Testate amoebae on fire

Contributed by Katarzyna Marcisz

How do testate amoeba communities respond to fire? How does burning affect this group of microorganisms and the environment where they occur in exceptionally high abundance – Sphagnum peatlands?

Answering these questions is crucial, as fire has a significant impact on peatland ecosystems. Peatlands are an important carbon pool, containing 1/3 of the global soil carbon (Parish et al., 2008). It has been shown that even a moderate drop in water table influences vegetation composition in peatlands and disturbs carbon accumulation (Kettridge et al., 2015). Peatlands experiencing drying are also more often ignited, and the frequency of fire and the extent of peat fires often increases (Turetsky et al., 2015). Peat fires can be deceptive: peat often burns by smouldering combustion that can persist for long periods of time rather than by more visible large fires as in the case of forests. The consequence is nevertheless that this burning affects the peat carbon stock.

The relationships between testate amoebae and fire are still quite a mystery. In studies from North America, Clifford and Booth (2013) and Clifford and Booth (2015) revealed peaks in microscopic charcoal accumulation rates that corresponded to drought periods as reconstructed with the use of a testate amoeba-based transfer function. However, these charcoal peaks were likely primarily derived from regional, upland fires rather than fire on the peatlands themselves. The effects of peatland fires on testate amoeba communities, and the response of individual species, have not been adequately examined. For example, Trigonopyxis arcula was suggested to be a possible fire indicator by Warner (1990) and Turner et al. (2014), and Hyalosphenia subflava was correlated with fire in the work by Turner and Swindles (2012). However, Turner et al. (2014) later suggested it should rather not be regarded as a reliable local fire indicator.

Fig. 1. Linje mire in the spring 2013.

Fig. 1. Linje mire in the spring 2013.

Recently the relationships between individual testate amoeba species and fire were examined in a study by Marcisz et al. (2015), which was conducted within the RE-FIRE Sciex project (http://www.swissuniversities.ch/en/topics/sciex) and CLIMPEAT project (www.climpeat.pl), in northern Poland at a beautiful Linje mire (Fig. 1).

The analyses revealed that the peatland was wet before the onset of the Little Ice Age (ca. AD 1300-1850), when a rapid drop in water table occurred. Drying was also correlated with human migrations in the region, and with changing agricultural practices. Anthropogenic fires preceded hydrological changes on the mire; the response of the mire recorded as hydrological changes towards drier conditions was delayed in relation to the surrounding fire-related vegetation changes.

Fig. 2. Cross-correlation diagrams between testate amoeba species and macroscopic charcoal particles (from Marcisz et al., 2015)

Fig. 2. Cross-correlation diagrams between testate amoeba species and macroscopic charcoal particles (from Marcisz et al., 2015)

Individual testate amoeba species were correlated with macroscopic charcoal particles recorded in the peat profile (Fig. 2). Testate amoebae indirectly responded to vegetation removal in the catchment driven by fire. While all the wet indicator species were negatively correlated with fire activity, most of the dry indicators (as well as Arcella discoides) were positively correlated. Although no explicit local fire indicator was found, from all the testate amoeba species Nebela tincta s.l. had the highest positive correlation recorded (Fig. 3)…..


Fig. 3. Nebela tincta s.l. on fire.

Fig. 3. Nebela tincta s.l. on fire.

Additional work is needed to better quantify the relationships between testate amoebae, hydrological change, and fire, and experimental work is needed to assess the causes of these relationships. Such work may provide insight into the underlying processes controlling microbial community responses to fire and hydrological change. However, fire activity likely affects peatland microbial food webs, both directly (combustion of surface Sphagnum and peat layers) and indirectly (surrounding vegetation and surface run-off changes).

Literature cited

Clifford, M.J., Booth, R.K., 2013. Increased probability of fire during late Holocene droughts in northern New England. Climatic Change 119, 693–704.

Clifford, M.J., Booth, R.K., 2015. Late-Holocene drought and fire drove a widespread change in forest community composition in eastern North America. The Holocene 25, 1102-1110.

Kettridge, N., Turetsky, M.R., Sherwood, J.H., Thompson, D.K., Miller, C.A., Benscoter, B.W., Flannigan, M.D., Wotton, B.M., Waddington, J.M., 2015. Moderate drop in water table increases peatland vulnerability to post-fire regime shift. Scientific Reports 5.

Marcisz, K., Tinner, W., Colombaroli, D., Kołaczek, P., Słowiński, M., Fiałkiewicz-Kozieł, B., Łokas, E., Lamentowicz, M., 2015. Long-term hydrological dynamics and fire history over the last 2000 years in CE Europe reconstructed from a high-resolution peat archive. Quaternary Science Reviews 112, 138-152.

Parish, F., Sirin, A., Charman, D.J., Joosten, H., Minayeva, T., Silvius, M., Stringer, L., 2008. Assessment on peatlands, biodiversity and climate change: main report. Global Environment Centre, Kuala Lumpur and Wetlands International, Wageningen 179 pp.

Turetsky, M.R., Benscoter, B., Page, S., Rein, G., van der Werf, G.R., Watts, A., 2015. Global vulnerability of peatlands to fire and carbon loss. Nature Geosci 8, 11-14.

Turner, T.E., Swindles, G.T., 2012. Ecology of Testate Amoebae in Moorland with a Complex Fire History: Implications for Ecosystem Monitoring and Sustainable Land Management. Protist 163, 844-855.

Turner, T.E., Swindles, G.T., Roucoux, K.H., 2014. Late Holocene ecohydrological and carbon dynamics of a UK raised bog: impact of human activity and climate change. Quaternary Science Reviews 84, 65-85.

Warner, B.G., 1990. Testate amoebae (Protozoa). Methods in Quaternary ecology no. 5. Geosci. Can. 5, 65-74.

Tropical testate amoebae as hydrological indicators?

Sampling testate amoebae in a tropical peatland. A recent paper in Microbial Ecology by Swindles et al. suggests that testate amoebae have potential as hydrological indicators in tropical peatlands.

Sampling testate amoebae in a tropical peatland. A recent paper in Microbial Ecology by Swindles et al. suggests that testate amoebae have good potential as hydrological indicators in tropical peatlands.

Testate amoebae have been successfully used as indicators of past changes in peatland hydrology, particularly ombrotrophic (i.e., nutrients derived exclusively from precipitation) peatlands of north-temperate and boreal regions.  Over the past couple decades, many ecological studies of testate amoebae have been performed in these northern bogs, allowing empirical relationships between community composition and surface moisture to be described. Because the shells of testate amoebae preserve well in the acidic and anaerobic environment of bogs, these modern relationships have been used to infer past changes in the relative wetness of the bog surface from the composition of subfossil communities.  Much recent work has focused on the validation and interpretation of testate amoeba paleohydrological records from bogs, and their application to pressing global change questions.

Surveying along a transect across the peatland.

Surveying along a transect across the peatland.

However, very little is known regarding the potential paleoenvironmental applications of testate amoebae in tropical peatlands.  Peatlands in this region contain a large reservoir of soil carbon, and are extremely vulnerable to environmental changes including changes in land use (e.g., drainage, deforestation) as well as ongoing climate change. How have these peatlands responded to past environmental changes? How similar are communities of testate amoebae in these systems to those of northern bogs? Can testate amoebae be used as indicators of past hydrological changes in tropical peatlands? A recent study by Swindles et al. published in the journal Microbial Ecology set out to find preliminary answers to some of these questions.

One of 100 sampling sites.  Testate amoeba communities and several environmental variables were examined at each site.

Testate amoeba communities and several environmental variables were examined at 100 sites.

The study examined the present-day testate amoeba communities on a ombrotrophic peatland in Peruvian Amazonia. Surface samples of testate amoeba communities and measurements of water-table depth, pH, litter-moisture content, vegetation, and loss-on-ignition were taken along a transect across the peatland. The field work sounds like it was challenging, as the authors succinctly noted that the orientation of the transect had to be a bit flexible given certain obvious constraints:

A slight change in direction was needed half-way along the transect to avoid working in an area containing snakes.

Clearly the work was not well suited for ophidiophobes.

The research group also collected a peat core from the site and assessed changes in testate amoeba communities for the last several thousand years, using the results of their modern survey to inform their interpretation of the paleoecological record.

Tropical testates!

Tropical testates!

Results of the study indicated that the species composition of testate amoebae on this tropical peatland was most strongly related to measurements of water-table depth, with secondary relationships to pH. Forty-seven testate amoeba taxa were encountered, including one species only found in the southern hemisphere. Some taxa were clearly diagnotic of particular habitats (e.g., pools on the peatland surface). Testate amoebae were also preserved in the peat core,  and the authors applied the results of their modern study to infer changes in past water-table depth for approximately the last 3000 years. Although there is clearly much more work needed to describe the ecology of testate amoebae in peatland systems of the tropics, the results of this work indicate good potential to use testate amoebae in paleoenvironmental studies of these important, vulnerable, and unique peatland systems. Hopefully more work will follow soon.