Photosymbiosis is a type of symbiosis where one of the organisms is capable of photosynthesis.
Examples of photosymbiotic relationships include those in lichens, plankton, ciliates, and many Marine life organisms including corals, fire corals, Tridacninae, and jellyfish.
Photosymbiosis is important in the development, maintenance, and evolution of terrestrial and aquatic ecosystems, for example in biological soil crusts, soil formation, supporting highly diverse microbial populations in soil and water bodies, and coral reef growth and maintenance.
When one organism lives within another symbiotically it's called endosymbiosis. 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.
Occurrence
Lichens
represent an association between one or more
Fungus 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
associated with lichens may also have functional significance to lichens.
Most mycobiont partners derive from the Ascomycota, and the largest class of lichenized fungi is Lecanoromycetes.
Currently described photobiont species number about 100, far less than the 19,000 described species of fungal mycobionts, and factors such as geography can predominate over mycobiont preference.
Sponges
(phylum Porifera) have a large diversity of photosymbiote associations. Photosymbiosis is found in four classes of Porifera (
Demosponge,
, Homoscleromorpha, and Calcarea), and known photosynthetic partners are cyanobacteria,
Chloroflexota,
, and red (
Red algae) and green (Chlorophyta) algae. Relatively little is known about the evolutionary history of sponge photosymbiois due to a lack of
Genomic library data.
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.
The evolution of photosymbiosis in corals was likely critical for the global establishment of .
Bilaterians
In basal
, 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
Zooxanthellae.
Some species have a symbiotic relationship with the chlorophyte
Tetraselmis convolutae while others have a symbiotic relationship with the dinoflagellates
Symbiodinium,
Amphidinium klebsii, or
in the genus Licomorpha.
In freshwater systems, photosymbiosis is present in Flatworm belonging to the Rhabdocoela group.
Molluscs
Photosymbiosis is taxonomically restricted in
Mollusca.
Tropical marine
Bivalvia in the Cardiidae family form a symbiotic relationship with the dinoflagellate
Symbiodinium.
This family boasts large organisms often referred to as
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.
However, how it has evolved in Cardiidae could have occurred through different gains or losses in the family.
Gastropods
In
Gastropoda, 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.
Another group of gastropods, Heterobranchia sea slugs, have two different systems for symbiosis. The first, , acquire their symbionts through feeding on prey that are in symbiotic relationships.
Chordates
Photosymbiosis is relatively uncommon in
chordate species.
One such example of photosymbiosis is in
Ascidiacea, the sea squirts. In the genus
Didemnidae, 30 species establish symbiotic relationships.
The photosynthetic ascidians are associated with
cyanobacteria in the genus of
Prochloron as well as, in some cases, the species
Synechocystis trididemni.
The 30 species with a symbiotic relationship span four genera where the congeners (species within the same genus) are primarily non-symbiotic, suggesting multiple origins of photosymbiosis in ascidians.
In addition to sea squirts, embryos of some amphibian species ( Ambystoma maculatum, Ambystoma gracile, Ambystoma jeffersonium, Tiger salamander, Hynobius nigrescens, Wood frog, and Lithobates aurora) form symbiotic relationships with the Green algae in the genus of Oophila.
Protists
Photosymbiosis has evolved multiple times in the protist taxa
Ciliate,
Foraminifera,
Radiolaria,
Dinoflagellate, and
.
Foraminifera and Radiolaria are
taxa that serve as
Autotroph in open ocean communities.
Photosynthetic plankton species associate with the symbiotes of dinoflagellates, diatoms,
Red algae,
Chlorophyta, and
Cyanobacteria that can be transferred both vertically and horizontally.
In Foraminifera,
Benthic zone species will either have a symbiotic relationship with
Symbiodinium or retain the chloroplasts present in algal prey species.
The planktonic species of Foraminifera associate primarily with
Pelagodinium.
These species are often considered indicator species due to their bleaching in response to environmental stressors.
In the Radiolarian group
Acantharea, 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
