Prokaryotes are small, single-celled organisms that lack a nucleus and other organelles. Prokaryotes are found in nearly every environment on Earth, from hot springs to arctic tundra. Prokaryotes are incredibly diverse, with an estimated 10 million different species. Prokaryotes are thought to have evolved before eukaryotes, which are cells that have a nucleus and other organelles. Prokaryotes are essential for the recycling of carbon and nitrogen in the global ecosystem. Prokaryotes play a key role in the global carbon cycle by fixing carbon dioxide from the atmosphere. Prokaryotes are also responsible for the release of methane gas into the atmosphere. Prokaryotes are essential for the recycling of nitrogen in the global ecosystem. Prokaryotes play a key role in the global nitrogen cycle by fixing nitrogen from the atmosphere. Prokaryotes are essential for the decomposition of organic matter, recycling important nutrients back into the ecosystem. Prokaryotes break down complex organic molecules into simpler inorganic molecules.