Bacterial proteolytic activity in sediments of the Subantarctic Indian Ocean sector

Organic material entering the oceanic benthic zone can be permanently buried or recycled to CO2 in the sediment. Therefore it is important to know the carbon flux across the sediment-water interface to determine the initial and rate-limiting step in carbon oxidation, a bacterial enzymatic activity. Bacterial density and ectoproteolytic activity, determined using a fluorogenic substrate analog (L-leucine-7-amino-4-methyl coumarin, Leu-MCA) were investigated in the water column and in sediments during the ANTARES 1 JGOFS cruise in the Indian sector of the Southern Ocean. A strong decrease in ectoproteolytic activity was observed with increasing water depth. Peak activity in surface water was three orders of magnitude less than in surface sediment. Analysis of experimental data revealed, for most sediment bacterial communities, the existence of a biphasic mechanism with different velocities for organic matter degradation and different affinities of enzymatic systems for substrates. To explain this, we hypothesize a strategy of bacterial communities that use the episodic supplies of organic matter reaching the sediment. Microbial ectoproteolytic activities were highest in surface sediment horizons and decreased progressively with depth. As benthic microbial activities reflect the quantity and quality of organic matter reaching the sea floor, high potential ectoproteolytic activities (PEA) measured in the sediment of the Polar Front Zone could indicate a direct and rapid coupling of relatively high surface productivity and deep ocean water by sinking particle fluxes. Lower Values of PEA were found in sediment in the northern study area where lateral processes associated with ocean circulation (Antarctic Circumpolar Current) have an important influence on settling particles. The lowest sediment PEA values were measured at 52 degrees S, a region in which low primary production provides a poor supply of organic matter for the sea floor. (C) 1997 Elsevier Science Ltd.