Bacterial distribution and activity at the water-sediment boundary layer on NW Mediterranean continental margin

We studied the distribution of bacterial populations and their catabolic activities [C-14-glutamate respiration and Leu-MCA (L-leucine-7-amido-4-methyl-coumarin hydrochloride) hydrolysis] in a set of samples collected in the near bottom waters and in the superficial sediments (0 to 41 cm depth) originating from 9 cores collected between 585 and 2065 m depth in diverse NW Mediterranean continental margins. Bacterial densities and respiration rates in the sediment generally exceeded counts and rates in the near bottom water by 4 orders of magnitude. The relative contributions of benthic and pelagic bacteria to the organic matter turnover in this oceanic area are discussed. Data showed that the highest bacterial densities, as well as the highest rates of glutamate mineralization, were usually in the most superficial layers of sediment, whereas proteolysis rates were frequently maximal nearly 5 cm deeper. This pattern for proteolysis differs from the decreasing gradient usually described.