UPPSC 2023 Prelims GS Paper-I · Question 77 of 150

Correct — C, Azolla. Azolla is a small floating water fern that harbours the nitrogen-fixing cyanobacterium Anabaena azollae in cavities of its leaves. Grown on the standing water of submerged (water-logged) paddy fields, this symbiosis fixes atmospheric nitrogen and, when the fern decomposes, releases it into the soil — which is why Azolla is a classic bio-fertilizer for rice. It also multiplies quickly and adds organic matter.

• (a)Wolfia — Wolffia is the world's smallest flowering plant (a rootless duckweed); it is used locally as fodder/food but does not fix nitrogen and is not a paddy bio-fertilizer.
• (b)Trapa — Trapa is the water chestnut (singhara), grown for its edible fruit — a food crop, not a nitrogen-fixing bio-fertilizer.
• (d)Lemna — Lemna (common duckweed) is useful for wastewater treatment and as fodder, but it does not fix atmospheric nitrogen and is not the rice bio-fertilizer meant here.

Bio-fertilizers are living organisms that enrich soil nutrients. In paddy, the Azolla-Anabaena symbiosis is the key nitrogen-fixing bio-fertilizer: the fern floats on the field's standing water while its cyanobacterial partner converts atmospheric N2 into plant-available nitrogen.

All four options are aquatic plants, so the trap is picking any floating plant. The distinguishing feature is biological nitrogen fixation through a cyanobacterial symbiont — a property unique here to Azolla.

• Azolla is a floating aquatic fern; its symbiont is the cyanobacterium Anabaena azollae.
• The symbiosis fixes atmospheric nitrogen, making Azolla a bio-fertilizer for rice/paddy.
• Wolffia (smallest flowering plant), Trapa (water chestnut) and Lemna (duckweed) do not fix nitrogen.
• Blue-green algae (cyanobacteria) such as Anabaena and Nostoc are the usual paddy bio-fertilizers.

• Choosing a floating plant on appearance (Lemna/Wolffia) instead of the nitrogen-fixer
• Thinking the fern itself fixes nitrogen — the fixer is the cyanobacterium Anabaena living inside it

UPSC/UPPSC repeatedly ask 'which organism is a bio-fertilizer for rice/paddy', sometimes routed through Anabaena, Nostoc or blue-green algae.

• practice — not a real PYQ

  The nitrogen-fixing cyanobacterium that lives in symbiosis with the Azolla fern is:

  • (a)Rhizobium leguminosarum
  • (b)Anabaena azollae
  • (c)Azotobacter
  • (d)Nostoc muscorum

  Answer(b) Anabaena azollae — it lives in the leaf cavities of Azolla and fixes nitrogen.

• practice — not a real PYQ

  Rhizobium bacteria fix atmospheric nitrogen in association with the roots of:

  • (a)cereal crops
  • (b)leguminous plants
  • (c)mosses
  • (d)aquatic ferns

  Answer(b) leguminous plants — via a root-nodule symbiosis.