Which of following organisms cannot fix nitrogen?
A. Azotobacter B. Oscillatoria
C. Anabaena D. Volv

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Question

Which of following organisms cannot fix nitrogen?
A. Azotobacter B. Oscillatoria
C. Anabaena D. Volvox
E. Nostoc
Choose the correct answer from the options given below:
(1) E only
(2) A only
(3) D only
(4) B only

Accepted Answer

Correct Answer: C — D only (Volvox)Step-by-step Solution:Nitrogen fixation is the biological conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or related compounds using the enzyme nitrogenase. Only prokaryotes (bacteria and archaea) can fix nitrogen — eukaryotes lack the nitrogenase enzyme.Evaluating each organism:Azotobacter — CAN fix nitrogen: Azotobacter is a free-living, aerobic, gram-negative soil bacterium (prokaryote). It is one of the most important non-symbiotic/free-living nitrogen fixers. It protects nitrogenase from oxygen damage through high respiration rates creating an anoxic microenvironment and through production of oxygen-scavenging proteins.Oscillatoria — CAN fix nitrogen: Oscillatoria is a filamentous cyanobacterium (blue-green alga; prokaryote). It can fix nitrogen using specialized cells called heterocysts (in some species) or under anaerobic conditions. Some species of Oscillatoria fix nitrogen.Anabaena — CAN fix nitrogen: Anabaena is a filamentous cyanobacterium that fixes nitrogen in specialized cells called heterocysts. Heterocysts provide an anaerobic environment (by lacking PSII and having thick walls) protecting the oxygen-sensitive nitrogenase. Anabaena azollae lives symbiotically in the water fern Azolla, providing nitrogen for rice fields — used as a biofertilizer in Asia.Volvox — CANNOT fix nitrogen (correct answer): Volvox is a colonial green alga — a eukaryote (Kingdom Protista). Eukaryotes do not possess the nitrogenase enzyme and therefore cannot fix atmospheric nitrogen. Volvox lacks heterocysts or any nitrogen-fixing machinery.Nostoc — CAN fix nitrogen: Nostoc is a filamentous cyanobacterium with heterocysts for nitrogen fixation. It can fix nitrogen both free-living and in symbiosis: symbiotic with Anthoceros (hornwort), Cycas (gymnosperm — in coralloid roots), lichens (with fungi), and Azolla.Extended Explanation:Types of biological nitrogen fixation: (1) Free-living (non-symbiotic): Azotobacter (aerobic), Clostridium (anaerobic), Beijerinckia (aerobic), cyanobacteria (Anabaena, Nostoc, Oscillatoria); (2) Symbiotic: Rhizobium with legume roots (most important agronomically — root nodules); Frankia with non-legume trees (alder — Alnus); Anabaena/Nostoc with Cycas (coralloid roots), Anthoceros, Azolla; (3) Associative: Azospirillum with grasses. Nitrogenase enzyme: requires anaerobic conditions (O₂ inactivates it); contains iron-molybdenum cofactor (FeMo-co); requires 16 ATP per N₂ fixed: N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi. The nitrogen cycle: Fixation → Ammonification → Nitrification (Nitrosomonas, Nitrobacter) → Denitrification (Pseudomonas) → back to N₂.Key Concepts: Nitrogen fixation, nitrogenase, prokaryotes only, Azotobacter (free-living), Anabaena/Nostoc/Oscillatoria (cyanobacteria with heterocysts), Volvox (green alga, eukaryote — cannot fix nitrogen), symbiotic vs free-living fixers.

Which of following organisms cannot fix nitrogen? A. Azotobacter B. Oscillatoria C. Anabaena D. Volvox E. Nostoc Choose the correct answer from the options given below: (1) E only (2) A only (3) D only (4) B only

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Which of following organisms cannot fix nitrogen? A. Azotobacter B. Oscillatoria C. Anabaena D. Volvox E. Nostoc Choose the correct answer from the options given below: (1) E only (2) A only (3) D only (4) B only

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