ABSTRACT
The solar system’s formation, including the earth, began about 4.6 billion years ago. About 2.7 billion years ago, cyanobacteria (blue-green algae) arose and began releasing oxygen into the atmosphere. Then the oxygen level in the atmosphere rose, and the evolution of eukaryotic organisms began, giving rise to humans. Today, algae in the world’s oceans, rivers, and lakes are thought to produce about half of all the oxygen produced on the planet.
Algae can be aquatic or subaerial when exposed to the atmosphere rather than being submerged in water. They include a variety of simple structures, from single-celled phytoplankton floating in the water to large algae attached to the ocean floor.
Algae occur worldwide in every biome (ecosystem) and colonize water bodies on every continent. Algae are mostly aquatic organisms, including marine algae and freshwater algae. In addition, several other algae are found in terrestrial habitats. The blue-green and green algae are generally the major components of terrestrial algal populations. Algae also form a mutually beneficial partnership with other organisms in a symbiotic relationship.
