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Identifying and explaining bottlenecks in organic carbon mineralization and the persistence of organic matter in marine sediments remain challenging. This study aims to illuminate the process of carbon flow between microorganisms involved in the sedimentary microbial food chain in anoxic, organic-rich sediments of the central Namibian upwelling system, using biogeochemical rate measurements and abundances of Bacteroidetes, Gammaproteobacteria, and sulfate-reducing bacteria at two sampling stations. Sulfate reduction rates decreased by three orders of magnitude in the top 20 cm at one sampling station (280 nmol cm−3 d−1 – 0.1 nmol cm−3 d−1) and by a factor of 7 at the second station (65 nmol cm−3 d−1 – 9.6 nmol cm−3 d−1). However, rates of enzymatic hydrolysis decreased by less than a factor of three at both sampling stations for the polysaccharides laminarin (23 nmol cm−3 d−1– 8 nmol cm−3 d−1 and 22 nmol cm−3 d−1– 10 nmol cm−3 d−1) and pullulan (11 nmol cm−3 d−1– 4 nmol cm−3 d−1 and 8 nmol cm−3 d−1– 6 nmol cm−3 d−1). Increasing imbalance between carbon turnover by hydrolysis and terminal oxidation with depth, the steep decrease in cell specific activity of sulfate reducing bacteria with depth, low concentrations of volatile fatty acids (less than 15 μM), and persistence of dissolved organic carbon, suggest decreasing bioavailability and substrate limitation with depth.
