| Neobodo | |
|---|---|
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| Neobodo designis (A) Schematic drawing; (B) Photo, where the arrow indicates the apicalflagellar pocket. Scale of 5 µm | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Discoba |
| Phylum: | Euglenozoa |
| Class: | Kinetoplastea |
| Order: | Neobodonida |
| Family: | Neobodonidae |
| Genus: | Neobodo Vickerman, 2004 |
| Species | |
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| Synonyms[1] | |
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Neobodo are diverseprotists belonging to the eukaryotic supergroupExcavata. They areKinetoplastids in the subclassBodonidae. They are small, free-living,heterotrophic flagellates with twoflagella of unequal length used to create a propulsive current for feeding.[3] As members of Kinetoplastids, they have an evidentkinetoplast[4] There was much confusion and debate within the class Kinetoplastid and subclass Bodonidae regarding the classification of the organism, but finally the new generaNeobodo was proposed byKeith Vickerman.[5] Although they are one of the most common flagellates found in freshwater, they are also able to tolerate saltwater[6] Their ability to alternate between both marine and freshwater environments in many parts of the world give them a “cosmopolitan” character.[6] Due to their relatively microscopic size ranging between 4–12 microns, they are further distinguished as heterotrophic nanoflagellates.[3] This small size ratio limits them as bacterivores that swim around feeding on bacteria attached to surfaces or in aggregates.[3]
The prefix ‘Neo-’ comes from the ancient Greek word for ‘neos’ which signifies 'young'. Attaching the prefix to the original bodonid species,neobodo literally means a “new” bodonid species.[5]
The order Neobodonida was proposed by a researcher, Keith Vickerman, based on significant characteristics that differed from the original bodonid species.[5] Differing characteristics included: beingphagotrophic, Polykinetoplastic/eukinetoplastic, biflagellate with usually both flagella lacking hairs, having a posterior flagellum attached to the body or free of it, and having anapical cytostome.[5] ManyNeobodo species derived fromBodo species, and by recognizing these differences, they were tentatively assigned to the new genusNeobodo by adding the ‘neo’ prefix.[5] Through studies on the ultrastructure ofBodo designis, researchers discovered the possession of a ‘microtubular prism’ supporting the cytostome–cytopharynx, as well as a significantly different feeding apparatus from other bodonids, thus proposing the new species asNeobodo designis.[5] Through this discovery, they were proposed as the type species of the new genusNeobodo.[5]Neobodo have very close connections with Kinetoplastid protists. Kinetoplastid protists belong together witheuglenids anddiplonemids, to the phylumEuglenozoa, and are grouped in the class Kinetoplastea.[5] The name of kinetoplastid is derived from the presence of a characteristic structure called the kinetoplast which is a mass of concentrated extranuclear DNA within a mitochondrion.[5] In the past, kinetoplastids were classified into two major suborder groups via morphology-based taxonomic criteria: either as parasitic uniflagellatetrypanosomatids, or biflagellate bodonids.[5] Originally, Vickerman proposed two families,Bodonidae and Cryptobiidae, but later on re-unified all bodonids within the single family, Bodonidae.[5] Based on comparisons of RNA sequences and molecular phylogenetic analyses, it was suggested that the trypanosomatids also emerged from within the bodonids.[5] Moreover, recent research of deep-sea hydrothermal vent samples at theMid-Atlantic Ridge and analysis viaPCR amplification reported several new kinetoplastid-like sequences.[5] Researchers David Moreira, Purificacion Lopez-Garcıa, and Keith Vickerman analyzed the phylogeny of these kinetoplastids and found a much more stable phylogeny that supported themonophyly of groups that typically emerged aspolyphyletic in the trees rooted using the traditional, distant outgroup sequences.[5] As a result, the classification of the class Kinetoplastea was divided as two new subclasses:
Through this process,Neobodo was created as a new genus, along with the revision of the classification of species formerly included in the genusBodo and the amendment of the genusParabodo.[5]
The new genusNeobodo is characterized as solitaryphagotrophic flagellates with a single discrete eukinetoplast. They are known for having anapical cytostome andcytopharynx supported by a prismatic rod ofmicrotubules.[5]
Neobodo cells are usually elongate andelliptical in shape and somewhat inflexible.[4] They range from 4 to 12 microns long, but are mostly 6 to 9 microns.[4] They have a nucleus near the middle of the cell and two unequal, heterodynamic flagella emerging from a shallow,subapical pocket.[4] The anterior flagellum appears inactive and just wraps around the anterior part of the cell. It is about the same length or slightly shorter than the cell.[4] It is held forward with a single anterior curve that is held perpendicular to the substrate and curves back over therostrum.[4] The acronematic posterior flagellum is trailed and sometimes forms an undulating membrane.[4] It is typically directed straight behind the cell and is about 2 to 4 times the length of the cell.[4] Theproximal part of the posterior flagellum is accompanied with a paraxial rod and sometimes non-tubularmastigonemes.[5] The cells use their posterior flagellum and rotate around theirlongitudinal axes to swim and glide along in rapid darts of straight lines.[7]
Along with their two flagella, they have two nearly parallelbasal bodies.[4] They also house discoid shapedmitochondrial cristae and a compact kinetoplast (a DNA-containing granule located within a single mitochondrion) that is associated with the flagellar bases.[4] The kinetoplasts are naked, but thecytoskeletal microtubules beneath thecell membrane are developed.[4] They have acytoplasm usually filled withsymbiotic bacteria and smallglycosomes that possessglycolyticenzymes.[4] Althoughsexual reproduction is unknown andcysts have not been found to date, they are able to reproduceasexually by means ofbinary fission.[4]
Bodonid flagellates (class Kinetoplastea) are abundant, free-livingbacterivores that occur in a wide variety of environments including freshwater, soil and marine habitats ranging from thetropics to theArctic.[6] Neobodo is one of the most common flagellates in freshwater environments, but can also tolerate marine environments with low salinities of 3–4 ppt.[4] Strains ofNeobodo species isolated from different environments fall exclusively into marine and freshwater lineages.[6] Studies show thatNeobodo is a complex and ancient species with a major marine clade nested among older freshwater clades.[7] This suggests that these lineages were constrained physiologically from moving between these environments for most of their long history.[7] Their broad physiological tolerance enables them to easily interchange between marine and freshwater environments, which gives them a cosmopolitan characteristic and a wide ecological tolerance.[6] Recent evidence forNeobodo designis suggested notable divergence between freshwater and marine strains and all strains exhibited extensivegenetic diversity.[7]Epifluorescent microscopy studies reported the abundance of several heterotrophic nanoflagellate groups (including bodonids) in theeuphotic zone of different marine areas.[3] Areas include theMediterranean Sea,Norwegian Sea, theIndian Ocean and around theAntarctic Peninsula.[3] Throughout the numerous oceans, large fractions of small heterotrophic flagellates with few morphological features remain unidentified.[3] Therefore there is a high possibility that there are many bodonids among the unidentified that have not yet been studied.[3]
AlthoughNeobodo are surface organisms, typically found in surface waters, studies have shown their ability to tolerate deep water conditions.[6] Due toadvection or attachment to sinking particles,microbes from the surface of the ocean are continuously transported to deeper areas.[6] The vast majority of the marine environment consists of dark, cold, high-pressure environments, which increases with depth.[6] When cultures ofNeobodo were isolated from surface waters and were put in different deep-sea temperatures and pressures, the abundance of protists declined in all treatments, with a significantly greater rate of mortality under combined cold temperature and high pressure conditions than in the cold temperature-only conditions.[6] However, an average of 6.1% ofN. designis cells survived in the high pressure treatments, indicating that some fraction of sinking protists can survive transport to the deep ocean.[6] In addition, after a period of acclimation, positive growth rates were measured in some cases.[6] This suggests that surface-adapted flagellates can not only survive under deep-sea conditions but are able to reproduce and potentially provide seed populations in cold, high-pressure environments.[6] AlthoughNeobodo are not abundant in the deep oceans, they are capable of surviving in the deep waters, tolerating high pressure and low temperature conditions.[6]
Neobodo are free-living and active microbial predators that swim around and feed on prey in aquatic ecosystems.[7] As free-living flagellates, they are the most important bacterivorous forms in aquatic environments.[4]Neobodo, like other bodonids, are heterotrophic flagellates (HF) which are a very diverse and heterogeneous group of protists with a size range between 1 and 450 microns.[3] They play an essential role in aquatic and terrestrial food webs as major consumers of bacterial biomass.[3] The predator to prey size ratio limits the maximal size difference between bacteria and their predator:Neobodo.[3] The marine environment presents additional constraints, imposed by the typical small size and low abundance of bacteria.[3] In these conditions, physical andhydrodynamic considerations theoretically restrict Neobodo’s feeding to graze on small bacteria, typically within thenanoplankton.[3] Most bacterivorous protists in the marinepelagic zone are generally in the size range of 2–5 microns and are classified as a functional group called heterotrophic nanoflagellates.[3] The predominance of heterotrophic nanoflagellates as marine bacterivores has been confirmed by manipulations with size-fractionated natural assemblages and by direct observation of protists with ingested fluorescent bacteria.[3] More specifically,Neobodo are interception feeders, meaning they feed on bacteria attached to surfaces/biofilms or in aggregates. They press their mouth against food and are often aided by apseudopod-like structure (pharynx) to detach bacteria.[3] Within this feeding mechanism, further variability in terms of feeding behavior and selection strategies can be observed among different species.[3]
Despite theecological andevolutionary significance of these organisms, many of their biological andpathological features are currently unknown. Throughmetatranscriptomics usingRNA-seq technology combined withfield-emission microscopy thevirulence factors of a recently described genus of Neobodonida that is considered to be responsible for Ascidian Soft Tunic Syndrome (AsSTS) was revealed.[8] AsSTS is a disease of the edibleascidian,Halocynthia roretzi, which has done enormous damage to the Korean and Japaneseaquaculture.[8] AsSTS is characterized by changes in the tunic (the outermost barrier against the environment), includingelasticity loss and subsequent rupture leading to thinner bundled tunicfibers and coarser tunicmatrices.[8] However, thepathogenesis is unclear and is still an area of research.[8]
Despite the considerable interest in free-living bodonids, their truebiodiversity has most likely been grossly underestimated by simplelight microscopy, as it does not differentiate most ‘species’ very well.[7]rRNA gene primers were used to testNeobodo’s globaldistribution andgenetic diversity.[7] The non-overlap betweenenvironmental DNA sequences and those from cultures suggests that there are hundreds, possibly thousands, of different rRNA gene sequences of free-livingNeobodo species globally.[7] Some of the species identified to date are: