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cyanobacteria

serve as a food source for other organisms
are used for medicinal purposes
produce oxygen
perform nitrogen fixation
cause disease

serve as a food source for other organisms

are used for medicinal purposes

produce oxygen

perform nitrogen fixation

cause disease

Explanation:

Serve as a food source for other organisms: The cyanobacteria serves as a food for marine organisms like zooplanktons and fishes.

Are used for medicinal purposes: The cyanobacteria are used for the purpose of medicine these produced biologically active materials such as antiviral, antifungal, antibacterial and anticancer activities.

Produce oxygen: The cyanobacteria produces oxygen by the process of photosynthesis.

Perform nitrogen fixation: The Cyanobacteria produces nitrogen form that is absorbed by the plants. It converts the atmospheric nitrogen into the ammonia absorbed by the plants.

Cause disease: The cyanobacteria produces cytotoxin which is responsible for neurodegenerative diseases like Parkinson's disease, and  Alzheimer's disease.