PHOTOSYNTHETIC CARBON ASSIMILATION IN AQUATIC MACROPHYTES

In submerged aquatic macrophytes the relationship between net photosynthesis and carbon concentration in the water often follows a less gradual pattern than anticipated assuming simple Michaelis-Menten kinetics. This indicates that other factors than the activity of the carboxylation enzymes are important in the regulation of photosynthesis in these plants. At low external concentrations of inorganic carbon, photosynthesis is restricted by the slow rate of diffusion from the bulk medium to the site of carboxylation, whereas the maximum photosynthetic capacity appears to be set by the enzyme activity or the turnover of intermediates in the carbon reduction cycle, including adenosine triphosphate (ATP) and reducing agents. The potential for active transport of inorganic carbon across the plasmalemma may also be of importance in some instances. Different strategies, which could be seen as adaptations to ameliorate the carbon constraints, have evolved in submerged macrophytes. These could be separated into physiological and exploitation strategies. Among the former, the most widespread is use of HCO3- in photosynthesis. The rationale for this strategy is the rather high concentration of HCO3- in most freshwaters and in seawater. Bicarbonate use has been found in approximately 50% of the species tested. Other physiological adaptations involve use of C4 acids in photosynthesis based on some kind of C4 or crassulacean acid metabolism (CAM). These strategies, however, appear to be of only minor importance, C4-like photosynthesis is found in a few freshwater and marine species and CAM is found mainly among the isoetids and in a few other species. The exploitation strategies all involve morphological features which allow the plants to avoid or reduce carbon limitation by exploiting alternative carbon sources in addition to those in the water. The most widespread is the development of floating or aerial leaves, which allow access to the atmospheric CO2 pool, where CO2 is more readily available owing to the higher diffusion rate and current velocity in air relative to water. Use of sediment-CO2, another exploitation strategy, has been recognized in several species, especially from the isoetid group. These species all have an extensive lacunal system with longitudinal channels from the roots to the leaves, providing an efficient transport route for sediment-CO2. In these species, sediment-derived CO2 can be responsible for more than 90% of the total carbon uptake. Sediment-CO2 concentrations are nearly always higher than the concentration in the water column; nevertheless, significant use of sediment-CO2 is not found in elodeid species. It has been suggested that this is due to the long diffusion path, low shoot/root ratio and a high, inherent rate of photosynthesis in these species. The carbon extraction capacity, which reflects the extent to which the plants use HCO3- or C4 acids in photosynthesis, is not a constant attribute among submerged macrophytes. However, some general trends are present. The extraction capacity is high in most macroalgae, intermediate in elodeid freshwater species and low in isoetids and in elodeid species with the potential of exploiting CO2 sources other than those in water. Accordingly, light-saturated photosynthesis in marine macroalgae is often carbon saturated at natural inorganic carbon concentrations, but rarely so among freshwater macrophytes. In addition to these apparently genetic differences in carbon extraction capacity among the plant groups, environmental conditions also influence the carbon uptake characteristics. When carbon is not limiting, i.e. at high CO2 and low light, high extraction capacity and carbon affinity are suppressed. This response seems appropriate as the plants thereby reduce investment in carbon acquisition systems when not needed and instead allocate energy to increase assimilation of more limiting resources. Thus, it is apparent that submerged macrophytes possess a homeostatic capacity that reduces the effects of imbalance among resources needed. However, it is evident that aquatic macrophytes, in particular freshwater species, often have an excess capacity for carbon assimilation not realized at ambient conditions of inorganic carbon. The significance of this overcapacity is unclear, but it stresses the importance of inorganic carbon as a potential limiting factor for photosynthesis and possibly growth of submerged aquatic macrophytes.