Rates and Regulation of Microbially Mediated Aerobic and Anaerobic Carbon Oxidation Reactions in Continental Margin Sediments From the Northeastern Arabian Sea (Pakistan Margin)

Rates of microbially mediated C oxidation were measured at sites above, within, and below the oxygen minimum zone (OMZ) on the Pakistan margin of the Arabian Sea, before and after the southwest monsoon, with the goal of assessing how low bottom water O-2 concentration affects microbial C oxidation processes. Rates of C oxidation coupled to aerobic and anaerobic processes were measured at five depths: 140 m (seasonally hypoxic), 300 m (OMZ core), 940 m (OMZ transition), 1200 m (OMZ transition), and 1850 m (non-OMZ). Rates and mechanisms of C oxidation did not vary significantly between seasons. However, an exception was found at the 140-m site, which became hypoxic during the southwest monsoon. Considering both seasons, C oxidation rates ranged from 0.73 to 4.86 mmol C m(-2) d(-1). Generally, OMZ sites and those on the OMZ transition had lower C oxidation rates (0.73-2.90 mmol C m(-2) d(-1)) than those located below the OMZ (3.13-4.86 mmol C m(-2) d(-1)). The relative importance of C oxidation via different terminal electron acceptors varied between sites according to the position and intensity of the OMZ. At all sites, a large proportion of measured O-2 consumption (30-100%) was coupled to the oxidation of reduced species; consequently, aerobic processes were essentially absent at low-O-2 sites. In contrast, under higher bottom water O-2 concentrations, aerobic processes accounted for 4-64% of C oxidation. Denitrification largely dominated carbon oxidation at all sites (36-99%). Rates of C oxidation coupled to microbial Mn4+ and Fe3+ reduction were quantitatively unimportant. Measured sulphate reduction rates at all sites across the margin were surprisingly low (0-0.45 mmol m(-2) d(-1)) compared to rates measured on other margin environments.