A method to assess the toxicity of pollutants on anaerobic microbial degradation activity in sediments

Anaerobic microbial degradation of organic compounds in sediments plays an important role in the cycling of nutrients and elements in aquatic systems. In principle, anaerobic mineralization of organic matter in sediments proceeds via two steps: microbial fermentation of organic substances to acetate and carbon dioxide and fermentation of acetate to methane and carbon dioxide during methanogenesis. In order to assess the toxicity of pollutants on anaerobic microbial degradation activity in sediments the influence of selected chemicals on the carbon dioxide and methane production rates was investigated. Sediment samples were placed in serum bottles and treated with different concentrations of either phenanthrene, isoproturon or one of the four known metabolites of isoproturon. The head-space was flushed with nitrogen. After incubation for 24 hours at 20 degree C the carbon dioxide and methane production was measured by gas-chromatography. For phenanthrene and isoproturon, only a slight i nhibition (smaller than 10%) of the methane production rate was observed for concentrations up to 10 mg/l and 6,25 mg/l, respectively. The four metabolites of isoproturon significantly inhibited the methane production rate (up to 70%) at concentration of 50 - 500 Myg/l. However, the carbon dioxide production rate was unaffected by isoproturon and its metabolites at corresponding concentrations. Phenanthrene increased the carbon dioxide production rate up to 20%