21. Which one of the following is NOT a product of denitrification in Pseudomonas?
(A) N2
(B) N2O
(C) NO2−
(D) NH4+
Answer: (D) NH4+
Denitrification in Pseudomonas species is a key anaerobic respiratory process that reduces nitrate (NO₃⁻) to gaseous nitrogen products, playing a vital role in the nitrogen cycle. This stepwise reduction involves intermediates like nitrite (NO₂⁻) and gases such as nitric oxide (NO), nitrous oxide (N₂O), and dinitrogen (N₂). Ammonium (NH₄⁺) is not produced, as denitrification removes nitrogen from ecosystems rather than assimilating it.
Option Analysis
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(A) N₂: This is the primary end product of complete denitrification in many Pseudomonas strains, formed by nitrous oxide reductase (NosZ). It represents full reduction of nitrate to atmospheric nitrogen.
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(B) N₂O: Often an intermediate or end product in incomplete denitrification, produced by nitric oxide reductase (Nor) and accumulated if NosZ activity is low, as seen in strains like P. fluorescens.
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(C) NO₂⁻: An early intermediate generated by nitrate reductase (Nar/Nap), which can build up transiently before further reduction.
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(D) NH₄⁺: Not a product; denitrification is dissimilatory (energy-yielding), distinct from assimilatory processes like nitrate ammonification that produce NH₄⁺ in other bacteria.
Denitrification products in Pseudomonas represent a critical microbial process where bacteria like Pseudomonas convert nitrates to gaseous nitrogen, aiding soil nitrogen balance. This MCQ—”Which one of the following is NOT a product of denitrification in Pseudomonas? (A) N2 (B) N2O (C) NO2− (D) NH4+”—tests understanding of this pathway, common in exams on microbiology and biogeochemistry.
Denitrification Pathway in Pseudomonas
Pseudomonas species, such as P. aeruginosa and P. fluorescens, perform denitrification under low-oxygen conditions with nitrate as an electron acceptor. The sequence is: NO₃⁻ → NO₂⁻ (nitrate reductase) → NO (nitrite reductase) → N₂O (nitric oxide reductase) → N₂ (nitrous oxide reductase). Gaseous products like N₂ and N₂O dominate, with NO₂⁻ as a transient intermediate.
Why NH4+ is Not a Denitrification Product
NH₄⁺ arises in nitrate ammonification (dissimilatory nitrate reduction to ammonium, DNRA), not denitrification. Pseudomonas favors denitrification for energy, expelling nitrogen as gas rather than retaining it as NH₄⁺, which would contradict the process’s role in nitrogen loss.
Practical Implications
In wastewater treatment and agriculture, Pseudomonas denitrification reduces nitrate pollution but can emit N₂O, a greenhouse gas. Optimizing conditions minimizes incomplete pathways.
