Vitamin B,2 binding by microalgal exocrines: dissociation constant of the vitamin-binder complex determined using an ultrafiltration technique

A simple, rapid ultrafiltration technique has been used to separate free vitamin B12 from that attached to the binding factor present in the media of cultures of the marine microalgae Mantoniella squamata and Pavlova lutheri (auxotrophic spec~es), and Dunaliella primolecta and Phaeodactylum tricomuturn (species with no B12 requirement).The dissociation constant (K,) for the B12-binder complex calculated from the results lies in the range 10-l2 to 10-l0 M indicating that the binder has a very high affinity for the vitamin.The ecological implications of the data are discussed in the light of our present understanding of B12 uptake by microalgae.