Question 8:
Sulfur bacteria are examples of which of the following:
Sulfur bacteria, such as those in genera like Thiobacillus and Chromatium, primarily oxidize inorganic sulfur compounds like H₂S for energy, classifying them as lithotrophs. The correct answer to the multiple-choice question is (C) Lithotrophs.
Correct Answer
Sulfur bacteria are lithotrophs (Option C). They derive energy by oxidizing inorganic substances, particularly reduced sulfur compounds like hydrogen sulfide (H₂S) or elemental sulfur (S⁰), through chemolithotrophy. This process generates ATP via electron transport chains, often fixing CO₂ for carbon needs, making many chemolithoautotrophs.
Option Breakdown
| Option | Definition | Applies to Sulfur Bacteria? | Example Microbes |
|---|---|---|---|
| (A) Organotrophs | Obtain energy by oxidizing organic compounds (e.g., glucose). | No—sulfur bacteria use inorganic sulfur, not organics. | Escherichia coli |
| (B) Autotrophs | Fix CO₂ into organic matter as carbon source (can be photo- or chemo-). | Partially—many are autotrophs, but this ignores their lithotrophic energy source. | Cyanobacteria |
| (C) Lithotrophs | Use inorganic compounds (e.g., rocks, minerals, sulfur) for energy/electrons. | Yes—core trait via sulfur oxidation (e.g., H₂S → SO₄²⁻). | Thiobacillus thiooxidans |
| (D) Phototrophs | Harvest light energy for metabolism (anoxygenic in some). | Some purple/green sulfur bacteria yes, but not all (e.g., colorless Thiobacillus are chemo-). |
Lithotrophs best captures the defining energy metabolism across sulfur-oxidizing bacteria, distinguishing them from purely organic or light-dependent types.
Key Implications
These bacteria thrive in extreme environments like hot springs or sediments, cycling sulfur globally and aiding bioremediation. For exams, note overlaps: many are photolithoautotrophs (purple sulfur bacteria) or chemolithoautotrophs (colorless), but “lithotrophs” targets the inorganic electron donor precisely.
