Answer:
There was no sulfur compound added to the medium, that could be used as an electron donor.
Explanation:
Bacteria that use light for energy requirements mean that they get their energy from photosynthesis and transferring energy through the molecules.
Bacteria play a major part in the reductive and oxidative cycle of the sulfur which is mediated by the help of photosynthetic bacteria, so the sulfur or sulfur compounds are major requirements for their growth in the habitat or the medium.
Thus, the correct answer would be- There was no sulfur compound added to the medium, that could be used as an electron donor.
