1. Food: Hydra consumes excess food produced by algae in the process of photosynthesis. When the algae die, the corpse is used by the hydra as food.
2. Obtaining O2: Hydra uses the O2 produced by algae in the process of photosynthesis. Dr. Siddiq Publications
3. Excretion of waste products: Hydra produces nitrogenous waste products in the process of respiration. Algae consume these waste materials and free the hydra waste.