NEET 2019 Botany Nitrogen Cycle MCQ Question

Explanation:

Thiocbacillus is a genus of bacteria that plays a vital role in the nitrogen cycle, particularly in the process of nitrification.

Correct Answer: A) Nitrification

Why This Answer is Correct:

Nitrification is a two-step process in the nitrogen cycle where ammonia ($NH_3$) is converted into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$). Thiobacillus species, especially Thiobacillus denitrificans, are important in this process as they oxidize inorganic compounds, contributing to the conversion of ammonia to nitrites and nitrates.

The overall reaction can be summarized as follows:

1. Ammonia Oxidation: $NH_3 + O_2 \rightarrow NO_2^- + 3H^+ + 2e^-$

2. Nitrite Oxidation: $NO_2^- + O_2 \rightarrow NO_3^-$

This process is essential for maintaining soil fertility, as nitrates are a primary source of nitrogen for plants.

Why the Other Options are Incorrect:

B) Denitrification: Denitrification is the process where nitrates ($NO_3^-$) are converted back into nitrogen gas ($N_2$), effectively removing nitrogen from the soil and returning it to the atmosphere. This process is primarily carried out by other bacteria, such as Pseudomonas and Bacillus species, and not Thiobacillus. Therefore, this option is incorrect.

C) Nitrogen Fixation: Nitrogen fixation is the conversion of atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$), which can be utilized by living organisms. This process is predominantly performed by certain bacteria (like Rhizobium) and cyanobacteria. Thiobacillus is not involved in this process, making this option also incorrect.

D) Chemoautotrophic Fixation: Chemoautotrophic fixation generally refers to the process by which certain organisms convert inorganic compounds into organic compounds using chemical energy. While Thiobacillus species can be considered chemoautotrophic, the term is too broad and does not specifically describe the role of Thiobacillus in the nitrogen cycle. Therefore, this option is not the most accurate choice.

Conclusion

In summary, Thiobacillus plays a crucial role in nitrification, converting ammonia into nitrites and nitrates, thus contributing to the nitrogen cycle and enhancing soil fertility. The other options do not accurately describe the functions performed by Thiobacillus, making them incorrect.