Background  Cable bacteria are filamentous sulphide-oxidisers capable of cm-scale electron transport. They are generally considered restricted to the upper few oxic–suboxic cm of marine sediments, where they couple sulphide oxidation to oxygen or nitrate reduction. Despite their influence on redox gradients, trace metal mobility, and nutrient cycling, their presence and activity in deeper anoxic sediment layers remain unknown. The presence and activity of marine cable bacteria (Candidatus Electrothrix) were investigated at four stations in Sweden and Finland, including deep vertical profiles of anoxic sediment layers, to assess their presence and activity under different environmental contexts.

Results  Using metatranscriptomic data for rRNA-based community profiling and gene expression combined with porewater geochemistry, evidence of abundant and active cable bacteria was found, peaking below 20 cm depth in deep anoxic sediment layers of Koljö Fjord on the Swedish West Coast. This zone coincided with elevated gene expressions related to sulphide oxidation (including sqr) and nitrate reduction (napA), as well as an abundant presence of sulphide and a sharp nitrate peak. Phylogenetic analyses revealed a diverse assemblage of Ca. Electrothrix includes several potential novel taxa. The co-occurrence of cable bacteria activity, sulphide availability, and a nitrate peak at depth suggests that these organisms may be supported by local nitrate production under anoxic conditions.

Conclusions  Our findings challenge the prevailing view that cable bacteria are restricted to shallow sediment horizons and demonstrate their activity and diversity in deep, anoxic layers. This expands the known ecological niche of cable bacteria and suggests that locally produced nitrate under anoxic conditions may facilitate their activity at depth. This discovery advances our understanding of ecology in anoxic marine environments, providing new insights into marine cable bacteria, sediment biogeochemistry, and analogues of early Earth microbial ecosystems.

Coastal ecosystems play a crucial role in greenhouse gas (GHG) dynamics but are less studied than open oceans or terrestrial systems. This study measured concentrations of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) in six shallow bays of the wider Stockholm Archipelago during spring (April) and autumn (September–October) 2024 using cavity ring-down spectroscopy combined with a water equilibration system. We explored how GHG levels relate to bay physical characteristics (i.e. topographic openness, sediment properties vegetation cover) and seawater properties (temperature, salinity, dissolved-oxygen saturation, chlorophyll-a, organic carbon, and nutrient concentrations), revealing significant seasonal variation of concentrations. Surface water pCO₂ ranged from 225–1372 ppm, CH₄ from 3.6–580 nmol L⁻¹, and N₂O from 8–20.8 nmol L⁻¹ with pCO₂ and CH₄ higher in autumn and N₂O higher in spring. CH₄ concentrations below 250 nmol L⁻¹ were negatively correlated with N₂O, while higher CH₄ levels showed a positive correlation, suggesting differences in the dominant sedimentary microbial pathways. Most bays acted as net GHG sinks in April and sources in September, with only one bay showing net source behaviour in both seasons. One bay that is subject to substantial human impacts (e.g. dredging, high nutrient loading, reduced vegetation cover) showed CO₂-equivalent CH₄ emissions that surpassed CO₂ uptake in this particular bay. CO₂-equivalent fluxes ranged from −195.2 to 793.6 mg CO₂ eq. m⁻² d⁻¹ (median: 131.5 mg CO₂ eq. m⁻² d⁻¹). This study is distinctive in simultaneously measuring all three major GHGs across multiple bays in relation to diverse environmental controls, offering a uniquely integrated understanding of coastal GHG dynamics. These findings highlight the variability and complexity of coastal ecosystems and demonstrate the importance of high-resolution measurements for accurate up-scaling of fluxes from these dynamic environments.

Östersjön är ett unikt ekosystem, i akut behov av bättre skydd och omsorg. Havet är utsatt för stora påfrestningar från föroreningar, övergödning, klimatförändringar och överfiske, och därför krävs fortsatt kraftfulla insatser. Här presenteras ett antal viktiga åtgärder som kan förbättra havsmiljön.

Vi människor är beroende av havet för vår existens, för syret vi andas och för ett klimat vi kan leva i. För många är också mat från havet en viktig del av födan. Transporter, fiske, turism och rekreation visar på havets stora ekonomiska och sociala värden, och mycket tyder på att välmående kustmiljöer också kan motverka klimatförändringarna. Havet behöver vårdas – både för vår egen skull och för de marina ekosystemens.

I Östersjön har mänskliga aktiviteter haft stor påverkan på miljön. Vissa bestånd av fisk, marina däggdjur och sjöfåglar har minskat kraftigt, andra är påverkade av miljögifter eller i dåligt skick, vilket bland annat syns i lägre tillväxt hos arter som sill och torsk. Syrefattiga bottnar, förändrad artsammansättning och skadade livsmiljöer är andra tydliga tecken på mänsklig påverkan. Klimatförändringarna förvärrar en del av miljöproblemen och gör det ännu mer angeläget att agera snabbt.

Det begränsade vattenutbytet i det inneslutna Östersjön gör att föroreningar blir kvar under lång tid och fördröjer effekten av olika åtgärder. Men vårt inneslutna hav med ett begränsat antal aktörer ger också möjlighet att agera unisont. I den här broschyren presenterar forskare och experter från Stockholms universitet ett antal viktiga åtgärder som kan förbättra miljön i Östersjön, från Bottenviken till Kattegatt.

Det behövs gemensamma ansträngningar och modiga beslut för att minska utsläppen av näringsämnen och farliga ämnen, och för att säkerställa en ekosystembaserad fiskeförvaltning som tar hänsyn till de ekologiska förutsättningarna. Bara så kan Östersjön bli basen för en hållbar blå ekonomi. Om hela havet förvaltas klokt, samtidigt som orörda områden skyddas och förstörda miljöer återställs, så finns goda möjligheter att på sikt återfå ett välmående och mer motståndskraftigt hav – till nytta för både människor och natur.

Excessive nutrient inputs have caused eutrophication of coastal ecosystems worldwide, triggering extensive algal blooms, oxygen-depletion, and collapse of local fisheries. In the Baltic Sea, inputs of nitrogen (N) and phosphorus (P) have been significantly reduced since the 1980s, but the environmental state shows little to no signs of recovery. However, a simulation with continued high loads from the mid-1980s demonstrates that while the state has not improved yet, it would be considerably worse today without the load reductions (e.g., 82% larger oxygen-free bottom areas and 104% and 58% higher wintertime concentrations of inorganic N and P, respectively, in the Baltic Proper). Additional simulations with current nutrient loads continuing into the future indicate that conditions will likely improve in the coming decades. This study underscores the significance of acting on early warning signs of eutrophication, and furthermore how sustained efforts to decrease nutrient loads can mitigate the severity of eutrophication.

Coastal areas including fjords, emit substantial amounts of methane (CH₄) and nitrous oxide (N₂O), that may partially offset their carbon dioxide (CO₂) sink potential. Expanding coastal marine aquaculture may impact greenhouse gas dynamics. The role of mussel farming as a CO₂ sink or source and its potential for nutrient removal is well investigated, but its effects on sea-air greenhouse gas emissions remain unclear. Here we resolve greenhouse gas emissions from a Swedish fjord with mussel farms, covering temporal (pre- and postharvest) and spatial (mussel farm versus control area) variability. The sediment-water fluxes of dissolved inorganic carbon, CH₄ and N₂O were on average 2.2 times (without the outlier in winter), 2.7 times and 6.0 times higher in the farm compared to the corresponding control site, leading to enhanced bottom water concentrations. The sea-air fluxes ranged from −8.3 to 12.8 mmol CO₂ m⁻² d⁻¹, from 2.9 to 39.2 μmol CH₄ m⁻² d⁻¹, and from −1.8 to 2.1 μmol N₂O m⁻² d⁻¹. CO₂ emissions were 15 % lower and 11 % greater in farms (small farm and large farm respectively) than control sites, CH₄ emissions were comparable between farms and controls and mussel farms lowered N₂O emissions by 46 % compared to controls. We estimated low aquatic emission intensities with −2.98 g CO₂ equivalents per kg edible meat for the smaller farm and 1.74 g CO₂ eq. for the larger farm. Overall, our findings highlight a large temporal and spatial variability of greenhouse gas emissions and reveal the relatively small impacts of mussel farms on total emissions.

In eutrophic coastal waters, aerobic methane-oxidizing bacteria (MOB) mitigate methane emissions by oxidizing benthic methane even in the stratified, anoxic water column. However, ongoing warming and eutrophication lead to extended stratification periods, enhancing anoxic and sulfidic conditions (euxinia), potentially affecting methane removal capacity. Here we compared overall water column methane removal between sites with irregular, seasonal and longer-term euxinia in the Stockholm Archipelago during summer 2022. The highest water–air methane emissions, bottom water–methane and sulfide accumulation, and the lowest methane oxidation potential were observed under longer-term euxinic bottom water conditions. While MOB relative abundance and potential activity indicated high functioning of the methane biofilter in the seasonally euxinic bottom water layer, the methane-filtering potential was much lower in the longer-term euxinic bottom water. Under persistent euxinic conditions, overall bacterial diversity and microbial network connectivity were lower, likely following a simultaneous shift in redox conditions and a shift toward anaerobic sulfur-cycling. This shift may force MOB to retreat from the euxinic bottom water into the narrow oxycline, reducing the capacity of the methane biofilter and resulting in higher methane emissions. These findings highlight the positive feedback loop that can further amplify oceanic methane emissions, particularly from eutrophic and shallow coastal waters prone to prolonged stratification under global warming.

Sedimentary concentrations of redox-sensitive trace metals are widely used to reconstruct past ocean redox conditions. Vanadium (V) has great potential as a (paleo)redox proxy, due to its strong redox-dependent speciation (+III, +IV, +V) and the increased sedimentary sequestration of its more reduced species. The geochemistry of V in sulfide-rich marine environments is not yet well understood, however, hampering the use of V as a (paleo)redox proxy. Here, we present V data for two coastal systems, with bottom water redox conditions ranging from oxic to euxinic, to further constrain V geochemistry. Our sedimentary record from a eutrophic coastal marine basin (Scharendijke basin, Lake Grevelingen, the Netherlands), covering the last decade, shows distinct enrichments in molybdenum (Mo) and organic carbon (C_org) but depletions in V during seasonal bottom water euxinia, which can be discerned due to the exceptionally high sedimentation rate at our study site (up to 20 cm yr⁻¹). A seasonal study for the same coastal basin confirms this trend and reveals the accumulation of V, iron (Fe) and manganese (Mn) in the water column during summer euxinia. We conclude that the slow kinetics of V reduction to V(III) and subsequent precipitation as (oxy)hydroxide V(OH)_3(s) likely provide the opportunity for V to escape sedimentary sequestration during summer euxinia, resulting in the observed sedimentary V depletion. Sediments from three sites with contrasting bottom water redox conditions (oxic, seasonally hypoxic, euxinic) in the eutrophic Stockholm Archipelago, show a similar trend as that of Lake Grevelingen, with decreasing V concentrations and increasing Mo and C_org concentrations as bottom water conditions become more reducing. This confirms that our findings for Lake Grevelingen are not site-specific and are likely a generic feature of euxinic coastal systems with high sulfide concentrations (> 0.5 mmol L⁻¹) near the sediment surface and high rates of anaerobic degradation of organic matter. Our results show that co-occurring sedimentary Mo and C_org enrichments and V depletion (or absence or suppression of an enrichment) are indicators of strongly sulfidic conditions in such settings. Finally, we show that maxima in sedimentary molar V/Mn ratios correlate with strongly reducing conditions. This finding contrasts with prior work on V/Mn ratios as a (paleo)redox proxy, implying that further research is necessary.

Wetlands are valuable and diverse environments that contribute to a vast range of ecosystem services, such as flood control, drought resilience, and carbon sequestration. The provision of these ecosystem services depends on their hydrological functioning, which refers to how water is stored and moved within a wetland environment. Since the hydrological functions of wetlands vary widely based on location, wetland type, hydrological connectivity, vegetation, and seasonality, there is no single approach to defining these functions. Consequently, accurately identifying their hydrological functions to quantify ecosystem services remains challenging. To address this issue, we investigate the hydrological regimes of wetlands, focusing on water extent, to better understand their hydrological functions. We achieve this goal using Sentinel-1 SAR imagery and a self-supervised deep learning model (DeepAqua) to predict surface water extent for 43 Ramsar sites in Sweden between 2020 and 2023. Clustering analysis grouped the wetlands based on the water extent predictions into five archetypes based on their hydrological similarity: “spring-surging”, “spring-flooded”, “summer-flooded”, “slow-drying”, and “summer-dry”. The archetypes represent great heterogeneity, with flashy regimes being more prominent at higher latitudes and smoother regimes found preferentially in central and southern Sweden. Additionally, many wetlands show exceptional similarity in the timing and duration of flooding and drying events, which only became apparent when grouped. We attempt to link hydrological functions to the archetypes, whereby headwater wetlands, such as spring-surging wetlands, have the potential to accentuate floods and droughts, while slow-drying wetlands, typical of floodplain wetlands, are more likely to provide services such as flood attenuation and water storage during low flow conditions. Additionally, although wetlands can be classified in a myriad of ways, we propose that classifying wetlands based on the hydrological regime derived from water surface extent is useful for identifying hydrological functions specific to the site and season and when discharge or water level data are not available. Lastly, we foresee that hydrological-regime-based classification can be easily applied to other wetland-rich landscapes to better understand the hydrological functions and identify their respective ecosystem services.

Coastal ecosystems dominate oceanic methane (CH₄) emissions. However, there is limited knowledge about how biotic interactions between infauna and aerobic methanotrophs (i.e. CH₄ oxidizing bacteria) drive the spatial–temporal dynamics of these emissions. Here, we investigated the role of meio- and macrofauna in mediating CH₄ sediment–water fluxes and aerobic methanotrophic activity that can oxidize significant portions of CH₄. We show that macrofauna increases CH₄ fluxes by enhancing vertical solute transport through bioturbation, but this effect is somewhat offset by high meiofauna abundance. The increase in CH₄ flux reduces CH₄ pore-water availability, resulting in lower abundance and activity of aerobic methanotrophs, an effect that counterbalances the potential stimulation of these bacteria by higher oxygen flux to the sediment via bioturbation. These findings indicate that a larger than previously thought portion of CH₄ emissions from coastal ecosystems is due to faunal activity and multiple complex interactions with methanotrophs. 

Coastal areas are an important source of methane (CH4). However, the exact origins of CH4 in the surface waters of coastal regions, which in turn drive sea-air emissions, remain uncertain. To gain a comprehensive understanding of the current and future climate change feedbacks, it is crucial to identify these CH4 sources and processes that regulate its formation and oxidation. This study investigated coastal CH4 dynamics by comparing water column data from six stations located in the brackish Tvärminne Archipelago, Baltic Sea. The sediment biogeochemistry and microbiology were further investigated at two stations (i.e., nearshore and offshore). These stations differed in terms of stratification, bottom water redox conditions, and organic matter loading. At the nearshore station, CH4 diffusion from the sediment into the water column was negligible, because nearly all CH4 was oxidized within the upper sediment column before reaching the sediment surface. On the other hand, at the offshore station, there was significant benthic diffusion of CH4, albeit the majority underwent oxidation before reaching the sediment-water interface, due to shoaling of the sulfate methane transition zone (SMTZ). The potential contribution of CH4 production in the water column was evaluated and was found to be negligible. After examining the isotopic signatures of δ13C-CH4 across the sediment and water column, it became apparent that the surface water δ13C-CH4 values observed in areas with thermal stratification could not be explained by diffusion, advective fluxes, nor production in the water column. In fact, these values bore a remarkable resemblance to those detected below the SMTZ. This supports the hypothesis that the source of CH4 in surface waters is more likely to originate from ebullition than diffusion in stratified brackish coastal systems.