STUDIES OF ELODEA-NUTTALLII GROWN UNDER PHOTORESPIRATORY CONDITIONS .2. EVIDENCE FOR BICARBONATE ACTIVE-TRANSPORT

Elodea nuttallii was grown under greenhouse conditions in domestic wastewater in an aquatic treatment system under conditions conducive to photorespiration. Initial research on the photosynthetic characteristics of E. nuttallii suggest that the submergent macrophyte possessed a carbon concentrating mechanism. Isotopic disequilibria H14CO3-uptake studies (5-80s) were used to assess the bicarbonate active-transport capabilities in E. nuttallii leaves. Using a range of substrate concentrations (50-50200 mmol m-3), the accumulation of label (mmol g-1 Chl) over time due to transport was found initially to exceed accumulation due to fixation until steady state rates were observed. Internal steady state pools of dissolved inorganic carbon (DIC) ranged from 40 to 80 mol m-3. The concentration factor (CF: the ratio of internal cyroplasmic [DIC] to external medium [DIC] decreased from 800 to 114 as external bicarbonate concentrations were increased. Inhibition of transport by uncouplers (2,4-dinitrophenol (DNP), carbonyl cyanide-m-chlorophenylhydrazone (CCCP)); ATPase inhibitors (dicylochexocarbodiamide (DCCD), phloridzin, arsenate); electron transport inhibitors (DCMU, Antimycin A), and carbonic anhydrase inhibitors (ethoxyzolamide, acetazolamide) suggest that bicarbonate transport required (1) a proton motive force, (2) a functional ATPase, (3) a chloroplast carbon sink, and possibly (4) a CA-like moiety associated with the transport protein. While plasmalemmasomes were not observed, the plasmalemma was vesiculated and acid and alkaline banding was observed when leaves were incubated under light in the presence of bicarbonate. These data are consistent with the operation of a bicarbonate-cation symport which concentrates substrate against a concentration gradient at the expense of metabolic energy. The presence of an active transport system for bicarbonate ensures that internal carbon concentrations are high when carbon dioxide is scarce and bicarbonate is the only carbon species available in aquatic treatment systems during photorespiratory conditions. Therefore, E. nuttallii is particularly well suited for use in these systems.