Even for those who are knowledgeable about peatlands, it may come as a surprise to know there is a huge diversity of tiny organisms busying themselves around the surface of a Sphagnum leaf or floating through the dark water of a bog pool. Much of our current survey and monitoring only considers the things we can observe with the naked eye, but microorganisms play a critical role in peatland function. There are thousands of species present in UK peatlands, many with fascinating life cycles and beautiful forms. In this showcase, we introduce one particular group: testate amoebae.
What are testate amoebae?
Testate amoebae (Protozoa) are unicellular microorganisms enclosed in a hard shell (‘test’) which protects them from predators. Globally, there are around two thousand known testate amoebae taxa. They vary in size, from 10 µm up to 800 µm – the largest are the size of a coarse grain of sand, whilst the smallest are invisible to the naked eye. You could fit just over 3000 of the smallest testates on the head of a pin!
Some testates build their protective shells from their own secretions. Others create their test by gluing together organic or mineral material from their surroundings, such as mineral grains or shells of other microorganisms like diatoms. Under a microscope, the tests can appear as scales, or even spikes, and the fascinating diversity of shapes, sizes and intricate ornamentation makes it possible to identify different species. The tests have an opening called an aperture, which the amoebae use to feed and move. Their pseudopodia - or “false feet” - can protrude from the aperture to capture prey or help them move around.
Testate amoebae are abundant in many peatlands, including fens and bogs, and other freshwater environments, such as rivers and lakes. Some testate species prefer a wetter environment, whilst others thrive in drier conditions. To adapt to drier surroundings, some amoebae have developed a thin, slit-like aperture that prevents dehydration. In peatlands, testates are often found living in the thin film of water that coats the leaves of mosses, such as Sphagnum. The high diversity of testates in healthy peatlands is due to a diverse microtopography, which creates various microhabitats suitable for different species1.
Why are they important?
Testate amoebae are the most abundant group of microorganisms in peatlands, making up half of the microbial biomass2. They are key predators and dominant microbial consumers in the microbial food web, feeding on bacteria, microalgae, fungi and other protozoans. As predators, they affect carbon and nutrient cycles by modifying the composition of the microbial community.
Some testates are mixotrophic, using a mix of different sources of energy and participating in photosynthesis by acting as a host to green algae. In fact, phototrophic or ‘light eating’ microbes (including mixotrophic testates) fix 10-30% of total carbon in peatlands3. Due to their abundance and function, testates play an important role in carbon and nutrient cycling in peatlands. The variety of microhabitats in healthy peatlands ensures there is high microbial diversity, enhancing overall carbon sequestration.
However, warming and drying of peatlands can lead to a reduction in the abundance of mixotrophic testates, which could limit the carbon uptake of peatlands2. Drying can also reduce the number of large testates, which can further affect the carbon cycle through the alteration of the food web2. Testates are very sensitive to changes in the environment, particularly to fluctuations in water availability, temperature and pH, but also to different types of pollution. Degradation can therefore have adverse effects on microbial species richness and community diversity in peatlands.
Testates have a short life cycle and reproduce rapidly. This means they respond quickly to changes in their surroundings - much faster than other peatland organisms, such as multicellular plants. For this reason, testates have often been used as bioindicators, as fluctuations in their abundance and distribution can be used as a proxy to indicate changes in the environment over short time scales4.
Testates are unique because they leave behind visible evidence of their presence - unlike many other peatland microorganisms, such as fungi, bacteria and archaea. After testates die, their soft bodies decompose, but the hard shell remains and persists in the peat.
Understanding the past
Peatlands are important historical archives that can store the fossil remains of plants, animals, pollen, spores and microorganisms for thousands of years. As peat accumulates, a stratigraphic record of material is produced. Peat cores can be collected to analyse this ecological record. Organic material that contains carbon can be dated, and this can tell us how old different layers of the peat are. The fossil remains extracted from peat can be identified and used to reconstruct past environments - they can tell us which plants and animals dominated the landscape in the past, and what the water levels and climate may have been like. This study of past ecosystems is known as palaeoecology.
Testate amoebae are used in such palaeoenvironmental reconstructions to investigate how peatlands have changed over time. Individual species of testates can be identified under a microscope, with the help of identification resources such as the one found here: Microworld. Knowing the ecological preferences of the identified testates allows scientists to piece together the past environment. Other plant material or pollen preserved at the same depth in peat further helps to construct a snapshot of past conditions.
Sometimes, a ‘functional traits’ approach is used: different species of testates can have similar functional traits (e.g. size or composition of the test) and each of these traits can provide key information about the functioning of the peatland6. For example, a change from large-sized testate taxa (usually found in wet conditions) to small taxa (that can survive in drier conditions) in the peat profile can indicate drying in the past environment. Due to their ecological preferences, testates are often used as hydrological proxies in palaeoecological studies.
In addition to revealing trends in past climate, testates can even pinpoint past disturbances, such as wildfires or volcanic activity4.Palaeoecological information can also be used to predict the effects of climate change on peatlands, or potentially inform conservation practices5. It is therefore vital that these invaluable historical archives are maintained as part of the conservation of peatlands.
Informing the future
The application of testate amoebae is not just limited to the past. Living testates can be collected from pools of water, or from Sphagnum moss by squeezing the water out into a container, for later analysis under a microscope. This can tell us something about the present conditions in the sampled ecosystem.
Scientific literature has explored the potential of using testates to monitor the success of peatland restoration projects and to inform restoration schemes in the UK2. The two main approaches are: palaeoecological studies to investigate past changes or to establish a pre-damage baseline, and the analysis of modern surface samples to identify the species currently living in a peatland7.
Recent studies have looked at how testate communities have changed after peatland restoration activities in the UK. Drain blocking has led to increased testate species diversity and the appearance of species that indicate wet conditions (e.g., Archerella flavum) within only a few years8,9. Other studies have found much longer delays - for example, it took 17 years before microbial assemblages fully recovered after tree removal from a former raised bog10.
Translational palaeoecology is a relatively new concept that refers to an increased dialogue between scientists and practitioners to encourage the use of palaeoecological data to inform current peatland management and conservation. There is a need to design research with this goal in mind, creating partnerships between scientists, practitioners, policy makers and relevant stakeholders to ensure research informs restoration practice.
Making the most of expert knowledge and skills
Using testates (or other bioindicators, such as diatoms, green algae or bacteria) for monitoring peatland restoration schemes can be challenging. First, the complete recovery of microbial assemblages after rewetting degraded peatlands can sometimes be a slow process that takes decades10. Second, the drivers of change in microbial communities may be more complex than assumed, meaning that these methods should be used with caution8.
Third, the process of sampling and identification can be costly and time consuming. Equipment and expertise are needed to identify testates to species level using a microscope, which is a slow process. Extracting tests from a peat core is even more expensive, requiring chemicals and laboratory equipment. There is potential to use DNA barcoding to identify testates in the future, which is less time consuming, but costly7. In the near future, microscopes are likely to remain the main tool for identification.
There is currently a lot we do not know about the distribution and conservation status of testate amoebae in peatlands. It is still unclear what the exact magnitude of their contribution to the carbon cycle in peatlands is, or how they interact with vegetation. However, we know that the specific ecological preferences and the high sensitivity of testates to hydrological conditions and nutrient balance make them vulnerable to drying, pollution and degradation. We also know that climate warming is predicted to cause higher rates of evapotranspiration, which can lead to drying in temperate peatlands11, making testates that prefer wet conditions particularly vulnerable.
Continued study of these diverse and fascinating organisms is essential to our understanding of how peatlands function and the consequences of human impacts on these precious environments. By developing the practice of translational palaeoecology, researchers can work with the wider peatland community to apply their specialist knowledge and expertise, informing the future of peatlands, as well as understanding their past.
This is part of our series of showcases celebrating peatland biodiversity. Our previous showcase introduced the Sphagnum mosses and their role in ecosystem function, informing restoration practice and inspiring creative collective action.
Useful resources
An illustrated children’s book on testate amoebae: The Hidden World of Testate Amoebae
Guide to how live testate amoebae are collected
Video: What is so special about testate amoebae?
References
1. Carballeira R., Pontevedra-Pombal, X. Diversity of testate amoebae as an indicator of the conservation status of peatlands in southwest Europe. Diversity (Basel). 2021; 13(6). https://doi.org/10.3390/d13060269
2. Kuuri-Riutta O., Väliranta M., Tuittila E-S. Literature review on testate amoebae as environmental indicators and as a functional part of the microbial community in northern peatlands. Mires and Peat. 2022; 28. https://doi.org/10.19189/MaP.2022.OMB.StA.2412
3. Hamard S., Céréghino R., Barret M., Sytiuk A., Lara E., Dorrepaal E. et al. Contribution of microbial photosynthesis to peatland carbon uptake along a latitudinal gradient. Journal of Ecology. 2021; 109(9): 3424–3441. https://doi.org/10.1111/1365-2745.13732
4. Freitas Y., Ramos B., Dos Santos Miranda V, Da Silva Y., Sampaio G., Da Silva Nascimento L. et al. Testate amoebae: a review on their multiple uses as bioindicators. Acta Protozoologica. 2022; 1-21. https://doi.org/10.4467/16890027AP.22.001.15671
5. Wingard G.L., Bernhardt C.E., Wachnicka A.H. The Role of Paleoecology in Restoration and Resource Management—The Past as a Guide to Future Decision-Making: Review and Example from the Greater Everglades Ecosystem, U.S.A. Frontiers in Ecology and Evolution. 2017; 5(11). https://doi.org/10.3389/fevo.2017.00011
6. Marcisz K., Jassey V.E.J., Kosakyan A., Krashevska V., Lahr D.J.G., Lara E. et al. Testate amoeba functional traits and their use in paleoecology. Frontiers in Ecology and Evolution. 2020; 8. https://doi.org/10.3389/fevo.2020.575966
7. Valentine J., Davis S.R., Kirby J.R., Wilkinson D.M. The Use of Testate Amoebae in Monitoring Peatland Restoration Management: Case Studies from North West England and Ireland. Acta Protozoologica. 2013; 52: 129–145. doi:10.4467/16890027AP.13.0013.1110
8. Swindles G.T., Green S.M., Brown L., Holden J., Raby C.L., Turner T.E. Evaluating the use of dominant microbial consumers (testate amoebae) as indicators of blanket peatland restoration. Ecological Indicators. 2016; 69: 318–330. https://doi.org/10.1016/j.ecolind.2016.04.038
9. Evans C.R.C., Mullan D.J., Roe H.M., Fox P.M., Gray S., Swindles G.T.. Response of testate amoeba assemblages to peatland drain blocking. Wetlands Ecology and Management. 2024; 32:1-8. https://doi.org/10.1007/s11273-023-09949-w
10. Creevy A.L., Wilkinson D.M., Andersen R., Payne R.J.. Testate amoebae response and vegetation composition after plantation removal on a former raised bog. European Journal of Protistology. 2023; 89:125977. https://doi:10.1016/j.ejop.2023.125977
11. Swindles G.T., Morris P.J., Mullan D.J., Payne R.J. et al. Widespread drying of European peatlands in recent centuries. Nature Geoscience. 2019; 12(11): 922–928. https://doi.org/10.1038/s41561-019-0462-z
