Thursday, 4 September 2014

Syringammina fragilissima: World's largest unicellular organism

Syringammina fragilissima, world's largest unicellular organism
Syringammina fragilissima
Credit:  Andy Gooday
Kingdom: Rhizaria
Phylum: Foraminifera
Class: Xenophyophorea
Order: Psamminida
Family: Syringamminidae
Genus: Syringammina
Species: Syringammina fragilissima
Common name(s): None

Hard to imagine that the sponge-like thing in the photo is actually comprised of just one cell, right? Meet Syringammina fragilissima, the world's largest unicellular organism, with a maximum diameter of at least 20 cm (~8 in)!

Discovery & History
The species was first described by two specimens collected by the Triton ship in 1882, in the sea north of Scotland, under the guidance of oceanographer John Murray (1841 -1914) who send the collected specimens to his colleague Henry Brady for examination. The specimens were in bad shape and broken in many pieces. Still, Brady identified them as a new species, S. fragilissima, which roughly translates to "fragile sand pipe". Murray and Brady had just discovered the first representative of the single-celled xenophyophores.

In 2006, in an undersea area known as Darwin Mounds, northwest of Scotland, researchers discovered the largest ever collected S. fragilissima specimens, as large as 20 cm in diameter.

Xenophyophores is a class of large, one-celled, sponge-like multinucleate organisms found at depths ranging from 800 and up to 10.000 meters on the ocean floor. Their characteristic trait is that they are composed of viscous fluid called cytoplasm, containing numerous nuclei distributed evenly throughout. This is contained within a ramose (highly branched) system of tubes called a granellare, itself composed of an organic cement-like substance.

Similarly for S. fragilissima, the test is a network of extremely fragile tubes. The cell branches and splits into hundreds of tubes, which ramify and interconnect in a hugely complex network. Each individual has multiple nuclei scattered throughout the tubes. As the test grows larger in size, the cell abandons parts of it, which may be taken over by a range of small animals, like nematodes.

The tests produced by Syringammina fragilissima are by far the largest structures created by any known one-celled organism.

Syringammina fragilissima sketch
Syringammina fragilissima sketch
Credit: World Register of Marine Species, Cedhagen, Tomas
 (CC BY-NC-SA 3.0)
Full of mysteries
Although abundant, Xenophyophores are poorly understood and we know very little about them. The same is true for Syringammina fragilissima. For example, we have no idea about the species' reproduction and their life cycle in general. Xenophyophores, is part of the Foraminifera phylum. Many Foraminifera organisms can switch between sexual and asexual reproduction. This may also be the case with S. fragilissima.

How the organism feeds is also unknown. However, research has shown that xenophyophores have high concentrations of lipids within their cytosol, indicating that they may feed on bacteria from the sediment that makes up the sand tubes. Research also suggests that they may have a limb-like structure that pokes out and grabs food.

And finally we have a third mystery. Scattered throughout the cell (as in most xenophyophores) are tiny crystals of barium sulphate (BaSO4). They may be a waste product, ballast, or something else. We really don't know.

Unfortunately, research on the species is very difficult, thanks to their deep-sea and brittle nature. Their limited distribution doesn't help either. This means that we probably aren't getting any answers any time soon!

Other Interesting Facts
- The name xenophyophore is Greek for "foreign body bearer."

References & Further Reading
- Laureillard, J., Méjanelle, L., & Sibuet, M. (2004). Use of lipids to study the trophic ecology of deep-sea xenophyophores Marine Ecology Progress Series, 270, 129-140 DOI: 10.3354/meps270129
- Hughes, J., & Gooday, A. (2004). Associations between living benthic foraminifera and dead tests of Syringammina fragilissima (Xenophyophorea) in the Darwin Mounds region (NE Atlantic) Deep Sea Research Part I: Oceanographic Research Papers, 51 (11), 1741-1758 DOI: 10.1016/j.dsr.2004.06.004
- Pawlowski J, Holzmann M, Fahrni J, & Richardson SL (2003). Small subunit ribosomal DNA suggests that the xenophyophorean Syringammina corbicula is a foraminiferan. The Journal of eukaryotic microbiology, 50 (6), 483-7 PMID: 14733441
- Hopwood, J., Mann, S., & Gooday, A. (2009). The Crystallography and Possible Origin of Barium Sulphate in Deep Sea Rhizopod Protists (Xenophyophorea) Journal of the Marine Biological Association of the United Kingdom, 77 (04) DOI: 10.1017/S002531540003856X
- Richardson, S. (2001). Syringaminna Corbicula Sp. Nov. (XENOPHYOPHOREA) From the Cape Verde plateau, E. Atlantic The Journal of Foraminiferal Research, 31 (3), 201-209 DOI: 10.2113/31.3.201

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