Freshwater angiosperm carbon concentrating mechanisms: processes and patterns

Aquatic angiosperms are derived from terrestrial ancestors and appear to have re-invaded water on many occasions. While removing problems of water supply and reducing the need for supporting tissue, freshwaters have a potentially low and fluctuating supply of CO2 for photosynthesis, as well as generally low light. This paper reviews the structural, morphological, physiological, and biochemical features of freshwater macrophytes in the context of maximising net carbon uptake underwater, and discusses how inorganic carbon may influence macrophyte ecology. Submerged leaves tend to have a low photosynthetic capacity on an area basis, matching the low rates of supply of CO2 and light. Morphological and structural strategies to overcome potential carbon limitation include possession of aerial or floating leaves, and lacunal connexions to high concentrations of sedimentary CO2 via the roots. Physiological and biochemical strategies include crassulacean acid metabolism, C4-like metabolism in Hydrilla and Egeria, and the ability to use HCO3^-. The activity of all these can be regulated by environmental conditions to maximize growth rate. Use of HCO3^-. is the most widespread carbon acquisition strategy, present in about half of the tested submerged angiosperms. It is more common in lakes of high alkalinity and in the elodeid growth form.