C4 characteristics of photosynthesis in the C3 alga Hydrodictyon africanum

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 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.