A Phylogenetic Assessment of the Acetogens

Abstract

Our first collaborative foray into molecular systematics was a phylogenetic examination of the acetogens, beginning in 1979. Following the discovery that a physiologically defined group of microorganisms, the methanogens, corresponded to a phylogenetically coherent group (Balch et al., 1977), it was natural to ask the question whether the acetogens, then limited to a handful of species, formed a phylogenetically distinct unit. The initial assessment based on examination of SSU (small subunit; 16S) rRNA catalogs of six acetogens and several other anaerobic microorganisms (Tanner et al., 1981; Tanner et al., 1982) indicated that the acetogens did not form a phylogenetically distinct group per se but that they were all members of a larger group of eubacteria more or less defined by the clostridia (Fox et al., 1980; Tanner et al., 1982). Today there exist at least 27 more or less well-described species of acetogens scattered among at least 12 different genera of eubacteria, based on the criteria currently used in microbial taxonomy. Our understanding of microbial systematics has kept pace with the great increase in described species of anaerobes over the past decade, and a phylogenetic assessment of the acetogens can be performed. The systematics of the acetogens and the clostridia are still basically intertwined.