Potential role of submerged macrophytes for oxic methane production in aquatic ecosystems

Methane (CH4) from aquatic ecosystems contributes to about half of total global CH4 emissions to the atmosphere. Until recently, aquatic biogenic CH4 production was exclusively attributed to methanogenic archaea living under anoxic or suboxic conditions in sediments, bottom waters, and wetlands. However, evidence for oxic CH4 production (OMP) in freshwater, brackish, and marine habitats is increasing. Possible sources were found to be driven by various planktonic organisms supporting different OMP mechanisms. Surprisingly, submerged macrophytes have been fully ignored in studies on OMP, yet they are key components of littoral zones of ponds, lakes, and coastal systems. High CH4 concentrations in these zones have been attributed to organic substrate production promoting classic methanogenesis in the absence of oxygen. Here, we review existing studies and argue that, similar to terrestrial plants and phytoplankton, macroalgae and submerged macrophytes may directly or indirectly contribute to CH4 formation in oxic waters. We propose several potential direct and indirect mechanisms: (1) direct production of CH4; (2) production of CH4 precursors and facilitation of their bacterial breakdown or chemical conversion; (3) facilitation of classic methanogenesis; and (4) facilitation of CH4 ebullition. As submerged macrophytes occur in many freshwater and marine habitats, they are important in global carbon budgets and can strongly vary in their abundance due to seasonal and boom-bust dynamics. Knowledge on their contribution to OMP is therefore essential to gain a better understanding of spatial and temporal dynamics of CH4 emissions and thus to substantially reduce current uncertainties when estimating global CH4 emissions from aquatic ecosystems.

Automated summary

Methane emissions from aquatic ecosystems, which contribute to about half of total global methane emissions, were traditionally attributed to methanogenic archaea in anoxic or suboxic conditions. However, recent studies have shown evidence of methane production in aerobic conditions in freshwater, brackish, and marine habitats. The role of submerged macrophytes in methane production in oxic waters has been largely overlooked, despite their presence in littoral zones of ponds, lakes, and coastal systems. Understanding the contribution of submerged macrophytes to methane emissions is crucial for estimating global methane emissions from aquatic ecosystems and improving our understanding of spatial and temporal dynamics of methane emissions.