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.

 

 

50 million-year-old testate amoebae

Testate amoebae have an extraordinarily long fossil record, with the oldest known fossils 750 to 700 million years old (Porter and Knoll, 2000; Bosak et al. 2011; Corsetti et al. 2003); however, knowledge of the evolutionary history of these single-celled organisms is surprisingly spotty.  In a recent paper in the journal Protist, Barber et al. help to fill this knowledge gap by describing some beautifully preserved testate amoeba remains from a ~50-million year old lake deposit.

A 163-meter long core was recovered from a kimberlite diatreme within the Slave Craton in the Northwest Territories of Canada in the Northwest Territories of Canada.

A 163-meter long core was recovered from a kimberlite diatreme within the Slave Craton in the Northwest Territories of Canada in the Northwest Territories of Canada.

Eocene-age scales probably representing testate amoebae in the genus Scutiglypha (from Barber et al. 2013).

Fossil scales above show the denticulate margin commonly found on plates bordering the aperture (from Barber et al. 2013).

The fossils represent plates of Euglyphid testate amoebae (Rhizaria: Euglyphida), a group of testate amoebae that construct siliceous shells, or tests, out of secreted plates. The plates are arranged on the shell in an overlapping fashion, and the morphology of individual plates varies by species and by location on the test. Barber et al. recovered several different types of plates in the deposit, and the remarkable preservation and abundance allowed even fine-scale features, like denticulate plate margins and pitting in the central portions of some plates, to be examined. Some of the fossil plates were morphologically indistinguishable from modern species of the genus Scutiglypha, suggesting that little evolutionary change in plate morphology has occurred for the last 50 million years, and perhaps much longer. Read the paper here.

Image of a modern specimen of Scutiglypha acanthophora showing the arrangement and morphology of plates on the test (from http://www.arcella.nl/).