Lake Rotsee © Eawag

The bacteria that could beat climate change

Lakes emit significant amounts of greenhouse gases, in particular methane, a greenhouse gas more that twenty times more potent than carbon dioxide (CO2). One square kilometre of lake can emit 800 times more methane than the same area of ocean.

By chance scientists in Switzerland discovered that damaging bacteria, commonly found in lake water, consume the potent greenhouse gas methane before it can be released into the atmosphere. 

Freshwater greenhouse gas emissions are enormous

Total freshwater greenhouse gas emissions are estimated to make up around 22 percent of total global methane emissions and around three quarters of all natural emissions. Natural emissions are those not directly connected to human activity, such as transport and food production.

Methane and other greenhouse gases are produced by the decomposition of organic material on lake floors. Low-oxygen conditions in lake beds promote the production of methane. The amount emitted is driven by the quantity of organic material, temperature, and various other factors such as water depth, soil and topographical conditions.

Rising temperatures are magnifying the problem

As temperatures rise, arctic lakes and pools are of special concern because of their vast combined area, their rich organic sediments, such as peat, and their rapid shift from ice to water as temperatures climb.

Methane testing on Lake Rotsee © Eawag

DieSwitzerland has 1,500 lakes and 6% percent of Europe’s freshwater reserves

Freshwater emissions matter in Switzerland. The nation contains six percent of Europe’s freshwater reserves, and its roughly 1,500 lakes and rivers cover around four percent of the country’s surface. Many of these lakes are natural but some were created for hydropower production, a process which increases emissions further as fast flowing agitated water passing through turbines releases more of the greenhouse gases contained in it.

Hydropower is not 100% green

Lake Wohlen, a Swiss lake created on the Aare river to generate electricity, has emissions equivalent to the carbon dioxide emissions from 25 million car kilometres. Environmental chemist Tonya Del Sontro says that in the summer the water in Lake Wohlen sometimes looks like champagne, with masses of gas bubbles rising to the surface.

Wohlensee © Martin Abegglen
Wohlensee © Martin Abegglen

"So hydropower isn't quite as climate-neutral as people have assumed in the past," says Del Sontro. "That's something which has previously been overlooked in greenhouse gas budgets", says Professor Bernhard Wehrli. At the same time a coal-fired power station with the same output as the Lake Wohlen hydro plant emits around 40 times as much carbon dioxide, so hydro electricity in Switzerland still delivers a substantial environmental benefit in greenhouse gas terms. And lakes like Wohlen have relatively high emissions. Substantial amounts of organic matter are transported by the river from a relatively large catchment area upstream. This settles rapidly and starts to decompose. In addition, the lake is relatively shallow allowing it to warm up quickly, something that accelerates decomposition. Emissions from higher, deeper lakes are typically lower.

An accidental discovery

In June 2017, while studying the Swiss Lakes Rotsee and Zug, a team of scientists from the Swiss Federal Institute of Aquatic Science and Technology (Eawag), accidentally discovered a powerful new ingredient in this underwater methane oxidization process5.

It turns out that filamentous bacteria of the genus Crenothrix is responsible for consuming much of the waterborne methane before it reaches the air. Also known as iron bacteria, for its ability to block iron water pipes, Crenothrix it normally considered a nuisance. However, this new discovery might turn it from a hindrance into a hero. It could turn out to be the most important methane consumer in freshwater lakes.

Crenothrix bacteria © Eawag
Crenothrix bacteria © Eawag

Dr. Carsten Schubert, who was part of the team said: “We seem to have completely underestimated this bacteria’s role in the biogeochemical cycle.” Scientists had not looked for these bacteria before in lake water and the Eawag team only found them by accident. The researchers were trying to quantify underwater methane removal with the aid of stable isotope labelling. To do this they labelled some methane molecules with heavy carbon-13 atoms. The labelled carbon atoms were then later found in bacteria, something which could be seen using mass spectrometry imaging. The team expected to see the labelled carbon in small round bacteria. However they were surprised to also find them in the long filamentous bacteria Crenothrix. They were also surprised by how many of these long bacteria contained the heavy carbon atoms. 

Finding the same bacteria found in two different kinds of lake is encouraging

The researchers said that their study unambiguously demonstrates a key role for these organisms in the mitigation of methane emissions from two stratified lakes.

Dr. Carsten Schubert, who was part of the team said: “Our discovery of Crenothrix has yet to be confirmed in other systems. So far we have found it in two lakes. Crucially these two lakes are different. One, Lake Zug, is deep and sunlight does not penetrate its oxycline, a line beyond which there is no oxygen, a higher concentration of methane, and cooler water. In the other shallower Lake Rotsee, sunlight penetrates the oxycline. Finding Crenothrix in these two different environments is encouraging.”

Carsten Schubert © Eawag
Carsten Schubert © Eawag

Watch this space

Could these bacteria be introduced to lakes to reduce their greenhouse gas emissions? Dr. Schubert is cautious. “Methane oxidizing bacteria are very special and live in very specific conditions. It is too soon to speculate whether these organisms could be artificially introduced into lakes to oxidize methane to reduce freshwater methane emissions.”

The next step is for researchers to see if they can find Crenothrix doing similar things in different environments. In addition, the bacteria need to be closely studied in these different environments to better understand how they work their methane munching magic.

Rotsee Switzerland © Eawag
Rotsee Switzerland © Eawag

 

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