Electron donors and trophic terms Microorganisms that can use reduced inorganic compounds (e.g., NH₃, H₂S, Fe²⁺, H₂) as electron donors are termed:

Introduction / Context:
Metabolic classifications include how organisms obtain electrons for energy (electron donors). Those that oxidize reduced inorganic compounds occupy key niches in biogeochemical cycles and wastewater treatment. The correct trophic term distinguishes them from microbes that oxidize organic molecules.

Given Data / Assumptions:

• Examples of inorganic electron donors: NH₃, H₂S, Fe²⁺, H₂.
• Alternative donors (organic compounds) define a different category.
• Trophic terms may combine with energy source (photo vs chemo) and carbon source (auto vs hetero).

Concept / Approach:
Lithotrophs (“rock eaters”) use reduced inorganic substances as electron donors. When they derive energy from chemical reactions, they are chemolithotrophs; many are autotrophs for carbon (CO₂-fixing), such as nitrifiers and sulfur oxidizers. In contrast, organotrophs use organic compounds as electron donors; phototrophs derive energy from light; chemotrophs from chemical reactions (may be organo- or litho-). The most precise term for using inorganic electron donors is lithotrophs.

Step-by-Step Solution:
Identify the defining feature: reduced inorganic electron donors.Map to term: lithotrophy.Select 'Lithotrophs' as the answer.

Verification / Alternative check:
Well-known lithotrophs include Nitrosomonas (ammonia), Acidithiobacillus (sulfur/iron), and hydrogen-oxidizing bacteria (H₂), validating the definition.

Why Other Options Are Wrong:

• Phototrophs: defined by light as energy source, not electron donor identity.
• Chemotrophs: too broad; includes both organo- and litho- variants.
• Photo-organotrophs: implies light energy with organic electron donors; does not match inorganic donors.
• Organotrophs: use organic compounds as electron donors.

Common Pitfalls:
Failing to distinguish the three axes of classification: energy (photo/chemo), electrons (organo/litho), and carbon (hetero/auto).

Final Answer:
Lithotrophs.