SUMMARY: A marine sp. was found to require a growth factor present in small quantities in natural sea water. Probably the same factor was produced in the medium by a bacterium with which the alga was associated when isolated. The factor could be removed from sea water by activated charcoal, and could be replaced by crystalline vitamin B (cobalamin).

The marine alga used in this investigation was collected in Great South Bay, Long Island, N.Y., by Mr J. H. Ryther (1954) and was purified as described below. There is some doubt as to the identity of the alga, which is a unicellular form about 2μ. wide and 2–6 μ. long. It has been tentatively assigned by the present author to the genus , and may be identical with Butcher (1952).


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