Degradation of dissolved organic monomers and short-chain fatty acids in sandy marine sediment by fermentation and sulfate reduction

The decay of a wide range of organic monomers (short-chain volatile fatty acids (VFA’s), amino acids, glucose and a pyrimidine) was studied in marine sediments using experimental plug flow-through reactors. The reactions were followed in the presence and absence of 10 mM SO42−. Degradation stoichiometry of individual monomers (inflow concentration of 6 mM organic C) was traced by measuring organic (VFA’s, amino acids) and inorganic (CO2, NH4+, SO42−) compounds in the outflow. Fermentation of amino acids was efficient and complete during passage through anoxic sediment reactors. Aliphatic amino acids (alanine, serine and glutamate) were primarily recovered as CO2 (24–34%), formate (3–22%) and acetate (41–83%), whereas only ∼1/3 of the aromatic amino acid (tyrosine) was recovered as CO2 (13%) and acetate (20%). Fermentation of glucose and cytosine was also efficient (78–86%) with CO2 (30–35%), formate (3%) and acetate (28–33%) as the primary products. Fermentation of VFA’s (acetate, propionate and butyrate), on the other hand, appeared to be product inhibited. The presence of SO42− markedly stimulated VFA degradation (29–45% efficiency), and these compounds were recovered as CO2 (17% for butyrate to 100% for acetate) and acetate (51% and 82% for propionate and butyrate, respectively). When reaction stoichiometry during fermentation is compared with compound depletion during sulfate reduction, the higher proportion CO2 recovery is consistent with lower acetate and formate accumulation. Our results therefore suggest that fermentation reactions mediate the initial degradation of added organic compounds, even during active sulfate reduction. Fermentative degradation stoichiometry also suggested significant H2 production, and >50% of sulfate reduction appeared to be fuelled by H2. Furthermore, our results suggest that fermentation was the primary deamination step during degradation of the amino acids and cytosine.