Citric acid biotechnology — The principal microorganism used worldwide for large-scale microbial production of citric acid is:

Introduction / Context:
Citric acid is among the highest-volume organic acids produced by fermentation. The process relies on microorganisms that channel carbon through the TCA cycle and secrete citrate under specific nutrient constraints (notably low metal availability).

Given Data / Assumptions:

• Several fungi and bacteria can produce organic acids, but productivity and robustness differ.
• Industrial processes emphasize strain stability and high yields.
• We must identify the dominant organism in modern citric acid production.

Concept / Approach:
Aspergillus niger is the global workhorse for citric acid, owing to its high yields, acid tolerance, and ability to thrive on diverse carbohydrate feedstocks. Other organisms listed are not standard for industrial citric acid production.

Step-by-Step Solution:

Verification / Alternative check:
Industry histories, FAO/WHO reviews, and patents uniformly cite A. niger as the primary producer.

Why Other Options Are Wrong:

• Penicillium notatum: penicillin producer, not citrate workhorse.
• Rhizopus spp.: can form organic acids (e.g., fumarate), not principal citrate platform.
• Lactobacillus delbrueckii: lactic acid bacterium.

Common Pitfalls:
Confusing general “acid-producing” capacity with the dominant industrial choice.

Final Answer:
Aspergillus niger