Testate amoebae
Testate amoebae are unicellular organisms that live mostly in the soil, mainly in wetlands and particularly in raised bogs. A very few species live in open water and are therefore part of ocean plankton.The clue is in the name. Amoebae means that their cell does not have a determined shape, and moves in a particular way, called amoeboid. It creates a protuberance called pseudopod, with which it propels itself forward. Test is the name of the shell they produce to protect themselves. The shell has a hole to let the pseudopods out, to move around or to collect food. Some species of testate amoebae produce themselves the material used to build their test. It can be organic, based on silica or calcareous compounds. Other species pick up debris or sediments from their surroundings, like for example abandoned diatom frustules, and glue them together to make the test. A few species are even able to switch between the two techniques, depending on what’s available in their environment.
When testate amoebae are extracted from soil samples, the test’s shape and size can be used to identify the different species, looking through a microscope. Although with the technological advances in genetic sequencing in the last couple of decades, it has been revealed that there is a whole lot of cryptic species1. It seems that different species look the same and cannot be identified just by their tests morphology.
Pseudopods
Different species of testate amoebae form different types of pseudopods. Three main types are usually determined, called lobopodia, filopodia and reticulopodia. Lobopodia are formed of one big broad pseudopod, filopodia are slender and pointed, while reticulopodia are long and thin pseudopods interconnected. Some testate amoebae are very fierce hunters of small rotifers, ciliates, and other small protists, bacteria, and cyanobacteria. Very recently it has been discovered that there are even cases of pack hunting of nematods2. Non-hunting testate amoebae feed on diatoms, mycelia and spores of fungi.
Carnivorous or not, once the amoebae get to their food of choice, their pseudopod engulfs the particles and absorbs them through the process called phagocytosis. A third way used by a large proportion of testate amoebae to feed is the symbiosis with photosynthetic microalgae. In particular, the alga called Chlorella variabilis is very commonly found in testate amoebae in Sphagnum dominated peatlands and is thought to be responsible for a considerable amount of atmospheric carbon fixation.
The use of testate amoebae to reconstruct past environmental conditions
After testate amoebae die, their test cannot be decomposed because of its mineral origin. Much as diatoms in the water, testate amoebae are highly sensible to their surrounding environmental conditions, meaning that small shifts that might not significantly impact other forms of life have a tremendous effect on the communities of testate amoebae. This strong correlation between the type of amoebae communities and the habitat is exploited by environmental scientists to reconstruct past climatic and environmental conditions. Once the necessary combination of soil conditions associated with a particular species or community of species is determined, the identification of said community in a soil sample can be used as an indicator of that combination of conditions. It is the subject of a science called paleolimnology, where biological indicators (or chemical or physical proxies) preserved in sediments are used to reconstruct the past. Concerning testate amoebae, this link is significantly stronger in raised bogs and becomes problematic in more minerals soils. Their use as bioindicators is therefore limited to these particular ecosystems.
Ubiquitous or endemic?
Testate amoebae are classified as being part of the large group of protists. Protists are, very simply put, unicellular organisms that are structurally more complex than bacteria. They can present characteristics similar to fungi, animal or plants, but at the micro-scale of one individual cell. Because of their size, they have been discovered relatively late, with Antonie van Leeuwenhoek describing them for the first time around 1665. For a long time it was assumed that, thanks to their size, protists, and all microorganisms alike, would have the ability to reach and colonise all suitable environments around the world4. Which would make them ubiquitous. More recently, accumulating studies are pointing towards a different story. It seems that even protists might encounter geographical barriers to colonisation and develop some degrees of endemism. Distribution, biogeography and endemism are at the centre of controversy, and current large scale projects aim to get to the bottom of it…
1 Kosakyan et al. (2012). DOI: 10.1016/j.protis.2011.10.003
2 Geisen et al. (2015). DOI: 10.1111/1462-2920.12949
3 Mitchell & Meisterfeld (2005). DOI:10.1016/j.protis.2005.07.001
4 Finley (2002). DOI: 10.1126/science.1070710