Anoxygenic photosynthesis: primary electron donor In anoxygenic photosynthetic bacteria, which substance is typically oxidized as the electron donor instead of water?

Introduction / Context:
Not all phototrophs release oxygen. Anoxygenic photosynthetic bacteria (for example, purple sulfur bacteria, green sulfur bacteria) use alternative electron donors and thus do not split water or produce O2. Identifying their common donors is key to environmental microbiology and biogeochemical cycles.

Given Data / Assumptions:

• Organisms: anoxygenic phototrophs.
• They lack photosystem II water-splitting oxygen-evolving complex.
• Common reduced substrates include hydrogen sulfide (H2S) or elemental sulfur; some can use H2 or organic acids.

Concept / Approach:

In oxygenic photosynthesis (plants, algae, cyanobacteria), water is oxidized to O2. In contrast, anoxygenic phototrophs often oxidize sulfide to sulfur or sulfate. Therefore, sulfide is the characteristic alternative electron donor in classic anoxygenic systems.

Step-by-Step Solution:

Verification / Alternative check:

Ecophysiology of purple sulfur bacteria shows oxidation of H2S to S0 and SO4^2- as hallmark metabolism in illuminated anaerobic zones of stratified waters.

Why Other Options Are Wrong:

• Water: Oxidized only in oxygenic phototrophs.
• Oxgyen (oxygen): Not an electron donor; it is an oxidant.
• Ammonia: Some chemolithotrophs oxidize ammonia, but that is not photosynthetic electron donation in these bacteria.
• Hydrogen peroxide: A reactive oxygen species, not the typical donor.

Common Pitfalls:

• Equating all photosynthesis with oxygenic processes; many bacteria are anoxygenic.

Final Answer:

sulfide