Unexpected Response to Vitamin B12of Dominant Centric Diatoms From the Spring Bloom in the Gulf of Maine (Northeast Atlantic Ocean)1,2

Twelve clones (seven species) representative of centric diatoms dominant in the spring phytoplankton bloom in the Gulf of Maine were isolated and rendered axenic. Genera included wereThalassiosira, PorosiraandChaetoceros.Unlike most centric diatoms studied previously, none of these has an absolute requirement for vitamin B12.However, B12(5ng.l‐1) stimulated growth of most clones by eliminating or reducing the lag phase and increasing the growth rate. Bloom population densities developed 4–54 days earlier with B12present. Several clones grown with B12removed more than 80% of the vitamin from the medium. When grown in vitamin‐free medium the cells put 0.01–0.7 ng.l‐1B12into the medium. We conclude that vitamin B12is of ecological significance even though the requirement for it is not absolute.