Photosynthesis and oxygen evolution: which of the following does NOT produce molecular oxygen (O2) as a product of photosynthesis?

Introduction / Context:
Photosynthesis can be oxygenic or anoxygenic. Oxygenic photosynthesis splits water to release oxygen, whereas anoxygenic photosynthesis uses other electron donors and does not generate oxygen. The question checks recognition of anoxygenic phototrophs.

Given Data / Assumptions:

• Oak trees and most phytoplankton (including many algae) perform oxygenic photosynthesis.
• Cyanobacteria are oxygenic prokaryotes.
• Purple sulfur bacteria are classic anoxygenic phototrophs.

Concept / Approach:
Organisms that use water as the electron donor produce O2. Purple sulfur bacteria instead oxidize reduced sulfur compounds such as H2S, depositing elemental sulfur and ultimately sulfate, without splitting water.

Step-by-Step Solution:

Verification / Alternative check:
Stromatolite formation and sulfur granules in purple sulfur bacteria confirm H2S usage and absence of oxygen evolution.

Why Other Options Are Wrong:
Oak trees / Phytoplankton / Cyanobacteria: All perform oxygenic photosynthesis and release O2.

Common Pitfalls:
Assuming all bacteria are anoxygenic; cyanobacteria are an important exception that are oxygenic.

Final Answer:
Purple sulfur bacteria