Acetogenesis from Carbon Dioxide in Termite Guts

Abstract

Since the isolation of Clostridium aceticum (Wieringa, 1940), the first bacterium ever shown to derive energy for growth by acetate synthesis from H2 + CO2, the phenomenon of acetogenesis from C1 compounds has been of intrinsic interest to microbiologists and biochemists. As seen from other chapters in this volume, work in various laboratories over the years has now led to the isolation of over two dozen different species of such acetogens and to the recognition that these bacteria, united by their unique metabolism, are actually quite diverse phenotypically and phylogenetically. Likewise, detailed studies on the biochemistry of acetogenesis from CO2, conducted mainly with Clostridium thermoaceticum by H. G. Wood and his students, have identified each step in the pathway and resulted in the purification and characterization of the relevant enzymes, and in some cases the genes encoding them. Nevertheless, the ecological significance of acetogenesis from CO2 has remained obscure. Certainly, the ability of most acetogens to use H2 as a reductant suggests that they might function as terminal or subterminal “electron sink” organisms in anaerobic microbial food webs, and they are often included in that position in diagrams depicting such webs (e.g., Zinder, 1984). Yet, rarely have habitats been identified in which acetogens outprocess, or are strongly competitive with, other potential H2 consumers such as methanogens and sulfate-reducing bacteria. Hence, their significance in the flow of carbon and reducing equivalents during anoxic decomposition processes has been debatable. In recent years, however, it has been found that the gastrointestinal tract of vertebrates and invertebrates is one type of habitat in which acetogens often appear to be major H2 consumers (Breznak and Kane, 1990; also see Wolin and Miller Chapter 13). During microbial fermentation in the gut of certain termites, in particular, acetogens not only appear to constitute the primary H2 sink, but their production of acetate from H2 + CO2 makes a major contribution to termite nutrition.