Foraminifera
Foraminifera are small marine organisms (around 1/2 millimeter) that live all over the planet, from the Arctic Sea to the warm tropical currents. They form the major part of the plankton.
Foraminifera have only one cell, protected by a shell called "test". This test is perforated with small pores, from where pass cell's extinctions called pseudopodes. The pseudopodes are used for transport and feeding of the foraminifera.
We can distinguish the planctonic foraminifera, which live in the water, from the benthic foraminifera, which live on or in the sediment, in the bottom water. Nowadays, it exists around 5000 benthic foraminifera species and 500 planctonic foraminifera.
Shape, composition and ornementations of the test are clues to identify the different foraminifera species, with the increasing help of genetic analyses. Among the benthic foraminifera, there are the agglutinated, the hyalin and the porcelaneous ones. The agglutinated forms use sediment grains to build their test, which they link together with a calcite cement, while hyalin and porcelaneous species synthesize their own calcite from sea water ions. The different arrangement of the calcite cristals allow to distinguish the hyalin forms, which are transparent, from the porcelaneous forms, which have a more opaque test.
A tool for paleontologists
Foraminifera have been used by geologists since decades, especially to date sedimentologic layers. Indeed, once the cell is dead, the test falls on the bottom water and can in some cases fossilize. Foraminiferal fauna composition is typical for every geological period and every type of environment, which allows precise sediment dating as well as past landscape reconstruction.
The concentration in various trace elements and isotopes of the test also allows the recontruction of past climates.
A tool for ecologists
Since the last years, foraminifera have been more and more used in environmental pollution studies. Indeed, some species are tolerant while other are very sensitive to pollution. To study which foraminiferal species are living on a site quickly gives information on the pollution state of this site.