The
Foraminifera, ("Hole Bearers") or
forams for short, are a large group of
amoeboidAmoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amoeba" are often used interchangeably even by biologists, and especially refers to a creature moving by using pseudopods. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
protists with reticulating
pseudopods, fine strands of
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
that branch and merge to form a dynamic net. They typically produce a
testA test is a term used to refer to the shell of sea urchins, and also the shell of certain microorganisms, such as testate foraminifera and testate amoebae.-See also:*sea urchins, also known as Echinoidea*foraminifera*testate amoebae...
, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are made of calcium carbonate (CaCO
3) or agglutinated sediment particles. About 275,000 species are recognized, both living and
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
. They are usually less than 1 mm in size, but some are much larger, and the largest recorded specimen reached 19 cm.
Although as yet unsupported by morphological correlates, molecular data strongly suggest that Foraminifera are closely related to the
CercozoaThe Cercozoa are a group of protists. They are sometimes described as a kingdom.-Characteristics:The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in...
and Radiolaria, both of which also include amoeboids with complex shells; these three groups make up the
RhizariaThe Rhizaria are a species-rich supergroup of unicellulareukaryotes. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, and these make up...
. However, the exact relationships of the forams to the other groups and to one another are still not entirely clear.
Living forams
Modern forams are primarily marine, although some can survive in brackish conditions. A few species survive in fresh water and one even lives in damp rainforest soil. They are most commonly
benthicBenthos are the organisms which live on, in, or near the seabed, also known as the benthic zone. They live in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths....
, and about 40 morphospecies are
planktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
ic. This count may however represent only a fraction of actual diversity, since many genetically discrepant species may be morphologically indistinguishable.
A number of forms have unicellular
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
as
endosymbiontAn endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s, from diverse lineages such as the green algae, red algae, golden algae,
diatomDiatoms are a major group of eukaryotic algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons Diatoms (Greek: (dia) = "through" + (temnein) = "to cut", i.e., "cut in half") are a major...
s, and
dinoflagellateThe dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth. About half of all dinoflagellates are photosynthetic, and these make up the...
s. Some forams are
kleptoplasticKleptoplasty or kleptoplastidy is a symbiotic phenomenon whereby plastids from algae are sequestered by host organisms. The alga is eaten normally and partially digested, leaving the plastid intact. The plastids are maintained within the host, temporarily retaining functional photosynthesis for use...
, retaining
chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s from ingested algae to conduct
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
.
Biology
The foraminiferal cell is divided into granular endoplasm and transparent ectoplasm from which a pseudopodial net may emerge through a single opening or through many perforations in the test. Individual pseudopods characteristically have small granules streaming in both directions.
The pseudopods are used for locomotion, anchoring, and in capturing food, which consists of small organisms such as
diatomDiatoms are a major group of eukaryotic algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons Diatoms (Greek: (dia) = "through" + (temnein) = "to cut", i.e., "cut in half") are a major...
s or bacteria.
The foraminiferal life-cycle involves an alternation between haploid and
diploidPloidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that compose the body are diploid , but sex cells are haploid...
generations, although they are mostly similar in form. The haploid or gamont initially has a single
nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
, and divides to produce numerous
gameteA gamete is a cell that fuses with another gamete during fertilization in organisms that reproduce sexually...
s, which typically have two
flagellaA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
. The diploid or schizont is
multinucleateMultinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...
, and after
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
fragments to produce new gamonts. Multiple rounds of
asexual reproductionAsexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes...
between sexual generations is not uncommon in benthic forms.
Tests
The form and composition of the test is the primary means by which forams are identified and classified. Most have calcareous tests, composed of
calcium carbonateCalcium carbonate is a chemical compound with the chemical formula CaCO
3. It is a common substance found in rock in all parts of the world, and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural...
. In other forams the test may be composed of organic material, made from small pieces of sediment cemented together (agglutinated), and in one genus of silica. Openings in the test, including those that allow cytoplasm to flow between chambers, are called apertures.
Tests are known as fossils as far back as the
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
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. Production estimates indicate that reef foraminifera annually generate approximately 43 million tons of calcium carbonate and thus play an essential role in the production of reef carbonates.
Genetic studies have identified the naked amoeba "Reticulomyxa" and the peculiar
xenophyophoreXenophyophores are marine protozoans, giant single-celled organisms found throughout the world's oceans, but in their greatest numbers on the abyssal plains of the deep ocean. They were first described as sponges in 1889, then as testate amoeboids, and later as their own phylum of Protista. A...
s 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.
Deep sea species
Foraminifera are found in the deepest parts of the ocean such as the
Mariana TrenchThe Mariana Trench is the deepest known part of the world's oceans, and the deepest location on the surface of the Earth's crust. It is located in the western Pacific Ocean, to the east of the Mariana Islands. The trench is about long but has a mean width of only...
, including the
Challenger DeepThe Challenger Deep is the deepest surveyed point in the oceans, with a depth of approximately . The error of measurement is less than a hundred metres. It is located at the southern end of the Mariana Trench near the Mariana Islands group...
, the deepest part known. At these depths, below the
carbonate compensation depthCarbonate compensation depth is the depth in the oceans below which the rate of supply of calcium carbonate lags behind the rate of solvation, such that no calcium carbonate is preserved....
, 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.
Four species have been found in the Challenger Deep that are unknown from any other place in the ocean, one of which is representative of an endemic genus unique to the region. They are
Resigella laevis and
R. bilocularis,
Nodellum aculeata, and
Conicotheca nigrans (the unique genus). All have tests that are mainly of transparent organic material which have small (~ 100
nmNM, nm, nM, or Nm may stand for:* nanometer, an SI unit of length, equal to 10-9 m * Nautical mile, a unit of length used for maritime and aviation purposes...
) plates that appears to be clay
Evolutionary significance
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 DrillingThe Deep Sea Drilling Project was anocean drilling project running from 1968 to 1983. The program is successful as evidenced by the data and publications that have resulted from it and is supported by Texas A&M.- See also :*Project Mohole...
,
Ocean DrillingThe Ocean Drilling Program was an international cooperative effort to explore and study the composition and structure of the earth's ocean basins. ODP, which began in 1985, was the direct successor to the highly successful Deep Sea Drilling Project initiated in 1968 by the United States...
, 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 by the millions. 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.
Larger benthic foraminifera with complex shell structure react in a highly specific manner to the different benthic environments and, therefore, the composition of the assemblages and the distribution patterns of particular species reflect simultaneously bottom types and the ligth gradient. In the course of Earth history, larger foraminifera are replaced frequently. In particular, associations of foraminifera characterizing particular shallow water facies types are dying out and are replaced after a certain time interval by new associations with the same structure of shell morphology, emerging from a new evolutionary process of adaptation. These evolutionary processes make the larger foraminifera prone to be fossil index for the Permian, Jurassic, Cretaceous and Cenozoic (e.g.
Lukas HottingerLukas Hottinger is a paleontologist, biologist and geologist. Hottinger is currently collaborating with the Natural History Museum of Basel ....
).
Uses of forams
Because of their diversity, abundance, and complex morphology,
fossil foraminiferal assemblages are useful for
biostratigraphyBiostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...
, and can accurately give relative dates to rocks. The oil industry relies heavily on microfossils such as forams to find potential oil deposits.
Calcareous fossil foraminifera are formed from elements found in the ancient seas they lived in. Thus they are very useful in
paleoclimatologyPaleoclimatology is the study of climate change taken on the scale of the entire history of Earth. It uses records from ice sheets, tree rings, sediment, and rocks to determine the past state of the climate system on Earth....
and
paleoceanographyPaleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation.- Source of information :...
. They can be used to reconstruct past climate by examining the
stable isotopeStable isotopes are chemical isotopes that are not radioactive . By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes. A list of these is given at the end of this article...
ratios of oxygen, and the history of the carbon cycle and oceanic productivity by examining the stable isotope ratios of carbon; see δ18O and δ13C. Geographic patterns seen in the fossil records of planktonic forams are also used to reconstruct ancient
ocean currentAn ocean current is a continuous, directed movement of ocean water generated by the forces acting upon the water, such as the wind, Coriolis force, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s. Because certain types of foraminifera are found only in certain environments, they can be used to figure out the kind of environment under which ancient marine sediments were deposited.
For the same reasons they make useful biostratigraphic markers, living foraminiferal assemblages have been used as
bioindicatorBiological indicators are species used to monitor the health of an environment or ecosystem. They are any biological species or group of species whose function, population, or status can be used to determine ecosystem or environmental integrity. An example of such a group are the copepods and other...
s 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 acidificationOcean acidification is the name given to the ongoing decrease in the pH of the Earth's oceans, caused by their uptake of anthropogenic carbon dioxide from the atmosphere...
.
Foraminifera can also be utilised in
archaeologyArchaeology or archeology is the science that studies human cultures through the recovery, documentation, analysis, and interpretation of material culture and environmental data, including architecture, artifacts, biofacts, and landscapes...
in the
provenancingProvenance, from the French provenir, "to come from", means the origin, or the source, of something, or the history of the ownership or location of an object, The term was originally mostly used of works of art, but is now used in similar senses in a wide range of fields, including science and...
of some stone raw material types. Some stone types, such as
chertChert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
, 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".
External links
General information:
- University College London's micropaleontology site has an overview of foraminifera, including many high-quality SEM
SEM can refer to:* Scanning electron microscope, a microscopy technique, primarily used in cell biology* Science and Engineering Magnet, a school in Dallas, Texas...
s
Online flip-Books:
Illustrated glossary of terms used in foraminiferal research by
Lukas HottingerLukas Hottinger is a paleontologist, biologist and geologist. Hottinger is currently collaborating with the Natural History Museum of Basel ....
(alternative version of the one published in
"Carnets de Géologie - Notebooks on Geology")
Resources:
- 3D models of forams, generated by X-ray tomography
- eForams is a web site focused on foraminifera and modeling of foraminiferal shells