of metals produced an immediate inhibition of methanogenesis, for several metals this was only a transient phenomenon followed by an overall stimulation. The initial suppression of methanogenesis may be relieved by precipitation, complexation or transformation of the metal (possibly by methylation), with the subsequent stimulation resulting from a sustained inhibition of competing organisms (e.g., sulfate-reducing bacteria). For several environmentally significant metals, severe metal pollution may substantially alter the flow of C in sediments.