[6][7][1] 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.

[6] Some species are model organisms for studies on human pathogenic bacteria, while others are sensitive and accurate bioindicators for toxic substances.

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.

[5] The genus Spirostomum was first mentioned by German researcher Christian Gottfried Ehrenberg in 1834[6] and then referred to later in his 1838 publication Die Infusionsthierchen als vollkommene Organismen, a collection of more than 350 species.

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.

[6][7] 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.

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.

[5][1][2] 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.

[3][4] 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.

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.

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.

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.

[5][1][2] 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.

Spirostomum’s macronucleus, possessing a double membrane with perforations, is the first record of a nucleus being highly extensile.

Two species, S. caudatum and S. semivirescens, have missing information in the table due to the limited amount of research completed.

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.

[1][4][9][11] Table 1 A summary of characteristics of the eight true morphospecies belong to the genus adapted from the information provided Boscaro et al.

[12] The following descriptions of the reproduction processes are based on observations made of Spirostomum ambiguum, the most investigated species in the genus.

Asexual division begins with the formation of the peristomal membranelle, AZM, of the new cytostome, indicated by the slight ridge in the posterior end of the specimen which eventually becomes more pronounced.

Mukhtar et al. (2021) suggested that the rhodoquinol dependent pathway that had been previously reported in multiple heterotrichs could be responsible for the ability to survive in these conditions.

Based on the analysis of the transcriptome, two or three RquA proteins were found in the species, providing evidence of the existence of the rhodoquinol dependent fumarate reduction pathway in these organisms.

Nalecz-Jawecki et al. (1993) determined that a duration of one hour was sufficient for results when estimating the toxicity of water polluted by heavy metals such as silver, copper, and mercury.

Tushmalova et al. formed a cost-effective bioassay that was also sensitive, simple, and easily performed using S. teres to provide a reasonable and affordable method of risk assessment.

As such, it is shown through multiple studies that Spirostomum’s ability to react to low concentrations of such physical and chemical stressors and factors make them suitable candidates for water quality indicators.