What organisms need oxygen to exist?
Most living organisms, including plants and animals, require oxygen for life. There are a few types of single-celled bacteria and protists, known as anaerobes, that don't. There is also a multi-cellular organism called Loricifera that is known to survive without any oxygen in the L'Atalante basin of the Mediterranean Ocean, where no oxygen reaches the dense salt brine beneath the surface. Loricifera don't have mitochondria, the cell organelles that use oxygen in energy production.
Organisms that use oxygen need it for cellular respiration, in which energy is released from glucose. Energy is needed by the organism for growth, mobility and reproduction. Plants only produce oxygen during photosynthesis when light is present, but they still use oxygen at night so they must obtain it from the atmosphere.
During aerobic cellular respiration oxygen acts as an electron acceptor, oxidizing sugar. Anaerobic respiration is less efficient because the substances that act as electron acceptors, usually sulfate and nitrate ions, release less energy per oxidized molecule.
Land animals and plants take in atmospheric oxygen. Aquatic organisms usually take in oxygen that is dissolved in the water in which they live.