Inducible Mechanisms for Hco3– Utilization and Repression of Photorespiration in Protoplasts and Thalli of Three Species of Ulva (Chlorophyta)1

Thalli of Ulva reticulata Forskaal, Ulva rigida C. Ag., and Ulva pulchra Jaasund were incubated at different concentrations of dissolved CO2. Incubation at a high CO2 concentration resulted in decreased oxygen evolution rate and lower affinity for inorganic carbon at high pH conditions, i.e. the ability to use HCO3– as a carbon source was reduced. This effect was reversible, and plants regained this HCO3– uptake capacity when transferred to air concentrations of CO2. The phytosynthetic oxygen evolution rate of plants grown at high CO2 concentration was reduced by high O2 concentrations, whereas thalli and protoplasts from cultures grown at air concentration were not affected. This is interpreted as a deactivation of the carbon‐concentrating mechanism during conditions of high CO2 resulting in high photorespiration when plants are exposed to high O2 concentrations. Protoplasts were not affected by high 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, but growth was suppressed when an inhibitor of external carbonic anhydrase was present. Assay of carbonic anhydrase activities showed that external and internal CA activities were lower in plants grown at a high CO2 concentration compared to plants grown at a low concentration of CO2. Possible mechanisms for HCO3– utilization in these Ulva species are discussed.