The role of volatile fatty acids and hydrogen in the degradation of organic matter in marine sediments

Volatile fatty acids (VFA) and H2, important intermediates in the anaerobic degradation of organic matter, were studied in three sets of experiments using permanently cold sediments from Svalbard. The response of sulfate reduction (SR) and VFA and H2 to a temperature shift was monitored in permanently cold and temperate sediments. Low concentrations of the intermediates revealed a close coupling of fermentation and SR up to the optimum temperature (26 and 33°C for the cold and temperate sites, respectively). Degradation of organic matter was studied following three major carbon pools at different levels of the microbial food web (hydrolysis of carbohydrates, sugar and VFA degradation) and by monitoring the response to carbon addition. The carbon addition experiment revealed a faster response of the fermenting bacteria compared to the terminal oxidizers reflected in transient increasing intermediate concentrations and delayed increase in the dissolved inorganic carbon. The initial and terminal steps of the degradation (hydrolysis and SR) were similar to rates reported for temperate sites. VFA turnover was lower than usually reported from temperate sites, but similar to rates from the cold season. Hence, temperature seems to have different effects on the different steps of the complex degradation pathway.Finally, the effect of the H2 concentrations on processes not directly involving H2 was investigated. At steady state H2 concentrations are thermodynamically controlled by the terminal electron accepting process, explaining the spatial separation between the H2 oxidation by different electron acceptors. Other substrates for the terminal oxidizing bacteria do not show a similar thermodynamic control. It was found that methanogensis from methylamine and methanol is controlled by ambient H2 concentrations. The responsable bacterial H2 leakage is a potential mechanism for spatial separation of oxidation reactions from substrates that do not show a thermodynamic control.