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Keywords:

  • Fermentation;
  • Homoacetogenesis;
  • Methanogenesis;
  • Hydrogen;
  • Gibbs free energy;
  • Radiotracer

Abstract

Washed excised roots of rice (Oryza sativa) produced H2, CH4, acetate, propionate and butyrate when incubated under anoxic conditions. Acetate production was most pronounced with a maximum rate (mean±standard error; four different root preparations) of 3.4±0.6 μmol h−1 g-dry weight−1 roots, compared to 0.45±0.13, 0.06±0.03, and 0.04±0.01 μmol h−1 g-dw−1 for propionate, butyrate and CH4, respectively. Hydrogen transiently accumulated to maximum partial pressures of >1 kPa after one day of incubation. Then it decreased and reached more or less constant concentrations of about 50–80 Pa after about 7–8 days. Hydrogen partial pressures were always high enough to allow exergonic methanogenesis (ΔG=−67 to −98 kJ mol−1 CH4) and exergonic homoacetogenesis (ΔG=−18 to −48 kJ mol−1 acetate) from H2 plus CO2. Radioactive bicarbonate/CO2 was incorporated into CH4, acetate and propionate. The specific radioactivities of the products indicated that CH4 was exclusively produced from H2/CO2 confirming a previous study. The contribution of CO2 to the production of acetate and propionate was 32–39% and 42–61%, respectively, assuming that each carbon atom was equally labeled. Propionate also became radioactively labeled, when the roots were incubated with either [1-14C]acetate or [2-14C]acetate accounting for 60–76% of total propionate production. Reductive formation of propionate was thermodynamically favorable both from H2 plus acetate plus CO2G=−15 to −38 kJ mol−1 propionate) and from H2 plus CO2G=−34 to −85 kJ mol−1 propionate). A substantial fraction of propionate was apparently reductively formed from acetate and/or CO2. In conclusion, our results demonstrate an intensive anaerobic dark metabolism of CO2 on washed rice roots with reduction of CO2 contributing significantly to the production of acetate, propionate and CH4. The CO2 reduction seemed to be driven by decay and fermentation of root material.