The Freshwater Cyanobacterium Synechococcus elongatus PCC 7942 Does Not Require an Active External Carbonic Anhydrase

Under standard laboratory conditions, <i>Synechococcus elongatus</i> PCC 7942 lacks EcaA^Syn, a periplasmic carbonic anhydrase (CA). In this study, a <i>S. elongatus</i> transformant was created that expressed the homologous EcaA^Cya from <i>Cyanothece</i> sp. ATCC 51142. This additional external CA had no discernible effect on the adaptive responses and physiology of cells exposed to changes similar to those found in <i>S. elongatus</i> natural habitats, such as fluctuating CO₂ and HCO₃^- concentrations and ratios, oxidative or light stress, and high CO₂. The transformant had a disadvantage over wild-type cells under certain conditions (Na⁺ depletion, a reduction in CO₂). <i>S. elongatus</i> cells lacked their own EcaA^Syn in all experimental conditions. The results suggest the presence in <i>S. elongatus</i> of mechanisms that limit the appearance of EcaA^Syn in the periplasm. For the first time, we offer data on the expression pattern of CCM-associated genes during <i>S. elongatus</i> adaptation to CO₂ replacement with HCO₃^-, as well as cell transfer to high CO₂ levels (up to 100%). An increase in CO₂ concentration coincides with the suppression of the NDH-1₄ system, which was previously thought to function constitutively.