Algae acquire vitamin B₁₂ through a symbiotic relationship with bacteria

Croft, Martin T., Lawrence, Andrew D., Raux-Deery, Evelyne, Warren, Martin J. and Smith, Alison G. (2005) Algae acquire vitamin B₁₂ through a symbiotic relationship with bacteria. Nature, 438, 90–93. (doi:10.1038/nature04056).

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.

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Contributors

Author: Andrew D. Lawrence

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