Using lake sediments, our study reveals that periods of past nutrient overload, known as eutrophication, and the active breakdown of organic matter likely boosted methane production in freshwater sediments. This process particularly supported certain methane-producing microorganisms (methylotrophic methanogens) in deeper sediment layers, which may use compounds released from decomposing algae. Interestingly, even though recent sediments lack oxygen, we found methane-oxidizing bacteria that typically need oxygen to survive. This discovery shows how eutrophication can lead to a complex balance: on one hand, it promotes methane production, a potent greenhouse gas, while on the other, it fosters microbial communities that help break down methane. This balance has a direct impact on greenhouse gas emissions from freshwater systems.

Iron is one of the most common elements in Earth’s crust, and microorganisms have been helping to cycle it for at least 3 billion years. These microorganisms can convert iron between two forms: an oxidized form (ferric iron or Fe(III)) and a reduced form (ferrous iron or Fe(II)). In environments without oxygen, certain bacteria, like Shewanella oneidensis, are expected to convert Fe(III) to Fe(II). However, in areas with oxygen, Fe(II) usually doesn’t last long because it quickly changes back to Fe(III). In our study, we used specialized lab setups to observe something surprising: microorganisms reducing Fe(III) to Fe(II) even in oxygen-rich conditions, creating a stable pool of Fe(II).