Intact lakes as methane-flinging

From oxygen-rich and nutrient-poor waters greenhouse gas is released

Reed belt at Stechlinsee © M. Feierabend
Read out

Overused, nutrient-rich waters have long been regarded as an important source for the greenhouse gas methane, which is harmful to the climate. Berlin scientists have now shown that methane is also formed in lakes with plenty of oxygen and an intact nutrient balance and can escape into the atmosphere.

Inland waters have so far been ignored as a source of greenhouse gases. However, researchers estimate that 70 percent of the natural methane content of the atmosphere comes from rivers and stagnant waters. Until now it has been thought that much methane is produced mainly in nutrient-rich and oxygen-poor lakes.

The team led by Hans-Peter Grossart of the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) has now been able to demonstrate for the first time in a study published in the "Proceedings of the National Academy of Sciences" (PNAS) that methane is also formed in nutrient-poor and oxygen-rich lakes becomes.

Microbes decompose sedimented substances

In the mud at the bottom of a lake, the oxygen penetrates only a few millimeters deep. Oxygenating, anaerobic bacteria decompose the sedimented substances there. This can include methane and carbon dioxide. These gases are used in the water by other microorganisms, but a certain amount escapes into the atmosphere, according to scientists.

Depending on the nutrient content of the water body, the activity of the bacterial communities occurring there and thus the amount of methane and carbon dioxide produced differs. display

On the trail of methane-forming and consuming bacteria

The IGB scientists have been working for some time on identifying methane producing and consuming bacteria in water bodies. In the Stechlinsee, a nutrient-poor lake, the researchers detected an enrichment of methane in the well-ventilated upper ten-meter water column. The maximum methane production, according to the new study, took place at a depth of six meters, where the oxygen concentrations were even supersaturated.

"The reason for the increased methane concentrations is that even in the oxygen-containing zones of the lake methane is produced and at the same time the production and the oxidation of methane are spatially separated, " explains Grossart. In the summer months, lakes such as Stechlin are heavily stratified, so that the methane-oxidizing bacteria could not degrade the methane formed in the well-ventilated upper layers of the lake. Therefore, increased methane concentrations were found there.

Laboratory results confirm field observations

These field observations were also demonstrated in the laboratory with seawater and the corresponding bacteria. Methane production was not affected by the addition of phosphorus as a nutrient, the researchers said. These results show that methane can be formed in lakes, regardless of the oxygen content. The scientists suspect that the bacteria responsible for this, which are potentially methane-producing archaea, were formerly known as ancient bacteria. These occur in close proximity to certain species of algae.

The close coupling of the microbes means that the direct transfer of molecular hydrogen and organic compounds to the archaea is possible without the free oxygen affecting methane formation.

This is in stark contrast to the long-standing scientific opinion that methane can only be formed in an oxygen-free environment. The conversion of hydrogen to methane in the presence of oxygen now offers interesting biotechnological possibilities, the scientists say.

Results important for climate research

DieIn addition, climate research is well advised to take greater account of the role of inland waters in the emission of greenhouse gases, says Grossart. Colleagues from his department had calculated in a previous study that in the summer per hectare of water surface of a nutrient-rich, oxygen-poor lake escapes every day 12, 000 liters of climate-relevant gas and pollutes the air with 6.2 kilograms of carbon,

"With an area of ​​2.5 million square kilometers that inland waters are taking over, the dimension of the problem becomes clear, " says Grossart. Even if methane formation could be detected in an intact watercourse such as the Stechlinsee, watercourses heavily polluted with nutrients are still a source for climate-relevant gases such as methane and carbon dioxide.

"Improving the ecological status of water bodies is therefore also a contribution to climate protection, " says Grossart. (Proceedings of the National Academy of Sciences (PNAS), 2011; doi: 10.1073 / pnas.1110716108)

(Forschungsverbund Berlin, 21.12.2011 - DLO)