Unusual regulation of the CO ₂ concentrating mechanism of marine chemolithoautotroph <i>Thiomicrospira pelophila</i>

Autotrophic organisms fix dissolved inorganic carbon (DIC: CO₂, HCO₃^-, CO₃^-2) into biomass, introducing organic carbon into the global carbon cycle. Many lineages of autotrophs developed CO₂ concentrating mechanisms (CCMs) to grapple with the catalytic constraints of the carboxylase of the Calvin-Benson-Bassham cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). Carboxysomes and DIC transporters act together to form CCMs, which facilitate DIC fixation by autotrophic bacteria when DIC is scarce. Autotroph <i>Thiomicrospira pelophila</i> has carboxysomes, with an unusual carbonic anhydrase, and encodes six potential DIC-transporters, more than other chemolithoautotrophs. We conducted experiments on how these carboxysomes and multiple transporters are integrated into a functioning CCM in <i>T. pelophila</i>. <i>T. pelophila</i> expresses a functional CCM. Four out of six transporters were capable of DIC uptake when expressed in <i>E. coli</i>. However, only one transporter was upregulated under DIC limitation. Transcript abundances from carboxysome-related genes in <i>T. pelophila</i> did not differ under DIC limitation, and carboxysome abundance in DIC-limited cells did not increase as markedly as in other species. This abundance of transporter genes and absence of dramatic changes in carboxysome abundance distinguish the CCM of <i>T. pelophila</i> from those studied in other organisms.IMPORTANCEAlthough the general composition of CCMs is conserved (carboxysomes and DIC transporters), the evolutionary origins of these components can differ (e.g., different lineages of carbonic anhydrase enzymes and transporters). Here, we show a new pattern of gene regulation in response to DIC limitation, suggesting an added level of diversity in CCM operation. Understanding these layers of diversity is key to discerning how these organisms function <i>in situ,</i> as well as how they or their CCM components could be engineered into organisms of industrial or agricultural importance.