Symbiosis
Archaeplastida, excavates, and chromalveolates can form beneficial associations with fungi, other eukaryotic microorganisms, insects, and higher animals. These symbioses can be specific and physically and physiologically intimate or they can be relatively non- specific and merely loose ecological associations.
Endosymbionts are symbionts that live inside other organisms. Photosynthetic species can occur within Alveolata and Euglenozoa (and others) and their presence allows the protistan dual organism to adopt a phototrophic habit. The photosynthetic partner is confined to a membrane-bound vacuole, but it is capable of cell division. There is a two-way exchange of materials where the products of nitrogen metabolism of the heterotrophic protistan are utilized by the photosynthetic partner and the products of photosynthesis are utilized by the heterotrophic partner.
The zooxanthellae are symbiotic dinoflagellates that are found as coccoid cells within animal cells. They are enclosed in intracellular double-membrane-bound vacuoles, which remain undigested. They are found in excavates and chromalveolates, hydroids, sea anemones, corals, and clams where they provide glycerol, glucose, and organic acids for the animal, and the symbiotic alga gains CO2, inorganic nitrogen, phosphates, and some vitamins from the animal. Reef-building corals are only able to build reefs if they have their symbiotic photosynthetic partner, and many of the animal hosts are at least partly dependent on the algal partner for carbohydrates. Radiolaria, responsible for massive primary productivity in the oceans, are wholly dependent on their photosynthetic symbiont for carbohydrate.
Many aerobic protista contain bacteria as endosymbionts. Amoeba proteus has a symbiotic Gram-negative bacterium that is essential to the survival of the ameba, and Paramecium aurelia has specific symbiotic algae that are responsible for the secretion of killer factors that are important in competition within environments and mating.There are a large number of Bacterial ectosymbionts. Spirochetes and many other species of Bacteria are often attached to the euglenozoal pellicle, arranged in very specific pat- terns. For example Mixotricha paradoxa is an inhabitant of termite guts. Its motility within this environment depends on the co-ordinated movement of adherent spirochetes.
Ecto- and endosymbiotic Bacteria are also found associated with anaerobic alveolata from sulfur-rich environments. Kentrophoros lanceolata has a dense mat of sulfur Bacteria on its dorsal surface. These symbionts provide alternative electron acceptors to the protistan.
Excavates and chromalveolates can be symbiotic with insects and higher animals, and are typically found within fermentative guts as mentioned above. They are part of a complex ecosystem where cellulose is broken down by fungi and prokaryotes, and their metabolic products are fermented by the protista and prokaryotes to provide fatty acids for their hosts. Some of the protistan population also predates the prokaryotic population to provide control of numbers.