Uptake of CO₂ by aquatic vegetation

Photosynthesis by aquatic plants based on the supply of CO 2 from air‐equilibrated solutions may be limited by the low diffusion coefficient of CO 2 in water. For plants in which the transport of CO 2 from the bulk medium is by diffusion, and the initial carboxylation uses RUBISCO, CO 2 supply can be increased by growth in habitats with fast water flow over the surface (reducing unstirred layer thickness), or with heterotrophically‐augmented CO 2 levels, including the direct use of sediment CO 2 . Many aquatic plants using RUBISCO as their initial carboxylase counter the limitations on CO 2 supply via the operation of biophysical CO 2 concentrating mechanisms which are based on active transport of HCO − 3 , CO 2 or H + at the plasmalemma, and use bulk‐phase HCO − 3 or CO 2 as the C source. A final group of aquatic plants use biochemical CO 2 concentrating mechanisms based on auxiliary carboxylation by PEPc: C 4 ‐like and Crassulacean Acid Metabolism–like processes are involved. These various mechanisms for increasing CO 2 supply to RUBISCO also help to offset the low specific reaction rate of aquatic plant RUBISCOs at low [CO 2 ] and low [CO 2 ]: [CO 2 ]. In addition to overcoming restrictions on CO 2 supply, the various methods of increasing inorganic C availability may also be important in alleviating shortages of nitrogen or photons.