Algae acquire vitamin B12 through a symbiotic relationship with bacteria

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Abstract
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.