Final answer:
Reduction in the sulfur cycle leads to the production of hydrogen sulfide, mainly through the activity of sulfate-reducing bacteria and archaea. This hydrogen sulfide is a critical component recycled in the sulfur cycle by various microorganisms, completing the cycle. Human activities have disrupted this cycle and contributed to the occurrence of acid rain.
Step-by-step explanation:
In the sulfur cycle, the reduction of sulfur principally leads to the production of hydrogen sulfide (H2S). This occurs when sulfate-reducing bacteria and archaea use sulfate as a final electron acceptor in anaerobic respiration. This process is essential because it helps in closing the loop of the sulfur cycle, as sulfate is converted back into hydrogen sulfide. Furthermore, hydrogen sulfide can be utilized by several types of microorganisms, including anoxygenic photosynthesizers and chemoautotrophs, as an electron donor, converting it to elemental sulfur, and then to sulfate - thus completing the cycle.
Human activities, such as the burning of fossil fuels, have significantly altered the balance of the global sulfur cycle. One key consequence of this alteration is the formation of acid rain, caused when sulfur dioxide (SO2) released into the atmosphere reacts with water, creating harmful sulfuric acid.