SUMMARY: Factors affecting methanogenesis in the sediments of a eutrophic lake were studied during late summer, a period during which CH gas production slowed down dramatically or stopped completely. The most active methanogenesis occurred in the surface sediments and the temperature optimum for the process in these deeper sediments was 30 °C. Addition of H or formic acid to sediment slurries stimulated CH production to a greater extent than did acetic or pyruvic acid. Analysis of the kinetics of the conversion of H to CH suggested that the sediments were severely limited in H, the concentration being considerably less than 2·5 μmol 1, the K for the process. Methanogenesis was not stimulated by the addition of trace quantities of Ni, Co, MoO or Fe ions but was inhibited by 0·5 mmol SO 1. Under natural conditions the sediments were also limited in SO and sulphate reducers acted as net H donors to the methanogens; addition of SO allowed the sulphate reducers to compete effectively for H. The addition of 20 mmol NaMoO 1 to sediments inhibited methanogenesis but this was not due entirely to its effect in the H transfer from sulphate reducers; it also inhibited CO uptake by sediments and the production of CH from CHCOOH and CO by cultures of methanogens. It is therefore inadvisable to use MoO at this concentration as a specific inhibitor of sulphate reducers in such freshwater sediments. Experiments with other inhibitors of methanogens suggested that they may interact with sulphate reducers, acetogens or anaerobic bacteria involved in fatty acid decomposition. Small, sealed sediment cores, which were used to reproduce natural conditions, particularly of available H concentration, were injected with trace quantities of HCO and CHCOOH. The results suggested that more than 75% of the CH was derived from CO and the remainder from CH. The overall rates of methanogenesis in the small cores agreed well with results from the field.


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