Bicarbonate utilization: Function and mechanism

A review is given of the occurrence, ecological significance and physiological mechanism of bicarbonate utilization by aquatic photosynthetic organisms. Following a short description of the carbon dioxide water system, the significance of the unstirred layer and the slow diffusion rate of CO2 in water is discussed. Three experimental procedures are discussed to establish the use of bicarbonate: establishing the relationship between pH and rate of carbon assimilation; the kinetic-reaction approach, which uses the slow equilibration between CO2 and HCO3− in water; the difference in discrimination between 12C and 13C during CO2 and HCO3− assimilation. The ability to use HCO3− occurs in many different groups of aquatic photosynthetic organisms, with the notable exception of Bryophytes and Pteridophytes, and is related to growth conditions. Polarity, that is the spatial separation of HCO3− influx and resulting OH− efflux, is observed in higher plant species and Characeae that assimilate HCO3−. These polar leaves or banded Characean cells are often used to study the mechanism. Three mechanisms for HCO3− assimilation apparently exist: H+HCO3− symport, external acidification of HCO3− into CO2 and an increased rate of conversion of HCO3− into CO2 by the action of the enzyme carbonic anhydrase. The role of transfer cells or plasmalemmasomes, often observed in HCO3− assimilating leaves, is still unclear. Their role may be to create sufficient space for membrane-bound enzyme systems by increasing the membrane surface area or to confine an extracellular space with a low pH value. The ecological significance of photosynthetic HCO3− utilization seems to be that it compensates for slow supply of CO2 by diffusion and/or suppresses photorespiration by creating a high intracellular CO2 concentration