Thalassiosira

Thalassiosira is a genus of centric diatoms, comprising over 100 marine and freshwater species. It is a diverse group of photosynthetic eukaryotes that make up a vital part of marine and freshwater ecosystems, in which they are key primary producers and essential for carbon cycling [1]

Thalassiosira
Temporal range: 11.608–0.0 Ma
Fossil species of Thalassiosira
Scientific classification
Domain:
Eukaryota
(unranked):
SAR
Superphylum:
Heterokonta
Class:
Coscinodiscophyceae
Order:
Thalassiosirales
Family:
Thalassiosiraceae
Genus:
Thalassiosira

P.T. Cleve, 1873 emend. Hasle, 1973
Species

See text

Thalassiosira is a diverse genus, however one species within the genus, T. pseudonana, has gained particular significance as the first marine phytoplankton to have its genome sequenced. T. pseudonana has since become a key model organism for studying diatom physiology. The T. pseudonana genome revealed novel genes for intracellular trafficking and metabolism in diatoms [2]. This species was again used to develop methods for genetic manipulation of diatoms [3] and for the study of silica biomineralization [4].

History of Knowledge

Thalassiosira was first described in 1873 by P.T. Cleve [5]. The genus was then characterized with transmission electron microscopy in the 1950s and scanning electron microscopy in the 1960s, which led to an enhanced understanding of the defining morphological features of the genus and the subsequent recognition of over 100 species [6]

A species of Thalassiosira, T. pseudonana, was selected as the first marine eukaryotic phytoplankton to undergo whole genome sequencing, due to its small 34Mb genome. As a result, T. pseudonana has served as a model organism for understanding diatom biology. Whole genome sequencing, transcriptomics and proteomics of T. pseudonana have revealed novel pathways for silicon biogenesis, phosphorus stress response, intracellular transport and metabolism in marine diatoms [2] [7] [8]. Recent genetic studies using ribosomal RNA gene sequences have also been able to better distinguish and classify Thalassiosira species [1].

Habitat and Ecology

Thalassiosira occupy diverse habitats, both marine and freshwater. Of note, they are a vital primary producers in temperate and polar regions [1]. Thalassiosira can thrive in low temperature and light, as well as mixed waters, and are therefore a large part of diatom blooms during spring in temperate regions, such as Canadian and Alaskan waters [9]. Species in this genus are also capable of assembling defensive threads against zooplankton, allowing them to survive the predation that normally keeps phytoplankton blooms in check.

Thalassiosira species are diverse in both their ecology and physiologies, with variable mechanisms for nitrogen storage or requirements for iron. Iron concentrations, temperature and macronutrient availability have been identified as important factors for the composition of Thalassiosira species communities in marine waters [1].

Description of Organism

Thalassiosira come in a variety of shapes, from box-shaped to cylindrical, discoid or spherical [10]. Some Thalassiosira cells are found alone while others form chains. Thalassiosira have a cell wall made of silica, known as a frustule [5]. Thalasiosira harbor several discoid plastids and a circular valve, which contains pores arranged in rows or arcs, opening outwards. The valve’s mantle edge is pattered with a series of bands. Different species of Thalassiosira can be identified by the morphological characteristics of their areolae and the processes on the valve [9]. During colony formation, Thalassiosira release chitin filaments through strutted processes known as fultoportulae. By extruding chitin fibers, and thereby increasing drag, Thalassiosira can slow the rate at which they sink [2].

Genetics

Phylogenetic studies based on 18S rRNA genes have revealed several clades within Thalassiosira, an overall paraphyletic group within the family Thalassiosirales [11]. The relationships of species within the clades remain to be elucidated.

The sequence of the T. pseudonana genome revealed a host of features which set apart diatoms from other eukaryotes. For instance, heat shock transcription factors make up the majority of transcription factors in the T. pseudonana genome, though they are less common in other eukaryotes [2]. Also setting it apart from other eukaryotic genomes is the relative absence of receptor kinases and G protein-coupled receptors. More specific to diatom biology, the understanding of silicon biochemistry in diatoms was enhanced by the discovery of genes involved in the uptake of silicic acid and proteins involved in vesicles for silica precipitation.

A surprising finding from the T. pseudonana genome was the presence of genes encoding enzymes for a complete urea cycle, which was unprecedented in a photosynthetic eukaryote [2]. Diatoms are known to utilize a urease enzyme to catalyze the breakdown of urea and were therefore not expected to need mechanisms to excrete it as waste. Curiously, the T. pseudonana urea cycle feeds into other metabolic pathways, which contribute to protein biosynthesis and possibly energy storage.

Life cycle

Thalassiosira can undergo both asexual and sexual reproduction in processes shared by other diatoms [12]. During asexual reproduction, the parent cell divides into two daughter cells of unequal size — one equal in size to the parent and one smaller. This constraint on size during mitotic division is due to the presence of the rigid silica cell wall. As a result, over multiple cell divisions, the cells size of each daughter cell will decrease. To cope with diminishing cell size, Thalassiosira can transition to sexual reproduction, which is triggered by an assortment of environmental factors, which are not well understood, once cells reach a critically small size [13]. In sexual reproduction, sperm and egg, which can arise from the same cell, fuse to form a diploid zygote, which is referred to as an auxospore. This progeny can then emerge from the parental frustule (silica wall) and reconstruct its own cell wall, thus becoming a cell of a larger size.

Fossil History

The geological record of Thalassiosira dates back to 13.82 million years ago [10]. The fossil record of both freshwater and marine Thalassiosira organisms is substantial, though the freshwater collection has been characterized more extensively [14].

Practical Importance

T. pseudonana has been particularly useful for molecular studies due to its small genome size. It has revealed novel pathways for silica biosynthesis, which involve uptake of monosilicic acid from the marine environment, intracellular transport to a specialized vesicle, and specialized enzymes and peptides driving silica synthesis [4].

Silica biosynthesis, for which the species T. pseudonana has come to serve as a model organism for study, has been of particular interest in the context of engineering silica nanotechnology [2]. For instance, a modified T. pseudonana has been explored as a drug-delivery vesicle in cancer treatment [15].

Thalassiosira, among other diatoms, have also been considered as potentially useful sources of lipids for biofuels. Specifically, T. weissflogii has been shown to reduce its silica synthesis while upregulating the production of triacylglycerols when cultured in nitrogen-limiting conditions [16].

Scientific classification

Chromista, Harosa, Heterokonta, Ochrophyta, Bacillariophyceae, Coscinodiscophycidae, Thalassiosiranae, Thalassiosirales, Thalassiosiraceae, Thalassiosira [17]

List of Species

Thalassiosira aculeata Proshkina-Lavrenko Thalassiosira aestivalis Gran Thalassiosira allenii H.Takano Thalassiosira ambigua O.G.Kozlova Thalassiosira andamanica A.D.Gedde Thalassiosira angulata (W.Gregory) Hasle Thalassiosira angustelineata (A.W.F.Schmidt) G.Fryxell & Hasle Thalassiosira antarctica Comber Thalassiosira australiensis (Grunow) G.R.Hasle Thalassiosira australis M.Peragallo Thalassiosira azovica Gogorev & Kovaleva Thalassiosira baldaufii P.Bodén Thalassiosira baltica (Grunow) Ostenfeld Thalassiosira binata Fryxell Thalassiosira bipora M.Shiono Thalassiosira bradburyi Khursevich & Kociolek Thalassiosira bramaputrae (Ehrenberg) Håkansson & Locker Thalassiosira brunii F.Akiba & Y.Yanagisawa Thalassiosira bulbosa E.E.Syvertsen Thalassiosira burckliana H.J.Schrader Thalassiosira californica Y.Tanimura Thalassiosira caspica I.V.Makarova Thalassiosira castanea F.Akiba & Y.Yanagisawa Thalassiosira catharinensis García Thalassiosira cedarkeyensis A.K.S.K.Prasad Thalassiosira centra M.Shiono Thalassiosira complicata R.Gersonde Thalassiosira concava H.J.Semina Thalassiosira condensata Cleve Thalassiosira conferta Hasle Thalassiosira convexa V.V.Muchina Thalassiosira coronifera Proshkina-Lavrenko Thalassiosira cryophila Kisselev Thalassiosira cuitzeonensis I.Israde, K.Serieyssol & F.Gasse Thalassiosira curviseriata Takano Thalassiosira decipiens (Grunow ex Van Heurck) E.G.Jørgensen Thalassiosira delicatula Ostenfeld Thalassiosira densannula Hasle & Fryxell Thalassiosira depressa M.Shiono Thalassiosira depressa Shiono Thalassiosira dichotomica (O.G.Kozlova) G.A.Fryxell & G.R.Hasle Thalassiosira diporocyclus Hasle Thalassiosira dolmatovae Oreshkina ex A.Gladenkov Thalassiosira dubia Leuduger-Fortmorel Thalassiosira dubiosa H.J.Schrader Thalassiosira duostra C.Pienaar Thalassiosira eccentrica (Ehrenberg) Cleve Thalassiosira echinata H.J.Semina Thalassiosira elliptipora J.Fenner ex A.D.Mahood & J.A.Barron Thalassiosira elsayedii Fryxell Thalassiosira exceptiuncula M.Shiono Thalassiosira exceptiuncula Shiono Thalassiosira exigua G.Fryxell & Hasle Thalassiosira fasciculata D.M.Harwood & T.Maruyama Thalassiosira faurii (Gasse) Hasle Thalassiosira feibelii Brindle & J.Mohan Thalassiosira ferelineata Hasle & G.A.Fryxell Thalassiosira flexuosa (J.Brun) F.Akiba & Y.Yanagisawa Thalassiosira fraga H.J.Schrader Thalassiosira fragilis G.Fryxell Thalassiosira frenguelliopsis G.A.Fryxell & J.R.Johans Thalassiosira fryxelliae Sunesen & Sar Thalassiosira gasseae Brindle & J.Mohan Thalassiosira gerloffii Rivera Ramirez Thalassiosira gersondei J.A.Barron Thalassiosira gessneri Hustedt Thalassiosira gracilis (Karsten) Hustedt Thalassiosira gravida Cleve Thalassiosira grunowii F.Akiba & Y.Yanagisawa Thalassiosira hasleae Cassie & Dempsey Thalassiosira hendeyi Hasle & G.Fryxell Thalassiosira hibernalis A.M.Gayoso Thalassiosira hispida Syvertsen Thalassiosira hyalina (Grunow) Gran Thalassiosira hydra A.M.Gombos Thalassiosira hyperborea (Grunow) Hasle Thalassiosira ignota Makarova Thalassiosira incerta I.V.Makarova Thalassiosira insigna (A.P.Jousé) D.M.Harwood & T.Maruyama Thalassiosira intrannula Herzig & Fryxell Thalassiosira inura R.Gersonde Thalassiosira iraidae Khursevich & Kociolek Thalassiosira irregulata H.J.Schrader Thalassiosira jouseae F.Akiba Thalassiosira kanayae Y.Tanimura Thalassiosira khursevichiae D.Bennett, Laslandes, & Kociolek Thalassiosira kushirensis H.Takano Thalassiosira labimarginata P.Bodén ''Thalassiosira laevis Y.H.Gao & Z.D.Cheng Thalassiosira latimarginata Makarova Thalassiosira lentiginosa (Janisch) Fryxell Thalassiosira leptopus (Grunow) Hasle & G.Fryxell Thalassiosira levanderi Van Goor Thalassiosira licea G.Fryxell Thalassiosira lineata Jousé Thalassiosira lineoides Herzig & Fryxell Thalassiosira livingstoniorum A.K.S.K.Prasad, P.Hargraves & J.A.Nienow Thalassiosira longifultoportulata R.M.Gogorev & Z.V.Pushina Thalassiosira lundiana Fryxell Thalassiosira lusca H.J.Schrader Thalassiosira maculata G.A.Fryxell & J.R.Johansen Thalassiosira mahoodii J.A.Barron Thalassiosira makarovae V.V.Mukhina Thalassiosira mala Takano Thalassiosira manifesta Sheshukova-Poretskaya Thalassiosira mediaconvexa H.J.Schrader Thalassiosira mediterranea (Schröder) Hasle Thalassiosira mendiolana Hasle & Heimdal Thalassiosira minicosmica Lee & Park Thalassiosira minima Gaarder Thalassiosira minuscula Krasske Thalassiosira miocenica H.J.Schrader Thalassiosira mioplicata F.Akiba & Y.Yanagisawa Thalassiosira multipora M.C.Whiting & H.Schrader Thalassiosira nana Lohmann Thalassiosira nanolineata (A.Mann) Fryxell & Hasle Thalassiosira nativa Sheshukova-Poretzkaya Thalassiosira nodulolineata (Hendey) Hasle & Fryxell Thalassiosira nordenskioeldii Cleve Thalassiosira oceanica Hasle Thalassiosira oliveriana (O'Meara) I.V.Makarova & V.A.Nikolajev Thalassiosira oranica G.R.Hasle Thalassiosira ordinaria I.V.Makarova Thalassiosira ornata Proshkina-Lavrenko Thalassiosira pacifica Gran & Angst Thalassiosira partheneia Schrader Thalassiosira parva Proshkina-Lavrenko Thalassiosira parvula Makarova Thalassiosira perispinosa Y.Tanimura Thalassiosira plicata H.J.Schrader Thalassiosira poroirregulata Hasle & Heimdal Thalassiosira praeconvexa Burckle Thalassiosira praefraga Gladenkov & Barron Thalassiosira praegracilis Gogorev & Pushina Thalassiosira praenidulus F.Akiba Thalassiosira primalabiata A.M.Gombos Thalassiosira profunda (Hendey) Hasle Thalassiosira pseudonana Hasle & Heimdal Thalassiosira punctifera (Grunow) Fryxell, Simonsen & Hasle Thalassiosira punctigera (Castracane) Hasle Thalassiosira rudolfii (Bachmann) Hasle Thalassiosira sackettii Fryxell Thalassiosira salvadoriana Hustedt Thalassiosira sancettae F.Akiba Thalassiosira scotia Fryxell & Hoban Thalassiosira scrotiformis G.W.Chen & S.B.Qian Thalassiosira simonsenii Hasle & G.Fryxell Thalassiosira simplex Hustedt Thalassiosira sinica Y.Li. & Y.Q.Guo Thalassiosira spinoconvexa F.Akiba & Y.Yanagisawa Thalassiosira strelnikovae I.V.Makarova Thalassiosira striata D.M.Harwood & T.Maruyama Thalassiosira subsalina Proshkina-Lavrenko Thalassiosira subtilis (Ostenfeld) Gran Thalassiosira sundarbana Samanta & Bhadury Thalassiosira symbolophora H.J.Schrader Thalassiosira symmetrica G.A.Fryxell & Hasle Thalassiosira tealata Takano Thalassiosira tenera Proschkina-Lavrenko Thalassiosira tertiaria Sheshukova Thalassiosira thailandica Boonyapiwat Thalassiosira torokina Brady Thalassiosira transitoria Y.Tanimura Thalassiosira tubifera Fryxell Thalassiosira tumida (Janisch) Hasle Thalassiosira umaoiensis F.Akiba Thalassiosira urahoroensis F.Akiba Thalassiosira variabilis I.V.Makarova Thalassiosira visurgis Hustedt Thalassiosira vulnifica (Gombos) Fenner Thalassiosira webbii D.M.Harwood & T.Maruyama Thalassiosira wongii Mahood Thalassiosira yabei (T.Kanaya) F.Akiba & Y.Yanagisawa Thalassiosira yunnaniana Khursevich & Kociolek


References
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  • The influence of aqueous iron chemistry on the uptake of iron by the coastal diatom Thalassiosira weissflogii. MA Anderson, FMM Morel, Limnology and Oceanography, 1982


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