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The effect of CO2 concentration on the rate of photorespiratory ammonium excretion and on glutamine synthetase (GS) and carbonic anhydrase (CA) isoenzymes activities has been studied in Chlamydomonas reinhardtii cw‐15 mutant (lacking cell wall) and in the high CO2‐requiring double mutant cia‐3/cw‐15 (lacking cell wall and chloroplastic carbonic anhydrase). In cw‐15 cells, both the extracellular (CAext) and chloroplastic (CAchl) CA activities increased after transferring cells from media bubbled with 5% CO2 in air (v/v, high‐Ci cells) to 0.03% CO2 (low‐Ci cells), whereas in cia‐3/cw‐15 cells only the CAext was induced after adaptation to low‐Ci conditions and the CAchl activity was negligible. During adaptation to low‐Ci conditions in the presence of 1 mM of l‐methionine‐D,L‐sulfoximine (MSX), a specific inhibitor of GS activity, both mutant strains excreted photorespiratory ammonium into nitrogen free medium. In addition, the ammonium excretion rate by cw‐15 in the presence of MSX was lower in cells grown and kept at 5% CO2 than in high‐Ci cells adapted to 0.03% CO2. The double mutant cia‐3/cw‐15 excreted photorespiratory ammonium at a higher rate than did cw‐15. Total GS activity (GS‐1 plus GS‐2) increased during adaptation to 0.03% CO2 in both strains of C. reinhardtii. However, only the activity GS‐2, which is located in the chloroplast, increased during the adaptation to low CO2, whereas the cytosolic GS‐1 levels remained similar in high and low‐Ci cells. We conclude that: (1) cia‐3/cw‐15 cells lack chloroplastic CA activity; (2) in C. reinhardtii photorespiratory ammonium is refixed in the chloroplasts through the GS‐2/GOGAT cycle; and (3) chloroplastic GS‐2 concentration changes in response to the variation of environmental CO2 concentration. [ABSTRACT FROM AUTHOR]

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]