Which of the following groups does not use water as a proton donor?

Explanation

Purple sulfur bacteria use hydrogen sulfide (H₂S) as their proton and electron donor, distinguishing them from other groups like cyanobacteria, plants, and algae, which all use water (H₂O) in their photosynthetic processes. This unique characteristic of purple sulfur bacteria allows them to thrive in environments where hydrogen sulfide is more abundant than water.