Which one of the following bacteria obtain energy from the oxidation of inorganic substances ?

Correct option is B

Introduction

Microorganisms are classified by their energy source (light or chemical) and their carbon source (organic or inorganic CO₂).

This nutritional grouping distinguishes how bacteria power their metabolism and build biomass.

Information Booster

The correct answer is Chemoautotrophs. Chemoautotrophs (chemolithoautotrophs) obtain their energy from the oxidation of inorganic substances — such as ammonia, nitrite, hydrogen sulphide, elemental sulphur, ferrous iron, or hydrogen gas — rather than from light or organic compounds.

They use this chemically derived energy to fix carbon dioxide (CO₂) as their carbon source, building organic molecules autotrophically.

Classic examples include the nitrifying bacteria Nitrosomonas (oxidises NH₃ to nitrite) and Nitrobacter (oxidises nitrite to nitrate), sulphur-oxidising Thiobacillus, and iron-oxidising bacteria.

These organisms are ecologically crucial, driving key steps of the nitrogen, sulphur, and iron biogeochemical cycles, and they thrive in environments such as soils, hydrothermal vents, and deep subsurface habitats where light is unavailable.

The defining feature — energy from inorganic oxidation combined with CO₂ fixation — matches this option precisely.

Additional knowledge

Chemoheterotrophs — Incorrect. These obtain energy from chemical compounds but specifically from organic molecules (glucose, proteins, lipids), which serve as both their energy and carbon source. Most bacteria, fungi, and all animals fall here; they do not oxidise inorganic substances for energy.
· Photoheterotrophs — Incorrect. These derive energy from light but obtain carbon from organic compounds rather than CO₂. Examples include purple non-sulphur bacteria; their energy comes from sunlight, not inorganic oxidation.
· Photoautotrophs — Incorrect. These use light as their energy source and CO₂ as their carbon source (e.g., cyanobacteria, plants, algae). Although autotrophic like the correct answer, their energy comes from photosynthesis, not from oxidising inorganic chemicals.