Carbon cycling in a continental margin sediment: contrasts between organic matter characteristics and remineralization rates and pathways

Although particulate organic carbon (POC) in sediments is derived from diverse sources, characteristics of bulk POC are frequently used as indicators of the quality of organic matter potentially available to sedimentary microbial communities. In order to investigate the extent to which characteristics of POC relate to sedimentary metabolism, the rates of the initial and terminal steps of organic carbon remineralization (extracellular enzymatic hydrolysis, and sulfate, iron, and manganese reduction, respectively) were compared at three sites in Skagerrak dominated by different terminal remineralization processes. In parallel, dissolved organic carbon (DOC) inventories and bulk POC characteristics were assessed. At all three sites, bulk characteristics of POC were similar, with C/N close to 12, low sedimentary amino acid content, and moderate concentrations of total hydrolyzable carbohydrates. On average, just 12% of POC was characterizable as carbohydrates or amino acids. These characteristics are frequently considered typical of unreactive or low quality organic matter. At all three sites, however, organic carbon remineralization (measured as CO2 production and sediment O2 uptake) was quite high relative to other locations with similar bulk characteristics. A comparison of DOC inventories with rates of terminal remineralization demonstrated that at the three sites, on average 27, 8, and 31% of the sedimentary DOC pool must be turned over on a daily basis in order to support terminal respiration. Extracellular enzymatic activity, calculated as potential carbon turnover, was sufficient to support these rates. At these sites, standard chemical characterization of bulk POC does not reflect the reactivity and availability of substrates to the sedimentary microbial community. Carbon remineralization is likely fueled by a small fraction of POC, not distinguishable by measurement of bulk parameters, which is rapidly cycled through the DOC pool.