Option Analysis

(A) Hydrogen sulfide: Serves as the primary electron donor in purple sulfur bacteria, oxidized to elemental sulfur (S⁰) stored intracellularly, then further to sulfate.

(B) Thiosulfates: Commonly used as an alternative reduced sulfur compound for electron donation in photosynthesis by many species.

(C) Methane: Not utilized as an electron donor; methane oxidation links to methanotrophs, not purple sulfur bacteria photosynthesis.

(D) Sulfates: Acts as an oxidized end product, not a donor; sulfate-reducing bacteria produce it, but purple sulfur bacteria do not reduce it for electrons.

Purple sulfur bacteria perform anoxygenic photosynthesis using specific electron donors for CO2 reduction, primarily hydrogen sulfide and thiosulfates, in anaerobic environments rich in sulfide. These phototrophs oxidize reduced sulfur compounds to fix CO₂ without oxygen evolution, playing key roles in sulfur cycling.

Key Electron Donors

Hydrogen sulfide (H₂S) acts as the main donor, converted to elemental sulfur granules inside cells.

Thiosulfates (S₂O₃²⁻) serve as viable alternatives, supporting autotrophic growth via similar oxidation pathways.

Incorrect Options Explained

Methane requires methanotrophic pathways absent in purple sulfur bacteria, while sulfates represent the fully oxidized form unsuitable as donors.

Ecological Significance

These bacteria thrive in stratified lakes, recycling sulfur and nutrients like phosphorus through upwelling. Their electron donors for CO2 reduction highlight unique adaptations for CSIR NET life sciences studies.