Spirostomum is a genus of ciliated protists in the class Heterotrichea. It is known for being very contractile.Fernandes, N.M.; da Silva Neto, I.D. (2013). “Morphology and 18S rDNA gene sequence of Spirostomum minus and Spirostomum teres (Ciliophora: Heterotrichea) from Rio de Janeiro, Brazil”. Zoologia 30: 72-79. Having been first identified by Christian Gottfried Ehrenberg in 1834, further research has identified eight additional true morphospecies.Bishop, A. (1923). “Some observations upon Spirostomum ambiguum (Ehrenberg)”. Journal of Cell Science 267: 391-434. Fokin, S. (2004). “A brief history of ciliate studies (late XVII the first third of the XX century)”. Protistology 3: 283-296.Boscaro, V.; Carducci, D.; Barbieri, G.; Senra, M.V.X.; Andreoli, I.; Erra, F.; Petroni, G.; Verni, F.; Fokin, S.I. (2014). “Focusing on genera to improve species identification: revised systematics of the ciliate Spirostomum”. Protist 165: 527-541. This bacterivore genus mainly lives in the sediment deposits at the bottom of various aquatic habitats, and members possess rquA genes that could be responsible for their ability to survive in these hypoxic and anoxic environments.Shazib, S.U.A.; Vd’acny, P.; Slovak, M.; Gentekaki, E.; Shin, M.K. (2019). “Deciphering phylogenetic relationships and delimiting species boundaries using a Bayesian coalescent approach in protists: A case study of the ciliate genus Spirostomum (Ciliophora, Heterotrichea)”. Scientific Reports 9: 16360. Mukhtar, I.; Wu, S.; Wei, S.; Chen, R.; Cheng, Y.; Liang, C.; Chen, J. (2021). “Transcriptome profiling revealed multiple rquA genes in the species of Spirostomum (Protozoa: Ciliophora: Heterotrichea)”. Frontiers of Microbiology 11: 1-15. Hines, H.N.; Onsbring, H.; Ettema, E.J.G.; Esteban, G.F. (2018). “Molecular investigation of the ciliate Spirostomum semivirescens, with first transcriptome and new geographical records”. Protist 169: 875-886. Spencer, M.; Warren, P.H. (1996). “The effects of habitat size and productivity on food web structure in small aquatic microcosms”. Oikos 75: 419-430. They are identifiable by their relatively (to other ciliates) large tubular/flat vermiform bodies. Their life cycle consists of a growth stage, in which they mature, and asexual and sexual reproduction stages. Some species are model organisms for studies on human pathogenic bacteria, while others are sensitive and accurate bioindicators for toxic substances.
Etymology The etymology, origin and meaning of the genus name, has not been discussed in literature thus far. Although the meaning can be inferred, a translation is not provided here so the possibility of incorrect interpretations is minimal. However, it is known that the genus name Spirostomum refers to the spiral contraction ability that these ciliates possess. The presence of post-ciliary fibers at the anterior end of the organism allows for this type of contraction, resulting in a counterclockwise spiral movement.
History of knowledge The genus Spirostomum was first mentioned by German researcher Christian Gottfried Ehrenberg in 1834 and then referred to later in his 1838 publication Die Infusionsthierchen als vollkommene Organismen, a collection of more than 350 species. At that time, the genus did not have a definition or description. In 1841, in his book Histoire naturelle des Zoophytes—Infusoires, Félix Dujardin, provided a description of the genus, but only recognized Spirostomum ambiguum as a species.
Around this time, another researcher, Friedrich Stein, played a key role in the classification of ciliated protists. He proposed dividing the phylum into four orders: Holotricha, Hypotricha, Petritricha, and Heterotricha (the order to which Spirostomum belongs). In Stein’s summary monograph of the order Heterotricha, Der Organismus der Infusionsthiere (1867), he provided a detailed account and illustration for Spirostomum, as well as a description of the nomenclature. He recognized two species: S. ambiguum (Ehrenberg) and S. teres, identified by Edouard Claparède and Johannes Lachmann in their 1858 publication Étude sur les Infusoires et les Rhizopodes.
Maximilian Perty’s classification of the species S. semivirescens in his 1852 publication Zur Kenntniss kleinster Lebensformen, was dismissed by Stein, who claimed that it was simply a variation of the species Spirostomum ambiguum.
In agreement with Stein, Eugène Penard also recognized S. ambiguum and S. teres as species of the genus, with the addition of S. filum, described in his book Études sur les Infusoires d’eau douce (1922). This species had previously been described as Uroleptus filum by Ehrenberg but was considered as a member of the genus Spirostomum by researchers Claparède and Lachmann, as well as Otto Bütschli in his 1889 publication Protozoa. Part III. Infusoria. This had been a speculative placement as these researchers had relied on Ehrenberg’s illustrated figures to draw their conclusion. Penard, having had the opportunity to observe the live organism like Ehrenberg, felt justified in his classification of the S. filum being a species in the genus.
In 1974, in their article “The systematics of the genus Spirostomum Ehrenberg, 1838”, Repak and Isquith recognized nine Spirostomum species, but S. inflatum and S. loxodes were removed due to their ambiguous cell shape. They were never mentioned again in other articles. S. intermedium was synonymized with S. minus by Foissner et al. in 1992, and Vittorio Boscaro et al. argued in their 2014 article that S. ephrussi should also be synonymized with S. teres. There are currently eight accepted morphospecies in the genus: S. ambiguum, S. minus, S. teres, S. yagiui, S. dharwarensis, S. semivirescens, S. subtilis, S. caudatum.
Habitat and ecology Organisms from the genus Spirostomum are commonly found worldwide in diverse aquatic habitats such as fresh or brackish (low salinity) water environments in the nutrient-rich sediment deposits located at the bottom of the water bodies. Due to debris accumulation, biofilm, and low oxygen transport, these deposits result in microaerophilic or anaerobic conditions so the genus experiences hypoxia and even anoxia daily. However, due to evidence of the presence of the rhodoquinol dependent fumarate reduction pathway, an anaerobic pathway, these organisms have the capability of respiring under oxygen-depleted and anoxic conditions.
They are considered large ciliate bacterivores and are also capable of feeding on microflagellates; they are among the top consumers of prokaryotic microbes in biofilms. In a study conducted by Matthew Spencer and Philip H. Warren (1996), Spirostomum ambiguum was found to be more plentiful in high-energy systems in comparison to low-energy systems. Due to their relatively large size, it was found that they do not deal with predation by other species present in the culture in the lab setup. However, this genus is prey for bigger and more complex organisms sharing their habitat. They are placed near the top of the functional food chain amongst predatory protozoa and microcrustacea.
Description
Morphology and anatomy Spirostomum is a medium to large worm-like (vermiform) elongated ciliate, ranging from around 150 μm to 4 mm and is large enough to be confused with small helminths. It moves by gliding slowly over or swimming freely above the substrate. It possesses tubular or somewhat flattened bodies, with colours ranging from a faint brown or yellow, to green. The organism is bound externally by a thin layer of ectoplasm while the endoplasm consists of large vacuoles separated by a mesh of fluid protoplasm. Cells are often hosts of prokaryotic and eukaryotic symbionts. The oral region is surprisingly long, and is bounded by a long adoral zone of membranelles on the left side. A paroral kinety is located to the right of the AZM, thickened significantly at its proximal end. The genus also has 10-50 somatic kineties (depending on the species), distributed on the cell surface, that are positioned parallel to the main body axis. However, upon contraction, these kineties form clockwise spirals, and the cell takes on an ellipsoid shape. There are also 1-6 rows of packed cortical granules in between each pair of kineties.
When the organism is left undisturbed on a slide, it builds a protective layer of mucilaginous lorica that accumulates sediment particles from the debris located around the specimen as it moves. During filter feeding, the anterior end of the organism would protrude out of the completed lorica.Esteban, G.F.; Bradley, M.W.; Finlay, B.J. (2009). “A case-building Spirostomum (Ciliophora, Heterotrichida) with zoochlorellae”. European Journal of Protistology 45: 156-158.
A single contractile vacuole is positioned at the posterior end of the organism, attached to a collecting canal that extends towards the anterior end. When filled with fluid, the vacuole takes up a substantial portion of the posterior end. The distinction between the contractile vacuole and the rest of the cytoplasm is usually very clear.
Species can possess either a moniliform or single (in ellipsoid, filiform, or elongated form) macronucleus. This is not dependent on the life stages, but rather that some species have a certain type while others have the other. They also have a variable number of macronuclei that are around 1-3 μm in size. Spirostomum’s macronucleus, possessing a double membrane with perforations, is the first record of a nucleus being highly extensile. The macronucleus can initially measure at 18 μm and then be extended to 72 μm (fifteen times longer).Seshachar, B.R. (1958). “The macronucleus of Spirostomum”. Nature 182: 1614-1615. The number of micronuclei vary, but they are either ovoid or ellipsoid in shape and are usually located near the macronuclei. For “single” type macronuclei, there are depressions for the micronuclei, thus resulting in overlapping.
Characteristics of the eight morphospecies are outlined in Table 1, which provides a brief overview of specific characteristics. Two species, S. caudatum and S. semivirescens, have missing information in the table due to the limited amount of research completed. Spirostomum caudatum is characterized mainly by its long thin tail, while Spirostomum semivirescens is identified by the presence of zoochlorellae (endosymbiotic algae) in its cytoplasm, providing its bright green colour. Esteban et al. suggested that it is possible that S. semivirescens was able to survive in hypoxic environments due to the oxygenic photosynthesis performed by the endosymbiotic algae. Spirostomum yagiui has an elongated macronuclei that circle through three unique states. This differentiates it from Spirostomum dharwarensis that has a distinctive filiform macronucleus that is long and slender relative to the other single-type macronuclei. Spirostomum subtilis have the highest length:width ratios, which sets it apart from its counterparts. It also has a unique cortical granule pattern that consists of a single CG row.Chen, X.; Kim, J.H.; Shazib, S.U.A.; Kwon, C.B.; Shin, M.K. (2017). “Morphology and molecular phylogeny of three heterotrichid species (Ciliophora, Heterotrichea), including a new species of Anigsteinia”. European Journal of Protistology 61: 278–293.