Factors Affecting Methanogenesis and Associated Anaerobic Processes in the Sediments of a Stratified Eutrophic Lake

Abstract

SUMMARY: Factors affecting methanogenesis in the sediments of a eutrophic lake were studied during late summer, a period during which CH₄ 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 H₂ or formic acid to sediment slurries stimulated CH₄ production to a greater extent than did acetic or pyruvic acid. Analysis of the kinetics of the conversion of H₂ to CH₄ suggested that the sediments were severely limited in H₂, the concentration being considerably less than 2·5 μmol 1^-1, the K_m for the process. Methanogenesis was not stimulated by the addition of trace quantities of Ni²⁺, Co²⁺, MoO₄^2- or Fe²⁺ ions but was inhibited by 0·5 mmol SO₄^2- 1^-1. Under natural conditions the sediments were also limited in SO₄^2- and sulphate reducers acted as net H₂ donors to the methanogens; addition of SO₄^2- allowed the sulphate reducers to compete effectively for H₂. The addition of 20 mmol Na₂MoO₄ 1^-1 to sediments inhibited methanogenesis but this was not due entirely to its effect in the H₂ transfer from sulphate reducers; it also inhibited CO₂ uptake by sediments and the production of CH₄ from CH₃COOH and CO₂ by cultures of methanogens. It is therefore inadvisable to use MoO₄^2- 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 H₂ concentration, were injected with trace quantities of H¹⁴CO₃^- and ¹⁴CH₃COOH. The results suggested that more than 75% of the CH₄ was derived from CO₂ and the remainder from CH_3COOH. The overall rates of methanogenesis in the small cores agreed well with results from the field.