cercozoans articles - Encyclopedia of Life

Cercozoa

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Cercozoa is a phylum of diverse single-celled eukaryotes.^[3]^[4] They lack shared morphological characteristics at the microscopic level,^[5] and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin.^[6] They were the first major eukaryotic group to be recognized mainly through molecular phylogenies.^[7] They are the natural predators of many species of bacteria. They are closely related to the phylum Retaria, comprising amoeboids that usually have complex shells, and together form a supergroup called Rhizaria.^[8]

Characteristics

The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in many other protozoa. They show a variety of forms^[9] and have proven difficult to define in terms of structural characteristics, although their unity is strongly supported by phylogenetic studies.

Diversity

Some cercozoans are grouped by whether they are "filose" or "reticulose" in the behavior of their cytoskeleton when moving:^[10]

Filose, meaning their pseudopods develop as filopodia. For example:
- Euglyphids, filose amoebae with shells of siliceous scales or plates, which are commonly found in soils, nutrient-rich waters, and on aquatic plants.
- Gromia, a shelled amoeba.
- Tectofilosids, filose amoebae that produce organic shells.
- Cercomonads, common soil-dwelling amoeboflagellates.
Reticulose, meaning they form a reticulating net of pseudopods. For example:
- Chlorarachniophytes, set apart by the presence of chloroplasts bound by four membranes and still possess a vestigial nucleus, called a nucleomorph. As such, they have been of great interest to researchers studying the endosymbiotic origins of organelles.

Other important ecological groups are:

Granofilosea, comprising several groups traditionally considered heliozoa such as Heliomonadida, Desmothoracida and Gymnosphaerida.^[10]
Phaeodaria, marine protozoa previously considered radiolarians.^[11]
Phytomyxea, parasites of mostly plants, including the fungi-like plasmodiophores.
Ascetosporea, parasites of mostly marine invertebrates.

Ecology

As well as being highly diverse in morphology and physiology, Cercozoa also shows high ecological diversity.^[12] The phylum Cercozoa includes many of the most abundant and ecologically significant protozoa in soil, marine and freshwater ecosystems.^[7]

Soil-dwelling cercozoans are one of the dominant groups of free-living eukaryotic microorganisms found in temperate soils, accounting for around 30% of identifiable protozoan DNA in arid or semi-arid soils and 15% in more humid soils. In transcriptomic analyses they account for 40-60% of all identifiable protozoan RNA found in forest and grassland soils. They also comprise 9-24% of all operational taxonomic units found in the ocean floor.^[12]

Some cercozoa are coprophilic or coprozoic, meaning they use feces as a source of nutrients or as transport through animal hosts. The faecal habitat is an understudied reservoir of microbial eukaryotic diversity, dominated by amoeboflagellates from the phylum Cercozoa. Strongly coprophilic examples of cercozoa are the flagellates Cercomonas, Proleptomonas and Helkesimastix, and the sorocarpic amoeba Guttulinopsis. Many new cercozoan lineages, especially among sarcomonads, have been discovered through phylogenetic sampling of feces because they appear preferentially in this medium.^[13]

Cercozoan bacterivores (i.e. predators of bacteria) are highly diverse and important in the plant phyllosphere, the leaf surfaces of plants. Particularly sarcomonads, with their ability to cyst, feed and multiply within hours, are perfectly adapted to the fluctuating environmental factors in the phyllosphere. Their predation causes shifts in the bacterial communities: they reduce populations of alphaproteobacteria and betaproteobacteria, which are less resistant to their grazing, in favour of other bacterial populations such as gammaproteobacteria.^[14]

Phylogeny

Paraphyletic Cercozoa^[8] Rhizaria Filosa

(=Cercozoa) Retaria

Ectoreta

Monophyletic Cercozoa^[15] Rhizaria Cercozoa Filosa

Reticulofilosa

Monadofilosa

Endomyxa

Lapot gusevi

Retaria

Foraminifera

Polycystinea

Acantharea

The initial molecular phylogenetic analyses of Cercozoa, based on ribosomal RNA and tubulins, recognized two subphyla, Endomyxa and Filosa, and showed a close relationship with phylum Retaria.^[10]^[16]

However, the monophyly of the group was still uncertain. Posterior phylogenomic analyses consistently recovered Cercozoa as a paraphyletic group, and Endomyxa was often clustered with Retaria.^[17]^[18]^[19] As a result, the current taxonomy of Rhizaria places Endomyxa inside the phylum Retaria instead of Cercozoa, which has therefore become synonymous with Filosa.^[8]

Despite the taxonomic change of Endomyxa into phylum Retaria, thanks to better phylogenomic sampling a 2019 analysis recovered phylum Cercozoa as a monophyletic group, with Endomyxa being the sister group to Filosa. In the same analysis, Endomyxa, Filosa, Reticulofilosa and Monadofilosa are proven to be monophyletic too.^[15]

In addition to Endomyxa and Filosa, a variety of clades inside Cercozoa have been discovered in other analyses and have slowly been described and named, such as Tremulida (previously known as Novel Clade 11)^[16] and Aquavolonida (Novel Clade 10),^[20] although their specific positions among the two main cercozoan subphyla have yet to be refined.

Classification

The classification of Cercozoa as revised in 2018,^[8] with the addition of Endomyxa:^[15]

Phylum Cercozoa Cavalier-Smith 1998 emend. 2018
Clade Filosa
Subphylum Reticulofilosa Cavaler-Smith 1997
Class Chlorarachnea Hibberd & Norris 1984 (as Chlorarachniophyceae)
Class Granofilosea Cavalier-Smith & Bass 2009
Class Skiomonadea Cavalier-Smith 2012
Subphylum Monadofilosa Cavalier-Smith 1997
Superclass Eoglissa Cavalier-Smith 2011 emend. 2018
Class Metromonadea Cavalier-Smith 2007
Class Helkesea Cavalier-Smith 2018
Superclass Ventrifilosa Cavalier-Smith 2012 emend. 2018
Class Sarcomonadea Cavalier-Smith 1993 stat. nov. 1995 emend. 2018
Subclass Paracercomonada Cavalier-Smith 2018
Subclass Pediglissa Cavalier-Smith 2018
Class Imbricatea Cavalier-Smith 2003 emend. 2018
Subclass Placonuda Cavalier-Smith 2012
Subclass Placoperla Cavalier-Smith 2012
Subclass Krakenia Cavalier-Smith 2018
Class Thecofilosea Cavalier-Smith 2003 emend. 2012
Subclass Ventricleftia Cavalier-Smith 2018
Subclass Eothecia Cavalier-Smith 2012
Subclass Phaeodaria Haeckel 1879
Subclass Tectosia Cavalier-Smith 2012
Clade Endomyxa
Superclass Marimyxia Cavalier-Smith 2018
Class Gromiidea Cavalier-Smith 2003 emend. 2018
Class Ascetosporea Sprague 1979 stat. nov. Cavalier-Smith 2002
Superclass Proteomyxia Lankester 1885 ex Cavalier-Smith 2018
Class Vampyrellidea Cavalier-Smith 2018
Class Phytomyxea Engler & Prantl 1897