Anoxygenic phototrophs — Which microorganisms use hydrogen sulfide (H2S) as the electron donor to reduce carbon dioxide during photosynthesis?

Introduction / Context:
Anoxygenic phototrophic bacteria perform photosynthesis without producing oxygen, using electron donors other than water. In sulfur-rich, anoxic environments, specialized groups oxidize H2S, coupling it to carbon fixation and contributing to sulfur cycling in sediments and stratified waters.

Given Data / Assumptions:

• We compare purple sulfur, green sulfur, and purple non-sulfur bacteria.
• Electron donor of interest is hydrogen sulfide (H2S).

Concept / Approach:
Purple sulfur bacteria (e.g., Chromatium/Chromaticum) and green sulfur bacteria (e.g., Chlorobium) use H2S as electron donor, storing elemental sulfur intracellularly or extracellularly. Purple non-sulfur bacteria (e.g., Rhodomicrobium) prefer organic acids or hydrogen; they can tolerate sulfide but usually do not use high H2S concentrations as the main donor.

Step-by-Step Solution:

Verification / Alternative check:
Phototrophic stratification in microbial mats shows green sulfur layers in deepest light with high sulfide; purple sulfur occupy upper sulfide zones, both oxidizing sulfide to sulfur/sulfate.

Why Other Options Are Wrong:

• Only (a) or only (b): incomplete; both groups use H2S.
• Rhodomicrobium: typically uses organic donors; not the canonical H2S oxidizer.

Common Pitfalls:
Confusing “purple” groups; sulfur vs non-sulfur have distinct donor preferences and habitats.

Final Answer:
Both (a) and (b)