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 aceta te and H-2/CO2 as methanogenic substrates. Instead, homoacetogenic bac teria 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 degr ees 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 prese nce of chloroform. Hydrogen partial pressures were at the same low val ue of about 0.5 Pa as in the uninhibited control. The amount of accumu lated acetate was similar to that of CH4 in the uninhibited controls. Similar results were obtained with fluoroacetate which inhibits methan ogenesis 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 clea vage. At 20 degrees C, by contrast, accumulation of H-2 was observed i n addition to that of acetate, propionate, valerate, caproate, methano l and isopropanol, when CH4 production was inhibited by chloroform. Th ermodynamic 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 CH 4 originated from reduction of CO2. Experiments with (HCO3-)-C-14 indi cated that about 33% of the CH4 originated from H-2/CO2 at 20 degrees C. Thermodynamic calculations showed that homoacetogenesis from H-2/CO 2 was endergonic, whereas methanogenesis from acetate or H-2/CO2 was e xergonic at both 4 degrees C and 20 degrees C. Low sediment temperatur es obviously limited methanogenesis from H-2/CO2 for reasons other tha n thermodynamics. Simultaneously, degradation processes seemed to be d ominated by homoacetogenic degradation of organic matter followed by a cetoclastic methanogenesis. However, increase of temperature resulted in a dramatic shift of the degradation pathway enhancing the role of H -2 as an intermediate.