Vitamin B
12 compounds are synthesized by only certain bacteria, thaumarchaeota, and cyanobacteria in marine system. Cyanobacteria do not synthesize vitamin B
12 but rather produce pseudovitamin B
12 , which is inactive in some microalgae as well as humans. Thus, vitamin B
12 and pseudovitamin B
12 co-exist in the surface ocean (photic zone, depth 0–200 m). Thaumarchaeota is the major producer of vitamin B
12 in the deeper ocean (mesopelagic zone, depth 200–300 m). Pseudovitamin B
12 formed by cyanobacteria can be remodeled to vitamin B
12 by some microalgae. As almost half of the species of algae require vitamin B
12 , such biological interactions over vitamin B
12 play pivotal roles in algal growth. Vitamin B
12 synthesized by certain bacteria and thaumarchaeota is transferred via algae and then concentrated into fish and shellfish bodies in the marine food chain.
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