Vitamin B12 (cobalamin) biosynthesis — Which domain of life is the sole source of de novo vitamin B12 synthesis?

Introduction / Context:
Vitamin B12 (cobalamin) is a complex corrinoid cofactor essential for methionine synthase and methylmalonyl-CoA mutase in humans. Understanding its biosynthetic origin helps explain dietary requirements and why certain diets need supplementation.

Given Data / Assumptions:

• B12 is required in trace amounts in humans.
• Its structure includes a cobalt ion within a corrin ring.
• Only certain organisms possess the full pathway to synthesize B12.

Concept / Approach:
De novo biosynthesis of cobalamin is carried out exclusively by micro-organisms (bacteria and some archaea). Plants and animals do not synthesize B12; animals obtain it indirectly by consuming foods harboring microbial B12 or via fortified sources. Therefore, “micro-organisms” is the correct answer.

Step-by-Step Solution:

Verification / Alternative check:
Microbial ecology and nutrition literature consistently attribute cobalamin production to bacteria/archaea, explaining B12 presence in ruminant products and fermented foods.

Why Other Options Are Wrong:

• Fishes/Animals: they accumulate B12 but do not synthesize it de novo.
• Plants: generally lack B12 synthesis; plant foods need fortification to supply B12.

Common Pitfalls:
Confusing “present in animal foods” with “made by animals.” Animals are consumers, not producers, of B12.

Final Answer:
Micro-organisms