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Decomposing cattails producing large amounts of methane

Posted May 8, 2018 at 3:19 PM

Oily sheen on water is natural

Updated May 8, 2018 at 3:19 PM

A new positive feedback on climate change has been identified. The warming climate is causing wetland plants, like the common cattail, to thrive while at the same time forest cover is being lost, causing increased methane production by northern freshwater lakes.

A study recently published in Nature Communications (Emilson et al. “Climate-driven shifts in sediment chemistry enhance methane production in northern lakes”) looked at the amount of methane released during the decomposition of three types of plant debris commonly found in the sediment in freshwater lakes in the northern parts of North America: pine needles, leaves from deciduous trees and cattails.

The researchers found that the organic matter from the decaying pine needles and deciduous leaves inhibited methane production whereas decomposing cattails produced large amounts of methane (400 times more than the pine debris sediment and almost 2,800 times that of the deciduous leaf debris sediment!). Methane is an important greenhouse gas - 25 times more potent than carbon dioxide - and freshwater lakes are big methane producers (they contribute 16 percent of Earth’s natural methane emissions). With models predicting that northern lakes could double their methane emissions in the next 50 years as cattail populations increase, this could mean a significant unexpected pulse of methane into our already greenhouse gas heavy atmosphere.

I was thinking about this study while participating in my local marsh cleanup this weekend. We were trudging around the edge of the marsh picking up debris washed in by the flooding this past winter. This is a freshwater marsh that borders a saltwater marsh. It was a beautiful day. Red-winged blackbirds and marsh wrens were singing from the cover of dried out cattail spikes. New, bright green, baby cattails were poking up through last year’s dead grasses. I was telling friends about the cattail study when I noticed an oily sheen on the water around the cattail stems. I was so excited! This wasn’t pollution, there hadn’t been a nearby oil spill, this was natural oil formed by the bacteria responsible for decomposing plant debris - just like in the study!

Any time decomposition is happening you can be certain it is being done by either bacteria or fungi. Decomposition is the process of breaking down big molecules into their parts. In cattail marshes there are bacteria that live up near the surface where there is plenty of oxygen (aerobic bacteria) and also bacteria that live down in the muck where there isn’t any oxygen (anaerobic bacteria). Aerobic bacteria release carbon dioxide as they chew through those big molecules, and anaerobic bacteria release methane. Methane, a gas, bubbles out of these swamps into the atmosphere. However not all the methane escapes, some gets converted into heavier hydrocarbons (methane is the simplest hydrocarbon with a chemical formula of CH4) that don’t evaporate but instead float to the surface and produce an oily sheen that looks just like fossil fuel-based gasoline and oil spills because gas and oil are also hydrocarbons (they come from very old dead things). This oily sheen can also be formed just from the sheer amount of dead plant material lying around decomposing into its various parts -- there is a lot of natural oil in a plant cell - it is an important component of cell membranes and used as an energy source.

While I don’t love the news about yet another positive feedback on climate change, I do love this reminder of the importance of bacteria to virtually every living system on Earth. I love being able to look at a marsh and see evidence of the presence of all those bacteria with something as simple as that oily sheen.