Comparison of microbial dynamics in marine and freshwater sediments: Contrasts in anaerobic carbon catabolism1

The microbiota of freshwater and marine sediments serve similar roles in carbon degradation and nutrient regeneration. However, because of differences in the chemical environment between freshwater and marine systems, distinct physiological groups of bacteria dominate terminal carbon catabolism in each system. In general, the distribution and rates of microbial activities within a sediment are determined by availability of electron acceptors for respiration and metabolizable organic substrates. Sulfate ion is a primary factor in the distribution of microbial activities in anoxic sediments. At the high sulfate concentration found in seawater, sulfate reduction exceeds methanogenesis and is responsible for most of the organic carbon oxidation. The importance of methanogenesis in sediment metabolism increases as salinity and, hence, sulfate decreases. In freshwaters, methanogenesis is responsible for the bulk of terminal metabolism under anoxic conditions. The higher affinity of sulfate reducers for substrates that can be used by both groups (e.g. hydrogen, acetate, methanol), as well as the more favorable thermodynamic energy yields of sulfate respiration compared to methanogenesis, may account for the dominance of sulfate respiration under sulfate replete conditions. The sources of organic matter to marine and freshwater sediments can be qualitatively different. Complex structural polysaccharides and phenolic polymers (i.e. ligno-cellulose) may comprise a greater fraction of the organic input to freshwater systems. Organic compounds that act as osmoregulatory solutes in marine plants and animals may be unique substrates for bacteria of marine sediments. It is likely that these differences also result in distinct assemblages of microorganisms responsible for the breakdown of organic carbon. The quantity of organic matter present in the scdimcnts is also a major factor determining the magnitude and distribution of various microbial activities. Near the extremes of high and low organic loading, organic matter input may play a greater role than sulfate concentration in determining the relative importance of sulfate reduction or methanogenesis. In marine systems, methanogenesis occurs in the presence of sulfate ion, but only at the expense of substrates not utilized by sulfate reducers (“noncompetitive” substrates), such as methylamines and, possibly, methylated reduced sulfur compounds. Methanogenesis from noncompetitive substrates represents only a small fraction of the sulfate reduction or carbon catabolism observed in marine sediments. In marine systems with very high rates of organic matter deposition, sulfate can bc depleted to the extent that methanogenesis takes on quantitative significance. Although sulfate concentrations are usually insufficient to permit much sulfate reduction in lacustrine environments, in lakes with relatively low organic deposition, sulfate concentrations in the water (and that supplied to the sediments) can bc sufficient to allow for a more significant contribution of sulfate respiration to carbon catabolism.