There Must be an Acetogen Somewhere

In the beginning of 1983, when I was a post-doctoral student at the University of Illinois, Ralph Wolfe handed me an offprint of a long article and said: “Read this!” That paper was the review by Thauer et al. (1977) on “Energy conservation in chemotrophic anaerobic bacteria.” I have read it many times. I remember that the first and second time I understood very little. During the third and fourth reading I started to grasp the ideas expressed and to appreciate them. And after having read the paper for the fifth and sixth time I had become convinced that microbial metabolism and metabolic diversity can only be properly understood using the kind of thermodynamic analyses on which Thauer and his colleagues based their review. Until this day I use this thermodynamic approach to explain the functioning of the microbial world in the basic and more advanced microbiology courses I teach. At the same time Ralph Wolfe also introduced me to the world of the acetogens. I still remember how excited he was that the culture of Clostridium aceticum, isolated in the 1930s (Wieringa, 1936) but subsequently considered as lost, had been revived from a preparation of endospores of the original strain and thus became again available for study (Braun et al., 1981). The place of the acetogens in nature, and especially those species that live as autotrophs on hydrogen as their energy source, has always been enigmatic. The reason becomes immediately obvious when calculating the thermodynamics of the process, comparing the Gibbs free energy change under standard conditions of three competing processes: sulfate reduction, methanogenesis, and homoacetogenic metabolism: