Citric acid fermentation (Aspergillus niger): To favor high yields, the production medium is formulated to be deficient in which micronutrient (critical limiter)?

Introduction / Context:
Citric acid is a flagship organic acid produced by Aspergillus niger. Media composition strongly influences metabolic routing toward citrate accumulation rather than biomass or byproducts. This question probes the key micronutrient limitation used to bias metabolism toward citric acid.

Given Data / Assumptions:

• Organism: Aspergillus niger.
• Industrial objective: maximize citrate yield and productivity.
• Medium tuning: trace metals and nutrients are tightly controlled.

Concept / Approach:
Manganese limitation is well known to promote citric acid accumulation by reducing certain enzyme activities and morphologies associated with excessive growth, thereby steering carbon flux toward citrate overflow. While iron and phosphate levels are also managed, the most classically emphasized deficiency for high-yield citrate production is manganese.

Step-by-Step Solution:
Identify the hallmark lever: trace manganese concentration kept extremely low.Link low Mn to morphological/enzymatic states favoring citrate secretion.Select “manganese” as the single, most critical deficiency.

Verification / Alternative check:
Fermentation manuals specify Mn in the microgram-per-liter range; inadvertent Mn contamination (e.g., from equipment) can depress titers.

Why Other Options Are Wrong:
Iron and phosphate are controlled but are not the principal “deficiency lever” in foundational teaching; “all of these” overstates the requirement.

Common Pitfalls:
Assuming any trace metal limitation will work; manganese is uniquely sensitive in this process.

Final Answer:
manganese