Studies of Elodea nuttallii grown under photorespiratory conditions. I. 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 CO 2 (aq) and HCO 3 − from the reactors. A stoichiometry of 1.19:1 was observed between HCO 3 − removal during photosynthesis and OH − production during photosynthesis, consistent with theories regarding direct bicarbonate utilization. In laboratory experiments, the light compensation points ( г PPFD ) 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 (K m (CO 2 ) = 96mmol m −3 ; V max = 133mmol g −1 Chl h −1 ) and the CO 2 compensation point ( г = 44cm 3 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 CO 2 utilization at pH 8 could account for most of the observed phototsynthetic rates, an HCO 3 − component was present, suggesting two separate transport systems for HCO 3 − and CO 2 (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 enol pyruvate 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 CO 2 levels which is hypothesized to include carbonic anhydrase (CA) and an active transport system for HCO 3 − . CA levels were surprisingly low in E. nuttallii (14.2 EUmg Chl − ).