Energy Flow Through Ecosystems

Step 1: Understanding the nitrogen cycle.
In the nitrogen cycle, ammonia is first converted into nitrites by Nitrosomonas. This process is called nitrification.

Step 2: Identifying X.
The organism responsible for converting ammonia into nitrites is Nitrosomonas. Hence, X = Nitrosomonas.

Step 3: Identifying Y.
The process shown at Y involves conversion of nitrates back into atmospheric nitrogen, which is carried out by nitrate-reducing (denitrifying) bacteria.

Step 4: Final matching.
Thus, the correct combination is: X – Nitrosomonas Y – Nitrate reducing bacteria

Step 1: Understanding the Question.
The question addresses the critical role nitrogen-fixing bacteria play in the ecosystem, particularly in the nitrogen cycle.

Step 2: Analyzing the Options.
(A) They convert nitrogen into a usable form for plants: Correct — Nitrogen-fixing bacteria convert atmospheric nitrogen into compounds that plants can absorb and use, which is essential for plant growth.
(B) They decompose plant material: This is not their primary function. Decomposers are fungi and certain bacteria, but not nitrogen fixers.
(C) They produce oxygen: Nitrogen-fixing bacteria do not produce oxygen. Oxygen is produced by plants through photosynthesis.
(D) They cause soil erosion: Nitrogen-fixing bacteria do not cause soil erosion. They actually contribute to soil health by enriching it with nitrogen.

Step 3: Conclusion.
The correct answer is (A) They convert nitrogen into a usable form for plants, as this is the primary role of nitrogen-fixing bacteria in the ecosystem.