STUDIES OF ELODEA NUTTALLII GROWN UNDER PHOTORESPIRATORY CONDITIONS. I, PHOTOSYNTHETIC CHARACTERISTICS
Abstract. 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 (гPPFD) were similar (31–35μmol m−2 s−1) to reported values for other macrophytes; however, the light saturation level was high (1100μmol m−2 s−1) and similar to values reported for aerial portions Of heterophyllous macrophytes. The kinetics of photosynthetic oxygen evolution (Km (CO2) = 96mmol m−3; Vmax= 133mmol g−1 Chl h−1) and the CO2 compensation point (г= 44cm3 m−3) suggested an adaptive, low photorespiratory state in response to low carbon concentrations. Photosynthetic Vmax 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 β-carboxylating enzymes phopho enolpyruvate carboyxlase (PEPcase), 24.1 mmol g−1 Chl h−1; phosphor enol pyruvate 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 EUmg Chl−).