INDUCIBLE MECHANISMS FOR HCO3- UTILIZATION AND REPRESSION OF PHOTORESPIRATION IN PROTOPLASTS AND THALLI OF 3 SPECIES OF ULVA (CHLOROPHYTA)

Thalli of Ulva reticulata Forskaal, Ulva rigida C. Ag., and Ulva pulch ra Jaasund were incubated at different concentrations of dissolved CO2 Incubation at a high CO2 concentration resulted in decreased oxygen e volution rate and lower affinity for inorganic carbon at high pH condi tions, i.e. the ability to use HCO3- as a carbon source was reduced. T his effect was reversible, and plants regained this HCO3- uptake capac ity when transferred to air concentrations of CO2. The phytosynthetic oxygen evolution rate of plants grown at high CO2 concentration was re duced by high 02 concentrations, whereas thalli and protoplasts from c ultures grown at air concentration were not affected. This is interpre ted as a deactivation of the carbon-concentrating mechanism during con ditions of high CO2 resulting in high photorespiration when plants are exposed to high O2 concentrations. Protoplasts were not affected by h igh O2 to the same extent and were not able to utilize HCO3- from the medium. The algae were able to grow at very low CO2 concentrations, bu t growth was suppressed when an inhibitor of external carbonic anhydra se was present. Assay of carbonic anhydrase activities showed that ext ernal and internal CA activities were lower in plants grown at a high CO2 concentration compared to plants grown at a low concentration of C O2. Possible mechanisms for HCO3- utilization in these Ulva species ar e discussed.