Rhizaria Definition Essay
Rhizaria is a very diverse supergroup in a genetic, morphological, and ecological way that unites phototrophic and heterotrophic flagellates, amoebae-flagellates, and amoebas. The name reflects the presence of the root-like filose and reticular pseudopods and/or axopodium. This supergroup’s representatives have a common feature in the form of bikonts with characteristic tubular mitochondrial cristae. A single root of a microtubular band is inherent to each centriole (Oren & Papke, 2011). This supergroup was allocated by Thomas Cavalier-Smith in 2002, basing only on the molecular data. Moreover, the connection between the majorities of its representatives is not sufficiently clear (Oren & Papke, 2011). However, the term Rhizaria has been completely and totally used by a scientific community over the last years, making a significant contribution to the further development of biological science. Thus, it is a large group of unicellular and colonial eukaryotes with unique morphological and ecological features.
In various embodiments of the classification, Rhizaria is considered as a domain or infrakingdom. In other cases, this group is not assigned to a specific taxonomic rank. Rhizaria is united in the SAR supergroup together with Stramenopiles and Alveolata. Two main groups are distinguished as a part of Rhizaria, i.e. Cercozoa and Retaria. Thus, Rhizaria consists of:
- Groups of the uncertain systematic position:
- Genuses: Actinolophus, Biomyxa, Cholamonas, and others (Oren & Papke, 2011).
The essence and main species of the Rhizaria supergroup can be seen on an example of the Cercozoa kingdom.
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The Cercozoa kingdom includes Chlorarachniophyte algae, Phytomyxea (Phytomyxea slime molds previously associated with mushrooms or Myxogasteromycetes), Ascetosporea, Thaumatomonadida (previously associated with Heterokonta), and Cercomonadidae (previously associated with Bodonidae), and Euglypha. There is a view that Foraminifera is a part of Cercozoa kingdom (Schaechter, 2011). General ultrastructural features of their representatives include tubular cristae in mitochondria and reticular (in a form of network) pseudopodia. A more structural analysis and general characteristics of Rhizaria representatives can be seen as an example of the Chlorarachniophyta division.
Chlorarachniophytes are amoeboid/flagellate eukaryotes that harbor the reduced green algal endosymbionts. Their general characteristics can be organized as follows:
- The flagellates’ stage has one smooth flagellum, spiraling around the cell during swimming.
- The chloroplasts contain chlorophyll a and h. Their nature is unknown.
- The chloroplasts are surrounded by four membranes: two chloroplasts’ own membranes and two membranes of chloroplast endoplasmic reticulum (second and third membranes may coalesce). A girdle lamella is absent.
- Periplasmic space (between the second and third chloroplast membrane) has a nucleomorph, which is surrounded by an envelope consisting of two membranes with pores. This nucleomorph contains the DNA, a nucleolus-like structure, eukaryotic ribosomes, and three small chromosomes. The nucleomorph is interpreted as a reduced core of endosymbiotic eukaryote.
- The chloroplasts have pyriform pyrenoids, which do not enter into the thylakoids.
- The principal storage carbohydrate is β-1,3 glucan (or probably paramylon), which is stored in cytoplasmic vesicles outside of the chloroplast.
- Mitochondrial cristae are tubular.
- Reproducing is represented by a simply division in half or a nonsexual way by means of zoospores. However, a couple of species has a sexual reproduction.
- They have trichocysts and inhabit the seas (Oren & Papke, 2011).
Chlorarachniophytes attract evolutionary biologists in terms of their origin and evolution of chloroplasts. These algae are a result of secondary endosymbiosis, when a non-photosynthetic eukaryote has captured eukaryotic alga. The latter one then evolved into the photosynthetic organelle (Schaechter, 2011). The confirmation of this provision can be considered the presence of the nucleomorph (reduced core of algal endosymbiont, which became chloroplast) in chlorarachniophytes and cryptomonads. These two groups of algae are a key to an understanding of how different groups of eukaryotes have acquired chloroplasts.
Chlorarachniophytes are a small group of marine mixotrophic flagellates and amoebae-flagellates. The division was first described 30 years ago by D. J. Hibberd and R. E. Norris, in 1984. Now, this division consists of five genuses (Chlorarachnion, Lotharelia, Gymnochlora, Cryptochlora, Bigelowiella) with six species. Several species have not yet received a name. Chlorarachniophytes’ births differ in ultrastructure of pyrenoids and location of nucleomorph (Oren & Papke, 2011).
Chlorarachniophytes’ cells are bare, single-core, with numerous chloroplasts. The four membranes, which surround chloroplasts, consist of the two membranes of the chloroplast envelope, a symbiont plasmalemma membrane, and the fourth membrane that comes from the membrane of the digestive vacuole of the host. The outer membrane of chloroplast is not linked to the outer membrane of the core. It is free of the ribosome. Such specie as Chlorarachnion reptans develops filopodias that merge in a number of underlying cells and form a reticulate plasmodium, i.e. meroplasmodium. It connects the bodies from several to hundreds of cells (Schaechter, 2011). Meroplasmodium is not formed in other Chlorarachniophytes.
The structure of the Chlorarachniophytes’ cells is of the particular interest due to the presence of nucleomorph, i.e. ancestral endosymbiont core. It is contained in a periplasmic space, which also has endosymbiont cytoplasm and eukaryotic ribosomes. Nucleomorph has its own genome (about 380 thousands of nucleotide pairs) with three linear chromosomes. Both ends of each chromosome end with telomeres and ribosomal RNA genes cistrons. It is estimated (Oren & Papke, 2011) that the nucleomorph genome encodes about 300 proteins. These genes are located in the genome very compactly, yet contain a number of very small introns. Almost all genes, which are encoded by the nucleomorph genome, maintain the nucleomorph itself. Only some of them encode chloroplast proteins.
Chlorarachniophytes have vegetative (division of the cell in half), asexual (zoospores), and sexual (iso- and anisogamy) reproduction. The life cycle may include amoeboid, coccoid, and monadic stages. For example, L. amoeboformis has all of these three stages, while one or two stages may be absent in other species. Coccoid stages are often regarded as cysts. Sexual reproduction is indicated for such species as Ch. reptans (amoeboid and coccoid cells merge with the formation of a zygote) and C. perforans (two amoeboid cells merge) (Hirt & Horner, 2004).
Chlorarachniophytes live in warm (tropical and temperate) waters, possibly worldwide. Amoeboid and coccoid representatives are found in coastal waters, while monadic forms live in the picoplankton of oceanic waters. Chlorarachniophytes are mixotrophy, thus, these photosynthetic organisms are able to absorb bacteria, flagellates, and eukaryotic algae.
Interconnection of Chlorarachniophytes with other organisms is very variable. All data, including the pigment composition and molecular phylogeny, indicate that the green algae were the Chlorarachniophytes’ endosymbiont, which has turned into a chloroplast. However, it is yet not clear what specific green algae has become the ancestors for the chloroplast. The composition of the xanthophyll pigments strongly suggests that this ancestor was probably some of the Prasinophyceae group. On the other hand, various molecular-phylogenetic analyzes (Hirt & Horner, 2004) have put forward the representatives of Ulvophyceae and Trebouxiophyceae green algae for this role. It especially makes sense due to the fact that most of Ulvophyceae inhabit the seas, which are the habitat for the Chlorarachniophytes.
Although Chlorarachniophytes’ chloroplasts contain chlorophyll a and b, these algae are phylogenetically separated from other eukaryotes that contain chlorophyll b (Euglenophytes and green algae) and a form independent of the phylum called Chlorarachniophyta. Cavalier-Smith put forward the theory that Chlorarachniophytes and Euglenophytes had evolved from a common photosynthetic ancestor, which acquired the chloroplast by a secondary endosymbiosis from a green alga (Hirt & Horner, 2004). However, a sequence analysis of a number of genes and the absence of any observed morphological and molecular relationships between Chlorarachniophytes and Euglenophytes suggests that these algae have acquired chloroplasts on an individual basis.
As for the origin of the Chlorarachniophytes’ host cell, then the molecular phylogenetic analysis (Hirt & Horner, 2004) has shown that they are closely related to a heterotrophic amoebae-flagellate line consisting of Cercomonadidae and Euglypha. However, morphological and ultrastructural data (Sapp, 2005) do not show a close connection of the host cell with a particular group of organisms. Phylogenetic relationships among Chlorarachniophytes were studied using karyogenes and SSU rRNA genes, encoded by nucleomorph (Sapp, 2005). Trees constructed by the results of this analysis (Sapp, 2005) have showed that Chlorarachniophytes is a monophyly group. However, the interrelation within the group is not entirely clear yet.
Rhizaria is a large group of unicellular and colonial eukaryotes which differs with the characteristic tubular cristae, root-like filose and other unique features. A group called Rhizaria was first proposed by the British biologist by Thomas Cavalier-Smith in 2002. Many representatives of this supergroup are characterized by the thin pseudopodia (a false foot) of different types: simple, branched or reticulate. Mitochondria have tubular cristae.
The origin of the eukaryotic microfossils and foraminiferans has not been resolved for years despite the numerous hypotheses. It seems that Rhizaria has brought together the oldest and the most diverse eukaryotic representatives of all times. One can only guess what evolutionary veils will be lifted for the further researches.