Effect of CO₂, CO₂ and Diamox on photosynthesis and photorespiration in <i>Chlamydomonas reinhardtii</i> (green alga) and <i>Anacystis nidulans</i> (Cyanobacterium, blue‐green alga)

Abstract

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO 2 uptake and the CO 2 compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO 2 , but CO 2 insensitive in cultures bubbled with air. Anacystis , even when cultured in 5% CO 2 , exhibited an CO 2 insensitive net photosynthesis. The CO 2 compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO 2 enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO 2 1 −1 ; while Chlamydomonas grown in 5% CO 2 enriched air for 4 days showed a high, but temporary CO 2 compensation point (60 μl CO 2 1 −1 ). After a two hour adaptation in low CO 2 , a stable, low CO 2 compensation point was reached. It seems that photorespiration can only be detected by the methods used in this study when the algae are cultured in high CO 2 , but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO 2 concentrations. When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO 2 (air), the cells behaved as if they had been grown in high CO 2 . They showed an oxygen sensitive net photosynthesis and a high CO 2 compensation point. This indicates that carbonic anhydrase plays an important role in the regulation of a measurable photorespiration in Chlamydomonas . The results are discussed in relation to previous observations of photorespiration measured by enzyme assay, metabolic products and gas exchange properties.