Plants and algae are photosynthetic organisms. They use light energy to make their own food, so they are called producers. Algae include seaweeds and the microscopic phytoplankton that float in the sea. On land the main producers are plants such as grass and trees.
During photosynthesis the producer makes glucose. It then uses the glucose to build other substances, such as cellulose for cell walls, starch for storage and proteins for growth. Together, the living material of an organism is its biomass.
Every food chain begins with a producer. In the chain seaweed, snail, crab, the seaweed is the producer. The snail gets its biomass by eating the seaweed, and the crab gets biomass by eating the snail. Because all of this biomass starts with photosynthesis, photosynthetic organisms are the main producers of food and biomass in almost every ecosystem.