Biofertilizers — Which organisms are considered a main source of biofertilizers for improving soil nitrogen, particularly in paddy fields and symbioses with water ferns?

Introduction / Context:
Biofertilizers are living microbial inoculants that enhance nutrient availability. Cyanobacteria (blue-green algae) contribute significantly to biological nitrogen inputs in flooded rice systems and in symbioses such as Azolla–Anabaena, reducing chemical fertilizer needs and improving sustainability.

Given Data / Assumptions:

• Focus on organisms widely deployed as biofertilizer inoculants.
• Use cases include paddy fields and aquatic/semiaquatic systems.

Concept / Approach:
Heterocystous cyanobacteria fix N2 under low oxygen microenvironments, supplying ammonia that benefits crops directly or via mineralization. Commercial biofertilizer formulations often contain mixed cyanobacterial cultures or Azolla with endosymbiotic Anabaena. By contrast, Bacillus and Streptococcus are not primary nitrogen-fixing biofertilizer sources (though some Bacillus spp. are plant-growth-promoting in other ways).

Step-by-Step Solution:

Verification / Alternative check:
Extension manuals and agronomy trials report yield gains and reduced N fertilizer when inoculating paddy fields with cyanobacterial cultures or Azolla mats.

Why Other Options Are Wrong:

• Bacillus: may aid plants via phosphate solubilization or biocontrol, but not a main nitrogen-fixing biofertilizer group.
• Streptococcus: unrelated to plant biofertilization; many species are animal/human associated.
• None of these: incorrect because cyanobacteria are a recognized source.

Common Pitfalls:
Conflating general plant growth promotion with nitrogen fixation; not all beneficial microbes are biofertilizers in the N-fixing sense.

Final Answer:
Cyanobacteria