Which of the following requires one or more organic nutrients in addition to the principal carbon source?

The term "auxotroph" refers to a microorganism that has lost the ability to synthesize one or more organic nutrients that are required for its growth. It generally requires supplementation with these specific organic compounds in addition to a primary carbon source in its growth medium. This can occur due to mutations in the genes that code for enzymes necessary for biosynthesis pathways, leading to a reliance on external sources for these nutrients.

In contrast, "prototrophs" are organisms capable of synthesizing all the organic compounds they need from simple substances, meaning they do not require any additional organic nutrients beyond their carbon source. "Autotrophs" can use inorganic carbon sources, such as carbon dioxide, to produce organic compounds, and therefore do not rely on organic nutrients. "Heterotrophs," while they do require organic compounds for growth, do not express the specific nutritional deficiency that defines auxotrophs. Thus, identifying auxotrophs as those requiring additional organic nutrients highlights their inability to biosynthesize certain essential molecules, making them distinct in their nutritional needs.