Objectives: In the present study, the effect of acute mercury contamination on the structure and activity of bacterial communities in intertidal mudflats was assessed through a microcosm experiment simulating the mobilization of highly contaminated sub-surface sediments.
Methods: Box-microcosms corresponding to different test conditions were constructed by mixing natural estuarine sediments with high and low concentrations of mercury in defined proportions. The effects on sediment bacteria were characterized by quantifying bacteria using fluorescent in situ hybridization (FISH) technique, assessing the community structural diversity by denaturating gradient gel electrophoresis (DGGE) and analysing descriptors of bacterial activity (extracellular enzymatic activity and leucine incorporation) at the beginning and at the end of a 7-days incubation period.
Results: At the end of the experiment, total abundance of Bacteria was significantly higher in the low-Hg microcosms than in the high-Hg and blended-sediment microcosms. DGGE patterns revealed that the structure of sediment bacterial communities responded to the experimental treatment and to the incubation time. Bacterial activity was inhibited by mercury and that the levels of arylsulfatase and biomass productivity were inversely related with the Hg concentration. The proportion sulfate-reducing in relation to total prokaryotes increased at the end of the experiment, which might indicate a differential response of Bacteria and Archaea to confinement and mercury contamination.
Conclusion: Mechanical disturbance of sediments historically exposed to mercury contamination, like bottom trawling or dredging, will cause the mobilization of deeper sediments highly contaminated with Hg which will impact the less contaminated surface sediments. These acute events will impact the structure and activity of bacterial communities and their contributions to the associated biogeochemical cycles, with expectable impacts at the ecosystem level.