Bacterial community structure and activity in fish farm sediments of the Ligurian Sea (Western Mediterranean)

The environmental impact of a well-established fish farm has been investigated in surface sediments of the Ligurian Sea in order to assess the biodeposition, bacterial community structure and dynamics at a mature stage of organic enrichment. The Biopolymeric carbon (BPC) fraction of organic matter and phytopigment concentrations displayed very high values beneath the fish cages. In particular lipid, carbohydrate and chlorophyll-a concentrations were higher in farm sediment while protein concentrations did not show significant change between farm sediment and control. Benthic bacteria were closely related to organic enrichment and their density was three times higher (up to 3 × 1010 cells g−1) in stations beneath the cages being positively correlated with BPC (n = 10, p < 0.05) and lipid (n = 10, p < 0.05) concentrations. Colony forming units (CFU) counts of heterotrophic bacteria indicate a shift in the relative importance of the gram negative bacterial fraction, displaying the predominance of the Cytophaga/Flexibacter-like bacteria (CBF), as well as the occurrence of pathogenic bacteria (such as Vibrio) in sediments beneath the farm. In contrast, Gram positive bacteria were more prevalent in control site where they represented up to 90% of total isolates. Aminopeptidase activity displayed higher values in sediment beneath the cages, whereas the enzymatic activity per bacterial cell was lower. These data suggest a functional stress of bacterial degradation rates and represent a potential valuable environmental index of imbalance between supply and removal of organic matter in eutrophicated environments. Data presented in this study also suggest that either the biochemical composition of sedimentary organic matter as well as the selected microbial variables may represent useful tools for evaluating the effects of organic enrichment due to fish farming and could be proposed as new environmental indices of aquaculture impact on marine sediment.