ABSTRACT
Dunaliella spp. live in a wide range of habitats with significant variations in inorganic carbon availability. As far as has been investigated, Dunaleilla spp. have the normal range of carboxylases for green algae and high plants, including Form IB Rubisco. Tracer carbon studies show that the biochemical pathway of CO2 assimilation in Dunaliella involves the photosynthetic carbon reduction cycle with no prior carboxylation-decarboxylation (C4-like) cycle except, perhaps, in stationary phase cells. All Dunaliella spp. so far examined, except perhaps D. acidophila, have inorganic carbon concentrating mechanisms (CCMs) based, apparently, on active transport of inorganic carbon (or protons) across one or more membranes. These CCMs, by accumulating CO2 at the site of Rubisco, increases the affinity of cells for inorganic carbon relative to what would be expected from the kinetics of Rubisco from relatives of Dunalialla. CCMs decrease, but do not eliminate, the occurrence of the oxygenase activity of Rubisco and the subsequent glycolate excretion and/or metabolism. Both CO2 and HCO3 − can enter both whole cells, and isolated, photosynthetically active, chloroplasts, of Dunaliella. The mechanism(s) of the CCM(s) of Dunalialla needs much further investigation. To the extent that microalgae can be used in carbon dioxide mitigation, Dunaliella spp. have the advantage of using water (saline or acidic) which would not be usable to grow most other algae (or for most other human uses), and of having high-value products such as β-carotene in addition to Dunalialla biomass that can be used as a fuel.
