The B12 Vitamins and Growth of the Flagellate Prymnesium parvum

SUMMARY Vitamin B12 is an essential nutrient for the flagellate Prymnesium parvum when growing in axenic culture and apparently can be replaced only by a few related ‘incomplete’ analogues, i.e. only those devoid of one or both benzimidazole-methyl groups; and, of the substituted analogues, some (e.g. the mono-acid, the dicarboxylic acid and the ethylamide of B12) can replace vitamin B12, but some others of this group, e.g. pseudovitamin B12, 2-methylmercaptoadenine cobamide, the ethylamide mono-acid, methylamide and anilide of the vitamin B12, act as competitive inhibitors of the growth of P. parvum.