Several investigations on the biology of deep-sea microorganisms showed that their comparatively low
activity is mainly caused by the limited availability of degradable organic matter rather than by the high
pressure and low temperature which is characteristic of the abyssal environment. Yet, in deep-sea
sediments, bacteria occur in high numbers and account for most of the biological turnover of elements.
Evidence of their impact was found to depths of 500 meters below the sediment surface, indicating that some of the bacterial populations are efficiently adapted to extreme oligotrophy. Likely, the critical step in the nutrition of deep-sea bacteria is the breakdown of refractory polymeric substances by extracellular enzymes, since only small molecules can pass through the cell membrane. Therefore, to study the adaptation of natural microbial assemblages to life under severe nutrient limitation, this investigation focused on microbial degradative potentials in deep-sea sediments of different trophic status and on the factors regulating the activities of several of their extracellular enzymes.

Strong trophic gradients can be found downward continental slopes, since the supply with particulate organic matter decreases largely with distance from the productive coastal areas and with increasing water depth. Concentrations of different organic compounds as well as microbial biomass and enzymatic activities were measured along several transects down the Mediterranean and the Arctic continental slope,
representative of warm (>10 degrees C) and cold deep-sea environments (<2 deg C), respectively. The enzymatic potentials of the bacterial assemblages changed substantially in quantity and quality with
increasing oligotrophy of their environment. The activity of several carbohydrate degrading enzymes was strongly correlated with variables indicating food availability as well as with oxygen consumption in the
sediments, decreasing substantially down the continental slope. Enrichment experiments indicated that these enzymes are regulated by substrate induction, allowing effective utilization of incoming pulses of particulate organic matter (POM). In contrast, the activity of protein degrading enzymes was severalfold lower at the shelf edge compared to the oligotrophic open ocean sediments. This is likely due to negative regulatory mechanisms repressing enzyme synthesis when adequate substrates are available, to prevent excess energy expenses of the cells. The results of several enrichment experiments suggested that bacterial assemblages are able to adapt their degradative potentials to food pulses of different quality within days. Furthermore, the composition of the POM available to the bacterial assemblages in deep-sea sediments has substantial effects on the hydrolysis rates and the assimilation efficiencies of the cells and thus on the recycling of organic carbon and nitrogen.