Metabolism of position-labelled glucose in anoxic methanogenic paddy soil and lake sediment


R. Conrad. Max-Planck-Institut für Terrestrische Mikrobiologie, Karl-von-Frisch-Str., D-3550 Marburg/Lahn, F.R.G.


Abstract Turnover times of radioactive glucose were shorter in paddy soil (4–16 min) than in Lake Constance sediment (18–62 min). In the paddy soil, 65–75% of the radioactive glucose was converted to soluble metabolites. In the sediment, only about 25% of the radioactive glucose was converted to soluble metabolites, the rest to particulate material. In anoxic paddy soil, the degradation pattern of position-labelled glucose was largely consistent with glucose degradation via the Embden-Meyerhof-Parnas (EMP) pathway followed by methanogenic acetate cleavage: CO2 mainly originated from C-3,4, whereas CH4 mainly originated from C-1 and C-6 of glucose. Acetate-carbon originated from C-1, C-2 and C-6 rather than from C-3,4 of glucose. In both paddy soil and Lake Constance sediment acetate and CO2 were the most important early metabolites of radioactive glucose. Other early products included propionate, ethanol/butyrate, succinate, and lactate, but accounted each for less than 1–8% of the glucose utilized. The labelling of propionate by [3,4-14C]glucose suggests that it was mainly produced from glucose or lactate rather than from ethanol. Isopropanol and caproate were also detectable in paddy soil, but were not produced from radioactive glucose. Chloroform inhibited methanogenesis, inhibited the further degradation of radioactive acetate and resulted in the accumulation of H2, however, did not inhibit glucose degradation. Since acetate was the main soluble fermentation product of glucose and was produced at a relatively high molar acetate: CO2 ratio (2.5:1), homoacetogenesis appeared to be the most important glucose fermentation pathway.