Blobby Shell-Dwellers Harness the Sun

Written by: Alexis Stansfield, Earth & Environmental Science, Lehigh University

Cover image: Peat moss with an inset photo of a testate amoeba with symbiotic algae inside (Source: Alexis Stansfield) 

In the last several decades, warming has resulted in numerous ecological changes in Arctic ecosystems. Understanding these changes is critical to anticipating potential climate feedbacks, particularly given the abundance of carbon-rich ecosystems in these regions¹. Although organic soils in the Arctic are typically much shallower than those further South, they are more vulnerable to climate change, so understanding their dynamics is vital.

As shown in Figure 1, peat is a type of organic soil made up of partially-decomposed plants. In other words, a peatland’s plant matter accumulates faster than it decays. Certain conditions can cause this slow decomposition rate, including very wet or very dry soil, acidic soil, poor availability of nutrients such as nitrogen, and a cold climate. In an Arctic peat bog, all of these conditions help to preserve the plant matter.

Paleo-ecology is the study of the composition and distribution of past ecosystems and their changes through time. A paleo-ecologist can learn a tremendous amount of information by studying peat, since it can typically provide a complete record of the plant community that has been accumulating at that site.

Figure 1. A peat core from the Brooks Range Foothills on the North Slope of Alaska. Note the fluffy living Sphagnum moss at the top (left in the photo) of the core and the highly-decomposed organic soil at the bottom (right in the photo) of the core.

Testate Amoebae

As part of my doctoral research in Dr. Bob Booth’s Lab, I document the changes over the past 250 years in both the plant and microbe communities of peatlands in the Arctic. I primarily focus on Sphagnum moss and several species of amoebae. An amoeba is any single-celled organism that is able to change its shape to engulf the organisms it eats. “Testa” is Latin for shell, and testate amoebae are amoebae that have a shell (see Figure 2).

Testate amoebae are found all over the world in every ecosystem, but they are especially abundant in the thin layer of water that covers moss leaves. When we look at Sphagnum moss in particular, testate amoebae make up as much as 30% of the microbial biomass in the water film². All testate amoebae consume other microorganisms as part of their diet. These food sources can include bacteria, fungi, algae, and fellow single-celled animals, even other testate amoebae.

In addition to eating food, some species of testate amoebae can also obtain carbon energy from the sun. Algae lives inside the amoeba and through photosynthesis provides energy to the amoeba. In return, the algae are protected from predators and short periods of drought. Thus, this is a beneficial relationship for both organisms.

The algae remain dormant whenever the amoeba is living in darkness, since light is required for photosynthesis. For a majority of the year, Arctic peatlands are covered with snow, meaning the testate amoebae must live exclusively off heterotrophy (eating other organisms), but during the short summer, they have access to both sources of energy.

Figure 2. Four different species of testate amoebae. The two on the left have symbiotic algae.

Increasing Temperature and its Effects

The global mean temperature has increased at an average rate of 0.08°C each decade since 1880. The rate since 1980, however, averages 0.18°C per decade, more than twice as fast³. Keeping in mind this increasing global average, consider that the Arctic has locally warmed 2-7 times faster than the global average over the past four decades⁴, a phenomenon known as Arctic amplification. As a result, the first snowfall on Arctic peatlands occurs later in the year, and Spring melting occurs sooner. The “snow-free season” has gotten, on average, one week longer each decade since the 1970’s⁵.

This prompts the following research question: if snow cover has changed over the past century, has the behavior of testate amoebae with symbiotic algae changed as well? 

It seemed reasonable to expect that the testates would get a larger portion of their energy from photosynthesis when exposed to longer periods of sunlight. I had to find out whether I could quantify that change in energy sources using carbon isotope chemistry.

Using a highly precise technique originally developed for analyzing the chemistry of individual grains of pollen, I measured the presence of two stable isotopes of carbon – Carbon 13 and Carbon 12 – in the amoeba species with symbiotic algae. I compared this to the carbon signature for the Sphagnum moss from the same depth in the soil core, to get a baseline of their environment. The Sphagnum, and therefore food sources, have a lower signature (-26‰ to -31‰) than carbon captured through photosynthesis (-14‰ to -15‰). I confirmed the carbon isotope signature of amoebae with symbiotic algae reflects their usage of both energy sources (-19‰ to -26‰), as shown in Figure 3. I also found that the prevalence of these species increased dramatically compared to testate amoebae without symbiotic algae, implying that the algae provide a bigger advantage than they used to when the globe was cooler.

Figure 3. Preliminary results showing the approximate percentage of carbon contributed to these testate amoebae from photosynthetic algae⁶.

Looking Forward

The carbon isotope signatures of testate amoebae had never before been measured in a peat core. Previously, carbon isotopes had been used to compare testate amoebae from different places, but not back in time.  my data appears to be highly correlated to the record of snow-free season length and reconstructed temperature data for the Alaskan Arctic (Figure 4).

Understanding the dynamics of these ecosystems as the globe has warmed will be crucial to anticipating future changes, as well as the potential for climate feedback due to changes in carbon balance. The microbial carbon cycle is a vital component of the greater global carbon cycle. As rates of climate change continue to increase, it is uncertain whether this will lead to large-scale ecosystem transformations and associated changes in carbon balance. My research will improve our understanding of the implications of ecologically critical peatland processes.

Figure 4. Algal symbiont use plotted against a measured record of snow-free days on the North Slope of Alaska, and plotted against a reconstructed temperature record⁶.

Reference

1. Schuur, E.A.G. et al. (2015). Climate change and the permafrost carbon feedback. Nature 520, 171-179. https://doi.org/10.1038/nature14338
2. Mitchell E. A. D., Gilbert D., Buttler A., Amblard C., Grosvernier P., Gobat J.-M. (2003). Structure of microbial communities in sphagnum peatlands and effect of atmospheric carbon dioxide enrichment. Microb. Ecol. 46 187–199. 10.1007/s00248-002-0008-5
3. NOAA National Centers for Environmental Information, Monthly Global Climate Report for Annual 2021, published online January 2022, retrieved on September 19, 2023 from https://www.ncei.noaa.gov/access/monitoring/monthly-report/global/202113.
4. Rantanen, M., Karpechko, A.Y., Lipponen, A. et al. (2022). The Arctic has warmed nearly four times faster than the globe since 1979. Commun Earth Environ 3, 168. https://doi.org/10.1038/s43247-022-00498-3
5. Cox, C. J., Stone, R. S., Douglas, D. C., Stanitski, D. M., Divoky, G. J., Dutton, G. S., … & Longenecker, D. U. (2017). Drivers and environmental responses to the changing annual snow cycle of northern Alaska. Bulletin of the American Meteorological Society, 98(12), 2559-2577.
6. Stansfield, A., Booth, R.K., Nelson, D., Johnson, J. Increased phototrophy in testate amoebae associated with recent warming on Alaska’s North Slope. Poster presentation. Ecological Society of America, Portland, 8 August 2023.