What are Algae?
There are many species of algae, most of which live in watery environments. They also live on tree bark, stones and in mutualistic symbiosis with fungi: the so-called lichens. Almost all algae get their energy from sunlight (algae are photoautotrophic). Other algae get their energy by extracting carbon from organic matter. These algal species are called mixotrophs. The heterotrophs, on the other hand, depend on external supplies for their energy. These are single-celled algae. It is believed that the first plants evolved from freshwater algae. Fossils of algae between 1.6 and 1.7 billion years old have been found in the Vindhya Mountains. Prokaryotic Cyanobacteria (blue-green algae) are among the oldest organisms on Earth. There is still an ongoing debate as to whether blue-green algae should be counted as algae or not.