In the NCERT chapter, two types of nitrogen-fixing biofertilizers are discussed for rice and paddy cultivation: *Azotobacter* (a free-living soil bacterium) and *Anabaena* / *Nostoc* (filamentous cyanobacteria). Both fix atmospheric N₂. State **one key difference** between how *Anabaena* (a cyanobacterium) and *Rhizobium* (a soil bacterium) obtain the energy needed to power the nitrogenase enzyme during nitrogen fixation, and name the specialised cell in *Anabaena* where nitrogenase is located.
Answer
The key distinguishing feature is that cyanobacteria are **autotrophic** (they fix CO₂ via photosynthesis) and fix nitrogen **independently in free-living form**, whereas *Rhizobium* is **heterotrophic** (depends on the host plant for carbohydrates) and fixes nitrogen only **in a symbiotic association** inside root nodules of leguminous plants.
Step-by-step solution
- *Rhizobium* is an obligate/conditionally symbiotic heterotroph. It lives inside root nodules of a leguminous host (e.g.,, *Trifolium*, *Cicer*) and receives carbohydrates (sucrose/organic acids) from the plant for its energy needs. It is the host plant that ultimately pays the energy cost of nitrogen fixation.
- *Anabaena* (and *Nostoc*) is a **photosynthetic autotroph**. It possesses Photosystem I and II and a full photosynthetic apparatus, so it generates its own ATP from sunlight to power nitrogenase. No host plant is required.
- Because oxygen produced by photosynthesis would irreversibly inactivate nitrogenase, *Anabaena* fixes nitrogen in specialised, thick-walled cells called **heterocysts**, which are anaerobic (they lack photosystem II and maintain a low-O₂ environment via a gas-diffusion barrier of the glycolipid layer).
- State the one-line difference clearly: *Anabaena* is self-sufficient (autotrophic, uses light energy in heterocysts); *Rhizobium* depends on the legume host for organic carbon (heterotrophic, symbiotic association in nodules).
Common mistake: A very common error is to consider the presence of heterocysts as proof that cyanobacteria are not nitrogen-fixing, when in fact heterocysts are the specialised anaerobic cells that *create* the low-oxygen environment required for nitrogenase to work. Another mistake is to say that cyanobacteria form nodules, which is true only for certain lichens (e.g., *Nostoc* in *Cyathocactus*), not for typical crop fields.
Concept tested: Cyanobacteria as biofertilizers
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