Trace metals in vitamin B12 fermentations: Which element, at proper concentration, stimulates Ascomycete production of vitamin B12 and can counteract iron toxicity in Candida species?

Introduction / Context:
Vitamin B12 (cobalamin) contains a central corrin ring with a cobalt ion. Trace metal management is crucial in fermentations because micronutrients influence enzyme activity, cofactor biosynthesis, and metal toxicity balance.

Given Data / Assumptions:

• Organisms include Ascomycetes and Candida species involved in vitamin-related fermentations.
• Iron can be toxic at excess; some metals mitigate adverse effects.
• We seek the trace metal that stimulates B12 formation and alleviates iron toxicity.

Concept / Approach:
Cobalt is essential to the cobalamin structure; providing controlled cobalt supports corrinoid biosynthesis. Additionally, balanced cobalt supply can offset negative impacts of iron by influencing metal uptake/competition and cofactor assembly.

Step-by-Step Solution:
Identify the metal core of vitamin B12 → cobalt (Co). Relate metabolic need: supplying Co stimulates corrinoid pathway enzymes. Note side benefit: appropriate Co levels can counteract iron toxicity in some yeasts. Select Co.

Verification / Alternative check:
Biochemical pathways of cobalamin synthesis explicitly require cobalt; process media list Co salts as key supplements.

Why Other Options Are Wrong:
Ni, Zn, Ca, Mg play other roles but are not the defining metal of cobalamin or the primary countermeasure cited for iron stress in this context.

Common Pitfalls:
Confusing cobalt's essentiality with general metal supplementation; overdosing cobalt can also be inhibitory—dosage matters.

Final Answer:
Co.