It was fun to talk about spiders yesterday, but let’s get back to the carbon cycle now. In a previous post I described some of the team’s work on the role of sunlight in breaking down organic matter to make it easier for microbes to convert it to CO2. Now let’s talk about the microbes.

    The microbes play the key role in breaking down organic matter and converting it to the heat-trapping gas CO2, which promotes global warming. In fact, microbes are so numerous that they play a far more important role than animals or anything else in breaking down organic matter to produce CO2. And as I’ve discussed before, as permafrost thaws there’s a lot more organic matter available for the microbes to eat.

    To get a better understanding of how organic matter is being converted to CO2 now and in the future, the team is doing innovative and fascinating work. One facet of the work is to identify the types of microbes that are present in the land, lakes and rivers, while another is to measure the rate at which different microbial communities convert organic matter into CO2.

    Microbial DNA Analysis

    The team is using DNA analysis to identify different microbial communities. Here’s how it works:

    First, the team collects water samples. We did this last week when we helicoptered in to sample the I-Series lakes just south of Toolik.

    Collecting a water sample at the outlet of Lake I-8.
    Collecting a water sample at the outlet of Lake I-8.

    After we collect the samples, we filter the water through a 0.22 micron filter to collect the microbes.

    Filtering water samples with Michelle
    Tha'ts me on the left, filtering water samples with Michelle

    0.22 micron filter
    0.22 micron filter for collecting microbes.

    Back in the lab, Michelle cracks open the filters and dries them to collect the microbial DNA.

    Michelle processing filters in the lab.

    The DNA is processed and placed into small vials that are centrifuged until all that’s left of 1 liter of water is a tiny pellet of microbial material about the size of a pinhead.

    Michelle with a centrifuged vial containing a pellet of DNA material.
    Michelle with a centrifuged vial containing a pellet of DNA material.

    DNA pellet
    DNA pellet.

    The pellets are then sent off to a lab for DNA sequencing. Eventually the team will get back a huge computer file full of A’s, C’s, T’s and G’s that are compared to a library of microbial DNA to identify the different microbes that were present in the initial water sample.

    Before the pellets are sent off for DNA sequencing, however, they’re scanned under UV light to make sure the sample contains enough DNA to be sequenced. In the photo below, the DNA has been treated to glow bright green. Can you see which vials we might not want to send out for sequencing?

    DNA vials under UV light.
    DNA vials under UV light.

    One of the interesting findings from this work is that the microbial communities are very different in different parts of the landscape. The picture below summarizes the key findings, which are:

    1. The type of vegetation present determines the type of dissolved organic matter, or “DOM”, found in the groundwater. For example, you find one type of DOM associated with the dry, upland tussock plant community, and a different type of DOM associated with the wet sedge plant community.
    2. The type of DOM in the groundwater then determines the microbial community.
    3. They type of microbial community present determines the rate at which DOM is converted into CO2.

    Landscape Diversity and Microbial Dynamics Diagram.
    Landscape Diversity and Microbial Dynamics diagram. Courtesy of George Kling.

    You can see from the diagram that the lake contains a mixture of the different microbial communities from across the landscape. This makes sense, because the groundwater from across the landscape flows into the lake. What it means, though, is that the lake contains a heterogeneous community of bacteria that may be able to adapt quickly to consume any type of organic matter that happens to be flowing into it. Thus, lakes appear to have a very important role in cycling carbon from the landscape to the atmosphere.

    Yes, this is complicated, but all it really means is that if you want to figure out the rate at which dead plant material on the tundra is being converted to CO2, you have to know what type of microbial community is present, and how quickly it can consume the available organic matter. If we’re going to predict the impact of a warming Arctic and thawing permafrost on future climate change, it’s critical to understand what the microbes are doing.

    Microbial Production

    Another way the team is learning about the microbial community is to study the rates at which different microbial communities grow, also known as the rate of “production.” This was another test we did when we sampled the I-Series lakes last week. To measure the rate of production, Jason collected water samples and spiked them with leucine, an organic molecule that he tagged with carbon-14, a radioactive isotope.

    Jason spiking samples with radioactive leucine.
    Jason spiking samples with radioactive leucine.

    Leucine is an organic molecule that microbes love to eat, so when it’s placed in the water with the microbes they immediately start consuming it. Jason allows the microbes to consume his radioactive leucine for about 2 hours, then kills them. Since the leucine contains a radioactive label, when he gets back to the lab can measure the amount of leucine consumed by the microbes and incorporated into new cells. By comparing the leucine in experimental and control samples, he can calculate the rate at which the microbial cells in the sample grew.

    The team is conducting this test to get an idea of the rate at which different microbial communities break down organic matter to produce CO2 on different parts of the landscape. They’re testing ponds, lakes and rivers at different times of the year to start to understand impact of the different microbial communities and their rates of activity in converting organic matter to CO2.

    The microbes are the key ecosystem player in converting organic matter to CO2, and it’s not easy to figure out how things work on such a small scale. But the team is out on the tundra almost every day, using sophisticated techniques to increase our understanding of the fate of the carbon from thawing permafrost.

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