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Question

Difficulty: HardNutrient and Biogeochemical Cycles

In an agricultural soil plot rich in decaying organic matter, ammonium ions are rapidly produced by decomposers. Soil biochemical analysis reveals that while ammonium is converted into nitrites (NO2−NO_2^-), the subsequent transformation of nitrites into nitrates (NO3−NO_3^-) is completely blocked, leading to a toxic buildup of nitrites. Which soil bacterium is deficient or inactive in this ecosystem?

  1. NitrobacterAnswer
  2. B
    Rhizobium
  3. C
    Clostridium
  4. D
    Pseudomonas

Answer

Nitrobacter is the bacterium responsible for the second stage of nitrification, transforming nitrites (NO2−NO_2^-) into nitrates (NO3−NO_3^-).
Nitrobacter is the obligate aerobic chemoautotroph responsible for the second oxidation step in nitrification, converting nitrites (NO2−NO_2^-) into nitrates (NO3−NO_3^-). If Nitrobacter is inactive, nitrites accumulate because the pathway cannot proceed to completion.

Step-by-Step Solution

1
Identify the specific biochemical step that is blocked in the soil.
The reaction converting nitrites (NO2−NO_2^-) to nitrates (NO3−NO_3^-) is inhibited.
Ammonium has already been oxidized to nitrite, so the second stage of nitrification is where the block occurs.
2
Recall the micro-organism responsible for nitrite oxidation.
Nitrobacter chemoautotrophically oxidizes NO2−NO_2^- to NO3−NO_3^-.
Different nitrifying bacteria specialize in distinct steps of the nitrification pathway.
3
Match the missing biological function to the candidate bacteria.
Inactivity or absence of Nitrobacter leads directly to the accumulation of toxic nitrites.
Without Nitrobacter, nitrites cannot be converted to nitrates, which plants absorb for amino acid synthesis.

Key Concept

Nitrification Pathway and Microbial Roles
Estimated Time:1m 30s
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Topics

Nutrient and Biogeochemical Cycles

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