Long-term ocean warming impacts the marine environment, and these effects will be exacerbated by future climate change affecting, e.g., biogeochemical processes and microbial communities. However, how the sediment microbial cell abundance and live/dead ratio respond to warming is poorly understood. In this study, sediment core samples were collected from a Baltic Sea bay artificially heated on average 5°C for > 50 years above a nearby (control) bay unaffected by the heating. Contrary to the expected increased productivity in the heated bay, qPCR-based sediment cell abundances showed decreased cell numbers along the sediment depth gradient in the heated bay compared to the control bay. This could reflect that a portion of the cells' metabolic energy was diverted to a heat related stress response rather than being used for replication. In addition, live/dead cell ratios showed no clear differences in either bay suggesting the majority of the cells were alive. Finally, sediment depth gradient 16S rRNA gene sequencing confirmed previous studies, showing that prolonged warming shallows sediment biogeochemical zones and related microbial communities. In conclusion, future climate change related warming will likely decrease microbial cell abundances that form part of the food web base, potentially impacting the entire ecosystem.

Abstract Ice–ocean interactions at Greenland Ice Sheet outlet glaciers exert critical control on ice-sheet mass loss and, consequently, global sea-level rise. The GEOEO North of Greenland 2024 icebreaker expedition acquired a breadth of data in the unsurveyed Victoria Fjord, north-western Greenland, to assess the causes of the rapid break-up of the C.H. Ostenfeld ice tongue in 2002, and the environmental changes since. The observations reveal inflows of warm (>0 °C) Atlantic water reaching the grounded ice margin. We propose that Atlantic warm water pulses entering the Arctic Ocean reach north-western Greenland, where they interact with glacier fronts, enhance ice discharge, and amplify the climate-driven retreat of marine-terminating glaciers. The collapse of C.H. Ostenfeld’s ice tongue, unlike the slower retreats of the ice tongues of neighbouring Petermann and Ryder glaciers, reflects unimpeded Atlantic water inflow due to the absence of a shielding bathymetric sill, a function of the region’s bedrock geology.

Zooplankton are crucial for food webs and biogeochemical cycles. However, warming associated with climate change may alter their seasonal timing and reproductive strategies. This study investigated how long-term warming impacted zooplankton (mainly copepods) phenology and overwintering strategies by comparing a Baltic Sea bay, heated by warm water discharge for more than 50 yr, with an unaffected control bay. Field observations showed that copepod and phytoplankton population growth began earlier in the heated bay than in the control bay, suggesting that copepod abundance was driven by both temperature and food availability in the heated bay and by a stronger temperature dependence in the control bay. Resting eggs are normally produced as a life-history strategy to survive unfavorable environmental conditions. Our laboratory incubation experiment showed fewer dormant resting eggs hatched from the heated bay sediment compared with the control bay, supporting an evolutionary change in overwintering strategy. In conclusion, the results seemed to suggest that copepods adjusted their life-history in elevated temperatures by relying less on the strategy of using sediment-stored dormant eggs and instead started their spring development earlier, when phytoplankton food was available. Hence, this study suggests that climate change can shift copepod overwintering strategies, leading to potential cascading effects in the food web and affecting overall biodiversity and productivity.

Climate change driven ocean warming is a worldwide environmental issue that can impact cycling of greenhouse gases. However, how methane production in marine sediments as a potential contributor to atmospheric greenhouse gases versus its consumption at the sulfate–methane transition zone will be affected by climate change related warming is still not well constrained. In this study, sediments from two Baltic Sea bays with long-term temperature differences were collected during summer and winter. The primary difference between the two bays was that one had been heated by a nearby power plant for 50 years, resulting in a 5.1 °C increase in annual average temperature compared to an unheated control bay. The results showed that near-seafloor sediment methane concentrations were 50 times higher compared to present-day conditions. Furthermore, the sediment fluxes along with microbial community composition changes suggested that long-term warming may thin the sulfate reduction zone, such that methanotrophic archaea and sulfate reducing bacteria peaked at shallower sediment depths in the heated bay. Overall, the results from long-term warming in natural sediment environment indicated that future climate change warming may increase the risk of methane release to the water and eventually the atmosphere.

Microplastics (MPs) are a significant environmental concern due to their abundance, widespread occurrence, slow degradation rate, and toxicity. Although MPs have been found in various environments and organisms (including humans), few studies have investigated their historical accumulation in marine settings. Here we present the first chronological accumulation of MPs in Baltic Sea sediments over time. The high sediment accumulation rates (ca. 1 cm yr-1) and anoxic bottom conditions in the study area allowed for an exceptionally well-preserved record of MPs in sediment over the last 50 years. Despite potential biases and limitations associated with the methodologies (e.g., visual identification), our results suggest that MP accumulation began in the 1970s-1980s and has continued to increase until present-day, with up to 4450 particles per kg of dry sediment. The temporal distribution of MPs in sediment is consistent with local population growth in catchment areas and global plastic production rates, highlighting the impact of human activities on MP pollution in the region.

Microbial methane generation (methanogenesis) is an important metabolic process in the terrestrial deep biosphere and is an analog to early Earth as it is proposed to be one of the most ancient metabolisms on Earth. Signs of methanogenesis in meteorite impact craters are of particular interest in this respect as these settings are proposed hot spots for deep microbial colonization of the upper crust. Yet, reports of active deep rock-hosted methanogenesis are scarce, particularly for methylotrophic methanogenesis, while reports from terrestrial meteorite impact craters are completely lacking. Here, we used indigenous communities in cultures enriched from 400-m deep fluids to confirm and characterize active methane production from several carbon donors, including indigenous oil, in a terrestrial impact crater at Siljan, Sweden. Metagenomic and metatranscriptomic data of the methane-producing cultures revealed a consortium dominated by Acetobacterium sp. KB-1 and Candidatus Methanogranum gryphiswaldense, mediating methanogenesis solely via the methyl-reduction pathway, and resulting in a δ13Cmethanol-methane isotope enrichment of up to 98.6‰. These results provide insights into methylotrophic methanogenesis in deep subsurface environments in general, and in particular in fractured meteorite impact structures.IMPORTANCEThis study revealed that microbes enriched from groundwater in a 380-m deep borehole within the Siljan meteorite impact crater in Sweden were capable of producing methane, a key greenhouse gas. This is especially significant because it is the first proof of active methanogens in an impact crater and showing a specific pathway of methane production-methylotrophic methanogenesis-is present in the deep terrestrial subsurface, an environment that is typically hard to study. These findings shed light on life in extreme conditions on Earth and show that meteorite craters can be biological hotspots, rich with ancient life processes.

Deeply fractured rocks of meteorite impact craters are suggested as prime niches for subsurface microbial colonization. Methane can be a product of such microbial communities and seeps of methane from impact craters on Earth are of strong interest as they act as analogs for Mars. Previous studies report signs of ancient microbial methanogenesis in the Devonian Siljan meteorite impact structure in Sweden, but the proportion of microbial methane, metabolic pathways, and potential modern activity remain elusive. In this study, gas composition, hydrochemistry, oil organic geochemistry, and microbial community analyses are reported in 400 m deep fractures of the Siljan impact structure. The results showed a dominantly microbial origin for methane, which was supported by highly negative δ13CCH4 and positive δ13CCO2 values along with multiply substituted isotopologues (Δ13CH3D) that indicated disequilibrium fractionation due to microbial kinetic isotope effects. The presence of C2 to C5 hydrocarbons suggested a minor thermogenic input in the gas mix. Characterization of the microbial community via 16S rRNA gene amplicon sequencing and real-time PCR indicated a low abundance of several methanogenic archaeal populations, which is common for settings with active methanogenesis. Evidence of oil biodegradation suggested that secondary microbial hydrocarbon utilization was involved in the methanogenesis. Low sulfate and high alkalinity in the groundwaters also suggested a dominantly microbial methane formation driven by infiltration of freshwater that was coupled to sulfate reduction and secondary utilization of early mature thermogenic hydrocarbons.

A recently acquired multidisciplinary dataset comprising acoustic surveys (high-resolution sub-bottom profiles, multi-beam bathymetry, and broad band mid-water echo sounder), geochemistry (gas chemical and isotopic composition, porewater chemistry), and sedimentology (core lithology and X-ray CT) in the area of the Landsort deep (450 m of depth), south of Stockholm Archipelago, revealed the existence of an extensive (20 km2) region of the seafloor where massive gas release is occurring in the form of multiple bubble streams. This new discovery represents a major seafloor methane release site in Europe and is comparable in area to other large sites worldwide such as the ones in Svalbard and in the South Atlantic Ocean associated with gas hydrate provinces. The gas is formed mostly by methane of microbial origin. Surprisingly, bubbles rise 100’s of meters above the seafloor and reach surface waters above the halocline/oxycline at around 80 m of depth. Some bubbles appear to reach the sea-air interface and their potential methane contribution to the atmosphere is under investigation. Another surprising observation is the absence of major seafloor features like pockmarks in the gas release area. The reasons for the seafloor methane release in the Landsort deep are still not entirely clear, but our preliminary acoustic and sedimentological data suggest that bottom currents may have acted to facilitate the accumulation of organic-rich sediments in a thick drift deposit during the Holocene and the modern warm period (latest 100 years). Our data further suggest that the high sedimentation rate in the drift deposit continuously supplies fresh organic matter that is quickly buried below a thin sulphate reduction zone, fueling vigorous methanogenesis and abundant methane formation. Similar methane release sites might be discovered in other known large drift deposits in the Baltic Sea.

Gas migration and seafloor exudation are common phenomena in both deep and shallow water settings. However, the formation mechanisms and the relationships between different gas migration-related structures remain only partially understood. We constructed a reduced physical model of a submarine slope with two layers of varying permeabilities, subjected to a punctuated air injection to simulate gas migration and seafloor exudation. The air passage resulted in various structures, including mounds, pockmarks, chimneys, and different types of fractures (tensile, shear, irregular, radial, dendritic, and semicircular). Their processes, evolution, and interconnections were recorded and analyzed using image processing techniques. The results reveal the development of different stages of gas migration leading to seafloor exudation, from the initial fracturing stage to gas release into the water column, emphasizing the crucial role of impermeable layer thickness, the distribution of structures along the slope, and the impact of local topographic features. Our model provides robust insights into sediment deformation and the formation of structures associated with gas migration and exudation on the seafloor.

This study provides a baseline analysis of sediment pollution in Kalmar Guest Harbor, Sweden, focusing on metals and microplastics. The study site, a bustling coastal area, was chosen to investigate the connections between anthropogenic activities, bioturbation, and environmental contaminants. The results revealed that the pollution extends beyond 30 cm of depth below the seafloor, with elevated levels of copper (Cu), tungsten (W), cadmium (Cd), zinc (Zn), lead (Pb), and microplastics. Significantly, there is no previous publication alarming the W contamination in the Baltic Sea sediment, and therefore, our findings highlight the need for further investigation into tungsten contamination in the region. Furthermore, we explored the distribution patterns, potential sources and relationships of different contaminants. The K-means cluster analysis revealed that bioturbation is speculated to influences the depth concentration of pollutants, particularly at shallow depths (0‬–18 cm). Metal contaminants do not appear to be predominantly bound to MP particles. Graphical abstract

The Amazon River culminates in one a deep-sea fan up to 10 km thick, a dynamic setting in which the rapid deposition of organic-rich sediment drives linked processes of methanogenesis, fluid migration and venting, gas hydrate formation, and large-scale slope instability. Growth of the fan over the last 8 Ma has been accompanied by its gravitational collapse on shale detachments to form extensional and compressional belts across the shelf and upper slope (96%) are

Methane is an important greenhouse gas, and global methane emission has been estimated separately from the perspective of anthropogenic and natural factors. However, in heavily populated semi-closed bays, methane emissions may be governed by both or even significantly amplified by human activities. One of the main factors mitigating methane emission from marine sediments to seawater and the atmosphere is the anaerobic oxidation of methane (AOM). The sulfate-rich zone acts as a barrier to methane release from the subseafloor because sulfate-dependent AOM removes sulfate and methane dissolved in interstitial water in a 1:1 molar ratio. Due to significant riverine inputs of freshwater and restricted water exchange, the seawater in some of the semi-closed bays is potentially fresher and has lower sulfate concentration, leading to a less effective AOM barrier for methane. Furthermore, the influx of nutrient-rich wastewater to densely populated semi-enclosed bays frequently leads to severe eutrophication, greatly enhancing biological productivity, anoxia, and the accumulation of organic-rich sediments in these systems. The objective of this study is to gain a deeper understanding of how the methane cycle is changed by anthropogenic activities in two case studies, with geochemical datasets collected from Tokyo Bay and the Baltic Sea, both known as heavily populated semi-closed bays. We conducted sediment coring at the entrance of Tokyo Bay and offshore Stockholm in the Baltic Sea. Two cores (2.5 m in length) from Tokyo Bay and six cores (4 to 6 m in length) from the Baltic Sea were recovered, respectively. Organic matter in the surface of 1 m of sediment, which may have been strongly influenced by recent anthropogenic activities, showed 1.5 to 2% and 1.5 to 3.6% of total organic carbon (TOC) in Tokyo Bay and the Baltic Sea, respectively. These results indicate the Baltic Sea has a higher potential to generate more methane than the Tokyo Bay. The sulfate concentration at the seafloor was 27 mM in Tokyo Bay and 4 mM in the Baltic Sea and decreased with depth due to the AOM reaction reaching 0 mM at 2.5 mbsf in both bays. The thickness of the sulfate reduction zone was the same in both bays, even though they have a large difference in sulfate concentration in the bottom seawater. The iodine concentration, which has been used as a tracer for methane due to its close association with organic matter, increased with depth up to 74 µM at 2.5 mbsf in Tokyo Bay and 63 µM at 4.5 mbsf in the Baltic Sea. The iodine flux in Tokyo Bay was two times higher than in the Baltic Sea, indicating the possibility of strong methane flux from deeper sediments, which may not directly derive from Anthropocene organic-rich sediment. We will discuss and compare the details of the geochemical datasets in both Bays in the presentation.

The Baltic Sea, a brackish basin characterised by significant organic matter deposition, presents a crucial area for climate change research. This study examines the long-term evolution of methane geochemistry in sediments from four sites within the Gotland basins of the Baltic Sea, spanning the past 14,000 years since deglaciation. This timescale enables us to capture the full transition from lacustrine to marine conditions and link past organic matter accumulation with present-day methane dynamics. Using a transport-reaction model, we also explore future scenarios (2020-2100), aligned with climate projections, to assess how changes in bottom water temperature, organic matter loading, and freshwater input may influence methane production and emission from sediments. Our findings reveal that a 2 °C rise in bottom water temperature could increase free gas formation, though without directly impacting methane release into the sea. However, elevated organic matter loading significantly influences methane diffusion through the seafloor. Additionally, anticipated freshwater influx and subsequent reductions in sulphate concentrations will substantially enhance methane diffusion into seawater. The model projects that rising temperatures, eutrophication, and freshwater input will together drive increased methane emissions into the Baltic Sea, with potential consequences for climate change amplification.

Climate change related warming is a serious environmental problem attributed to anthropogenic activities, causing ocean water temperatures to rise in the coastal marine ecosystem since the last century. This particularly affects benthic microbial communities, which are crucial for biogeochemical cycles. While bacterial communities have received considerable scientific attention, the benthic eukaryotic community response to climate change remains relatively overlooked. In this study, sediments were sampled from a heated (average 5°C increase over the whole year for over 50 years) and a control (contemporary conditions) Baltic Sea bay during four different seasons across a year. RNA transcript counts were then used to investigate eukaryotic community changes under long-term warming. The composition of active species in the heated and control bay sediment eukaryotic communities differed, which was mainly attributed to salinity and temperature. The family level RNA transcript alpha diversity in the heated bay was higher during May but lower in November, compared with the control bay, suggesting altered seasonal activity patterns and dynamics. In addition, structures of the active eukaryotic communities varied between the two bays during the same season. Hence, this study revealed that long-term warming can change seasonality in eukaryotic diversity patterns. Relative abundances and transcript expression comparisons between bays suggested that some taxa that now have lower mRNA transcripts numbers could be favored by future warming. Furthermore, long-term warming can lead to a more active metabolism in these communities throughout the year, such as higher transcript numbers associated with diatom energy production and protein synthesis in the heated bay during winter. In all, these data can help predict how future global warming will affect the ecology and metabolism of eukaryotic community in coastal sediments.

The vast oxygen-depleted area of the central Baltic Sea is the largest human-induced dead zone in the world with 70,000 km2 or approximately three times the second largest one in the Gulf of Mexico. Methane occurs in high concentrations in bottom waters (3200 nM) and sediments (30 mM), and its dynamics is better constrained for the water column, but still poorly understood on sediments. Here we show that sediment accumulation rate plays a major role in regulating the quantity of organic matter and its residence time in the sulphate reduction and methanogenesis zones and, therefore, affects methane generation, consumption, and diffusive flux in sediments near the seafloor (< 1 m). High fluxes found in high sediment accumulation rate areas and competition for substrate (organoclastic sulphate reduction vs. anaerobic oxidation of methane with sulphate), compromise the ability of the thin microbial filter to consume and prevent methane diffusion through the seafloor.

The world’s oceans are challenged by climate change linked warming with typically highly populated coastal areas being particularly susceptible to these effects. Many studies of climate change on the marine environment use large, short-term temperature manipulations that neglect factors such as long-term adaptation and seasonal cycles. In this study, a Baltic Sea ‘heated’ bay influenced by thermal discharge since the 1970s from a nuclear reactor (in relation to an unaffected nearby ‘control’ bay) was used to investigate how elevated temperature impacts surface water microbial communities and activities. 16S rRNA gene amplicon based microbial diversity and population structure showed no difference in alpha diversity in surface water microbial communities, while the beta diversity showed a dissimilarity between the bays. Amplicon sequencing variant relative abundances between the bays showed statistically higher values for, e.g., Ilumatobacteraceae and Burkholderiaceae in the heated and control bays, respectively. RNA transcript-derived activities followed a similar pattern in alpha and beta diversity with no effect on Shannon’s H diversity but a significant difference in the beta diversity between the bays. The RNA data further showed more elevated transcript counts assigned to stress related genes in the heated bay that included heat shock protein genes dnaKJ, the co-chaperonin groS, and the nucleotide exchange factor heat shock protein grpE. The RNA data also showed elevated oxidative phosphorylation transcripts in the heated (e.g., atpHG) compared to control (e.g., atpAEFB) bay. Furthermore, genes related to photosynthesis had generally higher transcript numbers in the control bay, such as photosystem I (psaAC) and II genes (psbABCEH). These increased stress gene responses in the heated bay will likely have additional cascading effects on marine carbon cycling and ecosystem services.

Besides long-term average temperature increases, climate change is projected to result in a higher frequency of marine heatwaves. Coastal zones are some of the most productive and vulnerable ecosystems, with many stretches already under anthropogenic pressure. Microorganisms in coastal areas are central to marine energy and nutrient cycling and therefore, it is important to understand how climate change will alter these ecosystems. Using a long-term heated bay (warmed for 50 years) in comparison with an unaffected adjacent control bay and an experimental short-term thermal (9 days at 6–35 °C) incubation experiment, this study provides new insights into how coastal benthic water and surface sediment bacterial communities respond to temperature change. Benthic bacterial communities in the two bays reacted differently to temperature increases with productivity in the heated bay having a broader thermal tolerance compared with that in the control bay. Furthermore, the transcriptional analysis showed that the heated bay benthic bacteria had higher transcript numbers related to energy metabolism and stress compared to the control bay, while short-term elevated temperatures in the control bay incubation experiment induced a transcript response resembling that observed in the heated bay field conditions. In contrast, a reciprocal response was not observed for the heated bay community RNA transcripts exposed to lower temperatures indicating a potential tipping point in community response may have been reached. In summary, long-term warming modulates the performance, productivity, and resilience of bacterial communities in response to warming.