Manila clams from Hg polluted sediments of Marano and Grado lagoons (Italy) harbor detoxifying Hg resistant bacteria in soil tissues

A previous survey on Hg bioaccumulation in Manila clams (Ruditapes philippinarum) in the polluted Marano and Grado lagoons suggested an efficient mechanism of mercury detoxification. In this study we demonstrated that this event might be partly related to the detoxifying activities of Hg-resistant bacteria (MRB) harbored in clam soft tissues. Natural clams were collected in six stations from Marano and Grado Lagoons during two different periods (winter and spring). Siphons, gills and hepatopancreas from acclimatized clams were sterile dissected to isolate MRB. These anatomical parts were glass homogenized or used for whole, and they were lying on a solid medium containing 5 mg.l-1 HgCl2 and incubated at 30°C. A total of fourteen bacterial strains were isolated and further affiliated, through 16S rRNA genes sequencing and analysis, to eight bacterial genera, four of which were Gram-positive (Enterococcus, Bacillus, Jeotgalicoccus and Staphylococcus) and the other four were Gram-negative (Stenotrophomonas, Vibrio, Raoultella and Enterobacter). Plasmids and merA genes were found in some strains. Fluorescence in situ hybridization (FISH) using different molecular probes revealed the presence of Firmicutes, Actinobacteria and Gammaproteobacteria in siphon and gills. Bacterial clumps inside clam flesh were observed by optical and transmission electronic microscope and the mercury detoxifying activity was confirmed by the determination of mercuric reductase in glass-homogenized siphons and gills.