GROWTH OF VITAMIN B12-LIMITED CULTURES: THALASSIOSIRA PSEUDONANA,MONOCHRYSIS LUTHERI,AND ISOCHRYSIS GALBANA1,2

The growth rates of 3 species of phytoplankton were found to be dependent on the vitamin B₁₂ concentrations in the media. In batch cultures, the vitamin B₁₂ half-saturation constants and standard errors were 0.39 ± 0.042 μμg/ml for Thalassiosira pseudonana (clone 3H), 1.69 ± 0.24 μμg/ml for Isochrysis galbana, and 2.77 ± 1.65 μμg/ml for Monochrysis lutheri. A chemostat was used to grow T. pseudonana with vitamin B₁₂ as the controlling factor. In the chemostat the yield and standard deviation, 102 ± 21 × 10⁴ cells/μμg vitamin B₁₂, was the same as in the batch culture, 126 ± 13 ± 10⁴ cells/μμg. The chemostat half-saturation constant, 0.26 ± 0.068 μμg/ml vitamin B₁₂, and maximum growth rate were in agreement with those obtained in batch cultures. Vitamin concentrations for maximum growth, rates were greater than those calculated necessary from yield data to give observed population densities similar to those in natural waters. In the sea the effect of vitamin B₁₂ concentration on growth rates may be complicated by low concentrations of other nutrients or the presence of inhibitors.