Methane (CH
4
) is a key greenhouse gas. Coastal areas account for a major proportion of marine CH
4
emissions. Eutrophication and associated bottom water hypoxia enhance CH
4
production in coastal ...sediments. Here, we assess the fate of CH
4
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 CH
4
in the sediment through anaerobic oxidation with sulfate
(
S
O
4
2
-
)
is known to be incomplete in this system, as confirmed here by only slightly higher values of δ
13
C-CH
4
and δD-CH
4
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, CH
4
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, CH
4
accumulated to high concentrations (up to 73 µmol L
-1
) in the waters below the oxycline. The sharp counter gradient in oxygen and CH
4
concentrations at ~35 m depth and increase in δ
13
C-CH
4
and δD-CH
4
above the oxycline indicate mostly aerobic water column removal of CH
4
. 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 CH
4
removal. Water column profiles of
N
H
4
+
and
N
O
3
-
indicate removal of both solutes near the oxycline. Analyses of 16S rRNA gene sequences retrieved from the water column reveal the presence of aerobic CH
4
oxidizing bacteria (
Methylomonadaceae
) and anaerobic methanotrophic archaea (
Methanoperedenaceae
), with the latter potentially capable of
N
O
3
-
and/or metal-oxide dependent CH
4
oxidation, near the oxycline. Overall, our results indicate sediment and water column removal of CH
4
through a combination of aerobic and anaerobic pathways, which vary seasonally. Some of the CH
4
appears to escape from the surface waters to the atmosphere, however. We conclude that eutrophication may make coastal waters a more important source of CH
4
to the atmosphere than commonly assumed.
Accumulation of methane in oxic waters of lakes and the ocean has been widely reported. Despite the importance for the greenhouse gas budget, mechanistic controls of such “methane paradox” remain ...elusive. Here, we use a combination of CH4 concentration and isotopic (δ13CCH4, δDH2O and δ18OH2O) measurements, plankton incubations and microbial community assessments to demonstrate the existence of the methane paradox in oxygenated waters of a meromictic lake (Lake Cadagno, Switzerland). Following mass dynamics using water isotopes, we exclude the possibility that the accumulation of CH4 at the thermocline results solely from lateral transport. Interannual variability in the magnitude of the methane paradox (between 0.5 and 5 μmol L−1) is associated to stratification patterns, changes in zooplankton biomass and planktonic detritus accumulation along density gradients, as well as fluctuating microbial cell numbers. The links between hydrodynamic conditions, aggregation of planktonic detritus and its microbiome, as well as the accumulation of CH4 in the water column are further supported by high‐resolution echo‐sounder measurements revealing backscatter maxima at the top of the thermocline, where detritus is effectively trapped, and by oxic incubations showing that CH4 is produced in zooplankton detritus (0.046 nmol L−1 to 0.095 CH4 mg dry mass L−1 d−1). Our results also show that detritus‐hosted methanogenesis is stimulated through the addition of methylphosphonate, suggesting that zooplankton‐associated microbiomes exploit organic phosphorus compounds to release CH4. Understanding the variability of the methane paradox in relation to changing hydrodynamics and plankton communities will be crucial to predict the future role of lakes in the global methane budget.
Coastal environments are a major source of marine methane in the atmosphere. Eutrophication and deoxygenation have the potential to amplify the coastal methane emissions. Here, we investigate methane ...dynamics in the eutrophic Stockholm Archipelago. We cover a range of sites with contrasting water column redox conditions and rates of organic matter degradation, with the latter reflected by the depth of the sulfate–methane transition zone (SMTZ) in the sediment. We find the highest benthic release of methane (2.2–8.6 mmol m–2 d–1) at sites where the SMTZ is located close to the sediment–water interface (2–10 cm). A large proportion of methane is removed in the water column via aerobic or anaerobic microbial pathways. At many locations, water column methane is highly depleted in 13C, pointing toward substantial bubble dissolution. Calculated and measured rates of methane release to the atmosphere range from 0.03 to 0.4 mmol m–2 d–1 and from 0.1 to 1.7 mmol m–2 d–1, respectively, with the highest fluxes at locations with a shallow SMTZ and anoxic and sulfidic bottom waters. Taken together, our results show that sites suffering most from both eutrophication and deoxygenation are hotspots of coastal marine methane emissions.
Abstract
In coastal waters, methane-oxidizing bacteria (MOB) can form a methane biofilter and mitigate methane emissions. The metabolism of these MOBs is versatile, and the resilience to changing ...oxygen concentrations is potentially high. It is still unclear how seasonal changes in oxygen availability and water column chemistry affect the functioning of the methane biofilter and MOB community composition. Here, we determined water column methane and oxygen depth profiles, the methanotrophic community structure, methane oxidation potential, and water–air methane fluxes of a eutrophic marine basin during summer stratification and in the mixed water in spring and autumn. In spring, the MOB diversity and relative abundance were low. Yet, MOB formed a methane biofilter with up to 9% relative abundance and vertical niche partitioning during summer stratification. The vertical distribution and potential methane oxidation of MOB did not follow the upward shift of the oxycline during summer, and water–air fluxes remained below 0.6 mmol m−2 d−1. Together, this suggests active methane removal by MOB in the anoxic water. Surprisingly, with a weaker stratification, and therefore potentially increased oxygen supply, methane oxidation rates decreased, and water–air methane fluxes increased. Thus, despite the potential resilience of the MOB community, seasonal water column dynamics significantly influence methane removal.
Methane-oxidizing bacteria in coastal waters can mitigate diffusive methane emissions. Their metabolic versatility and resilience are potentially high. Yet, methane removal is insufficient and highly variable throughout the year.
The potential and drivers of microbial methane removal in the water column of seasonally stratified coastal ecosystems and the importance of the methanotrophic community composition for ecosystem ...functioning are not well explored. Here, we combined depth profiles of oxygen and methane with 16S rRNA gene amplicon sequencing, metagenomics and methane oxidation rates at discrete depths in a stratified coastal marine system (Lake Grevelingen, The Netherlands). Three amplicon sequence variants (ASVs) belonging to different genera of aerobic Methylomonadaceae and the corresponding three methanotrophic metagenome‐assembled genomes (MOB‐MAGs) were retrieved by 16S rRNA sequencing and metagenomic analysis, respectively. The abundances of the different methanotrophic ASVs and MOB‐MAGs peaked at different depths along the methane oxygen counter‐gradient and the MOB‐MAGs show a quite diverse genomic potential regarding oxygen metabolism, partial denitrification and sulphur metabolism. Moreover, potential aerobic methane oxidation rates indicated high methanotrophic activity throughout the methane oxygen counter‐gradient, even at depths with low in situ methane or oxygen concentration. This suggests that niche‐partitioning with high genomic versatility of the present Methylomonadaceae might contribute to the functional resilience of the methanotrophic community and ultimately the efficiency of methane removal in the stratified water column of a marine basin.
