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Similarly we can calculate for L. ferro oxidans D GCAT = — 8.03 kJ.

Threshold concentrations and minimally required Gibbs energy for anaerobic acetate consuming methanogens

CH4 can be produced anaerobically from acetate according to the catabolic reaction Acetate + H2O r CH4 + HCO—. For this reaction DG£at and DH°at at 298 K follow as

This provides, using the Van't Hoff equation, for DScat = DHr — DGr/T = 0.121 kJ/molK.

There are two organisms known to perform this reaction at 60 °C, both of which have threshold acetate concentrations. According to Ref. 32 Methano sarcina has a threshold concentration of 0.3 x 10 —3 M and Methano trixof 16 x 10 —5 M of acetate.

Using a CH4 pressure of 0.6 bar, T = 333 K, and a bicarbonate concentration of 0.03 M, we can calculate the following DGCAT of the catabolic reaction at the threshold acetate concentration (using the Van 't Hoff relation DG = DH — TDS to take the temperature effect into account):

Methanosarcina DGCAT = — 23 kJ Methanotrixe DGCAT = — 15.7 kJ

It appears indeed that at the threshold concentration the coupled system of catabolic reaction and proton translocation may be in equilibrium. This leads to the statement that a threshold concentration can be estimated from the requirement that

(DGCAT)t

This explanation is further supported by the observation that for anaerobic H2-consuming systems (36), the observed effect of temperature on H2 threshold values can be explained from correlation equation 18. Example 11 shows how the expected threshold concentration can be estimated for such systems.

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