Acantharia
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| Haeckel , 1881 |
The Acantharia , often Acantharea written, are composed of approximately 140 species taxon unicellular , eukaryotic organisms that the Strahlentierchen include (Radiolaria). All species live in the ocean as an extremely common part of zooplankton in the uppermost ocean layers. They have a mathematically regular skeleton made of the mineral celestine (= strontium sulfate) and are of great importance for the strontium cycle in the sea.
Ecology, biology and, in particular, the life cycle of animals are so far only insufficiently known.
features
Acantharia are spherical or oblong-round, occasionally flattened and have a diameter of 0.05 to 5 millimeters.
skeleton
The cell has a skeleton of 20 (for the Holacanthida 10) spines, which consist of monocrystalline , rhombic strontium sulfate , a building material unique among the Protists .
The spines are always arranged in the same way with mathematical regularity. From the imaginary "sphere" of the round cell, the tips of the four rows emerge at 60 ° N, 60 ° S, 30 ° N, 30 ° S and 0 ° latitude and 0 °, 90 °, 180 ° and 270 ° (polar or equatorial peaks) or 45 °, 135 °, 225 ° and 315 ° (tropical peaks) of geographical longitude (see illustration).
Johannes Müller recognized this regularity for the first time and formulated it in 1858 in the "Müller law" named after him:
" One therefore obtains [...] for the acanthometry with 20 spines the same formula that between two spineless poles there are 5 belts of spines, each of 4 spines, all directed towards the common center of the whole sphere, and that the spines of each belt with the previous alternate. "
cell
The cell body consists of the central endoplasm and the peripheral ectoplasm . The endoplasm containing the cell nucleus and most of the organelles is mostly pigmented brown, red or black , in all species except for those of the arthracantida the concentration of the pigmented endoplasm is particularly dense in the center, so the cell becomes outwardly with increasing proportion of the transparent ectoplasm clearer.
Endo- and ectoplasm are separated by a capsule wall, a fibrillar network that has openings for cytoplasmic protuberances and the axopodia . In the Arthracantida the capsule wall is significantly thicker than in the other orders, in which the capsule wall is partially barely recognizable.
The periplasmic cortex, also a fibrillar meshwork, serves as the “outer skin” of the ectoplasm and thus the entire cell . It consists of twenty polygons connected to one another by means of elastic connecting pieces , each of which is arranged around one of the skeletal spines.
The capsule wall and the periplasmic cortex lie against one another at the spines. In the small space between the capsule wall and cortex are the Myoneme , anchored to the capsule wall and the stinger fibrillar bundles of proteins , which are flat, ribbon-shaped, short cylindrical, or triangular, and 5 to 90 microns long. Depending on the species, there are between 40 and 1200 muonemes, i.e. 2 to 60 per sting. They serve as movement- generating organelles and can trigger three movements: a slow wave-like contraction , then a sudden contraction , which can push the periplasmic cortex to the tip of the sting, and a slow, subsequent relaxation. If an individual activates all the muonemes at the same time, this leads to a sudden increase in cell volume. This movement, observable every ten to twenty minutes, likely promotes buoyancy .
The organelles in the endoplasm are large mitochondria with tubular cristae , a rough endoplasmic reticulum , ribosomes , dictyosomes , peroxisomes and extrusomes similar to the kinetocysts of sun animals . The cell nuclei are usually numerous, small and round or oblong-round, with the exception of some species of the order Symphyacanthida with a single very large and probably polyploid nucleus.
In almost all species except those of Arthracanthida, the endoplasm with age Lithosomen on. These are small, oval, birefringent platelets that are produced by the Golgi apparatus . Transported to the cell surface, the outer skin of the cyst is formed from them as part of the encystation prior to gametogenesis ; the muonemes are secreted in the process.
Axopodia
Like all radiant animals and sun animals , the Acantharia have axopods , particularly long, thin and straight cell processes that protrude from the cell surface. They are formed from a thin cytoplasmic layer and the cell membrane and solidified by a special structure of microtubules , which are dodecagonally arranged in Holacanthida , otherwise hexagonally to axonemes , which arise from small MTOCs located in the endoplasm . The cytoplasm contains organelles such as extrusomes, mitochondria, and various types of vesicles . The axopodia are used to catch prey and react to stimuli, they retreat to chemical and physical (temperature, touch) stimuli and then slowly build up again.
Life cycle
The life cycle of Acantharia is not fully known. Problems with observation are that the animals can neither be cultivated under laboratory conditions nor kept alive for a long time.
In addition to the trophon stage described, only strongly skeletal cysts and swarm stages are known from individual observations of freshly caught animals . Tens of thousands of single-core , flagellated swarmers are released through the latter . No information is available about other life stages. Surprisingly, however, at depths below 900 m, i.e. well below the zone in which Acantharia live, DNA from Acantharia could be detected in both water and seabed samples . It is assumed that these are traces of previously unknown life stages of Acantharia, since the trophons cannot be detected microscopically there.
Way of life
distribution
Acantharia belong to the so-called zooplankton , so they are not themselves photosynthetic parts of the plankton in the sea. They are widespread in all oceans worldwide, but especially in tropical and subtropical waters, but they are only found scattered in moderate or even polar latitudes. Coastal areas are largely avoided, as are eutrophic waters. Due to their photosynthesizing symbionts in particular, they live mainly in the light-flooded few hundred meters near the surface; on calm days, large acantharia gather in large groups just a few meters below. The highest population densities are found at depths of 50 to 200 meters, but for the purpose of gametogenesis , some species can sink to depths of 300 to 400 meters. A few individuals were rarely found on the ocean floor several thousand meters deep.
There are probably no clear seasonal dependencies, some studies have shown clusters in spring and summer, according to other observations clusters could be observed in spring, but in summer they appeared to be less frequent. In coastal regions, apparent seasonal accumulations were found in connection with the regularly recurring exchange of eutrophic coastal water for oligotrophic water from the open sea; in consistently oligotrophic waters of the tropics and subtropics, they are a common part of the microplankton all year round .
The animals are very common in near-surface water, random samples in the North Atlantic showed a density of up to 16 copies per liter for the upper 20 meters, between 40 and 120 meters still around 10 copies per liter. This means that they are around 10 to 16 times more common than planktonic foraminifera, for example . In some samples, Acantharia made up more than 30, occasionally even more than 70% of all living beings in the sample.
Catching prey
By means of the very dynamically changing and stimulus-sensitive void-rich network of anastomosing cytoplasmic processes as well as with the axopodia, microorganisms such as diatoms , silica flagellates , coccolithophores and tintinnids are preyed , but also small molluscs . In addition, traces of extremely small prey such as cyanobacteria or other bacteria could be detected, but it is unclear whether this prey is deliberately grazed or only represents “ bycatch ”.
Zooxanthellae
In many Acantharia species there are optionally zooxanthellae as symbionts , which supply the Acantharia with energy through photosynthesis. At the same time, only the medium-sized 50% of all individuals in the population have zooxanthellae, the largest and smallest individuals lacking them. However, all Arthracanthida species have them in certain phases of the life cycle. The zooxanthellae are absent in gamonts and young trophons. They are absorbed during or just before gametogenesis , and their number increases during the trophon stage. Before they reach the reproductive stage, however, they are rejected.
The zooxanthellae are mostly haptophyta or dinoflagellates . Several symbiont species can occur simultaneously in a host. They can also continue to fix carbon near the surface, sometimes in quantities that are significantly higher than the host's needs. Frequently parasitic dinoflagellates of the genus Amoebophrya are also found in the endoplasm .
Role in the marine element cycle
Since the strontium sulphate, which is used by the Acantharia to build skeletons, is water-soluble, the animals have to continuously absorb material from the seawater. With their death the shell sinks and dissolves rapidly in layers of water around 900 m. Due to the high number of animals, there is a continuous transfer of strontium from higher to deeper water layers, so that the higher layers are literally depleted of strontium compared to the deeper layers. Acantharia are therefore considered to be the most important biological component of the marine strontium cycle. The same applies - albeit in a smaller amount - to the element barium , which makes up around 0.4% of skeletons. During the construction of the skeleton, Acantharia also accumulate trace elements such as lead , zinc , copper and iron in significant quantities and "transport" them to the middle layers of the sea by dissolving the skeleton.
Systematics
Since Ernst Haeckel's monographic treatment of the radiolarians based on the Challenger finds in 1887, the acantharia have been one of three subgroups of the radiolaria. Their exact systematic position was at times controversial. Wladimir Schewiakoff separated them completely from the Radiolaria in 1926, but this view did not prevail.
Molecular genetic studies confirmed the assignment of the group to the Radiolaria. The traditional line-up as a separate group was also largely confirmed. The internal system of the group, however, is no longer tenable and needs to be revised.
According to the traditional classification, the group comprises around 140 species in 50 genera and is divided into 4 orders with 18 families. The different connections of the spike attachments serve as a characteristic to distinguish the four orders:
- Order Arthracanthida
- Order Symphyacanthida
- Order Chaunacanthida
- Order Holacanthida
Fossil record
Since the mineral skeletons of Acantharia quickly dissolve in seawater, there are hardly any fossils of the group. The few surviving finds only date back to the Eocene ( Chiastolus amphicopium ).
proof
- ↑ a b c d e f g h i j k l m n o p Colette Febvre, Jean Febvre, Anthony Michaels: Acantharia. In: John J. Lee, GF Leedale, P. Bradbury (Eds.): An Illustrated Guide to the Protozoa . tape 2 . Allen, Lawrence 2000, ISBN 1-891276-23-9 , pp. 783-803 .
- ↑ Johannes Müller: About the Thalassicollen, Polycystinen and Acanthometren of the Mediterranean Sea , treatises of the Royal Academy of Sciences in Berlin, 1858, p. 12
- ↑ a b c Ilana C. Gilga, Linda A. Amaral-Zettler, Peter D. Countwaya, Stefanie Moorthi, Astrid Schnetzer, David A. Caron: Phylogenetic Affiliations of Mesopelagic Acantharia and Acantharian-like Environmental 18S rRNA genes off the Southern California Coast . In: Protist, 2010, (at the time of access in press ), doi : 10.1016 / j.protis.2009.09.002
- ↑ a b c Patrick De Deckker: On the celestite-secreting Acantharia and their effect on seawater strontium to calcium ratios. In: Hydrobiologia 517, pp. 1-13, 2004
- ^ Elsa Massera Bottazzi, Bruno Schreiber, Vaughan T. Bowen: Acantharia in the Atlantic Ocean, Their Abundance and Preservation. In: Limnology and Oceanography, Vol. 16, No. 4, pp. 677-684, 1971
- ^ GW Brass: Trace Elements in Acantharian Skeletons. In: Limnology and Oceanography, Vol. 25, No. 1, pp. 146-149, 1980
- ↑ Stephane Polet, Cédric Berney, José Fahrni, Jan Pawlowski: Small-Subunit Ribosomal RNA Gene Sequences of Phaeodarea Challenge the Monophyly of Haeckel's Radiolaria. In: Protist, Vol. 155, 2004, pp. 53-63
- ^ Jan Pawlowski, Fabien Burki: Untangling the Phylogeny of Amoeboid Protists. In: Journal of Eukaryotic Microbiology, 56: 1, pp. 16-25, 2009
- ↑ Klaus Hausmann, Norbert Hülsmann, Renate Radek: Protistology , 3rd edition, Schweizerbart, 2003, p. 171, ISBN 3-510-65208-8
- ↑ Arthur Shackleton Campbell: Radiolaria. In: Treatise on Invertebrate Paleontology, Part D Protista 3 (Chiefly Radiolarians And Tintinnines) , 1954, pp. D30-D42
