Foraminifera test
Foraminiferal tests are the tests of Foraminifera.
Foraminifera are single-celled predatory protists, mostly marine, and usually protected with shells. These shells, often called tests, can be single-chambered or have multiple interconnected chambers; the cellular machinery is contained within the shell. So important is the test to the biology of foraminifera that it provides the scientific name of the group—foraminifera, Latin for "hole bearers", referring to the pores connecting chambers of the shell in the multi-chambered species.
Foraminiferal tests are usually made of calcite, a form of calcium carbonate, but are sometimes made of aragonite, agglutinated sediment particles, chitin, or of silica. Other foraminifera lack tests altogether.
Over 50,000 species are recognized, both living and fossil. They are usually less than 1 mm in size, but some are much larger, the largest species reaching up to 20 cm. Most forams are benthic, but about 40 extant species are planktic. The hard nature of most foraminiferal tests leads to an excellent fossil record, and they are widely researched to infer information about past climate and environments.
Background
Foraminiferal tests serve to protect the organism within. Owing to their generally hard and durable construction, the tests of foraminifera are a major source of scientific knowledge about the group.Openings in the test that allow the cytoplasm to extend outside are called apertures. The primary aperture, leading to the exterior, take many different shapes in different species, including but not limited to rounded, crescent-shaped, slit-shaped, hooded, radiate, dendritic. Some foraminifera have "toothed", flanged, or lipped primary apertures. There may be only one primary aperture or multiple; when multiple are present, they may be clustered or equatorial. In addition to the primary aperture, many foraminifera have supplemental apertures. These may form as relict apertures or as unique structures.
Test shape is highly variable among different foraminifera; they may be single-chambered or multi-chambered. In multilocular forms, new chambers are added as the organism grows. A wide variety of test morphologies is found in both unilocular and multilocular forms, including spiraled, serial, and milioline, among others.
Many foraminifera exhibit dimorphism in their tests, with microspheric and megalospheric individuals. These names should not be taken as referring to the size of the full organism; rather, they refer to the size of the first chamber, or proloculus. Tests as fossils are known from as far back as the Ediacaran period, and many marine sediments are composed primarily of them. For instance, the limestone that makes up the pyramids of Egypt is composed almost entirely of nummulitic benthic foraminifera. It is estimated that reef foraminifera generate about 43 million tons of calcium carbonate per year.
Genetic studies have identified the naked amoeba Reticulomyxa and the peculiar xenophyophores as foraminiferans without tests. A few other amoeboids produce reticulose pseudopodia, and were formerly classified with the forams as the Granuloreticulosa, but this is no longer considered a natural group, and most are now placed among the Cercozoa.
Composition
The form and composition of their tests are the primary means by which forams are identified and classified. Most secrete calcareous tests, composed of calcium carbonate. Calcareous tests may be composed of either aragonite or calcite depending on species; among those with calcite tests, the test may contain either a high or low fraction of magnesium substitution. The test contains an organic matrix, which can sometimes be recovered from fossil samples.Some studies suggest a high amount of homoplasy in foraminifera, and that neither agglutinated nor calcareous foraminifera form monophyletic groupings.
Soft
In some forams, the tests may be composed of organic material, typically the protein tectin. Tectin walls may have sediment particles loosely adhered onto the surface. The foram Reticulomyxa entirely lacks a test, having only a membranous cell wall. Organic-walled forams have traditionally been grouped as the "allogromiids"; however, genetic studies have found that this does not make up a natural group.Agglutinated
Other forams have tests made from small pieces of sediment cemented together by either proteins, calcium carbonate, or Iron oxide. In the past these forms were grouped together as the single-chambered "astrorhizids" and the multi-chambered textulariids. However, recent genetic studies suggest that "astrorhizids" do not make up a natural grouping, instead forming a broad base of the foram tree.Textulariid foraminifera, unlike other living members of the globothalamea, have agglutinated tests; however, grains in these tests are cemented with a calcite cement. This calcite cement is made up of small globular nanograins, similar to in other globothalameans. These tests may also have many pores, another feature uniting them with the globothalamea.
Agglutinating foraminifera may be selective regarding what particles they incorporate into their shells. Some species prefer certain sizes and types of rock particles; other species are preferential towards certain biological materials. Certain species of foraminifera are known to have preferentially agglutinated coccoliths to form their tests; others preferentially utilise echinoderm plates, diatoms, or even other foraminiferans' tests.
The foraminifera Spiculosiphon preferentially agglutinates silica sponge spicules using an organic cement; it shows strong selectivity also towards shape, utilising elongated spicules on its "stalk" and shortened ones on its "bulb". It is thought to use the spicules as both a means of elevating itself off the seabed as well as to lengthen the reach of its pseudopodia to capture prey.The agglutinated tests of xenophyophores are the largest of any foraminifera, reaching up to 20 cm in diameter. The name "xenophyophore", meaning "bearer of foreign bodies", refers to this agglutinating habit. Xenophyophores selectively uptake sediment grains between 63 and 500 μm, avoiding larger pebbles and finer silts; type of sediment seems to be a strong factor in which particles are agglutinated, as particle type preferentially includes sulfides, oxides, volcanic glass, and especially tests of smaller foraminifera. Xenophyophores 1.5 cm in diameter have been recorded completely naked, with no test whatsoever.
Calcareous
Of those foraminifera with calcareous tests, several different structures of calcite crystals are found.Porcelaneous
Porcelaneous walls are found in the Miliolida. These consist of high-magnesium calcite organized with an ordered outer and inner calcite lining and randomly oriented needle-shaped calcite crystals forming a thick center layer. An organic inner lining is also present. The external surface may have a pitted structure, but it is not perforated by holes. "Cornuspirid" miliolids apparently lack any extrados.Monocrystalline
A "monocrystalline" test structure has traditionally been described for the Spirillinida. However, these tests remain poorly understood and poorly described. Some supposed "monocrystalline" spirillinids have been found to actually have tests consisting of a mosaic of very small crystals when observed with scanning electron microscope. SEM observation of Patellina sp. suggests that a truly monocrystalline test may indeed be present, with apparent cleavage faces.Fibre bundles
tests consist of "fibre bundles" that can reach tens of micrometres long; each "bundle" is formed from a single calcite crystal, is triangular in cross-section, and has a pore in the centre. There is also an internal organic layer, attached to the "cone" structure of the fibre bundles. As the crystalline structure varies significantly from that of other calcareous foraminifera, it is thought to represent a separate evolution of the calcareous test. The exact mineralisation process of lagenids remains unclear.Hyaline
tests are described as "hyaline". They are formed from low-to-high-magnesium calcite "nanograins" positioned with their C-axes perpendicular to the external surface of the test. Further, these nanograins can have higher-level structure, such as rows, columns, or bundles. The test wall is characteristically bilamellar and perforated throughout with small pores. The outer calcite layer of the test wall is referred to as the "outer lamina" while the inner calcite layer is referred to as the "inner lining"; this should not be confused with the organic inner lining beneath the test. Sandwiched between the outer lamina and the inner lining is the "median layer", a protein layer that separates the two. The median layer is quite variable; depending on the species it may be well-defined while in others it is not sharply delineated. Some genera may contain sediment particles within the median layer.The now-extinct Fusulinids have traditionally been considered unique in having tests of homogenous microgranular crystals with no preferred orientation and almost no cement. However, a 2017 study found that the supposed microgranular structure was actually the result of diagenetic alteration of the fossils, and that unaltered fusulinid tests instead had a hyaline structure. This suggests that the group is affiliated with the Globothalamea.
Robertinids have aragonitic tests with perforations; these are similar to the tests of rotaliids in that they are formed from nanograins, however, they differ in composition and in having well-organised columnar domains. As the earliest planktonic forams had aragonitic tests, it has been suggested that this may represent a separate evolution of a planktonic lifestyle within the Robertinida, rather than being close relatives of Globigerinans.
Hyaline aragonitic tests are also present in the Involutinida.