Obligate anaerobes are microorganisms that cannot survive in the presence of oxygen. Oxygen is toxic to them because they lack the enzymes necessary to detoxify harmful oxygen byproducts like superoxide radicals and hydrogen peroxide. These organisms obtain energy through anaerobic respiration or fermentation. They are commonly found in environments lacking oxygen, such as deep soil, sediments, and the human gut. Examples include certain bacteria like Clostridium and Bacteroides.