The centrohelids or centroheliozoa are a group of heliozoan protists, single-celled eukaryotes with stiff radiating arms (known as axopodia) supported by microtubules and bearing extrusomes (known as kinetocysts). Their cells are spherical, ranging from 3 to 150 μm. Unlike other heliozoa, centrohelids lack flagella, have flat ribbon-shaped mitochondrial cristae, and arrange their microtubules in hexagons or triangles. Their microtubule-generating organelle, the centroplast, has a unique shape with a central trilamellar disc surrounded by two hemispherical caps. Some are naked or covered in a mucous coat, but most centrohelids produce cell coverings, namely organic spicules and siliceous scales of various species-specific shapes. Several species form colonies.
Centrohelids are passive predators with a cosmopolitan distribution. They feed on bacteria, other protists, and invertebrate larvae by phagocytosis; they can merge several cells around a larger prey to ingest it. Although they have been studied in aquatic (mostly freshwater) environments, they are more diverse in soil habitats. They include both free-floating and benthic forms, some of which attach to the substrate by a stalk.
Centrohelids are the closest relatives of the haptophyte algae, together forming the clade Haptista. Both groups have mineralized scales and at least one thin microtubule-based appendage. The common ancestor of centrohelids lost flagella and probably produced the two known types of cell coverings, organic spicules and complex siliceous scales. Some species can still produce both, while others secondarily lost or simplified them.
Centrohelids compose the class Centroplasthelida, in reference to their centroplast. Around 130 species have been described, but they only represent about 10% of the total estimated diversity, according to environmental DNA surveys. They are classified in 11 families grouped into two major clades, Pterocystida and Panacanthocystida.
Characteristics
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. organelles involved in the capture of prey, known as kinetocysts in centrohelids. and by several cellular features. One is their flat ('lamellate'), ribbon-shaped mitochondrial cristae.
thumb|upright=2|center|Diagram of a centrohelid showing the different cellular compartments
A few centrohelids are naked (like Oxnerella) or with a mucous coat. The majority are able to produce two kinds of cell coverings: organic spicules and siliceous scales, The specific shape of the siliceous scales of many centrohelids helps identify the species. Some centrohelids bear both scales and spicules, either simultaneously (as in Raphidiophrys heterophryoidea) or throughout their life cycles (like Triangulopteris lacunata). Some form colonies, with individual cells connected by cytoplasmic bridges, each cell with their own layer of spicules or scales. An exception is Yogsothoth: its colonies lack any bridges, and instead contain a roundish mass of densley packed cells surrounded by a thick outer layer of scales that differ from the inner plate scales covering each cell. multiple centrohelid cells have been observed fusing their cytoplasms to engulf a larger prey, resulting in a multinucleated cell. 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.
Ecology
Centrohelids are free-living predatory protists with a ubiquitous distribution. They are found abundantly in global freshwater environments, and also occur widely in marine and soil habitats, where they are comparatively understudied. Despite their ubiquity, little is known about their biogeography. According to environmental DNA analyses, soil-dwelling centrohelids are twice as diverse as their freshwater counterparts, and ten times more than marine ones.
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. telonemids may branch inside Haptista, or may be sister to SAR (as 'TSAR'). Groups included in the paraphyletic Hacrobia are marked *. Over the 1990s and 2000s decades, smaller heliozoan groups were removed from this artificial taxon and into their true evolutionary lineages, particularly Stramenopiles and Rhizaria. The centrohelids, the largest heliozoan group, remained difficult to resolve, and could not be placed within any other phyla. but phylogenetic trees were generally inconsistent. They also branched with telonemids and cryptomonads in a clade known as CCTH, and Hacrobia was found to be paraphyletic. An elusive microheliozoan, Microheliella maris, previously suggested to be a possible centrohelid relative, branches next to Cryptista (forming Pancryptista) instead.
Traits present in haptophytes (specifically Prymnesiophyceae) are inferred as the ancestral state of centrohelids. Both have an outer coat of complex mineralized scales: A similar earlier name Centrohelida, established by Kühn in 1926, is not preferred because it includes both centrohelids and gymnosphaerids.
History
Traditionally, the classification and species identification of centrohelids has been based upon the morphology of their cell coverings (spicules and scales). According to the main cell covering types, three families were distinguished before molecular phylogenetics: "Heterophryidae", either naked or covered in organic spicules, proposed as the most primitive family; "Raphidiophryidae", interpreted as more derived, with tangential siliceous plate-scales; and "Acanthocystidae", interpreted as the most derived, with the most complex coverings, including a double layer of siliceous scales. and the total number of described species has increased from around 85 species in 1999 Listed below are the accepted centrohelid genera, grouped into 11 families: Sphaerastrum.
- Superorder Panacanthocystida
- Order Chthonida , suborder Yogsothothina , family Yogsothothidae — Yogsothoth.
- Order Acanthocystida <small>Cavalier-Smith 2011</small>
- Suborder Ricksolina , family Ricksolidae — Ricksol.
- Suborder Chalarothoracina <small>Hertwig & Lesser 1874 emend. Cavalier-Smith in Yabuki et al. 2012</small>
- Family Raphidocystidae — Raphidocystis.
- Family Acanthocystidae <small>Claus 1874 emend. Cavalier-Smith & von der Heyden 2007</small> — Acanthocystis.
The following taxa have an uncertain status among centrohelids:
- Heterophrys and other spicule-bearing genera — The genus Heterophrys was established to group species covered in needle-like spicules. Spicule-bearing forms are scattered across the centrohelid tree, and some have been described within the life cycle of scale-bearing species.
- Choanocystis — The initial description of this genus did not report any feature indicative of centrohelids, except possibly external spine scales and internal plate scales (reported as "sticks and scales"), which he mentioned without specifying their precise shape. These were already the diagnostic features of Acanthocystis, a genus described earlier, and he did not provide a comparison of both. with no explanation for its identity, and its diagnostic feature (a cardioid-shaped basal plate of spine scales) also leaves its affinity uncertain, as it appears in several centrohelid clades. The organism known as "C. lepidula" is similar to a sequenced strain belonging to Panacanthocystida, but cannot be traced to the "Choanocystis" genus. Two additional species with cardioid-shaped basal plates, initially placed under "Choanocystis"