Vitamin B₁₂ (cobalamin) was identified nearly 80 years ago as the anti-pernicious anaemia factor in liver¹, and its importance in human health and disease has resulted in much work on its uptake², cellular transport³ and utilization⁴. Plants do not contain cobalamin because they have no cobalamin-dependent enzymes. Deficiencies are therefore common in strict vegetarians⁵, and in the elderly, who are susceptible to an autoimmune disorder that prevents its efficient uptake⁶. In contrast, many algae are rich in vitamin B₁₂, with some species, such as Porphyra yezoensis (Nori), containing as much cobalamin as liver⁷. Despite this, the role of the cofactor in algal metabolism remains unknown, as does the source of the vitamin for these organisms. A survey of 326 algal species revealed that 171 species require exogenous vitamin B₁₂ for growth, implying that more than half of the algal kingdom are cobalamin auxotrophs. Here we show that the role of vitamin B₁₂ in algal metabolism is primarily as a cofactor for vitamin B₁₂-dependent methionine synthase, and that cobalamin auxotrophy has arisen numerous times throughout evolution, probably owing to the loss of the vitamin B₁₂-independent form of the enzyme. The source of cobalamin seems to be bacteria, indicating an important and unsuspected symbiosis.

1. Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, UK
2. Department of Biosciences, University of Kent, Canterbury, Kent CT2 7NJ, UK

Correspondence to: Alison G. Smith¹ Correspondence and requests for materials should be addressed to A.G.S. (Email: as25@cam.ac.uk).

Received 18 May 2005; Accepted 15 July 2005

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