SummaryTotal lipids were extracted from the cells of Candida utilis grown at a constant population density in continuous culture. At different steady states, the environment was controlled with respect to dissolved oxygen and glucose concentrations, pH and temperature. Gas liquid chromatography was used t o follow quantitative and qualitative changes in the fatty acid composition of the cells. Increasing glucose concentration resulted in higher lipid content; high oxygen concentrations increased the level of polyunsaturated fatty acids. The most significant changes in fatty acid composition took place when both glucose and oxygen concentrations were high, and under these conditions the amount of linolenic acid was a t its highest value.

The experiments described in this paper arose as a result of information pained during attempts to prepare large quantities of Escherichm coli for the purpose of investigation of eytochrome a^, and if possible to increase the cytochrome a2 content of the bacteria. It is generally held that cytochrome 82 has the function of a cytochrome oxidase. Carbon monoxide shifts the absorption band to 634 m/i, oxyf]:en to 645 m/i, and aeration in the presence of cyaiiido causes the band to disappear.Further evidence for the oxidase function of cytochrome a2 of Aerahacter aerogenes, which has a similar cytochrome absorption spectrum to E. coli, was provided by Tissieres (1951) who showed that on passing air into a cooled suspension of A. aerogenes cytochrome as is oxidized bofore bi, and on cessation of aeration cytocbrome bi is reduced before a^. However, Tissieres has also shown that oxygen uptake is not directly related to the intensity of the ao absorption band in A. aerogenes, and suggested that cytochrome ai may also function as cytochrome oxidase. The same worker (1952) was able to prepare a cell-free suspension of particles, containing cytochrome a, and bi but no visible amount of eytochrome a^-This suspension showed considerable respiratory activity.The adaptive development of the eytoehrome system of yeast has now been rejiorted by a number of workers (Chin, 1950; Eplirussi, Slonimski and Perrodin, 1950; Ephrussi and Slonimski, 1950). Schaeffer (1950) described an increase in the cytochromes of Bax-iUns ccreus on aeration, and found that the medium from anaerobic culture contained much more free porphyrin than mediiun from aerobic culture. This suggested tliat porphyrins not used in the synthesis of cytochrome were excreted. In tlie case of E. coU, however, very little free porphyrin was fonnd in the medium from anaerobic cultures, and none in aerobic culture.Adaptive formatiou of the eytochrom,es of E. coli, and increase of respiration in the presence of oxygen was also found by the author, independently.

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SummaryDissolved oxygen and glucose concentrations have been independently maintained a t various concentrations for extended periods during growth of Candida utilis in continuous culture. Simultaneous observations of cytochrome concentration, growth rate, rates of uptake of oxygen and glucose and rates of production of CO,, ethanol and acid have been made during steady states at various levels of oxygen and glucose. There is an inverse relationship between dissolved oxygen and cytochrome and between glucose concentration and cytochrome.Studies of the transient state following a step change from high to low dissolved oxygen show that there is a lag of about 10 hr during which there is no change in the above parameters. This is followed by rapid oscillatory changes in cytochrome content and a change to a more fermentative metabolism.