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 Coccom yxa sp., grown at low inorganic carbon (C-i) concentrations, and compa red with Chlamydomonas reinhardtii, which had been grown so that the C O2 concentrating mechanism (CCM) was expressed, and with protoplasts i solated 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)(mmo l 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 f or Ci was also higher in Chlamydomonas, as the half maximum rate of ph otosynthesis [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. Eth oxyzolamide (EZ), an inhibitor of the enzyme carbonic anhydrase (CA) a nd the CCM, caused a 17-fold decrease in the initial slope of the phot osynthetic C-i-response curve in Chlamydomonas, but only a 1.5- to two -fold decrease in Coccomyxa and barley. The photosynthetic light-respo nse curve showed further similarities between barley and Coccomyxa. Th e 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 gro wn under low and high light, respectively. In contrast, the maximum co nvexity of Chlamydomonas was 0.85. The intrinsically lower convexity o f Chlamydomonas is suggested to result from the diversion of electron transport from carbon fixation to the CCM. Taken together, these resul ts suggest that Coccomyxa does not possess a CCM and due to this appar ent lack of a CCM, we propose that Coccomyxa is a better cell model sy stem for studying C-3 plant photosynthesis than many algae currently u sed.