ISSN:
1365-3040
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
Notes:
Photosynthesis was characterized for the unicellular green alga Coccomyxa sp., grown at low inorganic carbon (Ci) 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 C3 plant barley (Hordeum vulgare). Chlamydomonas had a significantly higher Ci-use efficiency of photosynthesis, with an initial slope of the Ci-response curve of 0.7 mol(gChl)−1 h−1 mmol Cim−3)−1, as compared to 0.3 and 0.23 mol(gChl)−1 h−1 (mmol Cim−3)−1 for Coccomyxa and barley, respectively. The affinity for Ci was also higher in Chlamydomonas, as the half maximum rate of photosynthesis [K0.5 (Ci)] 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 Cj-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 C3 plant photosynthesis than many algae currently used.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-3040.1994.tb00266.x
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