Zooxanthella
Encyclopedia
Zooxanthellae are flagellate
protozoa
that are golden-brown intracellular endosymbiont
s of various marine animal
s and protozoa
, especially anthozoa
ns such as the scleractinia
n corals and the tropical sea anemone, Aiptasia
.
Zooxanthellae live in other protozoa (foraminiferans and radiolarians) and in some invertebrates. Most are autotroph
s and provide the host with energy in the form of translocated reduced carbon compounds, such as glucose
, glycerol
, and amino acid
s, which are the products of photosynthesis
. Zooxanthellae can provide up to 90% of a coral’s energy requirements. In return, the coral provides the zooxanthellae with protection, shelter, nutrients (mostly waste material containing nitrogen and phosphorus) and a constant supply of carbon dioxide
required for photosynthesis. Available nutrients, incident light, and expulsion of excess cells limits their population.
Hermatypic
(reef
-building) coral
s largely depend on zooxanthellae, which limits that coral's growth to the photic zone
. The symbiotic relationship enables corals' success as reef-building organisms in tropical waters. However, under high environmental stress, corals die after losing their zooxanthellae either by expulsion or digestion resulting in coral bleaching.
Benefits cited for the algae include protection from predation and enhanced provision with chemicals such as carbon dioxide and ammonium. However, a number of conditions are thought to be necessary in order to maintain a symbiotic relationship; on a very simple level, the symbiont's optimal reproductive strategy must be to remain in the host. It is not clear that these conditions are met in the coral-zooxanthella instance; reproduction of the zooxanthella is retarded by almost two orders of magnitude when it dwells inside a coral.
The relationship may be better thought of as a parasitic relationship, with the coral parasitic upon its enslaved algae. The coral ensnares the algae by secreting a chemical attractant, before ingesting the algae and incorporating it into its cells. They are then surrounded by a 'symbiosome' membrane and confined within the host cell, separated from its cytoplasm. The host cell then emits chemical signals that prevent the zooxanthella from reproducing.
The presence of the alga results in the production of excess oxygen, which must be removed from the cell quickly in order to avoid destructive oxidation. The corals have mechanisms whereby they can kill over-oxygenated cells if necessary.
Coral bleaching
occurs when zooxanthellae densities within coral tissue become low or the concentration of photosynthetic pigments within each zooxanthella decline. Color loss also comes from reduced concentrations of pigments produced by the cnidarian itself. The result is a ghostly white calcareous skeleton
. The coral will then die unless conditions improve enough to allow the zooxanthellae to return.
Zooxanthellae directly or indirectly experience the stress that their containing corals undergo. Exposure to air during low tides and damage from solar radiation in shallow water environments are two of the ecological stressors coral and zooxanthellae face. Temperature changes now provide the most stress to the zooxanthellae-coral relationship. A 1-2 degree Celsius
temperature rise for 5–10 weeks and a 3-5 degree decline for 5–10 days have produced a coral bleaching event. Such temperature changes induce cell adhesion dysfunction which detaches zooxanthellae from their cnidarian endodermal cells.
Some see several different species of zooxanthellae within the Symbiodinium monophylum. Symbiodinium is related to Gymnodinium
simplex, Gymnodinium beii, and Polarella
glacialis.
The scleractinian-zooxanthella relationship evolved multiple times independently in the mid-Triassic, , and probably enabled the corals to build carbonate skeletons in the adverse Triassic seas (which had caused the tabulate and rugose
corals to become extinct). However, genetic evidence suggests that Symbiodinium did not begin to diversify until the early-to-middle Eocene, just .
of zooxanthella from Cassiopea
. At that time he proved that symbodinia had a motile stage which resembled a "gymnodinioid" dinoflagellate
. Being both symbiotic and a dinoflagellate, he named the genus Symbiodinium, and the species epithet microadriaticium, after its resemblance to a similar free-living species.
Dr. Freudenthal demonstrated that zooxanthellae go through a vegetative stage, a cyst stage, and a motile stage as part of their life cycle.
e.
Zooxanthellae reproduce asexually by budding (one individual splitting into multiple descendants).
, clam
s, foraminifera
, sea slugs i.e. nudibranch
s such as Pteraeolidia ianthina
, and also ciliates and radiolaria.
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
that are golden-brown intracellular endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s of various marine animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s and protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
, especially anthozoa
Anthozoa
Anthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
ns such as the scleractinia
Scleractinia
Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...
n corals and the tropical sea anemone, Aiptasia
Aiptasia
Aiptasia is a genus of a symbiotic cnidarian belonging to the class Anthozoa . Other well known cnidarian groups include the jellyfish , the hydroids , and the box jellyfish...
.
Zooxanthellae live in other protozoa (foraminiferans and radiolarians) and in some invertebrates. Most are autotroph
Autotroph
An autotroph, or producer, is an organism that produces complex organic compounds from simple inorganic molecules using energy from light or inorganic chemical reactions . They are the producers in a food chain, such as plants on land or algae in water...
s and provide the host with energy in the form of translocated reduced carbon compounds, such as glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, glycerol
Glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
, and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s, which are the products of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. Zooxanthellae can provide up to 90% of a coral’s energy requirements. In return, the coral provides the zooxanthellae with protection, shelter, nutrients (mostly waste material containing nitrogen and phosphorus) and a constant supply of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
required for photosynthesis. Available nutrients, incident light, and expulsion of excess cells limits their population.
Hermatypic
Hermatypic coral
Hermatypic corals or "stony corals" are reef-building corals, while corals that do not deposit aragonite structures and contribute to coral reef development are referred to as ahermatypic species....
(reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
-building) coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s largely depend on zooxanthellae, which limits that coral's growth to the photic zone
Photic zone
The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...
. The symbiotic relationship enables corals' success as reef-building organisms in tropical waters. However, under high environmental stress, corals die after losing their zooxanthellae either by expulsion or digestion resulting in coral bleaching.
Relationship
The coral and zooxanthella relationship has traditionally been considered mutualistic -- that is, both partners benefit from the arrangement. However, whilst the benefit for the coral is clear in terms of its enhanced growth and calcification rate, the benefit for the algae has been called into question.Benefits cited for the algae include protection from predation and enhanced provision with chemicals such as carbon dioxide and ammonium. However, a number of conditions are thought to be necessary in order to maintain a symbiotic relationship; on a very simple level, the symbiont's optimal reproductive strategy must be to remain in the host. It is not clear that these conditions are met in the coral-zooxanthella instance; reproduction of the zooxanthella is retarded by almost two orders of magnitude when it dwells inside a coral.
The relationship may be better thought of as a parasitic relationship, with the coral parasitic upon its enslaved algae. The coral ensnares the algae by secreting a chemical attractant, before ingesting the algae and incorporating it into its cells. They are then surrounded by a 'symbiosome' membrane and confined within the host cell, separated from its cytoplasm. The host cell then emits chemical signals that prevent the zooxanthella from reproducing.
The presence of the alga results in the production of excess oxygen, which must be removed from the cell quickly in order to avoid destructive oxidation. The corals have mechanisms whereby they can kill over-oxygenated cells if necessary.
Coral bleaching
Coral bleaching
Coral bleaching is the loss of intracellular endosymbionts through either expulsion or loss of algal pigmentation.The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with unicellular flagellate protozoa, called zooxanthellae, that...
occurs when zooxanthellae densities within coral tissue become low or the concentration of photosynthetic pigments within each zooxanthella decline. Color loss also comes from reduced concentrations of pigments produced by the cnidarian itself. The result is a ghostly white calcareous skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
. The coral will then die unless conditions improve enough to allow the zooxanthellae to return.
Zooxanthellae directly or indirectly experience the stress that their containing corals undergo. Exposure to air during low tides and damage from solar radiation in shallow water environments are two of the ecological stressors coral and zooxanthellae face. Temperature changes now provide the most stress to the zooxanthellae-coral relationship. A 1-2 degree Celsius
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
temperature rise for 5–10 weeks and a 3-5 degree decline for 5–10 days have produced a coral bleaching event. Such temperature changes induce cell adhesion dysfunction which detaches zooxanthellae from their cnidarian endodermal cells.
Geological history
Whilst there are genetic differences between the zooxanthellae of different species of coral, there is not agreement as to whether these are significant enough to denote different species.Some see several different species of zooxanthellae within the Symbiodinium monophylum. Symbiodinium is related to Gymnodinium
Gymnodinium
Gymnodinium is a genus of dinoflagellates. It is one of the few naked dinoflagellates, or species lacking armor . Since 2000, the species which had been considered to be part of Gymnodinium have been divided into several genera, based on the nature of the apical groove and the biochemistry...
simplex, Gymnodinium beii, and Polarella
Polarella
Polarella is a genus of Alveolata.It includes the species Polarella glacialis....
glacialis.
The scleractinian-zooxanthella relationship evolved multiple times independently in the mid-Triassic, , and probably enabled the corals to build carbonate skeletons in the adverse Triassic seas (which had caused the tabulate and rugose
Rugose
Rugose means "wrinkled". It may refer to:*Idiosoma nigrum, more commonly, a black rugose trapdoor spider*Rugosa, an extinct order of coral, whose rugose shape earned it the name...
corals to become extinct). However, genetic evidence suggests that Symbiodinium did not begin to diversify until the early-to-middle Eocene, just .
Life cycle
The genus, Symbiodinium, was defined by Hugo Freudenthal in 1970, after his identification of the life cycleBiological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
of zooxanthella from Cassiopea
Cassiopea
The Cassiopea jellyfish belongs to the Order Rhizostomeae and mostly lives in sandy areas and seagrass beds. The Cassiopea jellyfish is also called "Upside Down Jellyfish", because it lies on its back, so that the bell touches the ground. In this position it resembles a sea anemone...
. At that time he proved that symbodinia had a motile stage which resembled a "gymnodinioid" dinoflagellate
Dinoflagellate
The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
. Being both symbiotic and a dinoflagellate, he named the genus Symbiodinium, and the species epithet microadriaticium, after its resemblance to a similar free-living species.
Dr. Freudenthal demonstrated that zooxanthellae go through a vegetative stage, a cyst stage, and a motile stage as part of their life cycle.
Coral acquisition
Polyps can acquire Zooxanthellae by direct ingestion. However, their hosts do not digest them. In other cases, zooxanthellae may be transmitted by coral eggs and planulaPlanula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
e.
Zooxanthellae reproduce asexually by budding (one individual splitting into multiple descendants).
Other animal relationships
Other organisms which can host zooxanthellae include jellyfishJellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, clam
Clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...
s, foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
, sea slugs i.e. nudibranch
Nudibranch
A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...
s such as Pteraeolidia ianthina
Pteraeolidia ianthina
Pteraeolidia ianthina is a species of sea slug, an aeolid nudibranch, a marine gastropod mollusc in the family Glaucidae.-Distribution:...
, and also ciliates and radiolaria.