C4 characteristics of photosynthesis in the C3 alga Hydrodictyon africanum

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 C₃ higher plants. The chlorophyte algae fix inorganic carbon by the photosynthetic carbon reduction cycle pathway, but have a low CO₂ compensation point in 250 μM O₂, a low inhibition of CO₂ fixation from 10 μM CO₂/250 μM O₂ when compared with 10 μM CO₂/zero O₂, and a low half-saturation constant for CO₂. These three characteristics are different from those of charophytes and C₃ higher plants, and resemble those of C₄ higher plants. It is suggested that these characteristics of chlorophyte algae are the result of a ‘CO₂ concentrating mechanism’ which increases the CO₂/O₂ ratio at the site of ribulose bisphosphate carboxylase-oxygenase action in a similar way to that achieved by the C₄?C₃ acid cycle in C₄ plants. In the chlorophyte algae, however, CO₂ concentration probably involves active HCO₃^? transport at the inner membrane of the chloroplast envelope. Active HCO₃^? 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 CO₂ production in the light, also seem inadequate to account for the different photosynthetic characteristics. The chlorophyte type of ‘C0₂ 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.