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Individual species have ranges that limit where they live–not all species are found everywhere. For over a century, naturalists and ecologists have worked to discern what governs the limits of those ranges, and therefore determines what species are in a given location or where a species might be found. This field is known as “biogeography” and it demonstrates the intricate linkages between ecology and evolutionary biology. The most obvious foundations for this field of study were set down by Darwin during his voyage on the Beagle (seeing all the different organisms spread across the globe helped to get him thinking about his theory of evolution by natural selection), as well as by Alfred Russel Wallace, a less well-known contemporary of Darwin’s who independently and near-simultaneously came up with the theory of evolution of species while studying birds in New Guinea (Wallace even has an imaginary boundary named after him that separates the very different organisms of Asia from those of the Australian and New Guinean areas).
Questions about what limits species ranges aren’t limited to organisms like birds, trees or mammals, though. Part of the work we do down here is looking at what controls the ranges of the species of soil animals we find, and how long those animals have been where they are. In order to do this, we have to get out of Taylor Valley (where most of our work, such as the LTER, is carried out) and collect samples from the other valleys and areas of exposed soils (such as nunataks, or mountaintops that stick out from glacial cover). When we do this, our aim is to try and cover a variety of different habitat conditions, such as available moisture, visible mosses, algae or lichen, the amount of salts in the soil, size of the soil particles, how much exposure to sun the area gets and other factors that may influence how habitable different places in the valley are, which can inform us as to why we do or don’t find certain species in a given location. Last year, Byron, Uffe, Diana and Ian Hogg (our colleague from Waikato University in New Zealand) were able to get down to the Beardmore glacier and collect samples at many different locations there, which is much further south than the Dry Valleys. This year, we were fortunate enough to have Ian and Jeb Barrett (another of our colleagues, from Virginia Tech) send us samples from this region again; in addition, a group of New Zealand researchers led by Craig Carey have been sending us samples from some of the more southern Dry Valleys such as Miers and Hidden Valley.
This year, Byron, Uffe and Zach were able to get to some less-visited parts of the Dry Valleys to collect samples. They first started by going to Mount Suess, which is further north in the Transantarctic Mountains than Taylor Valley. Mount Suess sticks up out of the surrounding Mackay Glacier, and has a lower ridge that projects from the east side of the mountain. This ridge is covered with soil and dotted with small meltponds, which harbor mats of algae and patches of moss. Here you can see a picture of the mountain, with the soil-covered ridge in the foreground.
And here is a picture of one of the small ponds.
The three of us each went a different way from the helicopter, while the pilot stayed by the helicopter for our return. Each of us had a radio so that we could check in periodically, and to make sure we were all okay–if something happened to one of us, we could let the others know. Uffe went downhill and sampled by some of the meltponds and surrounding area while Zach moved along a small rocky ridge and Byron moved along the top of the ridge toward some other small ponds. Below you can see an example of one of the patches of soil we sampled, and note the small patches of green moss along the bottom of the rocks in the top-center!
We spent an hour and a half on the ground here collecting samples, and then got back into the helicopter to travel to Wall Valley, named after our own Diana Wall! Wall Valley was a short 30 minute flight west and slightly south of Mount Suess, and we passed over some pretty stunning areas that make you realize how big the Dry Valleys are, as you can see here:
Right before we got to Wall Valley, we passed Virginia Valley, named after Ross Virginia, from Dartmouth College, whom Diana has worked down in Antarctica with for over 20 years! As we made our approach, we got a picture of Wall Valley:
Previous sampling at Wall Valley by the Wormherders wasn’t successful in recovering nematodes, as the soil down at the bottom of the valley is too high in salts, which the nematodes can’t tolerate. So this time, we went up along the edge of the valley, sampling in the scree piles that slope up along the valley’s walls. Here you can see the helicopter on the valley floor, and up to the top right stretches a scree slope that Uffe has gone up to sample:
Again, we collected samples for an hour and a half and then made our way back to the helicopter to travel to our last destination, Hawkins’ Cirque. The Cirque, named after the head helicopter pilot at McMurdo, is a small hemisphere-shaped break in the wall of Wright Valley, and sits nearly all the way back in the valley just above the glacier. Here you can see across the Cirque, with the glacier to the left:
After we finished collecting samples, we posed for a group photo: from left to right are Uffe, Byron and Zach.
Once we’ve extracted and had a chance to examine these samples under the microscope, we can look at these data together with the data from other samples collected by ourselves and our colleagues, and begin to put together a better picture of what governs why we find species of soil animals where we do in the Dry Valleys. By looking at overall patterns in distributions, and through use of several dating tools (both by examining age of the exposed rock surfaces as well as comparing the times that different populations of animals in different areas may have been separated by, using molecular genetics), we can start to explain how dispersal throughout the valleys may have occurred, and why some areas were colonized while others were not!
Dry Valley soils are cold and salty – basically the same environmental conditions that are used for long-term storage of DNA. Sure, Dry Valley soils may be diverse, but are all of these different microbes actively playing a role in soil ecosystems? Or do they just blow in here from other places, hang out in the soil, but never actually contribute anything? Because past studies inferred diversity based on the presence of DNA, it is it possible that the diversity of microbes that play active roles in Dry Valley ecosystem functioning is only a small subset of the microbes present in the soil? Dry Valley soils look diverse, but is the diversity functionally relevant?
Suppose for a moment that we were collecting DNA from Siberian ice cores and inferring ecological function based on the taxonomic affinities of the DNA sequences. When we encounter a chunk of frozen wooly mammoth tissue what would we infer about biological diversity and ecological function? That giant herbivores are running around and shaping Siberian ecosystem processes? Clearly our inferences would be a poor reflection of what actually goes on!
Long-term climate observations by the McMurdo Long-Term Ecological Research group are revealing increased frequency and magnitude of ‘pulse- events’ – periods of rapid warming and ice melt that lead to increased liquid water moving across/through the Dry Valley landscape. Which microbes are active in dry soils? Which microbes are responding to pulse events? Which microbes are completely dormant, waiting for more favorable conditions?
To answer these questions we set up a contained experiment that will capture the microbial response to a pulse wetting event. In the experiment we wet some soils with stable isotope labeled water (O18). The microbes that respond to the wetting event by taking up the O18 will incorporate the label into their DNA. We then extract the DNA from all the microbes in the soil and separate the strands of DNA that have incorporated the label from the strands of DNA that did not incorporate the label. We then sequence the two pools of DNA to reveal which taxa responded to the pulse and which did not – essentially, when it comes to identifying who responds to pulse events, we’ll be able to distinguish the players from the poseurs. Additionally, in order to link the response to actual ecosystem processes, we are measuring CO2 flux in the soils as a surrogate of microbial activity. For example, soils pulsed with water are expected to increase in ecological activity, and thus an increase in CO2 flux through the soil ecosystem.
This involved several steps. First, the experiment was set up conducted near the McMurdo Long-term ecological research (LTER) stoichiometry plots at F6 (Lake Fryxell, Taylor Valley). We outlined a patch of soil 8 m long and 1 m wide, with 8 replicates of 1 m2. Six PVC collars were placed in each of these 8 replicates, color coded to make it easy to tell which sampling period each represented – a control that wasn’t treated at all, so we’d know what the background of each little plot was, and collars to be sampled at 12, 24, 48, 72 and 144 hours after the treatments were added. You can see the collars here:
These collars were placed into the soil, one of each color-coded sample per plot—the locations of these colors within the small plot were randomized beforehand, and here you can see Uffe setting the collars into the soil as Zach reads off where each is supposed to go.
Once the collars were in place, Diana went around with a ruler to measure how much space was between the surface of the soil and the top of the collar—this is important because the machine that Byron would later use to measure how much CO2 was emitted from each plot needs to know how much space is being measured!
Once this information was collected, Uffe and Zach added water to the plots (the treatment), and Byron added the O18 to the experimental plots. Here you can see Uffe and Zach carefully treating their collars, and in the background Byron is taking respiration measurements:
Byron then had to stay out all week (and will have to go out once more) in order to take the respiration measurements at each time period—one 12 hours after the treatments were conducted, one 24 hours later, and so on until all the appropriate readings were taken. The microbial responses are collected in a time series of 12, 24, 48, 72, and 144 hours. The DNA sequencing and analyses will be done off the ice at Brigham Young University. Here you can see him set up to take these measurements.
Happy New Year, everyone! The Wormherders are back on the ice!
We began our field season with the necessary (but not exactly pleasant) travel from the United States on the 31st of December. Our field team for the season met once again in Los Angeles, with Diana Wall and Uffe Nielsen flying from Denver, Colorado, Byron Adams from Sacramento, California and Zach Sylvain from Portland, Maine. From LA, it was roughly a 13 hour flight to Auckland, New Zealand and then another hour to Christchurch. Although we left at 11:30pm on December 31st, we didn’t arrive into New Zealand until the 2nd of January because of the International Date Line–a whole day, lost! Fortunately we didn’t have to wait long before heading down to Antarctica, and so late at night on the 3rd, we caught our C17 flight (seen here landing on the ice runway at the end of last field season) down to McMurdo.
Once we had arrived in Antarctica, we spent the morning going through a wide variety of updates about the facilities and responsibilities down here, and then began to set up the lab. The flight had arrived very early in the morning on the 5th (we started our first briefings upon arrival at 5:30am!), and so most of our group opted to take a quick nap before getting to work. What sort of work goes into setting up the lab, you ask? First, many pieces of equipment must be picked up and moved into our empty lab, such as microscopes and a variety of chemicals as well as glassware such as beakers, flasks, and vials. How much glass (and plastic) ware do we need to carry out all the extractions we do while we’re down here? Quite a bit!
On the left is a cart full of plastic falcon tubes that we use to collect the nematodes during extractions (and the caps to the tubes), with a pile of plastic spoons and scoops below–to the right are all of the plastic beakers we need for our extractions, as we mix soil with water in these prior to sieving (more on extractions later!).
After two days of gathering everything we required to start working, we finally were able to begin extracting soil samples in order to see what animals (nematodes, rotifers and tardigrades) we can find. This is the good stuff–taking the soil and running it through all the steps we do in order to see what lives within it is exciting, letting us explore the mystery of where we might find life on this harsh continent. Fortunately, we had many samples from a colleague stored in a freezer waiting for us, so we were able to get to work today without having had to go into the field just yet! Each of us helped in extracting the samples in order to get the animals out and under the microscope. Zach began by weighing out 100g of each soil sample into one of those plastic beakers shown above: we weigh the soil samples so that we can compare each sample to every other sample more easily.
After a sample has been weighed, it gets mixed with water in the plastic beaker, and the water is then poured over two sieves stacked together–this helps strain out some of the soil and rocks and collects liquid with the soil animals on the sieve at the bottom, which has very small holes in it to catch the animals but let most of the water flow through. The sieve is then rinsed over a funnel into one of those small plastic falcon tubes to collect the soil animals (and some residual soil). Here you can see Diana rinsing the sieves between samples, which she is doing in order to prevent one sample from contaminating another: if she didn’t do this, we may end up with animals from one sample being transferred to another, which would provide incorrect data of what lives in each area the samples were collected from.
Samples in the falcon tubes are then passed to someone operating the centrifuge. Here, the samples from the first run of the sieves are checked to make sure the water levels are roughly equal and then are added into the centrifuge four at a time: this first run lasts five minutes, and helps to move all the soil animals down onto a pack of soil. Once this first spin through the centrifuge is complete, all but the very last remnants of water are poured out as waste–the vast majority of soil animals are all tightly packed along with the soil at the bottom, and not in the water in the tube. We then add a solution of water and sugar, mix the soil up into this to re-suspend the nematodes into the solution, and then replace the tubes into the centrifuge for an additional minute. In this last centrifugation, the soil is spun into the bottom of the tubes while the soil animals remain suspended in the sugar solution. Here you can see Byron checking the level of water in the falcon tubes prior to operating the centrifuge:
When the second spin through the centrifuge is complete, the samples are passed back to be sieved once more, this time over a small sieve with an extremely fine mesh. This sieve is then rinsed once more over a new falcon tube, and all of the soil animals are concentrated into this tube for examination under the microscope. In this image, Uffe is looking at a sample under the microscope, where he will count the number of living and dead nematodes from each species present, as well as the number of rotifers, tardigrades and other soil animals.
These numbers will be entered onto a data sheet and then be checked so that we can conduct analyses on our data later. Once we have completed looking at all the samples from a given experiment, the samples will be preserved with formalin and then packaged to be shipped back to the US, along with the unused soil from each sample that wasn’t used in the extractions or for obtaining characteristics of the soil such as soil moisture or nutrient levels (such as carbon, nitrogen and phosphorus).