Final answer:
An organism that cannot produce an essential nutritional molecule due to a mutation is called an auxotroph.
Step-by-step explanation:
A microorganism that carries a mutation known to prevent the organism from producing an essential nutritional molecule is called a auxotroph. An auxotroph is a nutritional mutant that has a loss-of-function mutation in a gene encoding the biosynthesis of a specific nutrient such as an amino acid. This type of organism requires that specific nutrient to be added to its diet or growth medium because it can no longer synthesize it on its own.
For example, in the classic experiments by Beadle and Tatum involving mold spores, when these spores were grown on minimal medium, it was possible to determine which amino acid or vitamin the mutant was unable to produce. Through these experiments, the concept of the one gene-one enzyme hypothesis was proposed, suggesting that each gene is responsible for the production of a single enzyme.