Answer:
This suggests that the electron donor is not water
Explanation:
In photoautotrophs that produce oxygen, water is the electron donor in electronic chain: H2O dissociates in ½ O2 + 2H+ + 2e-, thereby oxygen is produced (superior plants, cyanobacteria, algae).
In some primitive prokaryotes, other molecule is used instead of water as electron donor: H2S and sulfur is generated.