The nutrient required in large quantities by microorganisms for cell structure and metabolism is carbon.

Here's why:

* Cell Structure: Carbon is the backbone of all organic molecules, including carbohydrates, lipids, proteins, and nucleic acids. These molecules form the building blocks of cell walls, membranes, and internal structures.

* Metabolism: Carbon is essential for energy production through processes like respiration and fermentation. It is also used in the synthesis of essential molecules like amino acids, vitamins, and enzymes.

While other nutrients are important, carbon is uniquely crucial for both structural and metabolic functions in microorganisms.