B,,-Vitamins and Growth of the Flagellate Ochromonas ma1 hamensis

SUMMARY: The flagellate Ochromonas rnalhamensis requires vitamin B,, and re-sponds only to those natural forms of the vitamin which are active also for higher animals. The relation between the rate of growth and the concentration of cyano-cobalamin is described by an equation of the form of an adsorption isotherm. The ‘inactive ’ analogues, pseudovitamin B,, and Factor A, were taken up by 0. mathamensis to about the same degree as cyanocobalamin, and inhibited com-petitively the growth response to cyanocobalamin, apparently by blocking a cell mechanism for ‘binding ’ the vitamin. At least half the marine algal flagellates and diatoms which have so far been studied in pure culture require an exogenous supply of vitamin B,, (Droop, 1957a), and in the natural orders Chrysomonadina and Euglenida the require-ment is probably characteristic (cf. Hutner, in Ford & Hutner, 1955). That so many of these planktonic organisms require vitamin B,, explains the present keen interest among marine biologists in the ecological role of the vitamin, particularly about the question whether the concentration of available vita