No abstract

: The addition of exogenous to poultry manure and synthetic NH 4Cl medium was used to study the e †ect of ammonia-nitrogen on the activity and composition of a methanogenic consortium. Results indicated that the production of biogas and methane was not a †ected by the variation in con-NH 4 Cl centration within the range 2È10 g dm~3 (0É5È2É6 g dm~3). At higher N-NH 4 values of ammonium (10È30 g dm~3 or 2È8 g dm~3) a signiÐcant N-NH 4 decline in both parameters (by 50È60% for biogas and 80È90% for methane) was observed. A signiÐcant decrease in the numbers of bacteria of all physiological groups (especially proteolytic and methanogenic) was observed when more than 30 g dm~3 (7É8 g dm~3) was added to the fermentation medium. NH 4 Cl N-NH 4 The addition of 10% (w/v) of powdered phosphorite ore enhanced the production of biogas and methane at concentrations up to 30 g dm~3, and NH 4 Cl also changed the composition of the methanogenic consortium. A partial recovery in the numbers of proteolytic and methanogenic bacteria coupled with the decrease in the density of sulphate-reducers was observed. High concentrations (more than 50 g dm~3) of seemed to cause irreversible inhibition of NH 4 Cl methanogenesis which could not be eliminated by the addition of phosphorites.

Propionate is syntrophically degraded in methanogenic paddy soil via a randomizing pathway. To study the thermodynamic conditions of this syntrophy, propionate degradation was measured in the presence of different H2 partial pressures (1–20 000 Pa) using methanogenic soil slurries taken from planted Italian paddy soil. The logarithmic decrease of [1‐14C]propionate or [2‐14C]propionate was measured during an incubation period of about 2–3 h to determine degradation rate constants (k). The change of the H2 partial pressure was measured during the same period. Values of k decreased with increasing H2 partial pressures (averaged over the incubation period). However, k was still relatively high, although the Gibbs free energy (ΔG) of syntrophic propionate conversion to acetate, bicarbonate and H2 was already strongly endergonic reaching ΔG values of +60 kJ mol−1 propionate. Assuming propionate conversion to acetate plus formate resulted in the same or even higher ΔG values indicating that this degradation pathway was not realistic. We therefore assume that propionate was degraded within microbial aggregates in which syntrophic propionate degraders were shielded from thermodynamically unfavorable H2 by methanogenic bacteria consuming H2. Gibbs free energies for H2 formation from propionate correlated negatively with the ΔG values for H2 conversion to CH4, but the latter values were generally <−5 kJ mol−1 H2 so that methanogenesis from H2 was always possible. Addition of sulfate did not result in a significant decrease of the ΔG values for H2 formation from propionate demonstrating that H2 consumption by sulfate reducers was not relevant during the short incubation period. Nevertheless, propionate degradation was less strongly inhibited by H2 when sulfate was present indicating that propionate was then mainly degraded by sulfate reduction rather than by syntrophy. The major degradation product of [2‐14C]propionate was 14C‐acetate (followed by 14CO2 and 14CH4) showing that the sulfate reducers oxidized propionate primarily to acetate, bicarbonate and H2. As a conceptual model we therefore speculate that propionate was degraded within methanogenic bacterial aggregates both in the presence and the absence of sulfate and that propionate degraders operated either as sulfate reducers or as H2‐producing syntrophs.

Propionate is syntrophically degraded in methanogenic paddy soil via a randomizing pathway. To study the thermodynamic conditions of this syntrophy, propionate degradation was measured in the presence of different H2 partial pressures (1–20 000 Pa) using methanogenic soil slurries taken from planted Italian paddy soil. The logarithmic decrease of [1‐14C]propionate or [2‐14C]propionate was measured during an incubation period of about 2–3 h to determine degradation rate constants (k). The change of the H2 partial pressure was measured during the same period. Values of k decreased with increasing H2 partial pressures (averaged over the incubation period). However, k was still relatively high, although the Gibbs free energy (ΔG) of syntrophic propionate conversion to acetate, bicarbonate and H2 was already strongly endergonic reaching ΔG values of +60 kJ mol−1 propionate. Assuming propionate conversion to acetate plus formate resulted in the same or even higher ΔG values indicating that this degradation pathway was not realistic. We therefore assume that propionate was degraded within microbial aggregates in which syntrophic propionate degraders were shielded from thermodynamically unfavorable H2 by methanogenic bacteria consuming H2. Gibbs free energies for H2 formation from propionate correlated negatively with the ΔG values for H2 conversion to CH4, but the latter values were generally <−5 kJ mol−1 H2 so that methanogenesis from H2 was always possible. Addition of sulfate did not result in a significant decrease of the ΔG values for H2 formation from propionate demonstrating that H2 consumption by sulfate reducers was not relevant during the short incubation period. Nevertheless, propionate degradation was less strongly inhibited by H2 when sulfate was present indicating that propionate was then mainly degraded by sulfate reduction rather than by syntrophy. The major degradation product of [2‐14C]propionate was 14C‐acetate (followed by 14CO2 and 14CH4) showing that the sulfate reducers oxidized propionate primarily to acetate, bicarbonate and H2. As a conceptual model we therefore speculate that propionate was degraded within methanogenic bacterial aggregates both in the presence and the absence of sulfate and that propionate degraders operated either as sulfate reducers or as H2‐producing syntrophs.

Samples from planted Italian paddy soil exhibited most probable numbers (MPN) of about 107 anaerobic propionate utilizers. In anoxic soil slurries that were either unamended or amended with rice straw production of CH4 was measured together with concentrations of H2, acetate and propionate. After a lag phase, during which ferric iron was depleted, CH4 was produced at a constant rate which was slightly higher in the straw‐amended than in the unamended soil. Propionate concentrations were relatively low at about 5–15 μM. However, in the straw‐amended soil propionate transiently accumulated to about 35 μM just after onset of methanogenesis. During the period of propionate accumulation H2 partial pressures were elevated and the Gibbs free energy (ΔG) of propionate consumption to acetate, bicarbonate and H2 was endergonic or higher than −3 kJ mol−1 propionate. Propionate concentrations decreased again when the ΔG decreased to more negative values. In unamended paddy soil, propionate did not accumulate transiently and ΔG was always <−6 kJ mol−1 propionate. Propionate radiolabelled in the C‐1 or C‐2 position was utilized with turnover times of 30–60 min. Propionate turnover rates approximately accounted for the rates of H2/CO2‐dependent methanogenesis that were measured in experiments with [14C]bicarbonate. The only radioactive product of [1‐14C]propionate was 14CO2. However, [2‐14C]propionate was converted to radioactive acetate, CO2 and CH4. This observation indicates that propionate was consumed via a randomizing pathway to CO2 and acetate, the latter being then further degraded by acetotrophic methanogens to CO2 and CH4. Turnover of [1‐14C]propionate was almost completely inhibited by high H2 concentrations, chloroform or molybdate. The MPN of bacteria that utilized propionate either in syntrophy with methanogens or by reduction of sulfate was identical. All these observations suggest that propionate was consumed by a syntrophic randomizing pathway, probably by bacteria that have also the capacity to reduce sulfate.