Growth of Vitamin B₁₂‐requiring Marine Diatoms in Mixed Laboratory Cultures With Vitamin B₁₂‐producing Marine Bacteria¹²

SUMMARY The vitamin B 12 requirement of several marine diatoms can be satisfied in B 12 − limited laboratory cultures by heterotrophic marine bacteria isolated from the same waters and from sediments. The bacteria can utilize diatom excretory products, or the remains of dead diatom cells, in the production of the vitamin. The growth of 12 B 12 1 − requiring diatoms (7 genera) in mixed cultures with 14 different bacteria (without added B 12 ) was compared to the growth of those same diatoms in axenic cultures with excess added B 12 . Diatom growth was generally rapid in the first few days, followed by sustained, slower growth. The diatom yields in mixed cultures ranged from 0.8 to 84% of the yields in axenic cultures with added B 12 . In a detailed study of one mixed culture, increases in diatom densities were paralleled by increases in cell densities of the bacterium during the first few days of exponential diatom growth. During the period of slow diatom growth, when diatom densities oscillated but steadily increased, the decreases in diatom densities were associated with increased bacterial growth. This suggests that death of a fraction of the B 12 ‐limited diatom population releases sufficient organic matter to stimulate growth of the bacteria and their subsequent excretion of B 12 ; this B 12 in turn stimulates further growth of the diatoms. Diatom‐bacteria interactions leading to the production of B 12 may be important in maintaining viable populations of B 12 ‐requiring diatoms in nutrient‐poor waters during periods between blooms, when conditions are unfavorable for rapid growth.