Methane production in littoral sediment of Lake Constance

Abstract

Maximum rates of CH 4 production in the littoral sediment were observed in 2–5 cm depth. The CH 4 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. CH 4 production rates showed a temperature optimum at 30°C and an apparent activation energy of 76 kJ mol −1. A large part of the seasonality of CH 4 production could be ascribed to the change of the sediment temperature. Most of the produced CH 4 was lost by ebullition. Gas bubbles contained about 60–70% CH 4 with an average δ 13C of −56.2% and δD of −354%, and 2% CO 2 with an average δ 13C of −14.1% indicating that CH 4 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 CH 4 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 μM, resulted in rates of acetate turnover which were comparable to the rates of CH 4 production. The respiratory index (RI) showed that [2− 14C]acetate was mainly used by methanogenesis rather than by respiratory processes, although the zone of CH 4 production in the sediment overlapped with the zone of sulfate reduction.