I have mentioned in previous journals that it is soil microbes that produce the methane gas that we are measuring. So why and how are they doing that? Obviously the 'how' and the 'why' are connected, but that is a big dose of chemistry all at once. So today I will try to explain the why…
The Soil Microbes Themselves
Soil is chock full of mostly microscopic organisms collectively called soil microbes. Most numerous are an assortment of species of bacteria, but there are also lots of fungi, algae, and protozoa.
Caption - Compost heap bacteria. Coloured scanning electron micrograph of a dividing spirochaete amongst other bacteria from a compost heap. Some saprophytic bacteria playing an important role in this degradation of plant remains are shown here (pink and green). Credit - EYE OF SCIENCE/SCIENCE PHOTO LIBRARY
Believe-it-or-not, this soil sample we took from Petsikko contains at least a billion soil microbes. Because conditions are so variable over space and time in a wetland, not all of these microbes are active at the same time.
Now that the wetland is a little drier, we can cut soil samples out with a large knife. You can't see them, but there are an amazing number of soil microbes in this sample!
Soil Microbes Have To Live, Too!
Like all living organisms, soil microbes have to get energy in order to survive. Just like humans, soil microbes cannot make their own food (only plants can do that through the process of photosynthesis). Soil microbes must therefore "feed" off something, and that something is dead organic material in the soil. Yes, soil microbes are the "workhorses" of the world of decomposers.
Oxidation, Reduction, and Wetland Soil Microbes
The process of turning "food energy" into energy that the organism can use is a matter of electrons moving in the oxidation-reduction (or 'redox') process. Remember in the May 27 journal entry when Scotty was checking out the DIY Redox Probe? We said then that soil microbial activity causes electrons to flow. Part 2 will explain the mechanics of this electron flow in more detail.
When electrons are flowing, something must be available to accept the moving electrons. This is called an electron acceptor. It is being reduced in the oxidation/reduction process. I know this sounds backwards, but the acronym OIL RIG helps to keep it straight:
OIL RIG: Oxidation Is Loss of electrons (OIL); Reduction Is Gain of electrons (RIG).
So focus on the RIG part: Reduction is Gain of electrons.
I put this table together from Kim's information and the Redox Ladder
As you can see from the table, oxygen is very good electron acceptor. The most energy is created through the oxidation/reduction process when oxygen is available as the electron acceptor. Humans must use oxygen as an electron acceptor, as we need so much energy. When we breathe in, it is the oxygen we need to run our metabolic processes. We breathe out a byproduct of the oxidation/reduction process, carbon dioxide.
Oxygen is only available, however, in aerobic conditions. Now think of wetlands. These represent anaerobic conditions – there is no oxygen in the spaces between soil particles, only water. In these anaerobic conditions where there is no oxygen, microbes are adapted to using a variety of different electron acceptors depending on local conditions. The table shows some of these different electron acceptors and the relative amounts of energy they make available through the oxidation/reduction process.
The methanogens (methane producing bacteria) and other methane producing soil microbes are able to use carbon dioxide as the electron acceptor, which produces methane gas as a byproduct. Very little energy is produced through this oxidation/reduction process as carbon dioxide is a poor electron acceptor, but it is enough to enable these particular soil microbes to survive in what would be considered difficult conditions for life.
Caption - Archaebacteria. Coloured transmission electron micrograph of the archaebacteria Methanospirillum hungatii. Methanospirillum hungatii is an anaerobe living in oxygen-free conditions. It is a methanogen in that it metabolizes carbon dioxide and hydrogen to produce methane. Credit - DR KARI LOUNATMAA/SCIENCE PHOTO LIBRARY
It is this methane gas that Kim is measuring, as methane gas, when free in the atmosphere, is a powerful greenhouse gas.
Field Day Bonus: IT HAPPENED
That's enough chemistry for one day! How about a video…
http://youtu.be/aDASllMmWxY
Additional Reading
This website has some reading on soil microbe groups. For more advanced readers, here is a lecture on microbes from the University of Michigan that may interest you.