Bacterial numbers and growth in surficial deep-sea sediments and phytodetritus in the NE Atlantic: Relationships with particulate organic carbon and total nitrogen

Bacteria in deep-sea sediments constitute the largest global fraction of total benthic bacteria, and play a major role in most biogeochemical cycles. However, as yet the relationship between bacterial production and substrate availability (particulate organic carbon and nitrogen) in deep-sea sediments is not well understood. Therefore the aim of this study was to explore the relationships between bacterial numbers and DNA synthesis (a measure of bacterial growth) and the concentration of bulk POC and total nitrogen in surficial deep-sea sediments and recently deposited phytodetritus, at three sites in the NE Atlantic. We found statistically significant positive exponential relationships between bacterial numbers and [3H]-thymidine incorporation rates (DNA synthesis) and sediment and phytodetritus % particulate organic carbon (POC) and % total nitrogen (%TN) from samples collected in the deep NE Atlantic at three different times at 3 contrasting sites. Mean bacterial numbers were in the range 5.6–53.2×1010 cell l−1 in surficial sediments and 9.2–12.9×1010 cells l−1 in the phytodetritus; [3H]-thymidine incorporation rates were 14.8–593.7 pmol l−1 h−1 in the sediment and 395.8–491.4 pmol l−1 h−1 in the phytodetritus. POC concentrations were 0.22–0.61% in the sediment and 0.92–0.99% in the phytodetritus; TN was 0.04–0.09% in the sediment and 0.13–0.14% in the phytodetritus. The C:N ratio was 6.7–9.0 in sediment and 7.6–7.9 in phytodetritus. In addition, a positive exponential relationship was still evident with the inclusion of other data from the NE Atlantic and Solomon and Coral Sea. The logarithmic regression for this combined relationship was not statistically different from the one derived from the current data alone. However, the relationship failed with the inclusion of data from the highly eutrophic Arabian Sea, which has a mid-water oxygen minimum zone that may inhibit heterotrophic bacterial production. This implies that such a relationship is not generic to all ocean systems, and additional research is required to further test the relationship between bacterial [3H]-thymidine incorporation rates and % POC from other oceanic deep-sea surficial sediments.