Cyclotella is a genus of diatoms often found in oligotrophic environments, both marine and fresh water. It is in the family Stephanodiscaceae and the order Thalassiosirales. The genus was first discovered in the mid-1800s and since then has become an umbrella genus for over 100 different species, the most well-studied and the best known being Cyclotella meneghiniana. Despite being among the most dominant genera in low-productivity environments, it is relatively understudied.Saros, J.E., Anderson, N.J. (2015). The ecology of the planktonic diatom Cyclotella and its implications for global environmental change studies. Biol Rev Camb Philos Soc. 90(2). 522-41.
Cyclotella habitat has traditionally been described as low-productivity mesotrophic or oligotrophic freshwater environments. However, it is unclear whether there is an archetypal aquatic setting for this genus as some species are more cosmopolitan, such as C. meneghiniana, which occurs in warm, nutrient-rich environments as well as low-productivity environments.
Etymology The name Cyclotella is derived from the Greek term kyklos, meaning "circle." While "circle" can be used to describe many diatoms, Cyclotella spp. are all circular and have a girdle band arrangement that makes the structure of the organism resemble a wheel.Brébisson, [L.] A. de (1838). Considérations sur les diatomées et essai d'une classification des genres et des espèces appartenant à cette famille, par A. de Brébisson, auteur de la Flore de Normandie, etc. pp. [i], -20, [4, err.]. Falaise & Paris: Brée l'Ainée Imprimeur-Libraire; Meilhac.
History The genus Cyclotella was described in 1838 by Louis Alphonse de Brébisson, a French botanist and photographer.Håkansson H. (2002). A compilation and evaluation of species in the genera Stephanodiscus, Cyclostephanos and Cyclotella with a new genus in the family Stephanodiscaceae. Diatom Research. 17(1): 1-139. Brébisson shares the credit of discovering the genus with Friedrich Traugott Kützing, a German pharmacist, botanist, and phycologist. This is in spite of the fact that neither one of these scientists ever worked together or even came in contact with one another. In 1849, Kützing published a comprehensive work describing 6000 different algae species, including the most known species of Cyclotella today, C. meneghiniana.Kützing, F.T. (1844). Die kieselschaligen Bacillarien oder Diatomeen. Nordhausen. 30. 1-152
Brébisson describes in the 1838 publication Flore de Normandie, Cyclotella "has a more or less elongated ovoid shape, it is swollen from both sides, and when its center is diaphanous, it resembles two tubular frustules united by their vertices ( translated from French )." Upon distinguishing Cyclotella from other diatom species, there have been over 100 different species of the genus described and taxonomically accepted.
Recent study has separated multiple genera from within Cyclotella, including Cyclostephanos, Discostella and Lindavia. Currently, there are 129 recognized species of Cyclotella and 72 species with uncertain status, but currently placed in Cyclotella. Cyclotella may still be an unnatural group (i.e. polyphyletic or paraphyletic).
Habitat and ecology Species of Cyclotella are most often found in oligotrophic (nutrient poor) environments. They are most often found in freshwater environments, but can also be found in brackish and marine habitats as well. Many of the freshwater species have been found throughout the United States in stagnant waters.Hasle, G.R., and E.E. Syvertsen. (1997). Marine Diatoms. In: Tomas, C.R. (Ed.) Identifying Marine Phytoplankton. Academic Press. Species that are most commonly found in marine environments are C. caspia, C. litoralis, C. meneghiniana, C. striata, and C. stylorwn.
In a study performed in 1974, it was determined that the optimal osmolar concentration for growth in C. meneghiniana in a medium of 0.5 Osm/L.Schobert, B. (1974). The influence of water stress on the metabolism of diatoms I. Osmotic resistance and proline accumulation in Cyclotella meneghiniana. Zeitschrift für Pflanzenphysiologie. 74(2). 106-120. For references, the osmolarity of seawater is on average, 1 Osm/L. Marine diatoms and algae in general tend to flourish in higher osmolar concentrations due to the increased presence of carbon dioxide and nutrients to be utilized as sustenance, but the low-solute environment Schobert found to be most optimal for the growth of C. meneghiana is consistent with most Cyclotella being found in low-productivity mesotrophic to oligotrophic environments. Species of cyclotella have been found in harsh aquatic environments such as coldwater regions in northern regions of the world.
Another study by Van de Vijver and Dessein found a new species of Cyclotella, C. deceusteriana, in the sub-antarctic region.Van de Vijver, Bart & Dessein, Steven. (2018). Cyclotella deceusteriana, a new centric diatom species (Bacillariophyta) from the sub-Antarctic Region. Phytotaxa. 333(1). One of the only ecological characteristics of Cyclotella that is consistent among most of its species is the fact that they are found in stagnant or near-stagnant waters and are immobile. Beyond that, there is a great deal of variation. Many of the Cyclotella species that have been studied have been shown to be found in aquatic environments that are either slightly or highly alkaline. C. distinguenda is known to prefer alkaline waters, and C. gamma has been found in lakes that have a pH range of 7.2 to 7.8. Nutrient concentration in the habitats of Cyclotella spp. varies. C. sensulato has been described as a dominant member of both mesotrophic and oligotrophic environments, as many are, but both C. atomus and C. meneghiniana are found to prefer nutrient-rich environments. Temperature ranges vary between species as well; it was mentioned earlier that C. deceusteriana was discovered in sub-antarctic regions, and C. gamma and C.quillensis have been found in the Northern United States and Saskatchewan, respectively. C. atomus, on the other hand, has been found in warmer lake sediments in California. Colonization patterns of Cyclotella spp. are relatively uniform, in the sense that most of them are solitary organisms. C. meneghiniana, however, has been described to occasionally live in colonies.Lowe, R.L. (1975). Comparative ultrastructure of the valves of some Cyclotella species (Bacillariophyceae) Journal of Phycology. 11(4): 415-424. Of course, the preference of nutrient rich environments of C. meneghiniana conflicts the findings mentioned earlier.
Morphology The size of Cyclotella varies by species. C. atomus has a diameter of 5-7 μm, whereas C. quillensis can have a diameter up to 24-54 μm.Bailey, L.W. (1922). Diatoms from the Quill Lakes, Saskatchewan, and from Airdrie, Alberta.Contributions to Canadian Biology 11(1): 157-165. The most studied species of the genus, C. meneghiniana, has a diameter of 6-18 μm. Like all other diatoms, Cyclotella spp. have transparent cell walls. They form biosilica shells using dissolved silicon and carbon acquired from various carbon partitioning pathways.
Other materials Cyclotella spp. use for cell wall biosynthesis are semiconductor metal oxides and extracellular fibers made of chitin. The primary allomorph of chitin that is found most often in diatoms is α-chitin, but Cyclotella and Thalassiosira contain the β-chitin allomorph. Poly N-acetyl glucosamine chains are oriented in a parallel manner and contain intermolecular hydrogen bonds.
The bond chains and hydrogen bonds between molecules form a paracrystalline matrix of β-chitin. This matrix contains pores large enough for whatever solvent is available in the aquatic ecosystem in which Cyclotella spp. reside in to enter the matrix and swell the structure.LeDuff, P., & Rorrer, G. L. (2019). Formation of extracellular β-chitin nanofibers during batch cultivation of marine diatom Cyclotella sp. at silicon limitation. Journal of Applied Phycology, 31(6), 3479–3490.
Diatoms are unique in the sense that they have valves, created by the two halves of a diatom's test. Cyclotella spp. are no exception, as they form the upper and lower portions of the wall. The girdle bands that support the valves are thin strips of silica and ultimately circumscribe the cell. Each valve has two central tubes traversing its surface, meeting in the middle at the central nodule. The morphology of the Cyclotella cell wall and its valves are important traits that distinguish species from each other. Each species has tangentially undulated valves all throughout their cell wall, regardless of their length, width, and concentration.Tesson, B., Hildebrand, M. (2010). Dynamics of silica cell wall morphogenesis in the diatom Cyclotella cryptica: Substructure formation and the role of microfilaments. Journal of Structural Biology. 169(1). 62-74. Frustules contain areolas, that is orifices that mediate the passage of nutrients and exudates across the cell wall for sustenance. The characteristics of these areolas are thought to cause differences in mechanical strength and metabolism among different cells.Shirokawa, Y., Shimada, M. (2016). Cytoplasmic inheritance of parent–offspring cell structure in the clonal diatom Cyclotella meneghiniana. Proceedings of the Royal Society B. 283(1842).
Like other monoraphid diatoms, Cyclotella frustules can be characterized as heterovalvar. The cell wall and cell membrane are what are known to this point as what distinguishes Cyclotella from other diatom genera. The cytoplasmic components are assumed to be similar to what other diatoms have. In C. meneghiniana, there are granules scattered and attached at the chromatophore all throughout the cytoplasm. The genus is photosynthetic like all other diatoms, so all species contain one or many pyrenoids traversed by a thylakoid membrane and a chloroplast within the endoplasmic reticulum.
Dictyosomes are also present in the cytoplasm, being in close proximity to the nucleus and making up the golgi complex. The nucleus has been found to change locations in C. meneghiniana throughout generations as a result of the cell diameter gradually decreasing.Hoops, H.J., Floyd, G.L. (1979). Ultrastructure of the centric diatom, Cyclotella meneghiniana: vegetative cell and auxospore development. Phycologia. 18(4). 424-435.
Life cycle Cyclotella meneghiniana divide into two daughter cells during asexual reproduction. The halves are separated by the distinction between the two valves for each cell. Each of the two offspring that arise as a result of cell division have one of the two valves from the parent cell. During the separation of the parent cell, the cytoplasm forms the two offspring valves that will end up complementing the inherited parent valves in the offspring once reproduction is complete.