Though we spend a good amount of time in the field, a lot of our time is also spent in the lab, analyzing the invertebrates in the soil samples for each field experiment. We do this by processing and counting nematodes and the other microorganisms we see down here.
Processing essentially just means extracting the nematodes from the soil matrix, so we can easily see them under the microscope. It’s a fairly simple process that involves three members of the team working together in an assembly line. First, each sample is weighed inside a laminar flow hood then divided for extraction for nematodes and soil moisture analysis. To measure the amount of water in the soil, a small amount of soil is weighed, placed in an oven, dried, then weighed again. Soil moisture is an important indicator of life in this ecosystem, and is very useful for interpreting the results of our experiments later on.
The other portion of soil goes off to a second person to be filtered: we mix the soil in water and then apply the muddy mixture to two sieves of different sizes, stacked on top of each other. The top sieve has holes large enough to let nematodes through, but too small for most of the dirt and rocks. The bottom sieve uses a mesh fine enough to catch nematodes and similarly sized dirt particles, but allows really fine grains to fall out of the sample.
Next, the third person in the line takes four filtered samples and centrifuges them all at once: the centrifuge pulls all the soil and the nematodes down to the bottom of the container so that we can pour out any leftover water. When that’s done, we add a sugar solution that is key to the whole process. The sugar separates the nematodes from the dirt and sand, pulling them up into a supernatant that can be selectively removed, leaving behind the unwanted soil. This is possible because of density: the nematodes are as dense as the sugar and float, while the rocks and the dirt are denser and sink. Once the sugar is added, the samples are passed back to the person doing the sieving, who removes sugar by filtering them in running water through a very fine sieve and placing the final solution (containing both water and nematodes)into tubes that are immediately refrigerated. The entire extraction process can be very stressful on the organisms in the samples, and so we try and do everything we can to get them through as quick as possible and back into a more natural environment (the fridge).
After processing, each sample is analyzed under the microscope and every animal is identified to species and life stage (male, female and juvenile) and living or dead and counted. This can take only a few minutes (if the sample is completely empty) or nearly an hour. There are three main species of nematode here in the Dry Valleys that we deal with in the lab: Scottnema lindsayae, Eudorylaimus antarcticus, and Plectus murrayi. Scottnema is by far the most common because most of the soils are very dry; the other two species need more moisture and thus show up much more rarely. Sometimes, w e also see rotifers and tardigrades, as well as tiny single-celled eukaryotes like ciliates and amoeba.
When we compare the species that are present in each sample to the moisture content of the sample, we begin to see patterns of how the physical factors of the soil affect species composition (which species are present in a given location). When we take these patterns from year to year and compare them to records of weather and climate we begin to see long-term patterns of how climate shifts and extreme events (like hot summers and subsequent valley “flooding” events) influence the ecology of the organisms living in these valleys. This is one of the main tools, and objectives, of the McMurdo Dry Valley LTER.
This is a species of Eudorylaimus; they are usually much longer than Scottnema or Plectus nematodes.
This is Scottnema lindsayae; these are only found in Antarctica and like the other nematodes there (Plectus, Eudorylaimus) are capable of surviving due to anyhydrobiosis, which allows them to expel the water in their body and roll into a tight ball and reduce their surface exposure to wind and cold. Tardigrades and rotifers can also enter anhydrobiosis.
This is a species of rotifer; compared to the nematodes in these valleys, very little is known about Rotifers. They get their name because the cilia on their mouths move in a way that it looks like they have rotors on their heads.
This is the front half of a tardigrade. Tardigrades have 8 legs and are related to nematodes and arthropods (like insects and spiders). They are also known as Water Bears and can be found in mosses almost anywhere in the world.
This is an unidentified protist. Protists are single-celled eukaryotes (meaning they have a nucleus), and are even less understood than rotifers in these valleys. Unlike the other pictures, I took this photo this season, with just my phone camera and a microscope. I have no idea what species it is! It could even be new to science, which would make it an exciting discovery.