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Thalli of Ulva reticulata Forskaal, Ulva rigida C. Ag., and Ulva pulchra Jaasund were incubated at different concentrations of dissolved CO[SUB2]. Incubation at a high CO[SUB2]2 concentration resulted in decreased oxygen evolution rate and lower affinity for inorganic carbon at high pH conditions, i.e. the ability to use HCO[SUP-][SUB3] as a carbon source was reduced. This effect was reversible, and plants regained this HCO[SUP-][SUB3] uptake capacity when transferred to air concentrations of CO[SUB2] The phytosynthetic oxygen evolution rate of plants grown at high C02 concentration was reduced by high O[SUB2] 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 CO[SUB2] resulting in high photorespiration when plants are exposed to high O[SUB2] concentrations. Protoplasts were not affected by high O[SUB2] to the same extent and were not able to utilize HCO[SUP-][SUB3] from the medium. The algae were able to grow at very low CO[SUB2] concentrations, but growth was suppressed when ah 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 CO[SUB2] concentration compared to plants grown at a low concentration of CO[SUB2] possible mechanisms for HCO[SUP-][SUB3] utilization in these Ulva species are discussed. [ABSTRACT FROM AUTHOR]