© Andrew Henderson, Natural History Museum (London)
The largest known protist is the xenophyophore Syringammina fragilissima. Xenophyophores are one-celled, multinucleate creatures found at depths of 800 to 6000 meters on the ocean floor. A characteristic feature of xenophyophores is that they construct a sort of artificial exoskeleton by gluing together particles of sediment and the microscopic shells of foraminifera. (The name “xenophyophore” is Greek for “foreign body bearer.”) These agglomerations are called tests.
In the case of S. fragilissima, the test is a network of tubes. The tests are extremely fragile, and specimens are rarely recovered.
In 2006 in an area called the Darwin Mounds, northwest of Scotland, researchers discovered specimens of Syringammina fragilissima with tests as large as 20 centimeters in diameter.
How can they grow so big? Usually, the size of protists is limited by the surface-volume relationship. Multicellular creatures have specialized structures for transporting nutrients and wastes. Diffusion is the main mode of transport in one-celled organisms, and at large sizes diffusion is not enough to sustain the cell's interior. Some xenophyophores minimize the problem by having a coin-like shape; Stannophyllum venosum is 25mm across, but only 1mm thick. Syringammina fragilissima is more of a blob. However, its surface area is increased by having a structure of branching tubes, supported by the test.
Another factor that helps to account for its gigantic size is that it lives in water. It is thought that xenophyophores are filter feeders. The large specimens are found on sea bottoms where there is a current, and lie beneath surface waters rich in food.
However, although they were discovered in the 19th century, very little is known about these creatures.
Building shells from bits of sand picked up from the environment is not limited to protists on the scale of S. fragilissima. Much smaller protists also construct shells, perhaps the most amazing example being the amoeba Difflugia coronata, about 150 micrometers in diameter.
For a description of the Darwin Mounds: www.deepseascape.org/darwin.php
Driesch has laid particular stress upon this principle of a ‘fixed cell-size’, which has, however, its own limitations and exceptions. Among these exceptions, or apparent exceptions, are the giant frond-like cell of a Caulerpa [a genus of green algae, some species being perhaps the largest single-celled organism] or the great undivided plasmodium of a Myxomycete [a true slime mold]. The flattening of the one and the branching of the other serve (or help) to increase the ratio of surface to content, the nuclei tend to multiply, and streaming currents keep the interior and exterior of the mass in touch with one another.
D’Arcy Wentworth Thompson.
Of Growth and Form.
Cambridge (UK): Cambridge University Press, 1917.
© Hervé Moreau, Laboratoire Arago
The smallest known free-living eukaryotes are marine picoplankton, of which the best-studied is Ostreococcus tauri. These organisms are so small (about 1 micrometer in diameter) that they are near the limit of resolution of ordinary light microscopes. O. tauri has a single chloroplast and a single mitochondrion. Despite their size, these organisms are extremely important contributors to the productivity of the oceans.
Chrétiennot-Dinet, M-J., C. Courties, A. Vaquer, J.
Neveux, H. Claustre, J. Lautier and M. C. Machado .
A new marine picoeucaryote: Ostreococcus tauri gen et sp. nov. (Chlorophyta, Prasinophyceae).
Phycologia vol. 34, pages 285-292 (1995).
Evelyne Derelle, Conchita Ferraz, Stephane Rombauts,
Pierre Rouzé, Alexandra Z. Worden, Steven Robbens,
Frédéric Partensky, Sven Degroeve, Sophie Echeynié,
Richard Cooke, Yvan Saeys, Jan Wuyts, Kamel Jabbari,
Chris Bowler, Olivier Panaud, Benoît Piégui, Steven G.
Ball, Jean-Philippe Ral, François-Yves Bouget,
Gwenael Piganeau, Bernard De Baets, André Picard, Michel
Delseny, Jacques Demaille, Yves Van de Peer,
and Hervé Moreau.
Genome analysis of the smallest free-living eukaryote Ostreococcus tauri unveils many unique features.
Proceedings of the National Academy of Sciences, vol 103, no. 31, pages 11647-11652. www.pnas.org/cgi/doi/10.1073/pnas.0604795103
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