Dinocysts or dinoflagellate cysts are typically 15 to 100 μm in diameter and produced by dinoflagellates as a dormant, zygotic stage of their lifecycle, which can accumulate in the sediments as microfossils. Organic-walled dinocysts are often resistant and made out of dinosporin. There are also calcareous dinoflagellate cysts and siliceous dinoflagellate cysts.
History
The first person to recognize fossil dinoflagellates was Christian Gottfried Ehrenberg, who reported his discovery in a paper presented to the Berlin Academy of Sciences in July 1836. He had observed clearly tabulate dinoflagellates in thin flakes of Cretaceous flint and considered those dinoflagellates to have been silicified. Along with them, and of comparable size, were spheroidal to ovoidal bodies bearing an array of spines or tubes of variable character. Ehrenberg interpreted these as being originally siliceous and thought them to be desmids (freshwater conjugating algae), placing them within his own Recent desmid genus Xanthidium. Though summaries of Ehrenberg's work appeared earlier, it was not published in full until 1837 or 1838; the date is uncertain.W.A.S. Sarjeant, 2002. 'As chimney-sweeps, come to dust': a history of palynology to 1970. pp. 273–327 In: Oldroyd, D. R. The earth inside and out: some major contributions to geology in the twentieth century. Geological Society (London) Special Publication no. 192.
A first relation between dinoflagellate thecae and cysts was made through morphological comparison of both by Bill Evitt and Susan E. Davidson.Evitt, W.R. and Davidson, S.E. 1964. Dinoflagellate studies. 1. Dinoflagellate cysts and thecae. Stanford university publications X (1), pp. 3–12. Further evidence came from detailed culture studies of dinoflagellate cysts by David Wall and Barrie Dale at Woods Hole Oceanographic Institution in the sixties.
Types of cysts
Ontologically, the term cyst can apply to (1) a temporary resting state (pellicle, temporary or ecdysal cyst), (2) a dormant zygote (resting cysts or hypnozygotes) or (3) a coccoid condition in which the cells are still photosynthetically active.Pfiester L.A. & Anderson D.M. 1987. Dinoflagellate reproduction. In: The biology of dinoflagellates. Botanical monographs 21 (Ed. by F.J.R. Taylor), pp. 611–648., Blackwell Scientific Publications. For example, for this last special case, all cysts described from species of the order Phytodiniales (e.g. Cystodinium, Stylodinium, Hypnodinium, Tetradinium, Dinococcus, Gloeodinium), are coccoid stages.
Digestive cyst or digestion cysts denote pellicle cysts formed after feeding by phagocytosis as in Katodinium fungiforme.
Division cysts refer to non-motile division stages wherein asexual reproduction takes place through division.BRAVO I., FIGUEROA R.I., GARCÉS E., FRAGA S. & MASSANET A. 2010. The intricacies of dinoflagellate pellicle cysts: the example of Alexandrium minutum cysts from a bloom-recurrent area (Bay of Baiona, NW Spain). Deep-Sea Research Part II: Topical Studies in Oceanography 57: 166–174. These are not pellicle or resting cysts since they are not dormant. Similarly, palmelloid or mucilage stages are not pellicle or resting cysts, but stages in which the monad loses its flagella and becomes enveloped in multilayered mucilage wherein division takes place.POPOVSKÝ J. & PFIESTER L.A. 1990. Dinophyceae (Dinoflagellida). In: Süßwasserflora von Mitteleuropa. Begründet von A. Pascher. Band 6 (Ed. by H. Ettl, J. Gerloff, H. Heynig. & D. Mollenhauer). Gustav Fischer Verlag, Jena, 272 pp.
Taxonomy
Dinoflagellate cysts described in the literature have been linked to a particular motile stage through morphological similarities and/or co-occurrence in the same population/culture or through the technique of establishing the so-called cyst-theca relation by incubation of the cysts. Geologists use a cyst-based taxonomy, whilst biologists use a motile-stage based taxonomy. Therefore, cysts can have different names than the corresponding motile stages. Living cysts can be easily isolated from the sediment using sodium polytungstate, a heavy liquid. Another method, rarely used, uses a sucrose gradient. Recent times have brought about the possibility to get molecular sequences from single cysts or single cells. The proportion of cyst-forming species for marine dinoflagellates is between 15 and 20%HEAD M.J. 1996. Modern dinoflagellate cysts and their biological affinities. In: Palynology: principles and applications (Ed. by J. Jansonius & D. C. McGregor), pp. 1197–1248. American Association of Stratigraphic Palynologists Foundation, Dallas, Texas. and for freshwater dinoflagellates 24%. The tabulation of the Dinoflagellate is sometimes mirrored in the tabulation (previously called paratabulation) of the dinocyst, allowing species to be deduced from the cyst. It has previously been suggested that morphological characters from the cyst stage may be phylogenetically important in marine species and this may to an even greater extent be the case for freshwater dinoflagellates, confirmed by new observations and recently reviewed. Several books document general cyst taxonomy.Evitt, W.R., Lentin, J.K., Millioud, M.E., Stover, L.E. and Williams, G.L., 1977. Dinoflagellate cyst terminology. Geological survey of Canada, Paper 76-24, 1-11. There are few guides for determination of marine Quaternary dinocysts.Rochon, A., de Vernal, A., Turon, J.-L., Matthiessen, J., and Head, M.J., 1999. Distribution of recent dinoflagellate cysts in surface sediments from the North Atlantic Ocean and adjacent seas in relation to sea-surface parameters. AASP Contribution Series, 35, 146 pp.MATSUOKA, K. & FUKUYO, Y. 2000. Technical guide for modern dinoflagellate cyst study. WESTPAC-HAB/WESTPAC/IOC, Japan Society of the Promotion Science, Tokyo, 29 pp. Many new species are still being described for the Neogene, which covers the Miocene,Soliman, A., Head, M.J., and Louwye, S. In press. Morphology and distribution of the Miocene dinoflagellate cyst Operculodinium? borgerholtense Louwye 2001, emend. Palynology. the PlioceneHead, M.J., 1999. The Late Pliocene St. Erth Beds of Cornwall: a review of the palynology and reappraisal of the dinoflagellates. In: Scource, J. and Furze, M.F.A. (eds.), The Quaternary of West Cornwall. Field Guide, Quaternary Research Association, Durham, U.K., p. 88–92.Head, M.J. 2000. Geonettia waltonensis, a new goniodomacean dinoflagellate from the Pliocene of the North Atlantic region, and its evolutionary implications" Journal of Paleontology 74(5): 812–827, 6 pls. and the Quaternary, which covers the Pleistocene and recent.
Size
Quaternary dinocysts are typically between 15 and 100 μm in diameter. One of the smallest recent cysts is the cyst of Pentapharsodinium dalei, which can be as small as 19 μm in length. One of the largest recent cysts is the cyst of Protoperidinium latissimum, which can be as large as 100 μm in length.
Composition
The walls of organic-walled dinocysts are composed of the resistant biopolymer called dinosporin.Fensome, R.A., Taylor, F.J.R., Norris, G., Sarjeant, W.A.S., Wharton, D.I., and Williams, G.L., 1993. A classification of modern and fossil dinoflagellates, Sheridan Press, Hanover. . This organic compound has similarities to sporopollenin, but is unique to dinoflagellates.
In addition to organic-walled cysts, there are also calcareous dinoflagellate cysts and siliceous dinoflagellate cysts.
Morphological terms
In pure morphological terms, a dinocyst can be described as the body formed by the cyst wall, as well as the space it encloses and all the spaces within it. Cysts may develop their wall immediately within the theca, and such cysts are called proximate. Alternatively, the cyst may comprise a more or less spherical central body with processes or crests, and such cysts are termed chorate or proximochorate. Cysts may have a single-layered wall (autophragm), a two-layered wall (comprising an outer periphragm and an inner endophragm) or a three-layered wall (ectophragm, periphragm and endophragm if the outer wall is structurally supported, or otherwise periphragm, mesophragm and endophragm). Cysts with two or more wall layers that define a cavity are termed cavate. Excystment usually results in loss of part of, or an opening in, the cyst wall, termed archeopyle, the shape and position of which may indicate the position and/or shape of one or more thecal plates.
Transmission electron microscopy (TEM) studies (e.g.) suggest that endophragm and periphragm are not morphologically separable. Therefore, the use of the terms pedium and luxuria are suggested instead. Within the cyst wall, a thick cellulose-like layer called the endospore is present which is birefringent under crossed nichols. Cysts may be identified using the overall body shape but more often based on the characteristic furrows housing the flagella (cingulum and sulcus) or details of the patterns of plates covering many motiles (thecal tabulation). The one distinctive feature common to all cysts is the excystment opening (archaeopyle) through which the emerging new motile stage exits. In many cases this reflects a recognizable part of the tabulation (one or more plates). However, one large group of dinoflagellates (athecate - or naked dinoflagellates) do not have thecal plates and therefore produce cysts lacking all forms of reflected tabulation.Dale, B. & Dale, A.L. 2002. Environmental applications of dinoflagellate cysts and acritarchs . In Quaternary environmental micropalaeontology (Haslett, S.K., editor), 207-240. Arnold, London.
Cyst ultrastructure
There have been very few ultrastructural studies of marine cysts with TEM, except for early on Hystrichosphaea bentorii, on Hystrichosphaeridium, Impletosphaeridium, Lingulodinium machaerophorum and Operculodinium centrocarpum and Bitectatodinium tepikiense and more recent work on Lingulodinium machaerophorum and Alexandrium.
Some freshwater cysts have been investigated with TEM, such as Ceratium hirundinella.
Relation to life cycle
Resting cysts are traditionally associated with the sexual cycle of dinoflagellates. Induced by particular triggers such as changes in temperature, nutrients,Pfiester, L. A. & Anderson, D. M. 1987. Dinoflagellate reproduction. In: The biology of dinoflagellates (ed. F. J. R. Taylor), pp. 611–648. - Blackwell, Oxford. etc., dinoflagellates undergo gamete formation. The gametes fuse to form the planozygote and undergo encystment: they form cysts within the thecae of the planozygote. These rapidly sink to the sediment. Many species may spend longer periods resting in the sediment than active in the water column.RENGEFORS K. 1998. Seasonal succession of dinoflagellates coupled to the benthic cyst dynamics in Lake Erken, Sweden. Archiv für Hydrobiologie, Special Issues, Advances in Limnology 51: 123–141. Resting stages also constitute a reservoir of genetic diversity, which increases the survival potential of the populations. Thus, dinoflagellate cysts have great ecological importance and act as "seed banks", comparable to those found in terrestrial ecosystems. The encysted forms may remain viable for up to 100 years. Sediment can be stored with live Lingulodinium cysts for at least 18 months. Cysts often need triggers to germinate ('excyst'), such as changes in temperature, nutrients, etc. Some cysts, such as Scrippsiella acuminata, require light to germinate.
Distribution and ecology of organic-walled dinocysts