Microbial biomass and activities in deep-sea sediments of the Eastern Mediterranean: trenches are benthic hotspots

Abstract

The relation of microbial biomass and extracellular enzymatic activities to substrate availability was investigated for the first time in deep-sea sediments of the Eastern Mediterranean basin, along a transect from the Gulf of Taranto (Italy) to the coasts of Egypt and Israel. Chloroplastic pigment equivalents (CPE), indicating the input of phytodetritus to the sediments, decreased from the shallow coastal stations around 200m water depth (4–6 μg cm −2) to the oligotrophic abyssal basins of the Ionian and Levantine Seas around 3000m water depth (0.1–1 μg cm −2). However, at two stations in the Hellenic Trench, at 4620 m and 3750 m, as well as in the Pliny Trench south of Crete at 4260 m, strikingly high accumulations of CPE above 10 μg cm −2 were recorded. The presence of relatively fresh detritus at these depths can only be explained by lateral flux of organic material down the steep slopes and its capture in the adjacent trenches. Consistently, microbial activity potentials in the trenches exceeded those at the abyssal plain by an order of magnitude. Microbial biomass was about 0.1 mg C cm −2 in the trenches and the coastal stations, compared to less than 0.05 mg C cm −2 at the abyssal plain stations. The increased availability of a potential food resource appears to have stimulated the deep-sea microbial assemblages to the same extent as those of shallower stations, suggesting that microbial activity in deep-sea sediments is mainly regulated by the level of food supply.