Pathways of methane removal in the sediment and water column of a seasonally anoxic eutrophic marine basin

Abstract

Methane (CH4) is a key greenhouse gas. Coastal areas account for a major proportion of marine CH4 emissions. Eutrophication and associated bottom water hypoxia enhance CH4 production in coastal sediments. Here, we assess the fate of CH4 produced in sediments at a site in a seasonally anoxic eutrophic coastal marine basin (Scharendijke, Lake Grevelingen, the Netherlands) in spring (March) and late summer (September) in 2020. Removal of CH4 in the sediment through anaerobic oxidation with sulfate (SO42-) is known to be incomplete in this system, as confirmed here by only slightly higher values of δ13C-CH4 and δD-CH4 in the porewater in the shallow sulfate-methane-transition zone (~5-15 cm sediment depth) when compared to deeper sediment layers. In March 2020, when the water column was fully oxygenated, CH4 that escaped from the sediment was at least partially removed in the bottom water through aerobic oxidation. In September 2020, when the water column was anoxic below ~35 m water depth, CH4 accumulated to high concentrations (up to 73 µmol L-1) in the waters below the oxycline. The sharp counter gradient in oxygen and CH4 concentrations at ~35 m depth and increase in δ13C-CH4 and δD-CH4 above the oxycline indicate mostly aerobic water column removal of CH4. Water column profiles of particulate and dissolved Fe and Mn suggest redox cycling of both metals at the oxycline, pointing towards a potential role of metal oxides in CH4 removal. Water column profiles of NH4+ and NO3- indicate removal of both solutes near the oxycline. Analyses of 16S rRNA gene sequences retrieved from the water column reveal the presence of aerobic CH4 oxidizing bacteria (Methylomonadaceae) and anaerobic methanotrophic archaea (Methanoperedenaceae), with the latter potentially capable of NO3- and/or metal-oxide dependent CH4 oxidation, near the oxycline. Overall, our results indicate sediment and water column removal of CH4 through a combination of aerobic and anaerobic pathways, which vary seasonally. Some of the CH4 appears to escape from the surface waters to the atmosphere, however. We conclude that eutrophication may make coastal waters a more important source of CH4 to the atmosphere than commonly assumed.

Versatile methanotrophs form an active methane biofilter in the oxycline of a seasonally stratified coastal basin

“…Salinity showed little change with depth (range of 31 to 32; Table S1). A detailed discussion of the water column chemistry at the time of sampling can be found in (Żygadłowska et al, 2023).…”

“…However, oxygen would still be required for the oxidation of methane to methanol (Liu et al, 1995; Wallar & Lipscomb, 2001). Despite the metabolic potential and indications for active methane oxidation by Methylomonadaceae under anoxic conditions in sediments and water columns of other aquatic ecosystems (Oswald et al, 2016; Steinsdóttir et al, 2022), the stable isotope signatures of the methane in Lake Grevelingen do not indicate high in situ activity of anaerobic methane removal in the anoxic bottom waters (Żygadłowska et al, 2023).…”

“…In the Scharendijke basin sediment in Lake Grevelingen, however, methane production outpaces methane consumption, likely because of the high sedimentation rate (~13 cm yr −1 ; Egger et al, 2016). This results in high benthic diffusive and ebullitive methane fluxes (Egger et al, 2016; Żygadłowska et al, 2023). Furthermore, in situ methane production in the anoxic bottom waters could add to these high methane concentrations, supported by the presence of methanogens, Methanosarcinaceae , Methanomicrobiaceae and Methanocorpusculaceae , as dominant archaeal families at these depths (Figure S1).…”

“…Lake Grevelingen is a former estuary in the southwest Netherlands. The lake has a surface area of 115 km 2 with an average water depth of 5.1 m. Here, we sampled Scharendijke basin, which is part of a former tidal channel and has a depth of 45 m. A more detailed description of the system and the study site can be found elsewhere (Egger et al, 2016; Żygadłowska et al, 2023). Lake Grevelingen was chosen to investigate microbial methane cycling as previous studies indicated that methanogenesis exceeded methane oxidation in the sediment during summer stratification, resulting in a periodic release of methane into the water column.…”

Anaerobic methanotrophy is stimulated by graphene oxide in a brackish urban canal sediment

“…The shallow SMTZ that is described at the Dijksgracht site could be due to a relatively high rate of OM deposition in this canal. Such a shallow SMTZ is not unusual for eutrophic coastal systems (Van Helmond et al, 2020; Żygadłowska et al, 2023). The sediment at the Dijksgracht site indeed was also very rich in organic carbon (5 wt%) and SRB were detected in the top 10 cm of the sediment (Figures S2 and S3).…”