Algae and seaweeds are by far the most important group of plants in the sea. Most of the oxygen in the air is produced by the marine algae and seaweeds. Land plants, including those in the tropical rain forests, only account for about one-fifth of the oxygen supply in the atmosphere. Even though the entire group falls under algae, according to the biological classification, most people think of one-celled plants when they hear the word ‘algae’. Seaweeds are multi-celled species. Algae and seaweed reproduction is often very complicated.