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ABSTRACT Using dissolved inorganic carbon (DIC) as a major carbon source, as autotrophs do, is complicated by the bedeviling nature of this substance. Autotrophs using the Calvin-Benson-Bassham cycle (CBB) are known to make use of a toolkit comprised of DIC transporters and carbonic anhydrase enzymes (CA) to facilitate DIC fixation. This minireview provides a brief overview of the current understanding of how toolkit function facilitates DIC fixation inCyanobacteriaand someProteobacteriausing the CBB and continues with a survey of the DIC toolkit gene presence in organisms using different versions of the CBB and other autotrophic pathways (reductive citric acid cycle, Wood-Ljungdahl pathway, hydroxypropionate bicycle, hydroxypropionate-hydroxybutyrate cycle, and dicarboxylate-hydroxybutyrate cycle). The potential function of toolkit gene products in these organisms is discussed in terms of CO₂and HCO₃⁻supply from the environment and demand by the autotrophic pathway. The presence of DIC toolkit genes in autotrophic organisms beyond those using the CBB suggests the relevance of DIC metabolism to these organisms and provides a basis for better engineering of these organisms for industrial and agricultural purposes. 

ABSTRACT Autotrophic bacteria are able to fix CO₂in a great diversity of habitats, even though this dissolved gas is relatively scarce at neutral pH and above. As many of these bacteria rely on CO₂fixation by ribulose 1,5-bisphospate carboxylase/oxygenase (RubisCO) for biomass generation, they must compensate for the catalytical constraints of this enzyme with CO₂-concentrating mechanisms (CCMs). CCMs consist of CO₂and HCO₃⁻transporters and carboxysomes. Carboxysomes encapsulate RubisCO and carbonic anhydrase (CA) within a protein shell and are essential for the operation of a CCM in autotrophicBacteriathat use the Calvin-Benson-Basham cycle. Members of the genusThiomicrospiralack genes homologous to those encoding previously described CA, and prior to this work, the mechanism of function for their carboxysomes was unclear. In this paper, we provide evidence that a member of the recently discovered iota family of carbonic anhydrase enzymes (ιCA) plays a role in CO₂fixation by carboxysomes from members ofThiomicrospiraand potentially otherBacteria. Carboxysome enrichments fromThiomicrospira pelophilaandThiomicrospira aerophilawere found to have CA activity and contain ιCA, which is encoded in their carboxysome loci. When the gene encoding ιCA was interrupted inT. pelophila, cells could no longer grow under low-CO₂conditions, and CA activity was no longer detectable in their carboxysomes. WhenT. pelophilaιCA was expressed in a strain ofEscherichia colilacking native CA activity, this strain recovered an ability to grow under low CO₂conditions, and CA activity was present in crude cell extracts prepared from this strain. IMPORTANCEHere, we provide evidence that iota carbonic anhydrase (ιCA) plays a role in CO₂fixation by some organisms with CO₂-concentrating mechanisms; this is the first time that ιCA has been detected in carboxysomes. While ιCA genes have been previously described in other members of bacteria, this is the first description of a physiological role for this type of carbonic anhydrase in this domain. Given its distribution in alkaliphilic autotrophic bacteria, ιCA may provide an advantage to organisms growing at high pH values and could be helpful for engineering autotrophic organisms to synthesize compounds of industrial interest under alkaline conditions.