Acantharia | |
---|---|
Acantharea species | |
Scientific classification | |
Domain: | Eukaryota |
Clade: | Diaphoretickes |
Clade: | SAR |
Phylum: | Retaria |
Subphylum: | Radiolaria |
Class: | Acantharia Haeckel, 1881, emend. Mikrjukov, 2000 |
Order | |
Synonyms [1] | |
Acantharea |
The Acantharia are a group of radiolarian [2] protozoa, distinguished mainly by their strontium sulfate skeletons. Acantharians are heterotrophic marine microplankton that range in size from about 200 microns in diameter up to several millimeters. Some acantharians have photosynthetic endosymbionts and hence are considered mixotrophs.
Acantharian skeletons are composed of strontium sulfate, SrSO4, [3] in the form of mineral celestine crystal. Celestine is named for the delicate blue colour of its crystals, and is the heaviest mineral in the ocean. [4] The denseness of their celestite ensures acantharian shells function as mineral ballast, resulting in fast sedimentation to bathypelagic depths. High settling fluxes of acantharian cysts have been observed at times in the Iceland Basin and the Southern Ocean, as much as half of the total gravitational organic carbon flux. [5] [6] [4]
The strontium sulfate crystals [3] are secreted by vacuoles surrounding each spicule or spine. Acantharians are unique among marine organisms for their ability to biomineralize strontium sulfate as the main component of their skeletons. [7] However, unlike other radiolarians whose skeletons are made of silica, acantharian skeletons do not fossilize, primarily because strontium sulfate is very scarce in seawater and the crystals dissolve after the acantharians die. The arrangement of the spines is very precise, and is described by what is called the Müllerian law in terms of lines of latitude and longitude – the spines lie on the intersections between five of the former, symmetric about an equator, and eight of the latter, spaced uniformly. Each line of longitude carries either two tropical spines or one equatorial and two polar spines, in alternation.
The cell cytoplasm is divided into two regions: the endoplasm and the ectoplasm. The endoplasm, at the core of the cell, contains the main organelles, including many nuclei, and is delineated from the ectoplasm by a capsular wall made of a microfibril mesh. In symbiotic species, the algal symbionts are maintained in the endoplasm. [8] [9] [10] The ectoplasm consists of cytoplasmic extensions used for prey capture and also contains food vacuoles for prey digestion. The ectoplasm is surrounded by a periplasmic cortex, also made up of microfibrils, but arranged into twenty plates, each with a hole through which one spicule projects. The cortex is linked to the spines by contractile myonemes, which assist in buoyancy control by allowing the ectoplasm to expand and contract, increasing and decreasing the total volume of the cell. [7]
The way that the spines are joined at the center of the cell varies and is one of the primary characteristics by which acantharians are classified. The skeletons are made up of either ten diametric or twenty radial spicules. Diametric spicules cross the center of the cell, whereas radial spicules terminate at the center of the cell where they either form a tight or flexible junction depending on species. Acantharians with diametric spicules or loosely attached radial spicules are able to rearrange or shed spicules and form cysts. [11]
The morphological classification system roughly agrees with phylogenetic trees based on the alignment of ribosomal RNA genes, although the groups are mostly polyphyletic. Holacanthida seems to have evolved first and includes molecular clades A, B, and D. Chaunacanthida evolved second and includes only one molecular clade, clade C. Arthracanthida and Symphacanthida, which have the most complex skeletons, evolved most recently and constitute molecular clades E and F. [7]
Many acantharians, including some in clade B (Holacanthida) and all in clades E & F (Symphiacanthida and Arthracanthida), host single-celled algae within their inner cytoplasm (endoplasm). By participating in this photosymbiosis, acantharians are essentially mixotrophs: they acquire energy through both heterotrophy and autotrophy. The relationship may make it possible for acantharians to be abundant in low-nutrient regions of the oceans and may also provide extra energy necessary to maintain their elaborate strontium sulfate skeletons. It is hypothesized that the acantharians provide the algae with nutrients (N & P) that they acquire by capturing and digesting prey in return for sugar that the algae produces during photosynthesis. It is not known, however, whether the algal symbionts benefit from the relationship or if they are simply being exploited and then digested by the acantharians. [12]
Symbiotic Holacanthida acantharians host diverse symbiont assemblages, including several genera of dinoflagellates ( Pelagodinium, Heterocapsa, Scrippsiella, Azadinium) and a haptophyte ( Chrysochromulina ). [13] Clade E & F acantharians have a more specific symbiosis and primarily host symbionts from the haptophyte genus Phaeocystis , [8] although they sometimes also host Chrysochromulina symbionts. [10] Clade F acantharians simultaneously host multiple species and strains of Phaeocystis and their internal symbiont community does not necessarily match the relative availability of potential symbionts in the surrounding environment. The mismatch between internal and external symbiont communities suggests that acantharians can be selective in choosing symbionts and probably do not continuously digest and recruit new symbionts, and maintain symbionts for extended periods of time instead. [10]
Adults are usually multinucleated. [7] Earlier diverging clades are able to shed their spines and form cysts, which are often referred to as reproductive cysts. [11] Reproduction is thought to take place by formation of swarmer cells (formerly referred to as "spores"), which may be flagellate, and cysts have been observed to release these swarmers. Non-encysted cells have also been seen releasing swarmers in laboratory conditions. Not all life cycle stages have been observed, however, and no one has witnessed the fusion of swarmers to produce a new acantharian. Cysts are often found in sediment traps and it is therefore believed that the cysts help acantharians sink into deep water. [14] Genetic data and some imaging suggests that non-cyst-forming acantharians may also sink to deep water to release swarmers. [15] Releasing swarmer cells in deeper water may improve the survival chances of juveniles. [14] Study of these organisms has been hampered mainly by an inability to "close the lifecycle" and maintain these organisms in culture through successive generations.
An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualistic relationship. Examples are nitrogen-fixing bacteria, which live in the root nodules of legumes, single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to insects.
Sponges or sea sponges are members of the metazoan phylum Porifera, a basal animal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and are one of the most ancient members of macrobenthos, with many historical species being important reef-building organisms.
Zooplankton are the heterotrophic component of the planktonic community, having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequently, they drift or are carried along by currents in the ocean, or by currents in seas, lakes or rivers.
The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elaborate mineral skeleton is usually made of silica. They are found as zooplankton throughout the global ocean. As zooplankton, radiolarians are primarily heterotrophic, but many have photosynthetic endosymbionts and are, therefore, considered mixotrophs. The skeletal remains of some types of radiolarians make up a large part of the cover of the ocean floor as siliceous ooze. Due to their rapid change as species and intricate skeletons, radiolarians represent an important diagnostic fossil found from the Cambrian onwards.
Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus Symbiodinium, but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. The true Zooxanthella K.brandt is a mutualist of the radiolarian Collozoum inerme and systematically placed in Peridiniales. Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.
Phaeodarea or Phaeodaria is a group of amoeboid cercozoan organisms. They are traditionally considered radiolarians, but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa. They are distinguished by the structure of their central capsule and by the presence of a phaeodium, an aggregate of waste particles within the cell.
A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil.
Endoplasm generally refers to the inner, dense part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer (non-granulated) layer of the cytoplasm, which is typically watery and immediately adjacent to the plasma membrane. The nucleus is separated from the endoplasm by the nuclear envelope. The different makeups/viscosities of the endoplasm and ectoplasm contribute to the amoeba's locomotion through the formation of a pseudopod. However, other types of cells have cytoplasm divided into endo- and ectoplasm. The endoplasm, along with its granules, contains water, nucleic acids, amino acids, carbohydrates, inorganic ions, lipids, enzymes, and other molecular compounds. It is the site of most cellular processes as it houses the organelles that make up the endomembrane system, as well as those that stand alone. The endoplasm is necessary for most metabolic activities, including cell division.
Symbiodinium is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known and have photosymbiotic relationships with many species. These unicellular microalgae commonly reside in the endoderm of tropical cnidarians such as corals, sea anemones, and jellyfish, where the products of their photosynthetic processing are exchanged in the host for inorganic molecules. They are also harbored by various species of demosponges, flatworms, mollusks such as the giant clams, foraminifera (soritids), and some ciliates. Generally, these dinoflagellates enter the host cell through phagocytosis, persist as intracellular symbionts, reproduce, and disperse to the environment. The exception is in most mollusks, where these symbionts are intercellular. Cnidarians that are associated with Symbiodinium occur mostly in warm oligotrophic (nutrient-poor), marine environments where they are often the dominant constituents of benthic communities. These dinoflagellates are therefore among the most abundant eukaryotic microbes found in coral reef ecosystems.
Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plants; as well as, algal and fungal species. They can reside within extracellular or intracellular structures of the host. In order for a cyanobacterium to successfully form a symbiotic relationship, it must be able to exchange signals with the host, overcome defense mounted by the host, be capable of hormogonia formation, chemotaxis, heterocyst formation, as well as possess adequate resilience to reside in host tissue which may present extreme conditions, such as low oxygen levels, and/or acidic mucilage. The most well-known plant-associated cyanobionts belong to the genus Nostoc. With the ability to differentiate into several cell types that have various functions, members of the genus Nostoc have the morphological plasticity, flexibility and adaptability to adjust to a wide range of environmental conditions, contributing to its high capacity to form symbiotic relationships with other organisms. Several cyanobionts involved with fungi and marine organisms also belong to the genera Richelia, Calothrix, Synechocystis, Aphanocapsa and Anabaena, as well as the species Oscillatoria spongeliae. Although there are many documented symbioses between cyanobacteria and marine organisms, little is known about the nature of many of these symbioses. The possibility of discovering more novel symbiotic relationships is apparent from preliminary microscopic observations.
Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.
Arthracanthida, a subclass of Acantharea, is a group of marine protozoans. They consist mainly of a gelatinous sheath filled with cytoplasm and a skeleton of up to 20 radially placed spicules made of celestine. While mostly found in the upper areas of the ocean, they are able to move vertically by using microfilaments attached to the spicules to expand and contract the sheath. They are plentiful in the Gulf Stream during the summer months, but little is known about their overall distribution.
Nassellaria is an order of Rhizaria belonging to the class Radiolaria. The organisms of this order are characterized by a skeleton cross link with a cone or ring.
Phaeocystis is a genus of algae belonging to the Prymnesiophyte class and to the larger division of Haptophyta. It is a widespread marine phytoplankton and can function at a wide range of temperatures (eurythermal) and salinities (euryhaline). Members of this genus live in the open ocean, as well as in sea ice. It has a polymorphic life cycle, ranging from free-living cells to large colonies.
Collodaria is a unicellular order under the phylum Radiozoa and the infrakingdom Rhizaria. Like most of the Radiolaria taxonomy, Collodaria was first described by Ernst Haeckel, a German scholar who published three volumes of manuscript describing the extensive samples of Radiolaria collected by the voyage of HMS Challenger. Recent molecular phylogenetic studies concluded that there are Collodaria contains three families, Sphaerozodae, Collosphaeridae, and Collophidilidae.
The genus Stylodictya belongs to a group of organisms called the Radiolaria. Radiolarians are amoeboid protists found as zooplankton in oceans around the world and are typically identified by their ornate skeletons.
Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as marine single-celled prokaryotes and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics because they are paraphyletic.
Richelia is a genus of nitrogen-fixing, filamentous, heterocystous and cyanobacteria. It contains the single species Richelia intracellularis. They exist as both free-living organisms as well as symbionts within potentially up to 13 diatoms distributed throughout the global ocean. As a symbiont, Richelia can associate epiphytically and as endosymbionts within the periplasmic space between the cell membrane and cell wall of diatoms.
Astracantha is a genus of planktonic phaeodaria and the only member of the family Astracanthidae. They are an unusual family of marine protists, but can be found across all oceans, from tropical to Arctic and Antarctic waters.
Photosymbiosis is a type of symbiosis where one of the organisms is capable of photosynthesis.