FORMATE PRODUCTION AND UTILIZATION BY METHANOGENS AND BY SEWAGE-SLUDGE CONSORTIA - INTERFERENCE WITH THE CONCEPT OF INTERSPECIES FORMATE TRANSFER

Pure cultures of H-2/CO2- and formate-utilizing methanogens or mixed c onsortia of sewage sludge generated some formate from H-2/CO2 at H-2 p artial pressure in the gas phase above 200 kPa. At decreasing H-2 part ial pressure the formate was taken up again and converted to methane. If methanogenesis was inhibited by bromoethanesulphonic acid (BESA) or a high redox potential (-180 to -200 mV), formate-utilizing methanoge ns produced high amounts of formate from H-2/CO2. No formate was excre ted by the species, which could only utilize H-2/CO2 for methanogenesi s. In contrast, H-2 formation from formate was observed in cultures of Methanobacterium thermoformicicum and M. formicicum. Measurable amoun ts were, however, only formed if its immediate utilization for methane production was inhibited by BESA. In the light of the data on formate formation from H-2/CO2 and its re-utilization by all formate-utilizin g methanogens, the concept of interspecies formate transfer of Thiele and Zeikus should be reconsidered. In pure cultures of methanogens or complex ecosystems with excess H-2, formate formation seemed to serve more as a means of disposal of surplus reducing power than for H-2 tra nsfer.