Environmental regulation of CO2-concentrating mechanisms in microalgae

Most microalgae possess a mechanism for actively transporting inorganic carbon that concentrates CO2 at the active site of the carbon fixing enzyme ribulose bisphosphate carboxylasenoxygenase (Rubisco). This review considers the effects of environmental factors on the capacity and activity of microalgal CO2-concentrating mechanisms. Limitation of energy supply by light availability decreases the rate of inorganic carbon transport and cells grown under light-limited conditions have a reduced capacity for CO2 accumulation. Phosphorus limitation also reduces the capacity of algal cells to accumulate CO2, whereas both the rate of supply of nitrogen and the form in which it is made available interact in various complex ways with carbon utilization. The potential role of other nutrients in modulating inorganic carbon transport is also discussed. The capacity of algae for carbon accumulation is also affected by CO2 supply, which, in turn, is a function of the interactions between ionic strength of the growth medium, pH, cell density in culture, aeration rate, and inorganic carbon concentration in the medium. The effects of these interacting parameters are discussed, together with an assessment of the possible roles and significance of CO2-concentrating mechanisms to microalgae in marine and freshwater ecosystems.