Centrohelid
Encyclopedia
The centrohelids or centroheliozoa are a large group of heliozoa
n protist
s. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.
A few genera have no cell covering, but most have a gelatinous coat holding scales and spines, produced in special deposition vesicles. These may be organic or siliceous and come in various shapes and sizes. For instance, in Raphidiophrys
the coat extends along the bases of the axopods, covering them with curved spicules that give them a pine-treeish look, and in Raphidiocystis there are both short cup-shaped spicules and long tubular spicules that are only a little shorter than the axopods. Some other common genera include Heterophrys, Actinocystis, and Oxnerella.
The axopods of centrohelids are supported by microtubule
s in a triangular-hexagonal array, which arise from a tripartite granule called the centroplast at the center of the cell. Axopods with a similar array occur in gymnosphaerid
s, which have traditionally been considered centrohelids (though sometimes in a separate order from the others). This was questioned when it was found they have mitochondria
with tubular crista
e, as do other heliozoa, while in centrohelids the cristae are flat. Although this is no longer considered a very reliable character, on balance gymnosphaerids seem to be a separate group.
occur among centrohelids, and genetic studies have been more or less inconclusive. Cavalier-Smith has suggested they may be related to the Rhizaria
, 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 cryptophyte
s and haptophyte
s (see Cryptomonads-haptophytes assemblage). They are currently classified as Hacrobia
, under the Plants+HC clade
.
Heliozoa
Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...
n protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.
Characteristics
Individuals are unicellular and spherical, usually around 30-80 μm in diameter, and covered with long radial axopods, narrow cellular projections that capture food and allow mobile forms to move about.A few genera have no cell covering, but most have a gelatinous coat holding scales and spines, produced in special deposition vesicles. These may be organic or siliceous and come in various shapes and sizes. For instance, in Raphidiophrys
Raphidiophrys
Raphidiophrys is a genus of centrohelid. R. intermedia is found in the bottom sludge of freshwater bodies in Canada, Chile, Argentina, Australia, New Zealand, Malaysia, Russia, and central Europe....
the coat extends along the bases of the axopods, covering them with curved spicules that give them a pine-treeish look, and in Raphidiocystis there are both short cup-shaped spicules and long tubular spicules that are only a little shorter than the axopods. Some other common genera include Heterophrys, Actinocystis, and Oxnerella.
The axopods of centrohelids are supported by microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s in a triangular-hexagonal array, which arise from a tripartite granule called the centroplast at the center of the cell. Axopods with a similar array occur in gymnosphaerid
Gymnosphaerid
The gymnosphaerids are a small group of heliozoan protists found in marine environments. They tend to be roughly spherical with radially directed axopods, supported by microtubules in a triangular-hexagonal array arising from an amorphous central granule.-Genera:There are only three genera, each...
s, which have traditionally been considered centrohelids (though sometimes in a separate order from the others). This was questioned when it was found they have mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
with tubular crista
Crista
Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...
e, as do other heliozoa, while in centrohelids the cristae are flat. Although this is no longer considered a very reliable character, on balance gymnosphaerids seem to be a separate group.
Classification
The evolutionary position of the centrohelids is not clear. Structural comparisons with other groups are difficult, in part because no flagellaFlagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
occur among centrohelids, and genetic studies have been more or less inconclusive. Cavalier-Smith has suggested they may be related to the Rhizaria
Rhizaria
The Rhizaria are a species-rich supergroup of unicellular eukaryotes. This supergroup was proposed by Cavalier-Smith in 2002. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods...
, 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 cryptophyte
Cryptophyte
Cryptophyte may refer to:*a plant which survives the unfavorable season underground or underwater in the Raunkiær plant life-form classification...
s and haptophyte
Haptophyte
The haptophytes, classified either as the Prymnesiophyta or Haptophyta, are a division of algae.The term "Haptophyceae" is sometimes used. This ending implies classification at a lower level...
s (see Cryptomonads-haptophytes assemblage). They are currently classified as Hacrobia
Hacrobia
thumb|upright|The [[coccolithophore]] [[Gephyrocapsa]] oceanicaThe Cryptomonads-haptophytes assemblage is a monophyletic grouping of chromalveolata that are not included in the SAR supergroup.The term "Hacrobia" has been proposed for this group...
, under the Plants+HC clade
Plants+HC clade
Plants+HC clade is a proposed group of eukaryotes.It includes:* Plants* Hacrobia...
.