Julia Catherine Engelmann

Estuaries are natural sources of methane (CH4) to the atmosphere and connect freshwater, brackish and marine environments. The pronounced salinity gradient in these environments imposes challenges to microbial life, yet its influence on methane oxidising bacteria (MOB) and methane oxidation remains unclear. Here, we investigated methane dynamics and MOB in the Scheldt estuary (City of Antwerp, Belgium to the North Sea). Water samples were collected at ten stations along the salinity gradient of the estuary. We analyzed CH4 concentrations, stable carbon isotope ratios, efflux to the atmosphere, as well as methanogenic and methanotrophic communities, and methane oxidation rates. Elevated CH4 concentrations of up to 110 nM with δ¹³C values around −46‰ were found in the freshwater zone near Antwerp. Instead of a gradual decrease towards the North Sea, a second CH4 maximum of 180 nM was observed in the marine zone, with a contrasting isotopic composition of −66‰, suggesting distinct methanogenic pathways and/or substrates. Methane oxidation rates showed no clear relationship with salinity. In contrast, methanotrophic community composition shifted markedly along the estuary, with Methyloparacoccus and Crenothrix prevailing in freshwater and Methyloceanibacter and PLTB-vmat-59 dominating the mixing and marine zones. Similar MOB community patterns in sediments suggest that sediments act as both a habitat and recruitment source for water-column MOB. Despite an active microbial filter, methane oxidation consumed only a minor fraction of estuarine CH4, whereas most CH4 escaped to the atmosphere and a smaller fraction was exported to the North Sea, thereby sustaining methane supersaturation in nearby coastal waters.

Estuaries are important natural sources of methane (CH₄) to the atmosphere. These transitional aquatic systems connect freshwater, brackish and fully marine environments. The pronounced salinity gradient, characteristic of estuaries, impose particular challenges to microbes and influence the biogeochemical processes they mediate. Salinity changes can inhibit methanotrophic activity. Here we analysed methane dynamics and microbial communities in the estuary of the Scheldt River, which flows through northern France, western Belgium, and the southwestern Netherlands, and finally discharges into the North Sea. During a research cruise conducted in June 2022 from the river mouth to Antwerp, water samples were collected at ten stations along the estuarine salinity gradient. We investigated water column CH₄ inventory and stable carbon isotope dynamics, together with methane oxidation rates. Elevated CH₄ concentrations of up to 110 nM with associated δ¹³C -values of about −46 ‰ were found in the freshwater zone of the upper estuary (in the area of the city of Antwerp). Rather than a gradual decrease in CH₄ towards the North Sea, we observed a second maximum of 180 nM in the marine zone, with a contrasting isotopic composition of -66 ‰ indicating distinct methanogenic pathways and/or substrates in the estuary. Methane oxidation rates showed no clear relationship with the salinity gradient. In contrast, the composition of the methanotrophic community shifted markedly along the estuary and the salinity gradient, with Methyloparacoccus and Crenothrix prevailing in the freshwater zone Methyloceanibacter and PLTB-vmat-59 dominating in the mixing and marine zones. Our results thus demonstrate that the salinity gradient is not the primary control on estuarine methane oxidation capacity but the community composition of the methane oxidizing bacteria (MOB). In sediments, the MOB community composition mirrored the patterns observed in the overlying water column, indicating that estuarine sediments represent both a key habitat and an important recruitment source sustaining methanotrophic communities in the water column. Despite an active microbial filter, methane oxidation accounted for only a minor fraction of the estuaries’ CH₄ budget. Most methane (about 98 %) was lost through advection and diffusive fluxes to the atmosphere, while a smaller fraction was exported to the North Sea, contributing to sustained methane supersaturation in coastal waters.

Estuaries are important natural sources of methane (CH₄) to the atmosphere. These transitional aquatic systems connect freshwater, brackish and fully marine environments. The pronounced salinity gradient, characteristic of estuaries, impose particular challenges to microbes and influence the biogeochemical processes they mediate. Salinity changes can inhibit methanotrophic activity. Here we analysed methane dynamics and microbial communities in the estuary of the Scheldt River, which flows through northern France, western Belgium, and the southwestern Netherlands, and finally discharges into the North Sea. During a research cruise conducted in June 2022 from the river mouth to Antwerp, water samples were collected at ten stations along the estuarine salinity gradient. We investigated water column CH₄ inventory and stable carbon isotope dynamics, together with methane oxidation rates. Elevated CH₄ concentrations of up to 110 nM with associated δ¹³C -values of about −46 ‰ were found in the freshwater zone of the upper estuary (in the area of the city of Antwerp). Rather than a gradual decrease in CH₄ towards the North Sea, we observed a second maximum of 180 nM in the marine zone, with a contrasting isotopic composition of -66 ‰ indicating distinct methanogenic pathways and/or substrates in the estuary. Methane oxidation rates showed no clear relationship with the salinity gradient. In contrast, the composition of the methanotrophic community shifted markedly along the estuary and the salinity gradient, with Methyloparacoccus and Crenothrix prevailing in the freshwater zone Methyloceanibacter and PLTB-vmat-59 dominating in the mixing and marine zones. Our results thus demonstrate that the salinity gradient is not the primary control on estuarine methane oxidation capacity but the community composition of the methane oxidizing bacteria (MOB). In sediments, the MOB community composition mirrored the patterns observed in the overlying water column, indicating that estuarine sediments represent both a key habitat and an important recruitment source sustaining methanotrophic communities in the water column. Despite an active microbial filter, methane oxidation accounted for only a minor fraction of the estuaries’ CH₄ budget. Most methane (about 98 %) was lost through advection and diffusive fluxes to the atmosphere, while a smaller fraction was exported to the North Sea, contributing to sustained methane supersaturation in coastal waters.

Microbial communities underpin ecosystem processes and biogeochemical cycles in marine ecosystems, yet their spatial and temporal dynamics at hourly scales remain poorly understood. We surveyed two stations from the North Sea (NS) and Wadden Sea (WS), generating six high‐frequency time‐series datasets across depths and seasons, complemented by sediment cores. Across seasons, the sites in the NS and the WS harbored distinct microbial communities shaped by contrasts in salinity, temperature, and potentially the quantity and lability of organic matter. Connectivity between communities was limited but favoured by known seasonal hydrographic exchanges. Despite taxonomic contrasts between sites, functional turnover remained low, with communities harbouring similar metabolic potential but being adapted to local conditions, suggesting potential functional redundancy. At hourly scales, community turnover was weaker and largely driven by vertical and horizontal mixing between water masses, occasional resuspension from sediments, or a summer bloom from a copiotroph. These shifts were transient and did not disrupt the coupling between taxonomic and functional composition. However, their immediate effects on ecosystem processes, such as organic matter remineralisation and nutrient recycling remain unclear. Continued high‐resolution microbiome monitoring, paired with biogeochemical flux measurements, is needed to better predict climate‐driven changes in coastal ecosystem functioning.

Estuaries are important natural sources of methane (CH₄) to the atmosphere. These transitional aquatic systems connect freshwater, brackish and fully marine environments. The pronounced salinity gradient, characteristic of estuaries, impose particular challenges to microbes and influence the biogeochemical processes they mediate. Salinity changes can inhibit methanotrophic activity. Here we analysed methane dynamics and microbial communities in the estuary of the Scheldt River, which flows through northern France, western Belgium, and the southwestern Netherlands, and finally discharges into the North Sea. During a research cruise conducted in June 2022 from the river mouth to Antwerp, water samples were collected at ten stations along the estuarine salinity gradient. We investigated water column CH₄ inventory and stable carbon isotope dynamics, together with methane oxidation rates. Elevated CH₄ concentrations of up to 110 nM with associated δ¹³C -values of about −46 ‰ were found in the freshwater zone of the upper estuary (in the area of the city of Antwerp). Rather than a gradual decrease in CH₄ towards the North Sea, we observed a second maximum of 180 nM in the marine zone, with a contrasting isotopic composition of -66 ‰ indicating distinct methanogenic pathways and/or substrates in the estuary. Methane oxidation rates showed no clear relationship with the salinity gradient. In contrast, the composition of the methanotrophic community shifted markedly along the estuary and the salinity gradient, with Methyloparacoccus and Crenothrix prevailing in the freshwater zone Methyloceanibacter and PLTB-vmat-59 dominating in the mixing and marine zones. Our results thus demonstrate that the salinity gradient is not the primary control on estuarine methane oxidation capacity but the community composition of the methane oxidizing bacteria (MOB). In sediments, the MOB community composition mirrored the patterns observed in the overlying water column, indicating that estuarine sediments represent both a key habitat and an important recruitment source sustaining methanotrophic communities in the water column. Despite an active microbial filter, methane oxidation accounted for only a minor fraction of the estuaries’ CH₄ budget. Most methane (about 98 %) was lost through advection and diffusive fluxes to the atmosphere, while a smaller fraction was exported to the North Sea, contributing to sustained methane supersaturation in coastal waters.

Global warming induces alterations in ocean temperature regimes, as well as in precipitation patterns. This leads to shifts in water column properties, which will increasingly impact coastal ecosystems. These changes may have profound implications for microbial communities such as methane-oxidizing bacteria (MOBs), which play a critical role in regulating methane fluxes and ecosystem dynamics. In this study, we investigate the resilience and adaptability of aerobic MOBs in response to changing environmental conditions. Microcosm incubations with waters from the North Sea and the Wadden Sea collected across different seasons and adjusted for methane availability, temperature, and salinity were used to assess how these factors shape MOB community structure and functional capacity. Our results reveal an increase in the relative abundance of MOBs to up to 57 % in experiments with elevated methane concentrations, highlighting the primary role of methane availability in MOB community development. Temperature and salinity variations, on the other hand, exerted lesser effects on MOB composition and relative abundance. A strong effect on MOB community development was furthermore caused by the origin of the inoculum (location and season). Our results thus suggest a functional redundancy in the variable pools of microbial inocula, enabling multiple MOB clades to cope with drastic changes in environmental parameters. The adaptability of MOB communities is key to understanding their role in mitigating methane emissions from coastal regions in a future ocean with potentially elevated methane and temperature and variable salinity levels.

Global warming induced alterations in ocean temperature regimes, and precipitation patterns are increasingly impacting coastal ecosystems, leading to shifts in water column properties. These changes may have profound implications for microbial communities such as methane-oxidising bacteria (MOBs), which play a critical role in regulating methane fluxes and ecosystem dynamics. In this study, we investigate the resilience and adaptability of aerobic MOBs in response to changing environmental conditions. Through microcosm incubation experiments with waters from the North Sea and the Wadden Sea collected during different seasons, we explore how variations in methane availability, temperature, and salinity influence the MOB community structure and functional capacity. Our results reveal an increase in the relative abundance of MOBs to up to 57 % in experiments with elevated methane concentrations, highlighting the primary role of methane availability for MOB community development. Temperature and salinity variations, on the other hand, exerted lesser effects on MOB composition and relative abundance. A strong effect on MOB community development was furthermore caused by the origin of the inoculum (location and season). Our results thus suggest a functional redundancy in the variable pools of microbial inocula enabling multiple MOB clades to cope with drastic changes in environmental parameters. The adaptability of MOB communities is key to understand their role in mitigating methane emissions from coastal regions in a future ocean with potentially elevated methane, temperature and variable salinity levels.

Global warming induced alterations in ocean temperature regimes, and precipitation patterns are increasingly impacting coastal ecosystems, leading to shifts in water column properties. These changes may have profound implications for microbial communities such as methane-oxidising bacteria (MOBs), which play a critical role in regulating methane fluxes and ecosystem dynamics. In this study, we investigate the resilience and adaptability of aerobic MOBs in response to changing environmental conditions. Through microcosm incubation experiments with waters from the North Sea and the Wadden Sea collected during different seasons, we explore how variations in methane availability, temperature, and salinity influence the MOB community structure and functional capacity. Our results reveal an increase in the relative abundance of MOBs to up to 57 % in experiments with elevated methane concentrations, highlighting the primary role of methane availability for MOB community development. Temperature and salinity variations, on the other hand, exerted lesser effects on MOB composition and relative abundance. A strong effect on MOB community development was furthermore caused by the origin of the inoculum (location and season). Our results thus suggest a functional redundancy in the variable pools of microbial inocula enabling multiple MOB clades to cope with drastic changes in environmental parameters. The adaptability of MOB communities is key to understand their role in mitigating methane emissions from coastal regions in a future ocean with potentially elevated methane, temperature and variable salinity levels.

Global warming induced alterations in ocean temperature regimes, and precipitation patterns are increasingly impacting coastal ecosystems, leading to shifts in water column properties. These changes may have profound implications for microbial communities such as methane-oxidising bacteria (MOBs), which play a critical role in regulating methane fluxes and ecosystem dynamics. In this study, we investigate the resilience and adaptability of aerobic MOBs in response to changing environmental conditions. Through microcosm incubation experiments with waters from the North Sea and the Wadden Sea collected during different seasons, we explore how variations in methane availability, temperature, and salinity influence the MOB community structure and functional capacity. Our results reveal an increase in the relative abundance of MOBs to up to 57 % in experiments with elevated methane concentrations, highlighting the primary role of methane availability for MOB community development. Temperature and salinity variations, on the other hand, exerted lesser effects on MOB composition and relative abundance. A strong effect on MOB community development was furthermore caused by the origin of the inoculum (location and season). Our results thus suggest a functional redundancy in the variable pools of microbial inocula enabling multiple MOB clades to cope with drastic changes in environmental parameters. The adaptability of MOB communities is key to understand their role in mitigating methane emissions from coastal regions in a future ocean with potentially elevated methane, temperature and variable salinity levels.

Global warming induced alterations in ocean temperature regimes, and precipitation patterns are increasingly impacting coastal ecosystems, leading to shifts in water column properties. These changes may have profound implications for microbial communities such as methane-oxidizing bacteria (MOBs), which play a critical role in regulating methane fluxes and ecosystem dynamics. In this study, we investigate the resilience and adaptability of aerobic MOBs in response to changing environmental conditions. Through microcosm incubation experiments with waters from the North Sea and the Wadden Sea collected during different seasons, we explore how variations in methane availability, temperature, and salinity influence the MOB community structure and functional capacity. Our results reveal an increase in the relative abundance of MOBs to up to 57% in experiments with elevated methane concentrations, highlighting the primary role of methane availability for MOB community development. Temperature and salinity variations, on the other hand, exerted lesser effects on MOB composition and relative abundance. A strong effect on MOB community development was furthermore caused by the origin of the inoculum (location and season). Our results thus suggest a functional redundancy in the variable pools of microbial inocula enabling multiple MOB clades to cope with drastic changes in environmental parameters. The adaptability of MOB communities is key to understand their role in mitigating methane emissions from coastal regions in a future ocean with potentially elevated methane, temperature and variable salinity levels.