METHANE PRODUCTION IN LITTORAL SEDIMENT OF LAKE CONSTANCE

Maximum rates of CH4 production in the littoral sediment were observed in 2-5 cm depth. The CH4 production rates increased during the year from about 5 mmol m-2d-1 in December to a maximum of about 95 mmol m-2d-1 in September. CH4 production rates showed a temperature optimum at 30-degrees-C and an apparent activation energy of 76 kJ mol-1. A large part of the seasonality of CH4 production could be ascribed to the change of the sediment temperature. Most of the produced CH4 was lost by ebullition. Gas bubbles contained about 60-70% CH4 with an average deltaC-13 of -56.2 parts per thousand and deltaD of -354 parts per thousand, and 2% CO2 with an average deltaC-13 of -14.1 parts per thousand indicating that CH4 was produced from methyl carbon, i.e. mainly using acetate as methanogenic substrate. This result was confirmed by inhibition of methanogenesis with chloroform which resulted in an accumulation rate of acetate equivalent to 81% of the rate of CH4 production. Most probable numbers of methanogenic bacteria were in the order of 10(4) bacteria g-1d.w. sediment for acetate-, methanol- or formate-utilizing, and of 10(5) for H-2-utilizing methanogens. The turnover times of acetate were in the order of 2.3-4.8 h which, with in situ acetate concentrations of about 25-50 muM, resulted in rates of acetate turnover which were comparable to the rates of CH4 production. The respiratory index (RI) showed that [2-C-14]acetate was mainly used by methanogenesis rather than by respiratory processes, although the zone of CH4 production in the sediment overlapped with the zone of sulfate reduction.