Lichens Lichens represent an association between one or more
fungal mycobionts and one or more photosynthetic
algal or cyanobacterial photobionts. The mycobiont provides protection from predation and
desiccation, while the photobiont provides energy in the form of fixed carbon. Cyanobacterial partners are also capable of
fixing nitrogen for the fungal partner. Recent work suggests that non-photosynthetic bacterial
microbiomes associated with lichens may also have functional significance to lichens. Most mycobiont partners derive from the
ascomycetes, and the largest class of lichenized fungi is
Lecanoromycetes. The vast majority of lichens derive photobionts from
Chlorophyta (green algae). Phylogenetic analyses in lichenized fungi have suggested that, throughout evolutionary history, there has been repeated loss of photosymbionts, switching of photosymbionts, and independent lichenization events in previously unrelated fungal taxa. Loss of lichenization has likely led to the coexistence of non-lichenized fungi and lichenized fungi in lichens. However, it has been shown that photosymbiotes are acquired
vertically (transmission from parent to offspring) and/or
horizontally (acquired from the environment). Photosymbiotes can supply up to half of the host sponge's respiratory demands and can support sponges during times of nutrient stress.
Cnidaria Members of certain classes in phylum
Cnidaria are known for photosymbiotic partnerships. Members of corals (Class
Anthozoa) in the orders
Hexacorallia and
Octocorallia form well-characterized partnerships with the dinoflagellate genus
Symbiodinium. Some jellyfish (class
Scyphozoa) in the genus
Cassiopea (upside-down jellyfish) also possess Symbiodinium. Certain species in the genus
Hydra (class
Hydrozoa) also harbor green algae and form a stable photosymbiosis. Corals are likewise adapted to eject damaged photosymbionts that generate high levels of toxic reactive oxygen species, a process known as
bleaching. The identity of the Symbiodinium photosymbiont can change in corals, although this depends largely on the mode of transmission: some species vertically transmit their algal partners through their eggs, while other species acquire environmental dinoflagellates as newly-released eggs. Since algae are not preserved in the coral fossil record, understanding the evolutionary history of the symbiosis is difficult.
Bilaterians In basal
bilaterians, photosymbiosis in marine or
brackish systems is present only in the family
Convolutidae. In the group
Acoela there is limited knowledge on the symbionts present, and they have been vaguely identified as
zoochlorella or
zooxanthella. Some species have a symbiotic relationship with the chlorophyte
Tetraselmis convolutae while others have a symbiotic relationship with the dinoflagellates
Symbiodinium,
Amphidinium klebsii, or
diatoms in the genus Licomorpha. In freshwater systems, photosymbiosis is present in
platyhelminths belonging to the
Rhabdocoela group.
Molluscs Photosymbiosis is taxonomically restricted in
Mollusca. This family boasts large organisms often referred to as
giant clams and their large size is attributed to the establishment of these symbiotic relationships. Additionally, the Symbiodinium are hosted extracellularly, which is relatively rare. The only known freshwater bivalve with a symbiotic relationship are in the genus
Anodonta which hosts the chlorophyte Chlorella in the gills and
mantle of the host. In bivalves, photosymbiosis is thought to have evolved twice, in the genus Anodonta and in the family Cardiidae.
Gastropods In
gastropods, photosymbiosis can be found in several genera. The species
Strombus gigas hosts
Symbiodinium which is acquired during the larval stage, at which point it is a
mutualistic relationship. However, during the adult stage, Symbiodinium becomes
parasitic as the shell prevents photosynthesis. Another group of gastropods,
heterobranch sea slugs, have two different systems for symbiosis. The first,
Nudibranchia, acquire their symbionts through feeding on
cnidarian prey that are in symbiotic relationships. In Nudibranchs, photosymbiosis has evolved twice, in
Melibe and
Aeolidida. Whether these kleptoplasts maintain their photosynthetic capabilities depends on the host species’ ability to digest them properly. In this group, functional kleptoplasty has been acquired twice, in
Costasiellidae and
Plakobranchacea.
Chordates Photosymbiosis is relatively uncommon in
chordate species. The photosynthetic ascidians are associated with
cyanobacteria in the genus of
Prochloron as well as, in some cases, the species
Synechocystis trididemni. In addition to sea squirts, embryos of some
amphibian species (
Ambystoma maculatum, Ambystoma gracile, Ambystoma jeffersonium, Ambystoma trigrinum, Hynobius nigrescens, Lithobates sylvaticus, and Lithobates aurora) form symbiotic relationships with the
green alga in the genus of Oophila. This algae is present in the egg masses of the species, causing them to appear green and providing oxygen and carbohydrates to the embryos. Similarly, little is known about the evolution of symbiosis in amphibians, but there appear to be multiple origins.
Protists Photosymbiosis has evolved multiple times in the protist taxa
Ciliophora,
Foraminifera,
Radiolaria,
Dinoflagellata, and
diatoms. Foraminifera and Radiolaria are
planktonic taxa that serve as
primary producers in open ocean communities. Photosynthetic plankton species associate with the symbiotes of dinoflagellates, diatoms,
rhodophytes,
chlorophytes, and
cyanophytes that can be transferred both
vertically and
horizontally. In Foraminifera,
benthic species will either have a symbiotic relationship with
Symbiodinium or retain the chloroplasts present in algal prey species. In the Radiolarian group
Acantharia, photosynthetic species inhabit surface waters whereas non-photosynthetic species inhabit deeper waters. Photosynthetic Acantharia are associated with similar microalgae as the Foraminifera groups, but were also found to be associated with
Phaeocystis,
Heterocapsa, Scrippsiella, and
Azadinium which were not previously known to be involved in photosynthetic relationships. In addition, several of the species present in symbiotic relationships with Acantharia were oftentimes identical to the free-living species, suggesting horizontal transfer of symbiotes. This provides insight into the evolutionary patterns responsible for these symbiotic relationships, suggesting that the selection for symbiosis is relatively weak and symbiosis is likely a result of the adaptive capacity of the host plankton species. == See also ==