Obligate anaerobes are organisms that cannot survive in the presence of oxygen, as it is toxic to them. They thrive in anoxic environments, such as deep soils or in the digestive tracts of some animals. In contrast, facultative anaerobes can utilize oxygen for respiration when available but can also switch to fermentation or anaerobic respiration when oxygen is scarce. This adaptability allows facultative anaerobes to thrive in varying environments, contributing significantly to the microbial diversity and functioning of ecosystems.