The primary aim of my thesis was to investigate the role of volatile fatty acids (VFA) and H2 in the degradation of organic matter in permanently cold environments. VFA and H2 are important intermediates, produced by fermenting bacteria they serve as substrates for the terminal oxidizing bacteria, thus linking the fermentation with the terminal oxidation.

In one experiment the effect of temperature changes on sulfate reduction rates and VFA and H2 concentrations were studied in permanently cold and temperate sediments. Low concentrations of the intermediates over a broad temperature range revealed a close coupling between fermen-tation and sulfate reduction that was not disturbed by the temperature change in both sediments. Only above the optimum temperature for sulfate reduction (~26 and 33°C for the cold and temperate sites, respectively) the concentrations increased, showing a higher thermostability of the fermenting compared to the sulfate reducing bacteria. The response to temperature changes is reflected the different in situ temperatures of the samples with respect the optimum for sulfate reduction. The permanently cold sediments, however, are not more sensitive to temperature changes than the temperate sediments, which experience seasonal temperature changes.


Degradation of organic matter in permanently cold anoxic sediments was studied in two experiments. In the first experiment the response of the microbial community to substrate addition was monitored. In the second experiment the turnover of three different carbon pools at different levels of the microbial food web, hydrolysis, sugar- and VFA degradation, was measured. 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 carbon degradation rates were similar to rates observed in temperate sediments at higher temperatures, showing that the permanently low temperatures do not rule out high metabolic rates. The initial and terminal steps of the degradation (i.e. hydrolysis and sulfate reduction) measured in the second experiment were similar to rates reported for temperate sites. VFA turnover was lower than usually reported from temperate sites, but similar to rates measured during the cold season. Hence, temperature seems to have different effects on the different steps of the complex degradation pathway, potentially indicating a shift in the degradation pathway with changing temperature.

In the last experiments, the effect of the H2 concentrations on processes not directly involving H2 was investigated. At steady state H2 concentrations are controlled by the terminal electron accepting process. Microorganisms using the most favourable electron acceptor lower the H2 concentration to values that make it inaccessible for other microorganisms using less favorable electron acceptors. This thermodynamic control of the H2 concentration explains the spatial separation between the H2 oxidation steps using different electron acceptors. Other substrates for the terminal oxidising bacteria do not show a similar separation on a thermodynamic basis. Incubation experiments of methanogensis from methylamine and methanol revealed, however, that H2 concentrations exert control even on reactions H2 is not directly involved in. This is accomplished via hydrogenases that catalyse the transfer of electrons from the cellular electron carrier to H2. This leakage of H2 is dependent on the external H2 concentration. The leakage likely represents a side effect of the presence of the hydrogenase, which is capable of catalyzing this reaction rather than an energy yielding reaction. H2 leakage by terminal oxidizing bacteria is a potential mechanism for spatial separation of oxidation reactions from substrates that do not show a thermodynamic control.
For more information email: nfinke@mpi-bremen.de