ACTIVE CO2 TRANSPORT IN CYANOBACTERIA

Cyanobacteria appear to possess an active transport system for molecular CO2. This system, first discovered by Badger and Andrews in 1982 (1982. Plant Physiol. 70: 517-523), is without reported precedence in the bacterial, animal, or plant literature. The transport system operates so efficiently that in dense cell suspensions the extracellular CO2 concentration is pulled far below the equilibrium value. This CO2 drawdown is not due to CO2 fixation but can be accounted for by a transport system that recognizes molecular CO2 and causes it to be transported into the cell. The fact that operation of the system causes a massive disequilibration of the extracellular CO2-HCO3- system means that there must be an expenditure of metabolic energy. The CO2 is actually moved against a considerable CO2 concentration gradient. In this review we discuss methods that can be used to monitor CO2 transport in cyanobacteria. We present evidence that CO2 transport is an active process. It is emphasized that little is known about the concomitant ion fluxes that must occur to ensure charge and pH regulation during CO2 transport.