Chemosynthesis Role in Sulphur Cycle

This question examines the relationship between chemosynthesis and the Sulphur ('S') cycle.

Assertion (A) Analysis

Assertion (A) states that chemosynthesis plays an important role in the 'S' cycle. Chemosynthesis is a process where organisms produce energy from chemical reactions. In the sulphur cycle, specialized bacteria (like sulfur-oxidizing bacteria) utilize inorganic sulphur compounds, such as hydrogen sulfide ($H_2S$), as an energy source. This process is vital for converting sulphur between different oxidation states, making it available in ecosystems, particularly in environments lacking light. Therefore, Assertion (A) is true.

Reason (R) Analysis

Reason (R) states that in chemosynthesis, some sulphur compounds act as a source of energy. This aligns directly with the mechanism described above. Bacteria oxidize reduced sulphur compounds (e.g., $H_2S$, elemental sulfur $S^0$, thiosulfate $S_2O_3^{2-}$) to obtain energy to fix carbon dioxide into organic matter. For example, the oxidation of $H_2S$ to sulfate ($SO_4^{2-}$) releases significant energy.

Example Reaction Snippet: $2H_2S + 3O_2 \rightarrow 2H_2SO_4 + Energy$

Thus, Reason (R) is true.

Relationship Between Assertion and Reason

Reason (R) explains *how* chemosynthesis contributes to the sulphur cycle. The ability of certain microorganisms to harness energy from the oxidation of sulphur compounds (R) is the very reason why chemosynthesis is considered important in the functioning and transformation of sulphur within the S cycle (A). Therefore, (R) provides the correct scientific explanation for (A).

Conclusion

Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation for Assertion (A).