Final answer:
The bacterium is a chemoheterotroph, obtaining energy and carbon from organic compounds.
Step-by-step explanation:
d. Chemoheterotroph, as it must rely on chemical sources of energy living in a lightless environment and a heterotroph if it uses organic compounds other than carbon dioxide for its carbon source.
A chemoheterotroph is an organism that obtains energy and carbon from organic compounds. In this case, the bacterium requires a particular amino acid as its organic nutrient, indicating that it relies on organic compounds for both its energy and carbon needs. Additionally, since it lives in a completely lightless environment, it cannot obtain energy from light and instead must rely on chemical sources for energy.