Cellular basis of B 12 uptake and remodelling in microalgae revealed using a novel bioassay

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Summary Vitamin B 12 , an essential micronutrient for many microalgae and humans, is synthesised only by certain prokaryotes. B 12 is a complex tetrapyrrole that can exist in many forms (vitamers), some more bioavailable than others. Some microalgae are able to interconvert, or remodel, different B 12 vitamers. As microalgae are important primary producers, it is crucial to understand how diverse microalgae acquire, utilise, and remodel this micronutrient. Through the development of a novel algal bioassay for B 12 quantification that distinguishes between B 12 vitamers with different lower axial ligands, and the generation of targeted knock-out lines, we characterised the role of proteins involved in algal B 12 uptake and remodelling. We found that the previously characterised protein CoBalamin-Acquisition protein 1 (CBA1) is also necessary for the acquisition of pseudocobalamin, a less bioavailable form of B 12 . In addition, we provide the first experimental evidence that COBT is required for Chlamydomonas reinhardtii to remodel B 12 . We apply the algal B 12 bioassay to show that the edible alga Chlorella vulgaris can accumulate pseudocobalamin but is unable to remodel it, highlighting the need for thorough investigation of the metabolic requirements and capabilities of microalgae, especially given the growing interest in microalgae-based food additives.

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