Uptake of CO2 by aquatic vegetation

Abstract Photosynthesis by aquatic plants based on the supply of CO₂ from air-equilibrated solutions may be limited by the low diffusion coefficient of CO₂ in water. For plants in which the transport of CO₂ from the bulk medium is by diffusion, and the initial carboxylation uses RUBISCO, CO₂ supply can be increased by growth in habitats with fast water flow over the surface (reducing unstirred layer thickness), or with heterotrophically-augmented CO₂ levels, including the direct use of sediment CO₂. Many aquatic plants using RUBISCO as their initial carboxylase counter the limitations on CO₂ supply via the operation of biophysical CO₂ concentrating mechanisms which are based on active transport of HCO^?₃, CO₂ or H⁺ at the plasmalemma, and use bulk-phase HCO^?₃ or CO₂ as the C source. A final group of aquatic plants use biochemical CO₂ concentrating mechanisms based on auxiliary carboxylation by PEPc: C₄-like and Crassulacean Acid Metabolism–like processes are involved. These various mechanisms for increasing CO₂ supply to RUBISCO also help to offset the low specific reaction rate of aquatic plant RUBISCOs at low CO₂] and low CO₂]: CO₂]. In addition to overcoming restrictions on CO₂ supply, the various methods of increasing inorganic C availability may also be important in alleviating shortages of nitrogen or photons.