Trophic mutualism
Trophic mutualism is a key type of ecological mutualism. Specifically, "trophic mutualism" refers to the transfer of energy and nutrients between two species. This is also sometimes known as resource-to-resource mutualism. Trophic mutualism often occurs between an autotroph and a heterotroph. Although there are many examples of trophic mutualisms, the heterotroph is generally a fungus or bacteria. This mutualism can be both obligate and opportunistic. History of research. Ecologists first began to understand trophic mutualisms in the mid-20th century with the investigation of nutrient abundance and distribution. One of the first trophic mutualisms was discovered in  1958 by Professor Leonard Muscatine of UCLA, the relationship between endozoic algae and coral. In this relationship, the algae provides the coral with a Carbon source to develop its CaCO skeleton and the coral secretes a protecting nutrient-rich mucus which benefits the algae. Perhaps one of the most famous discoveries made by Muscatine in the field of trophic mutualism came about 10 years later in another aquatic based system-the relationship between algae and water hydra. This work was significant in establishing the presence of mutualistic relationships in both aquatic and terrestrial environments. Perhaps the most widely acclaimed example  of a trophic mutualism was the discovery of the leafcutter ant that engage in trophic mutualism with a fungus. These ants cultivate a certain type of fungus by providing it with leaves and other nutrients. In turn, the ants will feed on a special nutrient that is only created by the fungus they nurture. This trophic mutualism was studied in detail in the 1970s and since.