These are the nitrogen fixer Cyanobacteria (Blue green algae) under order Nostocales. They have this ability due to a specialized cell, known as Heterocyst.

• Heterocyst identification key Features with respect to vegetative cells.
• Thick walled.
• Weak pigmentation.
• Present at both intercalary and terminal position.
• Intercalary Heterocyst have Polar granules at each pole while Terminal Heterocyst have polar granule at single pole.
• Neither divide nor Dedifferentiate.
• Fix nitrogen under Aerobic condition.