Foraminifera

The Foraminifera, ("Hole Bearers") 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 test, or shell, 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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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 discrepant 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^[6]

The 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 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]

Tests

Main article: Test (biology)

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^[7]. 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^[8].

Genetic studies have identified the naked amoeba "Reticulomyxa" and the peculiar xenophyophores as foraminiferans without tests^{[citation needed]}. 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 ^[9].

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 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 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.

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, and the history of the carbon cycle and oceanic productivity by examining the stable isotope ratios of carbon ^[10]; see δ18O, see δ13C. 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 susceptible to dissolution in acidic conditions, foraminifera may be particularly affected by changing climate and ocean acidification^{[citation needed]}.

References