Culture Conditions and Growth Rates

Because of the reactivity of oxygen with biologically impor tan t compounds, e.g. unsaturated lipids, SH-conta in ing proteins and cofactors, and redox compounds (see review by Haugaard , 1968), cells in aerobic envi ronments require some mean(s) for protect ion from indiscr iminate oxidation. Since the superoxide free radical, O2, is apparently a major product of the reaction of 02 with reduced compounds (Misra and Fridovich, 1972), Fridovich and co-workers have proposed an 02 protective mechanism based on the O~--scavenging enzyme, superox ide d i s m u t a s e (SOD) (McCord and Fridovich, 1969; McCord et al., 1971; Gregory and Fridovich, 1973 a). This report describes the combined occurrence of the following physiological characteris t ics in a m u t a n t s t ra in of the green alga Chlorella sorokiniana (Shihira and Krauss): (a) resistance to high part ial pressures of 02, (b) resistance to the O~--genera t ing antibiotic, streptonigrin, and (c) resistance to UV irradiat ion. The mutan t also contained a 3.5-fold higher level of SOD when grown under air than did the wild-type strain. These correlat ions provide substantial evidence tha t an 02 detoxification system (probably involving SOD) functions to protect the mutan t s train from, not only 02 toxicity, but also, some fract ion of UV-media ted damage.