Bacteria that perform anoxygenic photosynthesis use which of the following compounds as an electron donor?

Photosynthesis can be of two types:
Oxygenic photosynthesis (performed by plants, algae, cyanobacteria) → uses water (H₂O) as the electron donor, releasing oxygen (O₂).
Anoxygenic photosynthesis (performed by certain bacteria like purple sulfur bacteria, green sulfur bacteria, heliobacteria) → does not release oxygen.
In anoxygenic photosynthesis, bacteria use compounds other than water as electron donors, such as:
Hydrogen sulfide (H₂S)
Elemental sulfur (S⁰)
Ferrous iron (Fe²⁺)
Hydrogen gas (H₂)
Example:
Purple sulfur bacteria use H₂S and produce elemental sulfur instead of oxygen.
Green sulfur bacteria also use H₂S.
Therefore, the primary electron donor in anoxygenic photosynthesis is Hydrogen sulfide (H₂S) (though other reduced compounds may also be used depending on the bacterial species).