The lipid composition of S. cerevisiae cells was studied in different conditions of pH and of ItCO3-and CO s concentration.
Increasing the concentration of COs in the broth determines an increase of total lipids, of total fatty acids, and of the relative amount of unsaturated fatty acids.
Changing the concentration of bicarbonates at constant pCO2 does not affect the lipid composition of the cells. Also pH changes do not have significant effects.
The metabolic role of CO S in biological tissues (UTTER, 1961 ; CALVIN and Pox, 1959), and in the enzyme systems involved in the fatty acid biosynthesis (WAKIL, 1958) is well established. KLV, I~r (1957) gave the first demonstration of the participation of CO 2 in long-chain fatty acid synthesis from acetate in cell homogenates of Saccharomyces cerevisiae. The enzymic reactions of fatty acid synthesis were then elucidated with investigations "in vivo" (Lu 1961). The detailed mechanism of malonyl-CoA synthesis is not yet well established, although it seems that "in vitro" systems of Clostridium kluyveri (KAzmo, 1962)and of yeast (Lu 1961(Lu , 1967) bicarbonate rather than free CO~ is involved in the carboxylation reaction.
Nevertheless, it is not yet clear whether fatty acid metabolism "in vivo" may be related to variations either of total COs, or bicarbonates, or of pH of the culture medium (on the base of the known relationship between these parameters in aqueous solutions).
In the present study we investigated lipid and fatty acid modifications in S. cerevisiae cells cultured in different conditions. In one set of experiments the pit of the medium was varied by changing the concentration of bicarbonates or the partial pressure of CO S . In an other set of the experiments the pH of the medium was kept constant and the concentration of total COz was varied.
Methods
S. cerevisiae cells of the Fleishmann strain (ATCC 7754) were kept at 4 ~ C in a solid broth containing 2~ peptone, 1.5~ agar, 2~ glucose and 1~ Difco yeast extract (U~TE~, 1961).