| Paratrimastix pyriformis | |
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| Light microscopy image of P. pyriformis | |
| Scientific classification | |
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| Order: | Paratrimastigida |
| Family: | Paratrimastigidae |
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| Species: | P. pyriformis |
| Binomial name | |
| Paratrimastix pyriformis | |
| Synonyms | |
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Paratrimastix pyriformis is a species of free-living (non-parasitic) anaerobic freshwater bacteriovorous flagellated protists formerly known as Trimastix pyriformis and Tetramitus pyriformis. [1]
This species was first described by G. A. Klebs in 1892 as Tetramitus pyriformis. [2] [3] Under this name, it has been frequently discussed in the context of sewage, sewage treatment, and water quality during the 20th century. [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] It was also observed on Elephant Island, South Shetland Islands. [15] More than 100 years after its description, in 1999, it was transferred to the genus Trimastix based on its morphology. [16] The first ultrastructural study using transmission electron microscopy was published the same year, which reported a discovery of hydrogenosome-like organelles in the species. [17]
A molecular phylogenetic study based on small-subunit ribosomal RNA placed the genus Trimastix (then including P. pyriformis) as sister to the oxymonad Pyrsonympha in 2001 [18] and a close relationship to oxymonads was further supported in another study in 2005. [19] The clade uniting Trimastix and oxymonads was named Preaxostyla in 2003. [20] A more detailed molecular phylogenetic analysis in 2015 placed this species in a new genus Paratrimastix , even more closely related to oxymonads than Trimastix . [21] Preaxostyla (consisting of Trimastix , Paratrimastix , and oxymonads) is now considered one of the five major lineages of Metamonada. [22] [23]
The interest in P. pyriformis, and especially its reduced mitochondria, was largely driven by the possibility that oxymonads might be completely amitochondrial. [24] This was supported by a genomic analysis of Monocercomonoides exilis published in 2016, which demonstrated that this oxymonad is the first known eukaryote that has completely lost its mitochondria. [25] [26] [27]
A series of transcriptomic studies between the years 2006 and 2016 reported details of P. pyriformis glycolytic pathway [28] and arginine deiminase pathway, [29] as well as supported the mitochondrial ancestry of its hydrogenosome-like organelles [30] and uncovered their role in amino acid metabolism. [31] Preliminary results of a genomic project led to the characterization of the unusual preaxostylan type iron-sulfur cluster assembly machinery in P. pyriformis in 2018, [32] the role of its reduced mitochondria in the methionine cycle (2022), [33] and the experimental characterization of one of its mitochondrial carriers (2023). [34] The complete genomic assembly of P. pyriformis was published in 2023 in a large-scale comparative genomic study focused on the reductive evolution of mitochondria in Preaxostyla, which also identified two additional oxymonad species with no traces of mitochondria. [35]
Paratrimastix pyriformis has four flagella, one directed anteriorly, one posteriorly, and others laterally. The posterior flagellum has two vanes with thickened vane margins. Both vanes have a paracrystalline substructure. Ventral side of the cell is shaped in the form of a broad groove, a typical excavate feature, which is used as a feeding structure. The cells measure 9-17 μm in length and 5-13 μm in width. The single nucleus with a conspicuous central nucleolus is located in the anterior third of the cell. [17]
Dense network of rough endoplasmic reticulum extends from the nucleus towards the posterior end of the cell. A single stacked Golgi apparatus is located posterior and to the left of the basal bodies. The kinetid consists of four basal bodies, four microtubular roots, and various microtubules and fibers associated with the basal bodies and roots. The arrangement of the basal bodies is asymmetrical. Rod-shaped mitochondrion-related organelles resembling hydrogenosomes are 0.5-1.0 μm in length and bounded by a double membrane. The mitochondrion-related organelles are dispersed throughout the cell. [17]
Paratrimastix pyriformis may be distinguished from the marine Trimastix marina and the freshwater Paratrimastix eleionoma [1] by the non-thickened and discretely subapically inserting anterior flagellum, [16] from Trimastix inaequalis by the equal length of its lateral flagella, and from Trimastix convexa (most similar species) by its smaller size and ultrastructural details of the cytoskeleton. [17]
Paratrimastix pyriformis swims with the anterior and lateral flagella beating and rotates occasionally. It can attach to the substrate by the tip of the posterior flagellum. Its cell contains food vacuoles with bacteria. A small contractile vacuole is located posteriorly. [16] Bacteria are captured at the posterior end of the ventral groove. Flagella are retained throughout cell division. Some, but not all, strains of P. pyriformis produce cysts: rounded cells with thin walls and basal bodies and flagella preserved. [17]
Glycolysis in P. pyriformis includes at least four alternative enzymes that have likely been gained by lateral gene transfer from Bacteria. [28] P. pyriformis further produces additional ATP using the extended glycolysis pathway where pyruvate generated in glycolysis is metabolised into acetate, CO2, and H2. Alternatively, pyruvate can be produced by decarboxylation of malate through the activity of the malic enzyme (ME). Pyruvate is decarboxylated to acetyl coenzyme A by pyruvate:ferredoxin oxidoreductase (PFO). The last part of the pathway, which yields ATP, acetate, and coenzyme A is catalyzed by a single enzyme: acetyl-CoA synthetase (ACS) like in the diplomonad Giardia intestinalis. Activities of both ME and PFO produce excess electrons which are then consumed in reduction of protons to molecular hydrogen through the activity of [FeFe] hydrogenases. [35]
Based on an in-silico reconstructed amino acid metabolism, P. pyriformis is able to synthesize at least five protein-forming amino acids including selenocysteine. [35] Unlike other Preaxostyla, P. pyriformis doesn't have a complete arginine deiminase pathway, [29] and therefore is likely unable to produce ATP via arginine catabolism. However, other amino acids (cysteine, serine, tryptophan, and methionine) can hypothetically be utilized to produce ATP by their conversion of pyruvate and α-keto-butyrate, which can enter the extended glycolytic pathway. [35] [36]
A mitochondrion is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used throughout the cell as a source of chemical energy. They were discovered by Albert von Kölliker in 1857 in the voluntary muscles of insects. The term mitochondrion was coined by Carl Benda in 1898. The mitochondrion is popularly nicknamed the "powerhouse of the cell", a phrase coined by Philip Siekevitz in a 1957 article of the same name.
Giardia is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates, causing the disease giardiasis. Their life cycle alternates between a swimming trophozoite and an infective, resistant cyst. Giardia were first described by the Dutch microscopist Antonie van Leeuwenhoek in 1681. The genus is named after French zoologist Alfred Mathieu Giard.
Excavata is an extensive and diverse but paraphyletic group of unicellular Eukaryota. The group was first suggested by Simpson and Patterson in 1999 and the name latinized and assigned a rank by Thomas Cavalier-Smith in 2002. It contains a variety of free-living and symbiotic protists, and includes some important parasites of humans such as Giardia and Trichomonas. Excavates were formerly considered to be included in the now obsolete Protista kingdom. They were distinguished from other lineages based on electron-microscopic information about how the cells are arranged. They are considered to be a basal flagellate lineage.
The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiotic relationship in animals. These include a variety of forms found in the intestines of termites and cockroaches, many of which have symbiotic bacteria that help them digest cellulose in woody plants. Other species within this supergroup are known parasites, and include human pathogens.
The metamonads are a large group of flagellate amitochondriate microscopic eukaryotes. Their composition is not entirely settled, but they include the retortamonads, diplomonads, and possibly the parabasalids and oxymonads as well. These four groups are all anaerobic, occurring mostly as symbiotes or parasites of animals, as is the case with Giardia lamblia which causes diarrhea in mammals.
A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP in anaerobic conditions.
The Oxymonads are a group of flagellated protists found exclusively in the intestines of animals, mostly termites and other wood-eating insects. Along with the similar parabasalid flagellates, they harbor the symbiotic bacteria that are responsible for breaking down cellulose. There is no evidence for presence of mitochondria in oxymonads and 3 species have been shown to completely lack any molecular markers of mitochondria.
A symbiotic eukaryote that lives in the hindgut of termites, Streblomastix is a protist associated with a community of ectosymbiotic bacteria.
Breviata anathema is a single-celled flagellate amoeboid eukaryote, previously studied under the name Mastigamoeba invertens. The cell lacks mitochondria, much like the pelobionts to which the species was previously assigned, but has remnant mitochondrial genes, and possesses an organelle believed to be a modified anaerobic mitochondrion, similar to the mitosomes and hydrogenosomes found in other eukaryotes that live in low-oxygen environments.
Trimastix is a genus of excavate protists, the sole occupant of the order Trimastigida. Trimastix are bacterivorous, free living and anaerobic. It was first observed in 1881 by William Kent. There are few known species, and the genus's role in the ecosystem is largely unknown. However, it is known that they generally live in marine environments within the tissues of decaying organisms to maintain an anoxic environment. Much interest in this group is related to its close association with other members of Preaxostyla. These organisms do not have classical mitochondria, and as such, much of the research involving these microbes is aimed at investigating the evolution of mitochondria.
Anaeromonadea, also known as Preaxostyla, is a class of excavate protists, comprising the oxymonads, Trimastix, and Paratrimastix. This group is studied as a model system for reductive evolution of mitochondria, because it includes both organisms with anaerobic mitochondrion-like organelles, and those that have completely lost their mitochondria.
Jakobids are an order of free-living, heterotrophic, flagellar eukaryotes in the supergroup Excavata. They are small, and can be found in aerobic and anaerobic environments. The order Jakobida, believed to be monophyletic, consists of only twenty species at present, and was classified as a group in 1993. There is ongoing research into the mitochondrial genomes of jakobids, which are unusually large and bacteria-like, evidence that jakobids may be important to the evolutionary history of eukaryotes.
Proteromonas is a genus of single-celled biflagellated microbial eukaryotes belonging to the Superphylum Stramenopiles which are characterized by the presence of tripartite, hair-like structures on the anteriorly-directed larger of the two flagella. Proteromonas on the other hand are notable by having tripartite hairs called somatonemes not on the flagella but on the posterior of the cell. Proteromonas are closely related to Karotomorpha and Blastocystis, which belong to the Opalines group.
Mastigamoeba is a genus of pelobionts, and treated by some as members of the Archamoebae group of protists. Mastigamoeba are characterized as anaerobic, amitochondriate organisms that are polymorphic. Their dominant life cycle stage is as an amoeboid flagellate. Species are typically free living, though endobiotic species have been described.
Monocercomonoides is a genus of flagellate Excavata belonging to the order Oxymonadida. It was established by Bernard V. Travis and was first described as those with "polymastiginid flagellates having three anterior flagella and a trailing one originating at a single basal granule located in front of the anteriorly positioned nucleus, and a more or less well-defined axostyle". It is the first eukaryotic genus to be found to completely lack mitochondria, and all hallmark proteins responsible for mitochondrial function. The genus also lacks any other mitochondria related organelles (MROs) such as hydrogenosomes or mitosomes. Data suggests that the absence of mitochondria is not an ancestral feature, but rather due to secondary loss. Monocercomonoides sp. was found to obtain energy through an enzymatic action of nutrients absorbed from the environment. The genus has replaced the iron-sulfur cluster assembly pathway with a cytosolic sulfur mobilization system, likely acquired by horizontal gene transfer from a eubacterium of a common ancestor of oxymonads. These organisms are significant because they undermine assumptions that eukaryotes must have mitochondria to properly function. The genome of Monocercomonoides exilis has approximately 82 million base pairs, with 18 152 predicted protein-coding genes.
Andrew J. Roger is a Canadian-Australian molecular biologist and evolutionary bioinformatician. He is currently a professor in the Department of Biochemistry and Molecular Biology at Dalhousie University and was the founding director of the inter-departmental Centre for Comparative Genomics and Evolutionary Bioinformatics (CGEB).
Stygiella /ˌstɪ.d͡ʒiˈɛ.lə/ is a genus of free-living marine flagellates belonging to the family Stygiellidae in the jakobids (excavata).
Anaeramoeba is a genus of anaerobic protists of uncertain phylogenetic position, first described in 2016.
Stygiellidae is a family of free-living marine flagellates belonging to the order Jakobida, a deep-branching lineage within the eukaryotic supergroup Discoba. They are unicellular organisms that commonly inhabit anoxic, sulfide-rich and ammonium-rich marine habitats worldwide.
Paratrimastix is a genus of free-living freshwater anaerobic excavate protists from the group Metamonada, that was segregated from the genus Trimastix in 2015. The best studied species is Paratrimastix pyriformis.