In anoxygenic photosynthesis (as performed by green and purple bacteria), which one of the following is NOT used as the electron donor (reducing power source)?

Introduction / Context:
Green and purple photosynthetic bacteria carry out anoxygenic photosynthesis. Unlike plants, algae, and cyanobacteria, these bacteria do not split water and therefore do not release oxygen. The question probes which electron donor is not used by these organisms.

Given Data / Assumptions:

• Organisms considered: green sulfur bacteria and purple bacteria.
• Process: anoxygenic photosynthesis (no oxygen evolution).
• Common bacterial electron donors include reduced sulfur compounds and hydrogen gas.

Concept / Approach:
Anoxygenic phototrophs harvest light with bacteriochlorophylls and reduce their electron transport chains using donors other than water. Typical donors are hydrogen sulfide (H2S), elemental sulfur (S), thiosulfate, and molecular hydrogen (H2). Water is the hallmark donor in oxygenic photosynthesis only.

Step-by-Step Solution:

Verification / Alternative check:
Presence of photosystem architecture lacking the strong oxidant required to split water confirms these bacteria cannot oxidize water; many oxidize sulfide to elemental sulfur or sulfate instead.

Why Other Options Are Wrong:
H2: Common bacterial donor. H2S: Classic donor for sulfur phototrophs. S (elemental sulphur): Can serve as donor or intermediate in many of these bacteria.

Common Pitfalls:
Equating all photosynthesis with oxygen release. Only oxygenic photosynthesis splits water.

Final Answer:
H2O