THE CO2-CONCENTRATING MECHANISM IS ABSENT IN THE GREEN-ALGA COCCOMYXA - A COMPARATIVE-STUDY OF PHOTOSYNTHETIC CO2 AND LIGHT RESPONSES OF COCCOMYXA, CHLAMYDOMONAS-REINHARDTII AND BARLEY PROTOPLASTS

Photosynthesis was characterized for the unicellular green alga Coccomyxa sp., grown at low inorganic carbon (C-i) concentrations, and compared with Chlamydomonas reinhardtii, which had been grown so that the CO2 concentrating mechanism (CCM) was expressed, and with protoplasts isolated from the C-3 plant barley (Hordeum vulgare). Chlamydomonas had a significantly higher C-i-use efficiency of photosynthesis, with an initial slope of the C-i-response curve of 0.7 mol(gChl)(-1) h(-1)(mmol C(i)m(-3))(-1), as compared to 0.3 and 0.23 mol(gChl)(-1)h(-1) (mmol C(i)m(-3))(-1) for Coccomyxa and barley, respectively. The affinity for Ci was also higher in Chlamydomonas, as the half maximum rate of photosynthesis [K-0.5 (C-i)] was reached at 0.18 mol m(-3), as compared to 0.30 and 0.45 mol m(-3) for Coccomyxa and barley, respectively. Ethoxyzolamide (EZ), an inhibitor of the enzyme carbonic anhydrase (CA) and the CCM, caused a 17-fold decrease in the initial slope of the photosynthetic C-i-response curve in Chlamydomonas, but only a 1.5- to two-fold decrease in Coccomyxa and barley. The photosynthetic light-response curve showed further similarities between barley and Coccomyxa. The rate of bending of the curve, described by the convexity parameter, was 0.99 (sharp bending) and 0.81-0.83 (gradual bending) for cells grown under low and high light, respectively. In contrast, the maximum convexity of Chlamydomonas was 0.85. The intrinsically lower convexity of Chlamydomonas is suggested to result from the diversion of electron transport from carbon fixation to the CCM. Taken together, these results suggest that Coccomyxa does not possess a CCM and due to this apparent lack of a CCM, we propose that Coccomyxa is a better cell model system for studying C-3 plant photosynthesis than many algae currently used.