Final answer:
The middle layer of a Winogradsky column is typically inhabited by sulfur-oxidizing bacteria, which are capable of anoxygenic photosynthesis, using sulfur compounds as electron donors. Option d is correct.
Step-by-step explanation:
The middle level of a Winogradsky column is likely populated by sulfur-oxidizing bacteria. These bacteria perform anoxygenic photosynthesis, where sulfur compounds such as hydrogen sulfide (H2S) serve as electron donors. In the sulfur cycle, purple sulfur bacteria, like those of the genus Chromatium, oxidize hydrogen sulfide into elemental sulfur (S) and sulfuric acid (H2SO4), and they are strict anaerobes meaning they don't need oxygen to live.
These bacteria also use carbon dioxide (CO2) as their source of carbon, and they have the special capability to fix nitrogen. Furthermore, the green sulfur bacteria, such as the genus Chlorobium, also utilize sulfide during anoxygenic photosynthesis to produce elemental sulfur.