Lakes around the world emit methane to the atmosphere, accounting for up to 19% of total global emissions of this potent greenhouse gas. Most of the methane in lakes comes from microbes in the lake bed that consume dissolved organic matter for sustenance and release methane as a waste product. This process typically occurs in low-oxygen conditions; in environments with more oxygen, microbial metabolic processes that do not produce methane tend to dominate.
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Source: Phys.org