STUDIES OF ELODEA-NUTTALLII GROWN UNDER PHOTORESPIRATORY CONDITIONS .1. PHOTOSYNTHETIC CHARACTERISTICS

The photosynthetic characteristics of Elodea nuttallii grown in wastewater in continuous flow reactors in a greenhouse were investigated. The diurnal changes in dissolved inorganic carbon (DIC), dissolved oxygen (DO) and pH were monitored. Photosynthesis removed both CO2 (aq) and HCO3- from the reactors. A stoichiometry of 1.19:1 was observed between HCO3- removal during photosynthesis and OH- production during photosynthesis, consistent with theories regarding direct bicarbonate utilization. In laboratory experiments, the light compensation points (GAMMA-PPFD) were similar (31-35-mu-mol m-2 s-1) to reported values for other macrophytes; however, the light saturation level was high (1100-mu-mol m-2 s-1) and similar to values reported for aerial portions of heterophyllous macrophytes. The kinetics of photosynthetic oxygen evolution (K(m) (CO2) = 96 mmol m-3; V(max) = 133 mmol g-1 Chl h-1) and the CO2 compensation point (GAMMA = 44 cm3 m-3) suggested an adaptive, low photorespiratory state in response to low carbon concentrations. Photosynthetic V(max) values were slightly, but significantly higher (P < 0.001) at pH 8.0 compared to pH 4.5. While CO2 utilization at pH 8 could account for most of the observed phototsynthetic rates, an HCO3- component was present, suggesting two separate transport systems for HCO3- and CO2(aq) in E. nuttallii. The activity of RUBISCO (160.3 mmol g-1 Chl h-1 was one of the highest reported values for aquatic macrophytes. Compared to RUBISCO, we observed lower activities of the beta-carboxylating enzymes phopho enolpyruvate carboyxlase (PEPcase), 24.1 mmol g-1 Chl h-1; phosphoenolpyruvate carboxykinase (PEPCKase), 14 mmol g-1 Chl h-1. This suggests that the potential light-independent fixation of carbon in E. nuttallii was much less than RUBISCO-dependent fixation. The RUBISCO/PEPcase ratio was 6.6, indicating that E. nuttallii was similar to Myriophyllum sp. in possessing a physiological adaptation to low CO2 levels which is hypothesized to include carbonic anhydrase (CA) and an active transport system for HCO3-. CA levels were surprisingly low in E. nuttallii (14.2 EU mg Chl-1).