Biological fate of Diuron and Sea-nine® 211 and their effect on primary microbial activities in slurries of a contaminated sediment from Venice Lagoon

The individual fate of the industrial booster biocides Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and Sea-nine® 211 (4,5-dichloro-2-n-octyl-4-isothiazolin-3-one) in a marine sediment of Porto Marghera (Venice Lagoon, Italy) was investigated, under anaerobic and aerobic conditions, in slurry-phase microcosms consisting of sediment suspended in its own overlaying water, i.e. under biogeochemical conditions which closely mimic those occurring in situ. Biocides were singly supplied as methanol solutions to a final concentration of 50 mg/kg of dry sediment. No Diuron degradation occurred throughout 3 months of incubation under any of the conditions tested. Conversely, Sea-nine®211 was promptly and completely degraded, regardless of the presence of oxygen. Under anoxic conditions, neither sulphate reduction nor methanogenesis occurred significantly in the non-amended microcosms, probably because of the very low availability of substrates in the actual site sediment and water employed. In contrast, methanogenesis occurred in the biocide amended microcosms where methanol was supplied as the carrier for biocides. No sulphate consumption was observed in the same microcosms. The large methane production observed in the parallel biocide-free control microcosms supplemented with methanol suggested that methylotrophic methanogens were the main compounds responsible for the detected activities. Diuron was found to intensify methane production, probably because it exerted an inhibitory effect on some competitors of indigenous methylotrophic methanogens.