Cyanobacteria Blue Green Algae: Nitrogen Fixation and Uses

Cyanobacteria, popularly called blue-green algae, are photosynthetic bacteria found in ponds, rivers, moist soil and even hot springs. They are not true algae at all; they are prokaryotes, which means their cells lack a true nucleus. Their blue-green colour comes from the pigments chlorophyll a and phycocyanin, and they were among the first organisms on Earth to release oxygen through photosynthesis.

Cyanobacteria matter for two big reasons. First, many of them can fix atmospheric nitrogen, turning useless nitrogen gas into nutrients that plants can absorb, which makes them natural fertilisers. Second, some species like Spirulina are eaten as protein-rich health food, and farmers use cyanobacteria to enrich paddy fields without chemical fertilisers.

Key Concepts

• Photosynthetic prokaryotes: unicellular, colonial or filamentous forms that perform oxygenic photosynthesis like green plants, using chlorophyll a.
• Mucilaginous sheath: a slimy covering around the cells that protects them from drying out.
• Heterocysts: large, thick-walled specialised cells found in filamentous forms such as Nostoc and Anabaena; these are the sites of nitrogen fixation.
• Nitrogen fixation: converting atmospheric nitrogen (N2) into ammonia compounds that plants can use.
• Examples: Nostoc, Anabaena, Oscillatoria and Spirulina.

How It Works

Cyanobacteria photosynthesise in daylight, splitting water and releasing oxygen, exactly like plants do. The clever part is nitrogen fixation. The enzyme that fixes nitrogen, nitrogenase, is destroyed by oxygen, so the cell cannot fix nitrogen and release oxygen in the same place. Filamentous cyanobacteria solve this by using heterocysts: these special cells have thick walls, lack the oxygen-releasing part of photosynthesis, and create an oxygen-free chamber where nitrogenase safely converts N2 into ammonia, which is then shared with neighbouring cells.

Some cyanobacteria also live in partnership with other organisms. The water fern Azolla has tiny leaf cavities housing the cyanobacterium Anabaena; the fern gives shelter and the cyanobacterium supplies fixed nitrogen, a partnership rice farmers have used for centuries.

Worked Example

Question: A rice farmer adds Azolla carrying Anabaena to a paddy field. The Azolla biomass doubles every 5 days. Starting with 4 kg, how much will there be after 20 days?

Step 1: Number of doublings = 20 / 5 = 4.

Step 2: Growth factor = 2^4 = 16.

Step 3: Final biomass = 4 kg x 16 = 64 kg.

Answer: About 64 kg of Azolla, all fixing nitrogen for the rice crop, from just 4 kg in 20 days.

Real-World Applications

• Biofertilisers: Nostoc and Anabaena are added to paddy fields to supply nitrogen naturally and reduce chemical fertiliser use.
• Health food: Spirulina is dried and sold as a protein and vitamin-rich supplement.
• Soil reclamation: cyanobacteria help reclaim salty and alkaline wastelands by adding organic matter and nitrogen.
• Pioneer organisms: they are among the first to colonise bare rocks and start soil formation.

Cyanobacteria are photosynthetic bacteria with chlorophyll a and protective slime sheaths. Filamentous forms fix nitrogen inside special heterocyst cells, and species like Nostoc, Anabaena and Spirulina serve as natural fertilisers, health food and soil builders.