The use of stable isotopes in the study of photosynthesis in freshwater plants

The ratio of ¹³C/¹²C for photosynthetic tissues of 22 aquatic species was unrelated to photosynthetic pathway. In three aquatic environments CAM and non-CAM species were shown to have similar δ¹³C values. Although these CAM species derive up to half of their net carbon gain through dark fixation their δ¹³C values are similar to associated non-CAM species in part because the carbon source for dark CO₂ uptake is CO₂ released, through respiration or decomposition, from organic carbon. Thus, the carbon source for CAM reflects previous isotope discrimination events. As carbon isotopes are not able to distinguish the photosynthetic pathway, there is good evidence that they may prove invaluable in the study of diffusional resistance to photosynthesis. Such evaluations require careful analysis of the photosynthetic pathway, carbon species utilized and δ¹³C value of the source carbon. Although stable carbon isotope values do not allow differentiation between CAM and non-CAM aquatic species, there is evidence that hydrogen isotopes may be able to distinguish these two groups. Aquatic CAM species were shown to accumulate greater levels of deuterium than associated non-CAM species.