Carbonic anhydrase: Enzyme that has transformed the biosphere

The bases of modern type biosphere were laid down about two billion years ago during the predominance of prokaryotes on the Earth. Cyanobacteria changed radically the composition of the Proterozoic atmosphere by saturating it with photosynthetic oxygen. At the same time, large quantities of atmospheric CO₂ became sequestered in carbonates owing to mineralization of ancient cyano-bacterial communities; the latter have reached us in the form of laminated limestone deposits, termed stromatolites. The mechanism of carbonate depositing by cyanobacteria is still poorly understood. It is not yet clear whether physiological processes are involved in cell mineralization or if the outer membranes of cyanobacteria serve as a kind of crystallization center and arrange the structure for natural accumulation of sediments. We proposed that a key role in the mechanism of biomineralization belongs to the enzyme carbonic anhydrase (CA), which regulates the equilibrium between the inorganic carbon forms (Ci), including bicarbonate that participates in natural sedimentation of calcium. Since the deposition of calcium carbonate by prokaryotes occurs in the pericellular space and this deposition is controlled by pH, it seems likely that CA, localized on the periphery of cyanobacterial cells, is involved in stabilizing the external pH and in promoting cell mineralization. This review summarizes information concerning possible mechanisms of biogenic calcification (CaCO₃ deposition). The function of CA in the living cell and the role of this enzyme in biological processes are considered, and the data on localization of CA in cyano-bacterial cells are presented. Based on available evidence, a scheme is suggested to describe the role of extracellular CA in photosynthetic carbon assimilation and to relate this process with CaCO₃ deposition during mineralization of cyanobacteria.