Microbial activity at the deep water sediment boundary layer in two highly productive systems in the Western Mediterranean: the Almeria-Oran front and the Malaga upwelling

Microbiological and biogeochemical studies were carried out in sediments and near bottom waters in the upwelling area off the Malaga coast and in the Almeria-Oran frontal zone. In these nutrient-rich conditions, metabolic activity is potentially limited by oxygen availability through the sediment depth. In the surficial sediments of the frontal zone, however, oxygen penetrated to a depth of 4 cm, allowing oxic mineralisation processes to occur throughout the layer. In the surficial sediments of the upwelling area, oxygen penetration was limited to the top 2.5 cm, leading to anoxic conditions. Glutamate respiration and global oxygen uptake rates were clearly higher than in the frontal zone. In the superficial sediments of the frontal zone, bacteria were less abundant and showed the lowest potential rate for mineralisation processes, but the highest rate for proteolysis. This discrepancy is probably due to differences in the quality of organic inputs into the two areas, with labile organic compounds reaching the sea bottom in the shallower upwelling zone. Such inputs enhance the mineralisation of low molecular weight monomers, whilst inhibiting the polymer hydrolysis processes. Conversely, in the deeper frontal zone, labile monomers become exhausted, decreasing the mineralisation rates. Concomitantly, bacteria have to develop ectoenzymatic activities in order to extract their carbon and energy from the available semi-labile polymers. Consequently, the theoretical relationship between the mineralisation and hydrolysis processes are tightly coupled in the upwelling area, and uncoupled in the frontal zone.