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

Abstract Methane (CH 4 ) from aquatic ecosystems contributes to about half of total global CH 4 emissions to the atmosphere. Until recently, aquatic biogenic CH 4 production was exclusively attributed to methanogenic archaea living under anoxic or suboxic conditions in sediments, bottom waters, and wetlands. However, evidence for oxic CH 4 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 CH 4 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 CH 4 formation in oxic waters. We propose several potential direct and indirect mechanisms: (1) direct production of CH 4 ; (2) production of CH 4 precursors and facilitation of their bacterial breakdown or chemical conversion; (3) facilitation of classic methanogenesis; and (4) facilitation of CH 4 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 CH 4 emissions and thus to substantially reduce current uncertainties when estimating global CH 4 emissions from aquatic ecosystems.