The degree of unsaturation of fatty acids was higher in ChIorella vulgaris 1 h cells grown with air (low-CO2 cells) than in the cells grown with air enriched with 2% CO2 (high-CO2 cells). The change in the ratio of linoleic acid to a-linolenic acid was particularly significant. This change of the ratio was observed in four major lipids (monogalactosyldiacylglycerol, digalactosyldiacylglycerol, phosphatidylcholine, and phosphatidylethanolamine). The relative contents of lipid classes were essentially the same both in high-CO2 and low-CO2 cells. After high-CO2 cells were transferred to low CO2 condition, total amount of fatty acids remained constant but the relative content of a-linolenic acid increased during a 6-hour lag phase in growth with concomitant decreases in linoleic and oleic acids. When low-CO2 cells were transferred to high CO2 condition, total amount of fatty acids and relative content of oleic acid increased significantly. The amount of a-linolenic acid remained almost constant, while the amounts of palmitic, oleic, and linoleic acids increased. Similar, but smaller, changes in fatty acid compositions were observed in two species of green algae Chiamydomonas reinhardtii and Dunalieiia tertiolecta. However, no difference was found in Euglena gracilis, Porphyridium cruentum, Anabaena variabilis, and Anacystis nidulans.The affinity for inorganic carbon in photosynthesis of microalgae as well as submersed angiosperm is reduced when the concentration of C02 is elevated to 1 to 5% (e.g. refs. 3 and 19). It is generally assumed that this is due to decreases in the activity of carbonic anhydrase and in the capacity of accumulation ofinorganic carbon in high-CO2 cells.2 Likewise the development of pyrenoids by some green algae (12,18) and carboxysome by cyanobacteria (20) has been reported in '