Microbial respiration and diffusive oxygen uptake of deep-sea sediments in the Southern Ocean (ANTARES-I cruise)

Abstract

Benthic microbial respiration and diffusive oxygen uptake were measured, and used to calculate rates of aerobic mineralisation of organic matter and concomitant CO 2 production at the sediment water interface in the abyssal region of the Crozet Basin. This study was part of the ANTARES-I cruise on the R.V. Marion Dufresne, in the Permanent Open Ocean Zone of the Southern Ocean on a south to north transect from 52° to 42°S in the Indian Ocean. At all stations, oxygen penetrated much deeper than 10 cm. Aerobic respiration was maximal in the top 1 cm (10–96 nmol cm −3 day −1), was always detectable down to 5 cm depth, and at some stations even to 10 cm depth. Total depth-integrated oxygen consumption corresponded to within ±25% of the diffusive oxygen uptake across the sediment water interface. The shape of the profile indicated that a diffusive downward flow of oxygen occurred below 10 cm depth. Thymidine incorporation experiments suggested that bacteria, present at depths of between 15 cm and 1 m in the sediment, were in a dormant state or growing extremely slowly. However, rapid DNA-synthesis started within 4 h after thymidine was added, indicating a deep bacterial biosphere in Southern Ocean sediments. It is proposed that the diffusive downward flux of oxygen below 10 cm depth sustains aerobic bacterial metabolism and survival at greater depths. Along the transect, the total depth-integrated oxygen uptake peaked at 48°S close to the Polar Front, and at the Subantarctic and Subtropical Convergence Frontal Zone. Nevertheless, in general, the differences were not very pronounced. The average value of depth-integrated microbial O 2-consumption was 0.61 mmol m −2 day −1, which is equivalent to a carbon mineralisation rate of 2.3 g C m −2 year −1. These observations, together with relatively high Electron Transport System (ETS)-values (6.6 μl O 2 g −1 h −1) and bacterial numbers (4 × 10 9 g −1 dry weight) in the top centimeter, imply that this region is less oligotrophic than previously assumed.