Methanogenic Degradation of Sodium Benzoate in Profundal Sediments from a Small Eutrophic Lake

SUMMARY: Enrichments were established to examine the potential of Blelham Tarn profundal sediment to metabolize benzoate to CH4 and CO2. Long‘adaptation’ times were required before benzoate-dependent CH4 production occurred, though both increased inoculum size and prior methanogenic adaption to aliphatic fatty acids reduced the adaptation time. Benzoate was metabolized according to the stoichiometry: 4C6H5COOH + 18H2O 15CH4 + 13CO2. The optimum temperature for CH4 production from benzoate in the enrichments was 37 °C irrespective of the enrichment temperature. Methanogenic benzoate degradation was associated with a particulate‘floc’ in the enrichments and Methanobacterium soehngenii was tentatively identified as an important constituent of this floc by scanning electron microscopy. Anaerobic benzoate fermentation was observed after 4 h in undiluted sediment by the use of [14C]benzoate, and the temperature optimum for 14C-labelled gas formation was 28 °C. The 14CH4: 14CO2 ratio indicated that methanogenic fermentation of benzoate was occurring in situ. CO2 became the main gaseous product from [14C]benzoate when sulphate was added to sediment, and 20 mM-molybdate reversed this effect. Methanogenesis was slightly inhibited by addition of 20 mM-molybdate. Methanogenic benzoate fermentation in sediments was found to be inhibited by H2.