Cellular basis of B12 uptake and remodelling in microalgae revealed using a novel bioassay

Vitamin B12 is an essential micronutrient for many microalgae and humans, yet it is synthesized only by certain prokaryotes. B12 exists in multiple forms (vitamers) that differ in bioavailability, and some microalgae can interconvert or remodel these forms. Because microalgae are important primary producers, understanding how they acquire, utilize, and remodel B12 is crucial. This preprint reports the development of a novel algal bioassay that quantifies B12 while distinguishing between vitamers with different lower axial ligands, enabling more detailed analysis of B12 processing in algae.

Using targeted knockout lines and the new bioassay, the authors characterized the roles of proteins involved in algal B12 uptake and remodeling. They found that the previously identified protein CBA1 (Cobalamin-Acquisition protein 1) is also required for acquisition of pseudocobalamin, a less bioavailable B12 form. They also provide the first experimental evidence that COBT is necessary for Chlamydomonas reinhardtii to remodel B12.

Applying the bioassay to the edible alga Chlorella vulgaris, they showed that it can accumulate pseudocobalamin but is unable to remodel it into a more bioavailable form. This highlights the need for thorough investigation of the metabolic requirements and capabilities of microalgae, especially given the growing interest in microalgae-based food additives and supplements.