Anaerobic conversion of carbon dioxide to methane, acetate and propionate on washed rice roots

Washed excised roots of rice (Oryza sativa) produced H-2, CH4, acetate, pro pionate and butyrate when incubated under anoxic conditions. Acetate produc tion was most pronounced with a maximum rate (mean +/- standard error; four different root preparations) of 3.3 +/- 0.6 mu mol h(-1) g-dry weight(-1) roots, compared to 0.45 +/- 0.13, 0.06 +/- 0.03, and 0.04 +/- 0.01 mu mol h (-1) g-dw(-1) for propionate, butyrate and CH4 respectively. Hydrogen trans iently accumulated to maximum partial pressures of > 1 kPa after one day of incubation. Then it decreased and reached more or less constant concentrat ions of about 50-80 Pa after about 7-8 days. Hydrogen partial pressures wer e always high enough to allow exergonic methanogenesis (Delta G = -67 to -9 8 kJ mol(-1) CH4) and exergonic homoacetogenesis (Delta G = -18 to -48 kJ m ol(-1) acetate) from H-2 pills CO2 Radioactive bicarbonate/CO2 was incorpor ated into CH4, acetate and propionate. The specific radioactivities of the products indicated that CH4 was exclusively produced from H-2/CO2 confirmin g a previous study. The contribution of CO2 to the production of acetate an d propionate was 32-39% and 42-61%, respectively, assuming that each carbon atom was equally labeled. Propionate also became radioactively labeled, wh en the roots were incubated with either [1-C-14]acetate or [2-C-14]acetate accounting for 60-76% of total propionate production. Reductive formation o f propionate was thermodynamically favorable both from H-2 plus acetate plu s CO2 (Delta G = -15 to -38 kJ mol(-1) propionate) and from H-2 plus CO2 (D elta G = -34 to -85 kJ mol(-1) propionate), A substantial fraction of propi onate was apparently reductively formed from acetate and/or CO2. In conclus ion, our results demonstrate an intensive anaerobic dark metabolism of CO2 on washed rice roots with reduction of CO2 contributing significantly to th e production of acetate, propionate and CH4. The CO2 reduction seemed to be driven by decay and fermentation of root material. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All lights reserved.