Practice question

Genera Anabaena and Nostoc belong to order Nostocales within phylum Cyanobacteria, characterized by unbranched filamentous trichomes of vegetative cells capable of intercalary differentiation into heterocysts. Heterocysts develop thick glycolipid walls limiting oxygen diffusion, maintain microoxic environment for nitrogenase to reduce atmospheric dinitrogen to ammonia. Vegetative cells perform oxygenic photosynthesis supplying carbohydrate. Akinetes serve as resting spores. Their symbiotic associations with Azolla fern and cycads, and bloom formation in rice fields, highlight heterocystous cyanobacterial adaptation for combined photosynthesis and aerobic nitrogen fixation.