C⁴ characteristics of photosynthesis in the C₃ alga <i>Hydrodictyon africanum</i>

Abstract

Abstract Results obtained with Hydrodictyon africanum , and data from the literature, show that most green algae of the chlorophyte type (e.g. Chlorella, Chlamydomonas, Hydrodictyon ) differ in their photosynthetic C fixation characteristics from most green algae of the charophyte type (e.g. Spirogyra, Chara ) and from C 3 higher plants. The chlorophyte algae fix inorganic carbon by the photosynthetic carbon reduction cycle pathway, but have a low CO 2 compensation point in 250 μM O 2 , a low inhibition of CO 2 fixation from 10 μM CO 2 /250 μM O 2 when compared with 10 μM CO 2 /zero O 2 , and a low half‐saturation constant for CO 2 . These three characteristics are different from those of charophytes and C 3 higher plants, and resemble those of C 4 higher plants. It is suggested that these characteristics of chlorophyte algae are the result of a ‘CO 2 concentrating mechanism’ which increases the CO 2 /O 2 ratio at the site of ribulose bisphosphate carboxylase‐oxygenase action in a similar way to that achieved by the C 4 ?C 3 acid cycle in C 4 plants. In the chlorophyte algae, however, CO 2 concentration probably involves active HCO 3 − transport at the inner membrane of the chloroplast envelope. Active HCO 3 − transport can occur at the plasmalemma of charophyte algae and submerged aquatic higher plants as well as chlorophyte algae, so it is unlikely to explain the differences between the two groups of aquatic green plants. Differences in the properties of ribulose bisphosphate carboxylase‐oxygenase, and differences in CO 2 production in the light, also seem inadequate to account for the different photosynthetic characteristics. The chlorophyte type of ‘C0 2 concentrating mechanism’ appears to be common in other classes of eukaryotic algae, and in cyanophytes. Some of the ‘advanced’ members of these eukaryotic algal classes (including the chlorophytes) may lack the mechanism, while some ‘primitive’ charophytes may retain the mechanism which their ancestors presumably possessed.