IMPLICATIONS OF INORGANIC CARBON UTILIZATION - ECOLOGY, EVOLUTION, AND GEOCHEMISTRY

CO2-concentrating mechanisms based on active inorganic C influx across a membrane (or membranes) are widespread in aquatic phototrophs. They are apparently absent from, or poorly developed in, taxa such as bryophytes and freshwater red algae, but are retained in some terrestrial phototrophs, i.e., some lichens and free-living algae and cyanobacteria related to the lichen photobionts. The natural occurrence of CO2-concentrating mechanisms is negatively correlated with CO2 enrichment of aquatic habitats from respiration of organic C produced elsewhere and, less clearly, with low temperatures during the growing season, with low external pH, and with rapid water movement over the plant surface. Predictions based on laboratory data and on theoretical considerations as to the influence of light, nitrogen, and iron availability on the occurrence of CO2-concentrating mechanisms have yet to be rigorously tested in the field. The evolution of CO2-concentrating mechanisms is not well understood, but clearly the location of inorganic C pumps must have been influenced by successive symbiotic events. Increasing atmospheric CO2 levels, and other anthropogenic effects (acidification, eutrophication), may influence the selective advantage (or disadvantage) associated with possession of CO2-concentrating mechanisms in particular environments.