Kinetic Parameters of the Conversion of Methane Precursors to Methane in a Hypereutrophic Lake Sediment

Abstract

The kinetic parameters Ki, V,,,m, Tt (turnover time), and v (natural velocity) were determined for H2 and acetate conversion to methane by Wintergreen Lake sediment, using short-tern (a few hours) methods and incubation temperatures of 10 to 140C. Estimates of the Michaelis-Menten constant, Km, for both the consumption of hydrogen and the conversion of hydrogen to methane by sediment microflora averaged about 0.024 pmol g-' of dry sediment. The maximnal velocity, Vmax, averaged 4.8 ,umol of H2 g h-1 for hydrogen consumption and 0.64 ,umol of CH4 g-1 h-1 for the conversion of hydrogen to methane during the winter. Estimated natural rates of hydrogen consumption and hydrogen conversion to methane could be calculated from the Michaelis-Menten equation and estimates of Km, Vmax, and the in situ dissolved-hydrogen concentration. These results indicate that methane may not be the only fate of hydrogen in the sediment. Among several potential hydrogen donors tested, only formate stimulated the rate of sediment methanogenesis. Formate conversion to methane was so rapid that an accurate estimate of kinetic parameters was not possible. Kinetic experiments using [2-14C]acetate and sediments collected in the summer indicated that acetate was being converted to methane at or near the maximal rate. A minimum natural rate of acetate conversion to methane was estimated to be about 110 mnol of CH4 g-' h-', which was 66% of the V.,= (163 nmol of CH4 g-1 h-1). A 15-min preincubation of sediment with 5.0 x 10-3 atm of hydrogen had a pronounced effect on the kinetic parameters for the conversion of acetate to methane. The acetate pool size, expressed as the term Km + Sn (S. is in situ substrate concentration), decreased by 37% and Tt decreased by 43%. The Vmax remained relatively constant. A preincubation with hydrogen also caused a 37% decrease in the amount of labeled carbon dioxide produced from the metabolism of [U-14C]valine by sediment heterotrophs. Anaerobic decomposition is an important but little studied route of carbon flow in aquatic ecosystems. Methane production becomes the terminal step when alternative electron acceptors are absent, which is probably the case in most eutrophic lakes. For Wintergreen Lake, the subject of this study, we have estimated that as much as one-third of the primary production could be recovered as methane over a summer season (Strayer and Tiedje, Limnol. Oceanogr., in press), and Molongoski and Klug (unpublished data), in a 2-year study, have evidence that at least 39% of the sedimnenting carbon could be accounted for as methane. Thermodynamic considerations and reasoning from intert Journal Article no. 8256 of the Michigan Agricultural Experiment Station; contribution no. 344 of the W. K. Kellogg Biological Station.

Hydrogen distributions in marine sediments1,2

“…Hydrogen concentrations in the sediments we examined ranged from < 5 nM to about 60, with the lowest concentrations found near the sediment-water interface. These concentrations are an order of magnitude lower than those generally found in freshwater sediments (Strayer and Tiedje 1978;Michener 1985;Michener et al submitted) and are one to three orders of magnitude greater than concentrations found in anoxic water columns (Bullister et al 1982;Lilley et al 1982;Scranton et al 1984).…”

“…It seems reasonable to speculate that H2 concentrations in the deep methanogenic sediments of Skan Bay may bc the source of the higher levels of hydrogen at the bottom of the core. Concentrations in freshwater methanogcnic sediments are often quite high (> 150 nM;Strayer and Tiedje 1978;Michener 1985); however, no one has yet measured hydrogen concentrations in methanogenic marine sediments.…”

“…The rates of hydrogen production were determined with progress curve experiments (Strayer and Tiedje 1978;Robinson and Tiedje 1982). H, production during the 30-min equilibration period associated with the concentration measurements suggests an increase of about 5-10 nM above the actual in situ hydrogen levels at all depths during the equilibration period.…”

Anaerobic metabolism of particulate organic matter in the sediments of a hypereutrophic lake*

“…The relatively high acetate concentrations detected in Wintergreen Lake sediments, however, suggested that acetate was not a limiting factor for methanogenesis in these sediments. Kinetic experiments using (2-'*C) acetate and Wintergreen Lake sediments collected during the summer indicated that the rate of conversion of acetate to methane appeared to be very close to the maximal rate for this reaction in these sediments (Strayer & Tiedje, 1978a). These results suggest that anaerobic metabolism in Wintergreen Lake sediments closely resembles that found in anaerobic sewage digesters where decomposition may be limited by the rate at which methanogenic bacteria utilize acetate (Zeikus.…”

Taxonomy and ecology of methanogens

“…Many workers have illustrated the importance of acetate as a methane precursor in both frezhwaters and marine sediments [89,99,100], and demonstrated that H2 is a rate limiting factor in the process of methanogenesis in sediments [90,[101][102][103]. Landfills constitute another type of habitat from which methanogens have been isolated [104].…”