Symbiodinium: Difference between revisions
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The genus '''''Symbiodinium''''' encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known to science. These unicellular algae commonly reside in the endoderm of tropical cnidarians such as corals, anemones, and jellyfish, where they translocate photosynthetic material to the host and in turn receive inorganic nutrients (e.g. CO2, NH4) for growth (Fig. 1). They are also harbored by various species of sponges, flatworms, mollusks (e.g. giant clams), foraminifera (soritids), and some ciliates. With the exception of mollusks, these dinoflagellates enter the host cell through normal phagocytosis, persist as intracellular symbionts, reproduce, and disperse. Hosts that require high densities of Symbiodinium occur mostly in warm oligotrophic (nutrient-poor) marine environments where they are often the dominant constituents of benthic communities. These dinoflagellates are therefore among the most abundant eukaryotic microbes found in coral reef ecosystems. |
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Symbiodinium are colloquially called "zooxanthellae" (or "zoox"), and animals symbiotic with algae in this genus are said to be "zooxanthellate". The term was loosely used to refer to any golden-brown endosymbionts, including diatoms and other dinoflagellates. Continued use of the term in the scientific literature should be discouraged because of the confusion caused by overly generalizing many disparate and taxonomically diverse symbiotic relationships.<ref>Blank RJ, Trench RK (1986) Nomenclature of endosymbiotic dinoflagellates. Taxon 35:286-94</ref> |
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==Intracellular Symbionts== |
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[[File:Fig 1 SymbiodiniumCellLightandconfocal.png|thumb|right|350px|alt= |'''Figure 1.''' Light and confocal images of ''Symbiodinium'' cells present in hospite (living in a host cell) with scyphistomae of the jellyfish ''Cassiopea xamachana''. This animal requires infection by these algae to complete its life cycle. The extensive chloroplast organelle imaged in 3-D is highly reticulated and distributed around the cell’s periphery (photos by T.C. LaJeunesse).]] |
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''Symbiodinum'' are known primarily for their role as mutualistic endosymbionts. In host tissues they usually occur in high concentrations, ranging from 100s of thousands to millions per square centimeter.<ref>Stimson J, Sakai K, Sembali H (2002) Interspecific comparison of the symbiotic relationship in corals with high and low rates of bleaching-induces mortality. Coral Reefs 21:409-421</ref> The successful culturing of swimming gymnodinoid cells from coral led to the discovery that “zooxanthellae were actually dinoflagellates (Fig. 2A).<ref>Kawaguti S (1944) On the physiology of reef corals. VII. Zooxanthellae of the reef corals is Gymnodinium sp. Dinoflagellata; its culture in vitro. Palau Tropical Biological Station Studies 2:265-275</ref><ref>McLaughlin JJA, Zahl PA (1959) Axenic zooxanthellae from various invertebrate hosts. Ann NY Acad S 77:55-72</ref> Each ''Symbiodinium'' cell is coccoid in ''hospite'' (living in a host cell) and surrounded by a membrane that originates from the host cell plasmalemma during phagocytosis (Figs 2B and 3). This cell membrane probably undergoes some modification to its protein content, which functions to limit or prevent phago-lysosome fusion (Fig. 2B).<ref>Colley NJ, Trench RK (1983) Selectivity in phagocytosis and persistence of symbiotic algae by the scyphistoma stage of the jellyfish Cassiopeia xamachana. Proc R Soc Lond B 219: 61-82</ref><ref>Wakefield TS, Kempf SC (2001) Development of host- and symbiont-specific monoclonal antibodies and confirmation of the origin of the symbiosome membrane in a Cnidarian-Dinoflagellate symbiosis. Biol Bull 200:127-143</ref><ref>Peng S, Wang Y, Wang L, Chen WU, Lu C, Fang L, Chen C (2010) Proteomic analysis of symbiosome membranes in Cnidaria-dinoflagellate endosymbiosis. Proteomics 10:1002-1016</ref> The vacuole structure containing the symbiont is therefore termed the ''symbiosome''. Under normal conditions, symbiont and host cells exchange organic and inorganic molecules that enable the growth and proliferation of both partners. |
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[[File:Fig 2 Symbiodinium duel life.png|thumb|left|text-top|alt= |'''Figure 2.''' The life stages of dinoflagellates in the genus ''Symbiodinium''. (A) Electron micrographs of a ''Symbiodinium'' mastigote with characteristic ''gymnodinoid'' morphology (S. natans) <ref> Hansen G, Daugbjerg N (2009) Symbiodinium natans sp. nov.: a “free-living dinoflagellate from Tenerife (Northeast-Atlantic Ocean). J. Phycol. 45:251-63.</ref> and (B) the coccoid cell in hospite. As free-living cells the motile mastigote allows for short-range dispersal and can exhibit chemo-taxis toward sources of nitrogen. Once infected these symbionts rapidly proliferate the host endodermal tissues and in many cases dominate the cytoplasm of the host cell. ]] |
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[[File:Symbiodinium Host Tissue Section.png|thumb|right|'''Figure 3.''' ''Symbiodinium'' reach high cell densities through prolific mitotic division in the endodermal tissues of many shallow tropical and sub-tropical cnidarians. This is an SEM of a fractured internal mesentary from a reef coral polyp (Porites porites) that shows the distribution and density of symbiont cells.]] |
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==Natural Services and Economic Value == |
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Symbiodinium are among the most researched of the Dinophyceae. Their mutualistic relationships with reef-building corals form the basis of a highly diverse and productive ecosystem. Coral reefs have economic benefits – valued at hundreds of billions of dollars each year – in the form of ornamental, subsistence, and commercial fisheries, tourism and recreation, coastal protection from storms, a source of new bioactive compounds for pharmaceutical development, and more.<ref>Moberg F, Folke C (1999) Ecological goods and services of coral reef ecosystems. Ecol Econ 29:215-233</ref> The economic value of Symbiodinium is thus immeasurable, their continued productivity as symbionts and the functioning of tropical reef ecosystems are in serious jeopardy due to ocean warming. |
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==Symbiodinium and Coral Bleaching== |
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==Symbiodinium Diversity, Ecology, and Biogeography== |
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===Geographic distributions and patterns of diversity=== |
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Symbiodinium is a good group for studying micro-eukaryote physiology and ecology for several reasons. 1) Phylogenetic and population genetic markers are now available that allow for detailed examination of their genetic diversity over broad spatial and temporal scales. 2) Large quantities Symbiodinium cells are readily obtained through the collection of hosts that harbor them. 3) Their association with animals provides an additional axis by which to compare and contrast ecological distributions. |
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The earliest genetic methods for assessing Symbiodinium diversity relied on low-resolution molecular markers that separated the genus into a few evolutionarily divergent lineages, referred to as “clades” (see [[#Major phylogenetic disparity among Symbiodinium “Clades”|below]]). Previous characterizations of geographic distribution and dominance have focused on the clade-level of genetic resolution, however more detailed assessments of diversity at the species level are needed (Fig. 5). While members of a given clade may be ubiquitous, the species diversity within each group is potentially large, with each species often having different ecological and geographic distributions related to their dispersal ability, host biogeography, and external environmental conditions. A small number of species occur in temperate environments where few symbiotic animals occur. As a result, these high latitude associations tend to be highly specific. |
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[[File:CO1 Phylogeny.png|thumb|'''Figure 5''']] |
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===Symbiodinium diversity assigned to different ecological guilds=== |
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The large diversity of Symbiodinium revealed by genetic analyses is distributed non-randomly and appears to comprise several guilds with distinct ecological habits (Fig. 6). Of the many Symbiodinium characterized genetically, most are host-specific, mutualistic, and dominate their host.<ref>LaJeunesse TC (2002) Diversity and community structure of symbiotic dinoflagellates from Caribbean coral reefs. Mar Biol 141:387-400</ref> Others may represent compatible symbionts that remain as low-abundance background populations because of competitive inferiority under the prevailing external environmental conditions (e.g. high light vs. low light).<ref>Rowan, R., Knowlton, N., Baker, A. & Jara, J. (1997) Landscape ecology of algal symbionts creates variation in episodes of coral bleaching. Nature 388, 265–269.</ref> Some may also comprise opportunistic species that may proliferate during periods of physiological stress and displace the normal resident symbiont and remain abundant in the host’s tissues for months to years before being replaced by the original symbiont.<ref>Toller WW, Rowan R, Knowlton N (2001) Repopulation of zooxanthellae in the Caribbean corals Montastraea annularis and M. faveolata following experimental and disease-associated bleaching. Biol. Bull. 201:360-373</ref><ref name="Reference2">Thornhill DJ, LaJeunesse TC, Kemp DW, Fitt WK, Schmidt GW (2006) Multi-year, seasonal genotypic surveys of coral-algal symbioses reveal prevalent stability or post-bleaching reversion. Mar Biol 148:711-722</ref><ref>LaJeunesse TC, Finney JC, Smith R, Oxenford H (2009) Outbreak and persistence of opportunistic symbiotic dinoflagellates during the 2005 Caribbean mass coral ‘bleaching’ event. Proc. Roy Soc Lond, B 276: 4139-4148.</ref> Similarly there are those that rapidly infect and establish populations in host juveniles until being replaced by symbionts that normally associate with host adult colonies.<ref>Coffroth MA, Santos SR, Goulet TL (2001) Early ontogenic expression of specificity in a cnidarian-algal symbiosis. Mar Ecol Prog Ser 222:85-96</ref> Finally, there appears to be another group of Symbiodinium that are incapable of establishing endosymbiosis yet exist in environments around the animal or associate closely with other substrates where nutrients are available (i.e. macro-algal surfaces, surface sediment).<ref name="References4">Porto I, Granados C, Restrepo JC, Sanchez JA (2008) Macroalgal-associated dinoflagellates belonging to the genus Symbiodinium in Caribbean reefs. PLoS ONE 3:e2160</ref><ref name="Reference1">Reimer JD, Shah MMR, Sinniger F, Yanagi K, Suda S (2010) Preliminary analyses of cultured Symbiodinium isolated from sand in the oceanic Ogasawara Islands, Japan. Mar Biodiv 40:237-247</ref> Symbiodinium from functional groups 2, 3, and 4 are known to exist because they culture easily, however species with these life histories are difficult to study because of their low abundance in the environment (Fig. 6). |
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===Free-living and “non-symbiotic” Symbiodinium=== |
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There are few examples of documented populations of free-living Symbiodinium (see references<ref name="Reference1" />). Given that most host larvae must initially acquire their symbionts from the environment, Symbiodinium cells must occur commonly outside the host. It may be that the motile phase plays an important role in the external environment and in the infection of host larvae. The use of aposymbiotic host polyps deployed as "capture vessels" and the application of molecular techniques has allowed for the detection of environmental sources of Symbiodinium.<ref name="Reference2" /><ref>Coffroth MA, Lewis CF, Santos SR (2006) Environmental populations of symbiotic dinoflagellates in the genus Symbiodinium can initiate symbioses with reef cnidarians. Curr Biol 16:987-987</ref> With these methods employed, investigators may resolve the distribution of different species on various benthic surfaces <ref name="References4" /> and cell concentrations suspended in the water column.<ref>Manning MM, Gates RD (2008) Diversity in populations of free-living Symbiodinium from a Caribbean and Pacific reef. Limno. Oceanogr 53:1853-1861</ref> The genetic identities of cells cultured from the environment are often dissimilar to those found in hosts. Learning more about the "private lives" of these environmental populations and their ecological function will further our knowledge about the diversity, dispersal potential, and evolution among members within this genus. |
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==Molecular Systematics of Symbiodinium == |
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===Major phylogenetic disparity among Symbiodinium “Clades”=== |
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The earliest ribosomal gene sequence data indicated that Symbiodinium was comprised of lineages whose genetic divergence was similar to differences observed among dinoflagellates from different genera, families, and even orders.<ref>Rowan R & Powers DA (1991) A molecular genetic classification of zooxanthellae and the evolution of animal-algal symbiosis. Science 251:1348-51.</ref> This large genetic disparity among “clades” A, B, C, etc. was reconfirmed with mitochondrial gene sequences (CO1) analyzing the Dinophyceae (Fig. 7).<ref>Stern RF, Horak A, Andrew RL, Coffroth MA, Andersen RA, Kupper FC, Jameson I, Hoppenrath M, Veron B, Kasai F, Brand J, James ER, Keeling PJ (2010) Environmental barcoding reveals massive dinoflagellate diversity on marine environments. PLoS ONE 5:e13991</ref> Most of these “clade” groupings comprise numerous reproductively isolated, genetically distinct lineages (see below), exhibiting different ecological and biogeographic distributions (see [[#Geographic distributions and patterns of diversity|above]]). Given the over-simplified perceptions created by using clade-level taxonomic designations for grouping Symbiodinium, future taxonomic revision of this genus is required. Most likely, many of these “clades” will be reclassified into distinct genera. |
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===Species Diversity=== |
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The recognition of species diversity in this group remained problematic for many decades due the challenges of identifying morphological and biochemical traits useful for diagnosing species.<ref>Trench RK, Blank RJ (1987) Symbiodinium microadriaticum Freudenthal, S. goreauii sp. nov., S. kawagutii sp. nov. and S. pilosum sp. nov.: Gymnodinioid dinoflagellate symbionts of marine invertebrates. J Phycol 23:469-481</ref> Presently, phylogenetic, ecological, and population genetic data can be more rapidly acquired to resolve Symbiodinium into separate entities that are consistent with Biological, Evolutionary, and Ecological Species Concepts<ref name="Reference5" /><ref name="Reference3" /> Most genetics-based measures of diversity have been estimated from the analysis of one genetic marker (e.g. LSU, ITS2, cp23S), yet in recent studies these and other markers were analyzed in combination. The high concordance found among nuclear, mitochondrial and chloroplast DNA argues that a hierarchical phylogenetic scheme, combined with ecological data, can unambiguously recognize and assign nomenclature to reproductively isolated lineages, i.e. species (Fig. 8). |
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When analyzed in the context of the major species concepts,<ref>de Queiroz K (2007) Species concepts and species delimitation. Syst Bio. 56:879–86</ref> ITS2 sequence data provide a good proxy for species diversity.<ref name="Reference5">Sampayo E, Dove S, LaJeunesse TC (2009) Cohesive molecular genetic data delineate species diversity in the dinoflagellate genus Symbiodinium. Mol Eco. 18:500-519</ref><ref name="Reference3">LaJeunesse TC, Thornhill DJ (2011) Improved resolution of reef-coral endosymbiont (Symbiodinium) species diversity, ecology, and evolution through psbA non-coding region genotyping. PlosOne e29013</ref><ref>Thornhill DJ, LaJeunesse TC, Santos SR (2007) Measuring rDNA diversity in eukaryotic microbial systems: how intragenomic variation, pseudogenes, and PCR artifacts confound biodiversity estimates. Mol Ecol 24:5326-5340</ref> Currently ITS2 types number in the 100’s and most communities of symbiotic cnidaria around the world still require comprehensive sampling. Furthermore, there appear to be a large number of unique species found in association with equally diverse species assemblages of soritid foraminifera,<ref>Pochon X, Garcia-Cuestos L, Baker AC, Castella E, Pawlowski J (2007) One-year survey of a single Micronesian reef reveals extraordinarily rich diversity of Symbiodinium types in sorited foraminifera. Coral Reefs 26:867-82</ref> as well as many other Symbiodinium that are exclusively free-living and found in varied, often benthic, habitats.<ref name="Reference1" /> Given the potential species diversity of these ecologically cryptic Symbiodinium, the total species number may never be accurately assessed. |
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===Clone diversity and population genetics=== |
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===Culturing Symbiodinium=== |
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==Life cycle== |
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==Morphology and Anatomy== |
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===Morphological description of the genus Symbiodinium=== |
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===The mastigote cell=== |
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===The coccoid cell=== |
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===The cell wall=== |
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===The chloroplast=== |
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===The nucleus=== |
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===Other cytoplasmic organelles=== |
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==References== |
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{{Reflist|2}} |
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{{Uncategorized|date=March 2012}} |
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Revision as of 20:05, 13 March 2012
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