Question

Lithotroph is an organism that uses:

• A. Organic nutrient molecules
• B. Light energy
• C. Lithium molecules
• D. Inorganic molecules

Hint:

Lithotroph = Inorganic electron donor ($\text{H}_2\text{S}, \text{NH}_3, \text{Fe}^{2+}$).
Organotroph = Organic electron donor (Glucose, Amino acids).
Phototroph = Light energy source.
Chemotroph = Chemical energy source.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the nutritiol/metabolic definition of a lithotrophic organism based on its electron donor source.

Step 2: Key Formula or Approach:
Microorganisms are classified according to their source of electrons for biosynthesis and energy conservation:
• Organotrophs: Use reduced organic molecules (e.g., glucose, fatty acids) as electron donors.

• Lithotrophs: Use reduced inorganic molecules (e.g., $\text{H}_2, \text{NH}_3, \text{NO}_2^-, \text{H}_2\text{S}, \text{Fe}^{2+}$) as electron donors.

Step 3: Detailed Explation:

• Etymology and Definition: The term "lithotroph" derives from the Greek words *lithos* (meaning stone/rock) and *troph* (meaning feeder). Lithotrophs acquire reducing equivalents (electrons) from inorganic compounds.

• Metabolic Diversity: Lithotrophs can be chemolithotrophs (obtaining energy from the oxidation of inorganic chemicals) or photolithotrophs (using light energy with inorganic electron donors like $\text{H}_2\text{S}$ during anoxygenic photosynthesis).

• Examples: Nitrifying bacteria (Nitrosomos, Nitrobacter), iron-oxidizing bacteria (Acidithiobacillus ferrooxidans), and hydrogen-oxidizing bacteria.

• Clarification: The term lithotroph does not refer to lithium metal, nor does it refer exclusively to organic compounds or light energy alone.

Step 4: Fil Answer:
A lithotroph is defined as an organism that uses inorganic molecules as electron donors.