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SUMMARY: In cultures of the flagellate Ochromonas malharnensis the rate of growth is determined by the concentration of cyanocobalamin up to a limit at which a maximum growth rate is achieved. In organisms grown exponentially with different rate-limiting concentrations of cyanocobalamin, the amounts of protein and nucleic acids were approximately proportional to the rates at which the organisms had grown. With increasing growth rate the concentrations of riboflavin, nicotinic acid, pantothenic acid and chlorophylls increased. Carbohydrate showed little change, whereas fat and 'free ' amino-nitrogen diminished in concentration as the growth rate increased. In cultures of 0. malhamensis grown with limiting or near-limiting concentrations of cyanocobalamin and with increasing concentrations of chloram- phenicol, the rates of division declined asymptotically to a low value corresponding with 500 pg. chloramphenicol/ml. culture medium. Inhibition of growth caused by the antibiotic was annulled by increasing the concentration of cyanocobalamin in the culture medium. A variety of cultural and nutritional factors can influence the balance of com- position of living cells. Vitamin deficiencies, for example, may distort the cell metabolism and bring about characteristic ' biochemical lesions '. Vitamin B,, plays presumably a part in diverse metabolic processes, but in some of these its action is probably indirect. No single fundamental role can as yet be assigned to this vitamin, although there are'indications, mainly from work with animals and animal-tissue preparations, that its essential function is concerned with protein biosynthesis (see review by Smith, 1958). If indeed the vitamin acts in protein or nucleic acid synthesis, this action would give rise to a variety of secondary effects and explain an apparent multiplicity of functions. For example, it would explain the low concentrations of certain enzymes found in the tissues of deficient animals (Mistry et al. 1955; Wong & Schweigert, 1957; Murthy, Desikachar & Swaminathan, 1956) ; the sharp decrease in ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) in vitamin B,,-deficient Euglena gracilis (Soldo, 1955) and of DNA in Lactobacillus leichmannii (Wacker, Pfahl & Schroder, 1957); and the effect of vitamin B,, in reversing the actions of aminopterin and aureomycin on the synthesis of DNA in L. leichmannii (Rege & Sreenivasan, 1954a, b). In several species of bacteria the nucleic acid content of the organisms is related to the rate at which they have grown (Malmgren & Heddn, 1947). In Bacterium lactis aerogenes the growth rate under a variety of cultural conditions is indeed proportional to the cell content of RNA (Caldwell, Mackor & Hinshelwood, 1950). In cultures of the flagellate Ochromonas malhamensis the rate of growth is determined by the concentration of vitamin B,, up to a

The Influence of Vitamin B12 on Growth Rate and Cell Composition of the Flagellate Ochromonas malhamensis