Symbiobacterium Lost Carbonic Anhydrase in the Course of Evolution

Recent genetic studies have elucidated that carbonic anhydrase (CA; EC 4.2.1.1), a ubiquitous enzyme catalyzing interconversion between CO2 and bicarbonate, is essential for microbial growth under ambient air but not under high-CO2 air. The irregular distribution of the phylogenetically distinct types of CA in the prokaryotic genome suggests its complex evolutionary history in prokaryotes. This paper deals with the genetic defect of CA in Symbiobacterium thermophilum, a syntrophic bacterium that effectively grows on CO2 generated by other bacteria. Phylogenetic analysis based on 31 ribosomal protein sequences demonstrated the affiliation of Symbiobacterium with the class Clostridia with 100% bootstrap support. The phylogeny of beta- and gamma-type CA distributed among Clostridia supported the view that S. thermophilum and several related organisms lost this enzyme during the course of evolution. The loss of CA could be based on the availability of a high level of CO2 in their living environments.