Centrohelid

The centrohelids or centroheliozoa are a large group of heliozoan protists.^[5] They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.

Centrohelids are heliozoans or "sun animalcules", a type of single-celled organisms that have axopodia, narrow stiff projections radiating from the cell and granting a sun ray appearance.^[6] Their cells are spherical, ranging in size from 3 μm (as in Choanocystis minima) to 150 μm (in Acanthocystis turfacea).^[7] Unlike other heliozoa, centrohelids lack flagella.^[8][9] They are also distinguished by flat, ribbon-shaped mitochondrial cristae, and a unique organelle known as the microtubule-organizing center (MTOC) or microtubule-nucleating center (MNC), from which the axopodia are generated.^[10] The MTOC of centrohelids, known as the centroplast, is located at the center of the cell and has a unique inner differentiation of a central disc sandwiched between two dense caps, each around 0.1–1.5 μm in diameter. All other heliozoans lack this differentiation.^[6]

Some centrohelids are naked (like Oxnerella), but most are covered in siliceous or organic scales.^[6] Most centrohelids usually have siliceous scales with a specific shape that helps identify the species.^[11]

Centrohelids feed on bacteria, other protists, and even larvae of invertebrates, through phagocytosis. They are considered passive predators, capturing prey as they pass by.^[6] Experiments studying the feeding behavior of centrohelids are rare, but they have been observed consuming environmentally impactful strains of cyanobacteria, such as Microcystis aeruginosa and Aphanizomenon. This predation is interesting due to its potential to regulate harmful algal blooms caused by such cyanobacteria.^[11]

Centrohelids are free-living predatory protists with a ubiquitous distribution.^[10] They are found abundantly in global freshwater environments, and also occur widely in marine and soil habitats, where they are comparatively understudied.^[8] Despite their ubiquity, little is known about their biogeography.^[7] Most reports of centrohelids are done in temperate zones due to insufficient studies in tropical regions. Within temperate regions, the species diversity of freshwater centrohelids appears to be influenced by the type of body of water: highest diversity occurs in terrace forest lakes, while Sphagnum peat bogs have the lowest diversty.^[12] According to environmental DNA analyses, soil-dwelling centrohelids are twice as diverse as their freshwater counterparts, and ten times more than marine ones.^[7][13]

Like other heliozoans, most known species are found in aquatic benthic environments, where they prey on a variety of other microbes. Some float in the water column, while others attach to substrates by a stalk. Free-floating (planktonic) forms are well known, but the ecological niche of centrohelids is considered to be the benthos, inhabiting the superficial layer of detritus and interstitial spaces.^[6]

The centrohelids are an evolutionary lineage or clade of protists. They are the closest relatives of the haptophyte algae,^[12] together forming the clade Haptista.^[14] Before molecular phylogenetics, they were grouped with other axopodial protists in the polyphyletic taxon Heliozoa. Over the 1990s and 2000s decade, smaller heliozoan groups were removed from this artificial taxon and into their true evolutionary lineages—actinophryids to Stramenopiles, desmothoracids to Cercozoa, and Sticholonche with the Radiolaria.^[15] The centrohelids, the largest heliozoan group, remained difficult to resolve. Earlier analyses using the 18S rRNA gene weakly placed them with haptophytes;^[16] this was later confirmed with maximal support in larger-scale phylogenomic analyses in the 2010s.^[14]

Traits present in haptophytes (specifically Prymnesiophyceae) are inferred as the ancestral state of centrohelids. Both have an outer coat of complex mineralized dimorphic scales arranged in two successive layers:^[16] calcareous in haptophytes, siliceous in centrohelids. Another common feature is the presence of a thin microtubule-based appendage used for feeding: the axopodia in centrohelids, and the haptonema in haptophytes.^[1]

The evolutionary position of the centrohelids is not clear. Structural comparisons with other groups are difficult, in part because no flagella occur among centrohelids, and genetic studies have been more or less inconclusive. Cavalier-Smith has suggested they may be related to the Rhizaria,^[17] but for the most part they are left with uncertain relations to other groups. A 2009 paper suggests that they may be related to the cryptophytes and haptophytes (see Cryptomonads-haptophytes assemblage).^[18] They are currently classified as Hacrobia, under the Plants+HC clade, although some research studies have found evidence against the monophyly of this group.^[19] Centrohelids were previously divided into two orders with contrasting scale morphology and ultrastructure: Pterocystida and Acanthocystida.^[8] Posterior molecular studies of 2018 have rearranged the classification of centrohelids into two taxa: Pterocystida and Panacanthocystida, which includes both Acanthocystida and the family Yogsothothidae.^[2][1] These two taxa are considered superorders, each with two orders.^[3]

The modern classification of centrohelids divides them into two superorders, four orders, and 18 genera.^[2][1][3]

• Superorder Pterocystida Cavalier‐Smith and von der Heyden 2007, emend. Shɨshkin and Zlatogursky 2018
  • Order Raphidista Shɨshkin & Zlatogursky 2018
    • Family Choanocystidae Cavalier-Smith & von der Heyden 2007
      • Choanocystis Penard 1904 non Cognetti 1918
    • Family Raphidiophryidae Mikrjukov 1996 emend. Cavalier-Smith & von der Heyden 2007
      • Raphidiophrys Archer 1867 [Raphidiaphrys (sic) Greeff 1869]
  • Order Pterista Shɨshkin & Zlatogursky 2018
    • Family Oxnerellidae Cavalier-Smith & Chao 2012
      • Oxnerella Dobell 1917
    • Family Pterocystidae Cavalier-Smith & von der Heyden 2007
      • Chlamydaster Rainer 1968
      • Pseudoraphidocystis Mikrjukov 1997 [Pseudoraphidiocystis (sic)]
      • Pseudoraphidiophrys Mikrjukov 1997
      • Pterocystis Siemensma & Roijackers 1988 non Lohmann 1904
      • Raineriophrys Mikrjukov 2001 [Rainierophrys (sic); Raineria Mikrjukov 1999 non Osswald 1928 non de Notaris 1838; Echinocystis Mikrjukov1997 non Haeckel 1896 non Bhatia & Chatterjee 1925 non Torrey & Gray 1840 non Gregory 1897]
      • Triangulopteris Zagumyonnyi, Radaykina & Tikhonenkov 2021^[7]
    • Family Heterophryidae Poche 1913
      • Heterophrys Archer 1869
      • Parasphaerastrum Mikrjukov 1996
      • Sphaerastrum Greeff 1873
• Superorder Panacanthocystida Shɨshkin & Zlatogursky 2018
  • Order Chthonida Shɨshkin & Zlatogursky 2018
    • Suborder Yogsothothina Shɨshkin & Zlatogursky 2018
      • Family Yogsothothidae Shɨshkin & Zlatogursky 2018
        • Yogsothoth Shɨshkin & Zlatogursky 2018
  • Order Acanthocystida Cavalier-Smith 2011
    • Suborder Marophryina Cavalier-Smith 2011
      • Family Marophryidae Cavalier-Smith & von der Heyden 2007
        • Marophrys Cavalier-Smith & von der Heyden 2007
    • Suborder Chalarothoracina Hertwig & Lesser 1874 stat n. Cavalier-Smith 2011
      • Family Raphidocystidae Zlatogursky in Zlatogursky et al., 2018
        • Raphidocystis Penard 1904 [Raphidiocystis (sic) Doflein 1928; Rhaphidocystis (sic)]
      • Family Acanthocystidae Claus 1874 emend. Cavalier-Smith & von der Heyden 2007
        • Acanthocystis Carter 1863 non Kuehner 1926 non Bather 1889 non Haeckel 1896 nomen nudum

The following taxa are incertae sedis among centrohelids:

• Parasphaerastrum Mikrjukov 1996
• Heteroraphidiophrys Mikrjukov & Patterson 2000^[a]
• Family Spiculophryidae Shɨshkin & Zlatogursky 2018
  • Spiculophrys Zlatogursky 2015