Foraminifera
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Allogromiida |
The Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net[1]. They typically produce a shell, or test, which can have either one or multiple chambers, some becoming quite elaborate in structure. [2]. About 275,000 species are recognized, both living and fossil[citation needed]. They are usually less than 1 mm in size, but some are much larger, and the largest recorded specimen reached 19 cm[citation needed].
Although as yet unsupported by morphological correlates, molecular data strongly suggest that Foraminifera are closely related to the Cercozoa and Radiolaria, both of which also include amoeboids with complex shells; these three groups make up the Rhizaria[3] However, the exact relationships of the forams to the other groups and to one another are still not entirely clear.
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[edit] Living forams
Modern forams are primarily marine, although they can survive in brackish conditions[4] A few species survive in fresh water and one even lives in damp rainforest soil[citation needed]. They are very common in the meiobenthos, and about 40 morphospecies are planktonic[1]. This count may however represent only a fraction of actual diversity, since many genetically discrepent species may be morphologically indistinguishable[5] The cell is divided into granular endoplasm and transparent ectoplasm. The pseudopodial net may emerge through a single opening or many perforations in the test, and characteristically has small granules streaming in both directions[4].
The pseudopods are used for locomotion, anchoring, and in capturing food, which consists of small organisms such as diatoms or bacteria[1]. A number of forms have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates[1]. Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis[citation needed].
The foraminiferan 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 schizont is multinucleate, and after meiosis fragments to produce new gamonts. Multiple rounds of asexual reproduction between sexual generations is not uncommon in benthic forms[4].
[edit] Tests
The form and composition of the test is the primary means by which forams are identified and classified[citation needed]. Most have calcareous tests, composed of calcium carbonate[4]. 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 Cambrian period[citation needed], 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[citation needed]. Forams may also make a significant contribution to the overall deposition of calcium carbonate in coral reefs[citation needed].
Genetic studies have identified the naked amoeba Reticulomyxa and the peculiar xenophyophores as foraminiferans without tests[citation needed]. A few other ameoboids 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[citation needed].
[edit] Uses of forams
Because of their diversity, abundance, and complex morphology, fossil foraminiferal assemblages are useful for biostratigraphy, and can accurately give relative dates to rocks. The oil industry relies heavily on microfossils such as forams to find potential oil deposits[citation needed].
Calcareous fossil foraminifera are formed from elements found in the ancient seas they lived in. Thus they are very useful in paleoclimatology and paleoceanography. They can be used to reconstruct past climate by examining the stable isotope ratios of oxygen[citation needed]; see δ18O. Geographic patterns seen in the fossil records of planktonic forams are also used to reconstruct ancient ocean currents[citation needed]. 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[citation needed].
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[citation needed]. Because calcium carbonate is subsceptible to dissolution in acidic conditions, foraminifera may be particularly affected by changing climate and ocean acidification[citation needed].
[edit] References
- ^ a b c d Hemleben, C., Spindler, M.& Anderson, O.R. (1989). Modern Planktonic Foraminifera. Springer-Verlag, 363.
- ^ Kennett, J.P., Srinivasan, M.S. (1983). Neogene Planktonic Foraminifera: A Phylogenetic Atlas. Hutchinson Ross, 265.
- ^ Cavalier-Smith, T. (2003). "Protist phylogeny and the high-level classification of Protozoa". European Journal of Protistology 34 (4): 338-348.
- ^ a b c d Sen Gupta, B.K. (1983). Modern Foraminifera. Springer, 384.
- ^ Kucera, M., Darling, K.F. (2002). "Genetic diversity among modern planktonic foraminifer species: its effect on paleoceanographic reconstructions". Philosophical Transactions of the Royal Society of London A360 (4): 695-718.
[edit] External links
- The University of California Museum of Paleontology website has an Introduction to the Foraminifera
- The star*sand project (part of micro*scope) is a cooperative database of information about foraminifera
- The Smithsonian Institution's National Museum of Natural History has the largest type collection of foraminifera in the world
- Researchers at the University of South Florida developed a system using foraminifera for monitoring coral reef environments
- University College London's micropaleontology site has an overview of foraminifera, including many high-quality SEMs
- A glossary of terms used in foraminiferal research
- CHRONOS has several foraminifera resources, including a taxon search page and a micro-paleo section
- eForams is a web site focused on foraminifera and modeling of foraminiferal shells
- Benthic foraminifera information from the 2005 Urbino Summer School of Paleoclimatology
