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

Abstract

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.

Network analysis uncovers associations in the turnover of C1 molecules in a winter lake

“…All that is needed for Fenton-driven CH 4 release is the presence of methylated compounds e.g. DMSO, to explain the as yet unaccounted for component of (cyano/bacterial) aquatic methane (Bižić et al ., 2020; Hilt et al ., 2022). It is possible that all cellular lifeforms release CH 4 especially during phases of rapid growth (Ernst et al ., 2022) but in this study only bacteria with documented Fenton-driven release (Fig 3) are annotated.…”

Application of concentration and 2-dimensional stable isotope measurements of methane to constrain sources and sinks in a seasonally stratified freshwater lake

“…Moreover, it has been discovered that cyanobacteria produce CH 4 (Bižić et al, 2020b) and that light as well as temperature yield a significant control over oxic CH 4 production rates by influencing phytoplankton communities (Klintzsch et al, 2020). Furthermore, the role of submerged macrophytes for oxic methane production has so far been ignored, but might also contribute significantly (Hilt et al, 2022). Hence, oxic CH 4 production is most likely closely coupled to primary production, potentially contributing to an increase in CH 4 concentrations in the surface water layer during the stratification period as it was also observed at Lake Willersinnweiher.…”

Carbon sequestration in aquatic ecosystems: Recent advances and challenges

“…Al‐Haj et al (2022) measured negligible emissions of both CH 4 and N 2 O from seagrass meadows, adding further evidence that seagrass beds are effective at sequestering carbon. Hilt et al (2022) reviewed the mechanisms of methane emissions from macrophytes both in freshwater and marine habitats. They also set a new research agenda by highlighting how submerged macrophytes can be sources of methane to the atmosphere, partially counteracting some of their long‐term organic carbon sequestration.…”