Influence of temperature on pathways to methane production in the permanently cold profundal sediment of Lake Constance

The in situ temperature of the profundal sediment of Lake Constance is constant at 4 degrees C. Methanogenic bacteria could not be detected at 6 degrees C by the most probable number (MPN) technique using acetate and H-2/CO2 as methanogenic substrates. Instead, homoacetogenic bacteria were detected on H-2/CO2. At a higher temperature of 20 degrees C, however, methanogenic bacteria were found in numbers of about 10(5) cells ml(-1) with H-2/CO2 and about 5x10(4) cells ml(-1) with acetate. However, CH4 production was observed at both 4 degrees C and 20 degrees C. Production of CH4 was inhibited by chloroform and fluoroacetate and the accumulation of intermediary metabolites was measured. At the in situ temperature of 4 degrees C, only acetate accumulated in presence of chloroform. Hydrogen partial pressures were at the same low value of about 0.5 Pa as in the uninhibited control. The amount of accumulated acetate was similar to that of CH4 in the uninhibited controls. Similar results were obtained with fluoroacetate which inhibits methanogenesis from acetate. Addition of (HCO3-)-C-14 did nor result in the formation of (CH4)-C-14 after 28 days of incubation. However, [2-C-14]acetate was immediately converted to (CH4)-C-14. The results indicate that methanogenesis at 4 degrees C was exclusively due to acetate cleavage. At 20 degrees C, by contrast, accumulation of H-2 was observed in addition to that of acetate, propionate, valerate, caproate, methanol and isopropanol, when CH4 production was inhibited by chloroform. Thermodynamic calculations indicated that the accumulation of the fatty acids was a consequence of feedback inhibition by the accumulated H-2. Balance calculations indicated that at 20 degrees about 22% of the CH4 originated from reduction of CO2. Experiments with (HCO3-)-C-14 indicated that about 33% of the CH4 originated from H-2/CO2 at 20 degrees C. Thermodynamic calculations showed that homoacetogenesis from H-2/CO2 was endergonic, whereas methanogenesis from acetate or H-2/CO2 was exergonic at both 4 degrees C and 20 degrees C. Low sediment temperatures obviously limited methanogenesis from H-2/CO2 for reasons other than thermodynamics. Simultaneously, degradation processes seemed to be dominated by homoacetogenic degradation of organic matter followed by acetoclastic methanogenesis. However, increase of temperature resulted in a dramatic shift of the degradation pathway enhancing the role of H-2 as an intermediate.