Foraminifera facts for kids
Quick facts for kids Foraminifera |
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| Live Ammonia tepida (Rotaliida) | |
Scientific classification  |
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| Clade: | SAR |
| Phylum: | Retaria |
| Subphylum: | Foraminifera d'Orbigny, 1826 |
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"Monothalamea"
Tubothalamea
Globothalamea
incertae sedis
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Foraminifera ( FƏ-ram-Ə-nih-FƏ-rə; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of Cercozoan protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an external shell (called a "test") of diverse forms and materials. Tests of chitin (found in some simple genera, and Textularia in particular) are believed to be the most primitive type. Most foraminifera are marine, the majority of which live on or within the seafloor sediment (i.e., are benthic, with different sized species playing a role within the macrobenthos, meiobenthos, and microbenthos), while a smaller number float in the water column at various depths (i.e., are planktonic), which belong to the suborder Globigerinina. Fewer are known from freshwater or brackish conditions, and some very few (nonaquatic) soil species have been identified through molecular analysis of small subunit ribosomal DNA.
Foraminifera typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are commonly made of calcium carbonate (CaCO3) or agglutinated sediment particles. Over 50,000 species are recognized, both living (6,700–10,000) and fossil (40,000). They are usually less than 1 mm in size, but some are much larger, the largest species reaching up to 20 cm.
Contents
History of study
The earliest known reference to foraminifera comes from Herodotus, who in the 5th century BCE noted them as making up the rock that forms the Great Pyramid of Giza. These are today recognized as representatives of the genus Nummulites. Strabo, in the 1st Century BCE, noted the same foraminifera, and suggested that they were the remains of lentils left by the workers who built the pyramids.
Robert Hooke observed a foraminifera under the microscope, as described and illustrated in his 1665 book Micrographia.
Antonie van Leeuwenhoek described and illustrated foraminiferal tests in 1700, describing them as minute cockles; his illustration is recognizable as being Elphidium. Early workers classified foraminifera within the genus Nautilus, noting their similarity to certain cephalopods. It was recognised by Lorenz Spengler in 1781 that foraminifera had holes in the septa, which would eventually grant the group its name. Spengler also noted that the septa of foraminifera arced the opposite way from those of nautili and that they lacked a nerve tube.
Alcide d'Orbigny, in his 1826 work, considered them to be a group of minute cephalopods and noted their odd morphology, interpreting the pseudopodia as tentacles and noting the highly reduced (in actuality, absent) head. He named the group foraminifères, or "hole-bearers", as members of the group had holes in the divisions between compartments in their shells, in contrast to nautili or ammonites.
The protozoan nature of foraminifera was first recognized by Dujardin in 1835. Shortly after, in 1852, d'Orbigny produced a classification scheme, recognising 72 genera of foraminifera, which he classified based on test shape—a scheme that drew severe criticism from colleagues.
H.B. Brady's 1884 monograph described the foraminiferal finds of the Challenger expedition. Brady recognized 10 families with 29 subfamilies, with little regard to stratigraphic range; his taxonomy emphasized the idea that multiple different characters must separate taxonomic groups, and as such placed agglutinated and calcareous genera in close relation.
This overall scheme of classification would remain until Cushman's work in the late 1920s. Cushman viewed wall composition as the single most important trait in classification of foraminifera; his classification became widely accepted but also drew criticism from colleagues for being "not biologically sound".
Geologist Irene Crespin undertook extensive research in this field, publishing some ninety papers—including notable work on foraminifera—as sole author as well as more than twenty in collaboration with other scientists.
Cushman's scheme nevertheless remained the dominant scheme of classification until Tappan and Loeblich's 1964 classification, which placed foraminifera into the general groupings still used today, based on microstructure of the test wall. These groups have been variously moved around according to different schemes of higher-level classification. Pawlowski's (2013) use of molecular systematics has generally confirmed Tappan and Loeblich's groupings, with some being found as polyphyletic or paraphyletic; this work has also helped to identify higher-level relationships among major foraminiferal groups.
Anatomy
The most striking aspect of most foraminifera are their hard shells, or tests. These may consist of one of multiple chambers, and may be composed of protein, sediment particles, calcite, aragonite, or (in one case) silica. Some foraminifera lack tests entirely. Unlike other shell-secreting organisms, such as molluscs or corals, the tests of foraminifera are located inside the cell membrane, within the protoplasm. The organelles of the cell are located within the compartment(s) of the test, and the hole(s) of the test allow the transfer of material from the pseudopodia to the internal cell and back.
Foraminifera are unique in having granuloreticulose pseudopodia; that is, their pseudopodia appear granular under the microscope; these pseudopodia are often elongate and may split and rejoin each other. These can be extended and retracted to suit the needs of the cell. The pseudopods are used for locomotion, anchoring, excretion, test construction and in capturing food, which consists of small organisms such as diatoms or bacteria.
Aside from the tests, foraminiferal cells are supported by a cytoskeleton of microtubules, which are loosely arranged without the structure seen in other amoeboids. Forams have evolved special cellular mechanisms to quickly assemble and disassemble microtubules, allowing for the rapid formation and retraction of elongated pseudopodia.
Some species of foraminifera have large, empty vacuoles within their cells; the exact purpose of these is unclear, but they have been suggested to function as a reservoir of nitrate.
Mitochondria are distributed evenly throughout the cell, though in some species they are concentrated under the pores and around the external margin of the cell. This has been hypothesised to be an adaptation to low-oxygen environments.
Several species of xenophyophore have been found to have unusually high concentrations of radioactive isotopes within their cells, among the highest of any eukaryote. The purpose of this is unknown.
Ecology
Modern Foraminifera are primarily marine organisms, but living individuals have been found in brackish, freshwater and even terrestrial habitats. The majority of the species are benthic, and a further 50 morphospecies are planktonic. This count may, however, represent only a fraction of actual diversity, since many genetically distinct species may be morphologically indistinguishable.
Benthic foraminifera are typically found in fine-grained sediments, where they actively move between layers; however, many species are found on hard rock substrates, attached to seaweeds, or sitting atop the sediment surface.
Most foraminifera consume smaller organisms and organic matter; some smaller species are specialised feeders on phytodetritus, while others specialise in consuming diatoms. Some benthic forams construct feeding cysts, using the pseuodopodia to encyst themselves inside of sediment and organic particles. Certain foraminifera prey upon small animals such as copepods or cumaceans; some forams even predate upon other forams, drilling holes into the tests of their prey. One group, the xenophyophores, has been suggested to farm bacteria within their tests, although studies have failed to find support for this hypothesis. Suspension feeding is also common in the group, and at least some species can take advantage of dissolved organic carbon.
A few foram species are parasitic, infecting sponges, molluscs, corals, or even other foraminifera. Parasitic strategies vary; some act as ectoparasites, using their pseudopodia to steal food from the host, while others burrow through the shell or body wall of their host to feed on its soft tissue.
Foraminifera are themselves eaten by a host of larger organisms, including invertebrates, fish, shorebirds, and other foraminifera. It has been suggested, however, that in some cases predators may be more interested in the calcium from foram shells than in the organisms themselves. Several aquatic snail species are known to selectively feed upon foraminifera, often even preferring individual species.
Certain benthic foraminifera have been found to be capable of surviving anoxic conditions for over 24 hours, indicating that they are capable of selective anaerobic respiration. This is interpreted as an adaptation to survive changing oxygenic conditions near the sediment-water interface.
Foraminifera are found in the deepest parts of the ocean such as the Mariana Trench, including the Challenger Deep, the deepest part known. At these depths, below the carbonate compensation depth, the calcium carbonate of the tests is soluble in water due to the extreme pressure. The Foraminifera found in the Challenger Deep thus have no carbonate test, but instead have one of organic material.
Nonmarine foraminifera have traditionally been neglected in foram research, but recent studies show them to be substantially more diverse than previously known. They are known to inhabit disparate ecological niches, including mosses, rivers, lakes and ponds, wetlands, soils, peat bogs, and sand dunes.
Reproduction
The generalized foraminiferal life-cycle involves an alternation between haploid and diploid generations, although they are mostly similar in form. The haploid or gamont initially has a single nucleus, and divides to produce numerous gametes, which typically have two flagella. The diploid or agamont is multinucleate, and after meiosis divides to produce new gamonts. Multiple rounds of asexual reproduction between generations are not uncommon in benthic forms.
Foraminifera exhibit morphological dimorphism associated with their reproductive cycle. The gamont, or sexually reproducing haploid form, is proportionally large. The gamont is also known as the A form. Gamonts, despite having typically larger proloculi, also generally have smaller overall test diameter than do agamonts.
After reaching maturity, the gamont divides via mitosis to produce thousands of gametes which are also haploid. These gametes all have a full set of organelles, and are expelled from the test into the environment leaving the test undamaged. Gametes are not differentiated into sperm and egg, and any two gametes from a species can generally fertilize each other.
When two gametes combine, they create a diploid, multi-nucleated cell known as the agamont, or B form. In contrast to the gamont, the agamont is microspheric, with a proportionally small first chamber but typically larger overall diameter with more chambers. The agamont is the asexual reproduction phase of the foraminifera; upon reaching adulthood, the protoplasm entirely vacates the test and divides its cytoplasm meiotically via multiple fission to form a number of haploid offspring. These offspring then begin to form their megalospheric first chamber before dispersing.
In some cases the haploid young may mature into a megalospheric form which then reproduces asexually to produce another megalospheric, haploid offspring. In this case, the first megalospheric form is referred to as the schizont or A1 form, while the second is referred to as the gamont or A2 form.
Maturation and reproduction occur more slowly in cooler and deeper water; these conditions also cause forams to grow larger. A forms always seem to be much more numerous than are B forms, likely due to the reduced likelihood of two gametes encountering one another and successfully combining.
Tests
Foraminiferal tests serve to protect the organism within. Owing to their generally hard and durable construction (compared to other protists), 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 (star-shaped), dendritic (branching). 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 (past primary apertures from an earlier growth stage) or as unique structures.
Test shape is highly variable among different foraminifera; they may be single-chambered (unilocular) or multi-chambered (multilocular). 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 megalospheric and microspheric 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 pseudopods, 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.
Paleontological applications
Dying planktonic Foraminifera continuously rain down on the sea floor in vast numbers, their mineralized tests preserved as fossils in the accumulating sediment. Beginning in the 1960s, and largely under the auspices of the Deep Sea Drilling, Ocean Drilling, and International Ocean Drilling Programmes, as well as for the purposes of oil exploration, advanced deep-sea drilling techniques have been bringing up sediment cores bearing Foraminifera fossils. The effectively unlimited supply of these fossil tests and the relatively high-precision age-control models available for cores has produced an exceptionally high-quality planktonic Foraminifera fossil record dating back to the mid-Jurassic, and presents an unparalleled record for scientists testing and documenting the evolutionary process. The exceptional quality of the fossil record has allowed an impressively detailed picture of species inter-relationships to be developed on the basis of fossils, in many cases subsequently validated independently through molecular genetic studies on extant specimens
Because certain types of foraminifera are found only in certain environments, their fossils can be used to figure out the kind of environment under which ancient marine sediments were deposited; conditions such as salinity, depth, oxygenic conditions, and light conditions can be determined from the different habitat preferences of various species of forams. This allows workers to track changing climates and environmental conditions over time by aggregating information about the foraminifera present.
In other cases, the relative proportion of planktonic to benthic foraminifera fossils found in a rock can be used as a proxy for the depth of a given locality when the rocks were being deposited.
Since at least 1997, the Paleocene–Eocene thermal maximum (PETM) has been investigated as an analogy for understanding the effects of global warming and of massive carbon inputs to the ocean and atmosphere, including ocean acidification. Humans today emit about 10 Gt of carbon (about 37 Gt CO2e) per year, and at that rate will release a comparable amount to the PETM in about one thousand years. A main difference is that during the PETM the planet was ice-free, as the Drake Passage had not yet opened and the Central American Seaway had not yet closed. Although the PETM is now commonly held to be a case study for global warming and massive carbon emission, the cause, details, and overall significance of the event remain uncertain.
Foraminifera have significant application in the field of biostratigraphy. Due to their small size and hard shells, foraminifera may be preserved in great abundance and with high quality of preservation; due to their complex morphology, individual species are easily recognizable. Foraminifera species in the fossil record have limited ranges between the species' first evolution and their disappearance; stratigraphers have worked out the successive changes in foram assemblages throughout much of the Phanerozoic. As such, the assemblage of foraminifera within a given locality can be analyzed and compared to known dates of appearance and disappearance in order to narrow down the age of the rocks. This allows paleontologists to interpret the age of sedimentary rocks when radiometric dating is not applicable. This application of foraminifera was discovered by Alva C. Ellisor in 1920.
Calcareous fossil foraminifera are formed from elements found in the ancient seas where they lived. Thus, they are very useful in paleoclimatology and paleoceanography. They can be used, as a climate proxy, to reconstruct past climate by examining the stable isotope ratios and trace element content of the shells (tests). Global temperature and ice volume can be revealed by the isotopes of oxygen, and the history of the carbon cycle and oceanic productivity by examining the stable isotope ratios of carbon; see δ18O and δ13C. The concentration of trace elements, like strontium (Sr), magnesium (Mg), lithium (Li) and boron (B), also hold a wealth of information about global temperature cycles, continental weathering, and the role of the ocean in the global carbon cycle. Geographic patterns seen in the fossil records of planktonic forams are also used to reconstruct ancient ocean currents.
Modern uses
The oil industry relies heavily on microfossils such as forams to find potential hydrocarbon deposits.
For the same reasons they make useful biostratigraphic markers, living foraminiferal assemblages have been used as bioindicators in coastal environments, including indicators of coral reef health. Because calcium carbonate is susceptible to dissolution in acidic conditions, foraminifera may be particularly affected by changing climate and ocean acidification.
Foraminifera have many uses in petroleum exploration and are used routinely to interpret the ages and paleoenvironments of sedimentary strata in oil wells. Agglutinated fossil foraminifera buried deeply in sedimentary basins can be used to estimate thermal maturity, which is a key factor for petroleum generation. The Foraminiferal Colouration Index (FCI) is used to quantify colour changes and estimate burial temperature. FCI data is particularly useful in the early stages of petroleum generation (about 100 °C).
Foraminifera can also be used in archaeology in the provenancing of some stone raw material types. Some stone types, such as limestone, are commonly found to contain fossilised foraminifera. The types and concentrations of these fossils within a sample of stone can be used to match that sample to a source known to contain the same "fossil signature".
Gallery
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Foraminifera of Pag Island, Adriatic Sea -60 m, field width 5.5 mm
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Foraminifera of Pag Island, Adriatic Sea -60 m, field width 5.5 mm
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Foraminifera of Pag Island, Adriatic Sea -60 m, field width 5.5 mm
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Foraminifera of Pag Island, Adriatic Sea -60 m, field width 5.5 mm
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Foraminifera of Indian Ocean, south-eastern coast of Bali, field width 5.5 mm
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Foraminifera of Indian Ocean, south-eastern coast of Bali, field width 5.5 mm
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Foraminifera of Indian Ocean, south-eastern coast of Bali, field width 5.5 mm
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Foraminifera in Ngapali, Myanmar, field width 5.22 mm
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Foraminifera Heterostegina depressa, field width 4.4 mm
See also
In Spanish: Foraminifera para niños
