Photosymbiotic relationships where microalgae and/or cyanobacteria live within a heterotrophic host organism, are believed to have led to eukaryotes acquiring photosynthesis and to the evolution of plants.
[9] The co-evolutionary dynamics between mycobionts and photobionts are still unclear, as many photobionts are capable of free-living, and many lichenized fungi display traits adaptive to lichenization such as the capacity to withstand higher levels of reactive oxygen species (ROS), the conversion of sugars to polypols that help withstand dedication, and the downregulation of fungal virulence.
Photosymbiosis is found in four classes of Porifera (Demospongiae, Hexactinellida, Homoscleromorpha, and Calcarea), and known photosynthetic partners are cyanobacteria, chloroflexi, dinoflagellates, and red (Rhodophyta) and green (Chlorophyta) algae.
Members of corals (Class Anthozoa) in the orders Hexacorallia and Octocorallia form well-characterized partnerships with the dinoflagellate genus Symbiodinium.
Certain species in the genus Hydra (class Hydrozoa) also harbor green algae and form a stable photosymbiosis.
[18] Corals are likewise adapted to eject damaged photosymbionts that generate high levels of toxic reactive oxygen species, a process known as bleaching.
[38] Tropical marine bivalves in the Cardiidae family form a symbiotic relationship with the dinoflagellate Symbiodinium.
The species Strombus gigas hosts Symbiodinium which is acquired during the larval stage, at which point it is a mutualistic relationship.
[48] The second, Sacoglossa, removes chloroplasts from macroalgae when feeding and sequesters them into their digestive tract at which point they are called kleptoplasts.
[49] Whether these kleptoplasts maintain their photosynthetic capabilities depends on the host species ability to digest them properly.
[54] The 30 species with a symbiotic relationship span four genera where the congeners are primarily non-symbiotic, suggesting multiple origins of photosymbiosis in ascidians.
[56][57][58] This algae is present in the egg masses of the species, causing them to appear green and providing oxygen and carbohydrates to the embryos.
Photosymbiosis has evolved multiple times in the protist taxa Ciliophora, Foraminifera, Radiolaria, Dinoflagellata, and diatoms.
[61] Photosynthetic plankton species associate with the symbiotes of dinoflagellates, diatoms, rhodophytes, chlorophytes, and cyanophytes that can be transferred both vertically and horizontally.