How shall I build my test?

How do testate amoebae build their tests? How they chose the components to build it? How do they assemble organic or inorganic particles that shape their shell? These are questions I’m often asked. Unfortunately, these questions are very difficult to answer using common tools like the light microscope.

During the last couple decades, testate amoebae have been increasingly used as proxies for reconstructing Holocene environmental change in peatlands. Community composition primarily reflects surface wetness and pH, and can be used to study mire development, climate change and human impacts. However, little is known regarding the factors that may alter quantitatively or qualitatively the test composition of these organisms. Recently studies observed variations in shell composition of some testate amoebae in acidic environments, and suggested that a better understanding of how testate amoebae build their test may improve paleo-reconstruction models (Mitchell et al. 2008).

Historically, many researchers have worked on characterizing the shell of testate amoebae (e.g., Moraczewski 1971, Netzel 1972, Saucin Meulenberg et al. 1973, Eckert et al. 1974, Stout & Walker 1976, Hedley et al. 1976, Golemansky & Couteaux 1982, Ogden 1980a, b, 1983, 1984). Unfortunately, this line of research on testate amoebae has diminished over time.

Structural variability of the shell of testate amoebae (Source; Maxence Delaine)

Structural variability of the shell of testate amoebae (Source; Maxence Delaine)

So, when I heard that a PhD student – Maxence Delaine – had recently worked on this topic in France I was very curious. To satisfy my curiosity, I met Maxence Delaine and he explained me what they found in their recent paper (Armynot du Chatelet et al. 2013).

Across 14 sites situated in north-eastern France, Maxence collected samples from different microhabitats, such as mosses and soil, to study variations in testate amoeba shell composition.

Sites sampled for determination of shell construction of testate amoebae

Sites sampled for determination of shell construction of testate amoebae

3D representation of one species of testate amoebae, Difflugia oblonga. This picture is obtained by numerical recombination and correction of numerous 2D slides which are given by X-Ray microtomography. We can see on this picture the variability of the numerous grains constituting the shell: small vs big or smooth vs angular particules. (Source Maxence Delaine)

3D representation of one species of testate amoebae, Difflugia oblonga.
This picture was obtained by numerical recombination and correction of numerous 2D slides which are given by X-Ray microtomography.  The picture highlights the variability of the grains constituting the shell: small vs big or smooth vs angular particules. (Source Maxence Delaine)

The authors explored the potential application of 3D X-ray micro-tomography in addition to 2D techniques (Environmental Scanning Electron Microscope, Electron Probe Micro-Analysis, and cathodoluminescence) to characterize specimens such as Difflugia oblonga. The goal of this work was to test whether 3D morphology of testate amoebae in aqueous environments was governed by sediment size distribution and mineralogical composition.

From the 3D images, the authors calculated different parameters characterising the geometry of the specimens (size and mass) and of the individual grains forming the specimen (grain size distribution and volume). Combining chemical, mineralogical and morphological analyses allowed them to compare the grains forming the test with those of the sediment. Surprisingly, they found that Difflugia oblonga selectively picked up the small size fraction of the sediment with a preference for low-density silicates close to quartz density (~2.65). They also found that the maximum-sized grains are used for the pseudostome (i.e. shell aperture).

The following diagram shows that Difflugia oblonga is able to select the grains based on size, because the grain-size of the sediment is completely different from the grain-size of the particles constituting these amoeba shells. Moreover, no particles exceed the limit ‘‘αβ’’, which corresponds to the maximal measured size of the pseudostome of these 2 individuals. It seems likely that all the particles must pass through the pseudostome before being distributed by the amoeba for the shell’s construction.

Histograms of the particles size which constitute the shell of 2 individuals (Difflugia oblonga), compared to the grain-size curve of the sediment (in which these amoebae lived).

Histograms of the particles size which constitute the shell of 2 individuals (Difflugia oblonga), compared to the grain-size curve of the sediment (in which these amoebae lived).

Amazing isn’t it? A single-celled organism selecting the “bricks” for its house! Research on this topic is very promising, and these results highlight that there is still much to learn shell composition of these amazing organisms.

References

ARMYNOT DU CHATELET E., NOIRIEL C., DELAINE M. (2013). 3D morphological and mineralogical characterisation of testate amoebae.  – Microscopy and Microanalysis, 19, 1511-1522.

ECKERT B.S., MCGEE-RUSSELL S.M., 1974, Shell structure in Difflugia lobostoma observed by scanning and transmission electron microscopy. – Tissue & Cell, 6, 215-221.

HEDLEY R.H., OGDEN C.G. & MORDAN N.J., 1976, Manganese in the shell of Centropyxis (Rhizopodea: Protozoa). – Cell Tiss. Res., 171, 543-549.

GOLEMANSKY V. & COUTEAUX M.M., 1982, Etude en microscopie électronique à balayage de huit espèces de thécamœbiens interstitiels du supralittoral marin. – Protistologica, 18, 473-480.

MITCHELL E.A.D., PAYNE R.J & LAMENTOWICZ M., 2008, Potential implications of differential preservation of testate amœba shells for paleoenvironmental reconstruction in peatlands, – J. Paleolimnol., 40, 603-618.

MORACZEWSKI J., 1971a, La composition chimique de la coque de Arcella discoides Ehrbg. – Acta Protozool., 8, 407-422.

NETZEL H., 1972, Die schalenbildung bei Difflugia oviformis (Rhizopoda, Testacea). – Z. Zellforsch, 135, 55-61.

STOUT J. D. & WALKER G. D., 1976, Discrimination of mineral particles in test formation by thecamœbae. – Trans. Amer. Micros. Soc., 95, 486-489.

OGDEN C.G. & HEDLEY R.H., 1980a, An atlas of freshwater testate amœbæ, Oxford University Press, 113 p.

OGDEN C.G., 1980b, Shell structure in some pyriform species of Difflugia (Rhizopodea). – Arch. Protistenk., 123, 455-470.

OGDEN C.G., 1983, Observations on the systematics of the genus Difflugia in Britain. – Bull. Br. Mus. Nat. Hist. (Zool.), 44, 1-73.

OGDEN C.G., 1984, Shell structure of some testate amœbæ from Britain (Protozoa, Rhizopoda). – Journal of Natural History, 18, 341-361.