Mineralization rates of organic matter in freshwater sediments when different electron acceptors dominate.

Abstract: Microbial decomposition of organic matter in aquatic environments plays an important role in natural fluxes of methane and carbon dioxide because the gases are end-products in microbial energy metabolism of organic matter. Microbial metabolism depends on the use of electron donors and electron acceptors in redox reactions that generate energy for growth and maintenance. Energy yields can be used to envisage specific patterns of microbial redox reactions and these predictions depend on the hypothesis that, in a specified environment, the metabolic reaction that yields most energy will dominate over any competing reactions. The energy yield hypothesis indicates a sequential order in electron acceptor use by microbes and also make it tempting to conclude that degradation rate of organic matter is different depending on available electron acceptors. The main purpose of this thesis was to study how the presences of different electron acceptors in freshwater sediments influence organic matter decomposition. Mineralization rates of organic matter under six different conditions regarding the electron acceptor availability were investigated in a river sediment sample from Stångån, Sweden, by measuring carbon dioxide and methane production using gas chromatography. This was done during a fixed time period, in vials containing a mixture of water, sediment, buffer solution and a dominating electron acceptor. Six different metabolic processes; aerobic respiration, denitrification, manganese reduction, iron reduction, sulphate reduction and methanogenesis were included. The overall result indicates similar mineralization rates in both oxic and anoxic treatments. The result also indicates that methane formation was present in the iron reduction and methanogenesis treatments and not evident in the oxic treatments. Sulphate reduction, denitrification and manganese reduction seems to inhibit methanogenesis, but the result also indicates that no significant total mineralization was apparent when NO3- and Mn(IV) were the dominating electron acceptors. The similarities between oxic and anoxic mineralization rates indicates that organic matter degradation rates are not dependent on available electron acceptors and that degradation rates of organic matter are independent of the thermodynamically based energy yield.