Lakes emit as much methane (CH4) as agriculture. A team from the University of Liège has just shown that, in 79 African lakes, bacteria living on the surface of microalgae act as a natural filter. The greener the water becomes, the more these bacteria (known as methanotrophs) break down this powerful greenhouse gas.
Methane (CH4) is the second most potent greenhouse gas (GHG) after carbon dioxide (CO2). Lakes are a major source of GHGs to the atmosphere, equivalent to emissions from agriculture."Methane emissions from a lake result from the balance," explains Alberto Borges , an oceanographer and head of the CO2 Lab at University of Liège, "between, on the one hand, its production through methanogenesis in the sediments, and, on the other, its loss through microbial oxidation. Methane is, in fact, the preferred substrate of certain bacteria, known as methanotrophs, which specialise in using it as a source of energy and biomass."
To understand what regulates this oxidation, the scientific team carried out the largest study of its kind, taking 503 measurements in 79 African lakes, ranging from lakes with very low algal biomass production and clear waters to extremely productive lakes with green waters due to the presence of phytoplankton, constituted of microalgae.
The main finding is that methane oxidation increases sharply with the amount of microalgae present. The surface of these algae is colonised by bacteria, including methanotrophs, which coexist harmoniously in a mutually beneficial relationship. "The methanotrophs supply the algae with carbon dioxide for photosynthesis, whilst the algae produce the oxygen the bacteria need to oxidise methane," the researcher continues. "So the more abundant the microalgae are, the more abundant the methanotrophic bacteria are too, and therefore the more intense the methane oxidation is." In the richest lakes, it reaches levels tens of thousands of times higher than in the poorest lakes.
This mechanism is far from insignificant. This study shows that oxidation is the main 'fate' of methane dissolved in surface waters. It eliminates more than 70 per cent of it, which is far more than what ultimately escapes into the atmosphere. The lakes bordered by the flooded forests of the Congo Basin are also characterised by high levels of oxidation, fuelled by the influx of bacteria from the submerged forest soils.
"This finding enables us to better understand, but also to reassess, methane emissions from lakes. We had previously shown (https://doi.org/10.1126/sciadv.abi8716) that as phytoplankton biomass increased, so too did methane production in the sediments, with algal detritus serving as a feedstock for methane-producing microorganisms.
This new study therefore demonstrates that, in the event of water pollution (eutrophication) leading to an increase in phytoplankton, two opposing effects occur: methane production in the sediments increases, but the methanotrophic bacteria attached to the algae also multiply and utilise this methane. However, in the lakes studied, the increase in oxidation can outweigh the increase in production. In the absence of these bacteria attached to microalgae, methane emissions into the atmosphere would be much higher. "These methanotrophic bacteria thus act as natural and welcome filters, capable of partially mitigating the effect of lake pollution on global warming. This is an important piece of the puzzle that was missing and will enable us to better predict the future trend in methane emissions from lakes," concludes Alberto Borges.
