Abundance, vertical distribution, and community structure of benthic prokaryotes from permanently cold marine sediments (Svalbard, Arctic Ocean)

A detailed investigation of the benthic prokaryotic community from 3 permanently cold stations near Svalbard (Arctic Ocean) and 1 site near Tromso (northern Norway) was conducted. Prokaryotic abundances, determined by DAPI-staining, were in the range of ca 2 x 10(8) to 4 x 10(9) cells cm(-3) wet sediment. They showed little variation among sampling stations. Vertical profiles were characterized by a decrease of cell numbers with increasing sediment depth. The prokaryotic community composition was investigated employing rRNA (ribosomal RNA) slot-blot hybridization with domain-specific probes. Irrespective of station and vertical depth, Eubacteria always dominated the population, and the relative contribution of Archaea never exceeded 4 %. The measured total rRNA concentration and the prokaryotic cell counts in each sample were used to calculate per cell rRNA contents. Mean rRNA content (averaging all samples) was close to 3 fg rRNA cell(-1). None of our data showed considerable differences to comparable results from temperate or warm habitats; therefore our findings do not allow conclusions on special adaptations of the prokaryotic community to their existence in permanently cold systems. In all samples, but most pronounced in the 3 coldest stations, per cell rRNA contents showed steep vertical gradients with maximum values at the sediment surface. Taking into account all stations, rRNA concentration and prokaryotic abundance were strongly positively correlated below ca 5.5 cm (r(2) = 0.739), whereas in the upper sediment layers (0 to ca 5.5 cm) there was no significant correlation between these 2 parameters. This implies that there may be different mechanisms involved in the control of prokaryotic rRNA contents in different sediment horizons. Cellular rRNA concentrations can give an indication of growth rate and thereby the activity of prokaryotes. This is supported by the fact that we recorded the highest per cell, rRNA contents in those stations and sediment depths where other studies conducted simultaneously with our investigation demonstrated the highest rates of metabolic processes.