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

Abstract

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO2 uptake and the CO2 compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO2, but CO2 insensitive in cultures bubbled with air. Anacystis, even when cultured in 5% CO2, exhibited an CO2 insensitive net photosynthesis. The CO2 compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO2 enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO2 1−1; while Chlamydomonas grown in 5% CO2 enriched air for 4 days showed a high, but temporary CO2 compensation point (60 μl CO2 1−1). After a two hour adaptation in low CO2, a stable, low CO2 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 CO2, but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO2 concentrations.When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO2 (air), the cells behaved as if they had been grown in high CO2. They showed an oxygen sensitive net photosynthesis and a high CO2 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.