Why are algae described as the most important photosynthesizing organisms on earth?

Algae are described as the most important photosynthesizing organisms on Earth primarily because of their massive contribution to oxygen production and their role as the base of aquatic food webs. Microscopic algae, known as phytoplankton, are responsible for generating between 50% and 85% of the oxygen in our atmosphere through photosynthesis. They also form the foundation of nearly all marine and freshwater ecosystems, supporting the entire aquatic food chain.