section_path: Ecology/Interactions with other organisms/Mutualism
section_name: Mutualism
paragraph_text: Mutualism
  
  Mutualism is an interaction between individuals of different species that results in positive (beneficial) effects on per capita reproduction and/or survival of the interacting populations. One well-understood example of mutualism is the interaction between protozoa and methanogenic archaea in the digestive tracts of animals that digest cellulose, such as ruminants and termites. In these anaerobic environments, protozoa break down plant cellulose to obtain energy. This process releases hydrogen as a waste product, but high levels of hydrogen reduce energy production. When methanogens convert hydrogen to methane, protozoa benefit from more energy.
  
  In anaerobic protozoa, such as Plagiopyla frontata, Trimyema, Heterometopus and Metopus contortus, archaea reside inside the protozoa and consume hydrogen produced in their hydrogenosomes. Archaea associate with larger organisms, too. For example, the marine archaean Cenarchaeum symbiosum is an endosymbiont of the sponge Axinella mexicana.