Thermoanaerobacter species differ in their potential to reduce organic acids to their corresponding alcohols

The reduction of organic acids to their corresponding alcohols has been shown for some bacterial species within the Firmicutes super-phylum and a genetically modified strain of the hyperthermophilic archaeon Pyrococcus furiosus. In the latter strain, an aldehyde:ferredoxin oxidoreductase (AOR) catalyzed the reduction of a variety of organic acids to their corresponding aldehydes, as shown by the deletion of the corresponding aor gene. Here, we found that the genomes of a few thermophilic bacterial species within the genus Thermoanaerobacter which have been described to efficiently ferment sugars to ethanol harbor a copy of aor, while others do not. Specific AOR activity was only found in strains with aor, and the gene was highly expressed in Thermoanaerobacter sp. strain X514. The reduction of a variety of organic acids was observed for several Thermoanaerobacter sp.; however, strains with aor reduced, e.g., isobutyrate at much higher rates of up to 5.1 mM h^-1 g^-1. Organic acid reduction also led to increased growth rates in Thermoanaerobacter sp. strain X514 and in Thermoanaerobacter pseudethanolicus. Organic acid activation may proceed via acyl-CoA with subsequent NADH-dependent reduction by an aldehyde dehydrogenase (ALDH), or via direct reduction by AOR. Cell-free extracts of Thermoanaerobacter sp. strain X514 exhibited both enzyme activities at comparable rates. Therefore, the biochemistry of organic acid reduction to alcohols in Thermoanaerobacter sp. remains to be elucidated; however, relatively high specific activities and the correlation of AOR specific activities with alcohol production rates suggest a role for AOR.