Epoxiconazole (EPO) and fludioxonil (FLU) are fluorinated fungicides known for their extremely high environmental persistence and significant ecotoxicological impact. Given their decades-old use in the agrochemical sector, EPO and FLU became frequent pollutants of terrestrial and aquatic ecosystems. And yet, not much is known regarding how these pesticides biodegrade in the natural environment or how to develop suitable bioremediation approaches capable of tackling their inherent recalcitrance. As such, this work focused on providing new insights into the bacterial degradation of EPO and FLU, by surveying the catabolic activity of a previously obtained EPO-enriched bacterial consortium through chemical and metaproteogenomic analyses guided by different metabolic modelling tools. The bacterial consortium was capable of extensively degrading EPO and FLU in 21 days, with fungicide removals of over 90% and defluorination efficiencies of up to 80%, but none of the subproducts predicted in silico were identified for either pesticide. Despite this, the combination of metabolic modelling tools and metaproteogenomic surveys suggested that EPO and FLU were first attacked in their N-heterocyclic moieties and that the targets of defluorination were the resulting aromatic fluorinated intermediates. This catabolic cascade is consistent with the experimental data gathered in this study and with the existing literature on this topic. Also, the degrading consortium remained stable at the taxonomical and functional levels, highlighting its catabolic plasticity in biodegrading and defluorinating two chemically distinct fluorinated compounds. This work offers a conceptual framework with novel observations that can guide future efforts to further elucidate the pathways of microbial transformation of these pesticides, ultimately contributing to better environmental risk management practices for these pollutants.

Widely used synthetic polymers such as polyethylene persist in the environment and are challenging to remove or degrade. As a sustainable alternative, biodegradation using bacteria and their enzymes is increasingly being explored. This study evaluated the potential of three bacterial strains from the genera Rhodococcus (strains NC1 and NC8, from plastic net biofilms) and Pseudomonas (strain 1.7 L, a marine hydrocarbon-degrader) to degrade low-density polyethylene (LDPE), as sole carbon source. LDPE fragments were incubated in microcosm flasks in oligotrophic medium, under four conditions: each strain individually, and a consortium of the strains. An abiotic control with no strains was included. LDPE fragments and medium were sampled after one (T30), three (T90), and six (T180) months. Bacterial growth on plastic surface was assessed via bacterial counts and Scanning electron microscopy, while polymer surface changes were analyzed using ATR-FTIR. Higher biomass and noticeable biofilm formation were observed with Rhodococcus strains, particularly after 3 months, including biofilm-like aggregates structures. Overall, Rhodococcus qingshengii (NC1) showed potential to be further studied for LDPE degradation, with biomass growth along time, biofilm formation on LDPE pieces and weight loss values up to ∼7% after 3 months. Structural changes were detected in an LDPE sample after 30 days incubation with Rhodococcus fascians (NC8). Strain 1.7 L, also formed biofilms at T90, together with weight losses up to 6% after 6 months. In contrast, the bacterial consortium showed no effective colonization or degradation of LDPE, similar to abiotic controls. As the tested strains demonstrated individual LDPE-degrading potential, this study highlights the importance of exploring marine bacteria from the plastisphere and hydrocarbon-degraders for new insights into LDPE degradation.

Kongsfjorden, a glaciated fjord in the Arctic, is shaped by Atlantic water inflow and seasonal glacier melt. This study compared prokaryotic community dynamics during spring and summer in two contrasting years, 2019 and 2020. In spring 2019, warmer Atlantic water increased temperatures and nutrients, supporting a Phaeocystis pouchetii-dominated bloom. In contrast, spring 2020 was colder, with extensive sea ice and a bloom succession from diatoms to P. pouchetii. Summer surface waters showed strong glacier runoff influence and low-biomass, flagellate-dominated phytoplankton. Greater prokaryotic diversity was observed in 2019, particularly at the surface, which was supported by network analysis revealing fewer interactions among prokaryotes in surface waters during 2020 compared to 2019. Prokaryotic community composition clearly differed between the 2 years, showing vertically homogeneous communities and higher cyanobacterial abundance in 2019, while in 2020 communities were more vertically heterogeneous and potentially locally adapted. These patterns reflect greater Atlantic water influence in 2019 versus more stratified Arctic conditions in 2020. Surface and bottom communities showed contrasting trends, shaped by depth-specific environmental conditions. Our findings highlight the sensitivity of Arctic microbial communities to shifts in hydrography and bloom dynamics and provide important perspectives on the ecological stability and dynamics of Arctic microbial communities under changing environmental conditions.

The European Marine Omics Biodiversity Observation Network (EMO BON) is a long-term genomic observatory run by the European Research Infrastructure European Marine Biological Resource Centre (EMBRC). It was established in 2021 to support the challenges of biodiversity observation and unsystematic management of biodiversity data in the European seas. EMO BON introduced and coordinated the systematic and harmonised observation of biodiversity amongst more than fourteen marine stations in the European coastline. Here, we report the next release (Release 2) of shotgun metagenomic data from seawater and sediment microbial communities.

Background Fjords in Svalbard are undergoing significant changes due to climate warming.Those along the west coast of Spitsbergen are particularly affected by the increasing influence of "warm"Atlantic Water (AW), a process known as Atlantification. We compared Kongsfjorden, a relatively "warm"fjord on the west coast, with Rijpfjorden, a typical cold Arctic fjord on the north coast of Nordaustlandet, combining physical and biogeochemical data with 16S rRNA gene amplicon and shotgun metagenomic sequencing. We hypothesize that differences in fjords'water masses and prokaryotic communities provide insight into the effects of Atlantification as it expands eastwards along the shelf north of Svalbard. Results We found that warm AW dominated in Kongsfjorden, whereas Rijpfjorden was dominated by cold Arctic Water and Winter Cooled Water. Our results suggest that the Atlantic-influenced Kongsfjorden is a nutrient sink, whereas Rijpfjorden showed similar behavior only in 2016, a particularly warm year, otherwise no clear sink/source role could be identified. Analysis of 16S rRNA gene sequences revealed that Proteobacteria had higher relative abundances in Kongsfjorden while Bacteroidota dominated in Rijpfjorden. Ammonium and nitrite-oxidizing prokaryotes were most prevalent in deeper water masses of both fjords. The archaeal taxa of the ammonia-oxidizing community, mainly Nitrosopumilus and Nitrosopelagicus, were consistently more dominant than ammonium and nitrite-oxidizing bacteria. Denitrification and nitrogen fixation genes differed between the fjords, with Kongsfjorden having a higher coverage of diazotroph genes. Conclusions Kongsfjorden and Rijpfjorden displayed distinct hydrographic conditions, with Kongsfjorden being under a stronger influence of Atlantification. Our results suggest that warmer water masses are linked to higher nutrient uptake.The clear association between microbial communities and water masses offers insight into changes driven by Atlantification.

The study of ecological systems usually requires data on multiple taxa, under different environmental conditions, regions and temporal scales. However, taxonomic data is represented by many categories organized in multiple levels (e.g., multiple genera within a family), which is a non-trivial problem for model integration and interpretation. To solve this problem, we developed an algorithm for automatic interesting rule identification for taxa (AIRItaxa). This algorithm leverages rule-based machine learning to automatically extract consistent associations between taxa and multiple environmental variables, and prune out redundant or uninformative rules. We tested AIRItaxa on two case studies, using microbiome datasets: In case study 1 (Arctic Ocean data), AIRItaxa found associations between complex environmental patterns and taxonomic groups previously overlooked in marine Arctic microbiome research (e.g., Flavobacteriaceae was abundant in the epipelagic layer under low silica concentrations), as well as identifying distinct environmental patterns for closely related taxa (e.g., NS5 marine group was associated with very high silica and lowest temperatures, while NS9 marine group was associated with low salinity in the epipelagic layer). In case study 2 (seawater methods comparison), AIRItaxa identified methodological anomalies in the detection of specific taxa, which were previously undetected (e.g., an unclassified sequence of Bacillariophyta using the HiSeq 2500 sequencing platform). AIRItaxa is a useful addition to current ecological analysis, by automatically identifying interesting patterns between untargeted taxa, metadata and outcomes (like abundance), without relying on parametric or linear modeling assumptions.

Marine plastic litter, including microplastics, has a profound impact on the ocean and its wildlife, and strategies to remove/eliminate it are needed. Microbial biodegradation, particularly by bacteria, offers a potential solution, where a link between hydrocarbon and plastic-degradation has been hypothesized. This study screened the plastic-degrading potential of 18 bacterial strains isolated from 1-month-old biofilms developed in three submerged plastic fishing nets (braided polyethylene (PE), braided nylon, thin nylon). In addition, three highly efficient hydrocarbon-degrading strains were also tested. Strains were cultivated on solid minimal media with fishing net small pieces (new/unused nets) added as a carbon source for 1 month, followed by tributyrin-agar assays to assess esterase/lipase activity. Eleven bacteria exhibited enhanced growth with net polymers, mainly from the genera Sulfitobacter, Rhodococcus, Bacillus, and Pseudomonas, and eight of which bacteria also demonstrated esterase/lipase activity. Then, genes encoding hydrocarbon or plastic-degrading enzymes (alkB and almA homologs, PETase-like enzymes) were screened by PCR in the 21 mentioned bacteria and in ca.100 other strains found in submerged nets biofilm. Amplification of the investigated genes was predominantly observed in Actinomycetes strains. Genome mining of six promising strains revealed hits with enzymes linked to degradation of synthetic polymers like polyethylene terephthalate, low-density PE and nylon. The workflow developed here enabled the selection of marine bacteria with plastic-degrading potential, sourced from biofilms of submerged plastic fishing nets and hydrocarbon-enriched environments. • A comprehensive lab workflow was developed to assess plastic-degrading potential. • Genome mining in Rhodococcus and Pseudomonas strains revealed plastic-degrading enzymes. • Hydrocarbon-degrading bacteria could hold plastic-degrading capabilities.

Current life support and in situ resource utilization technologies remain insufficient to ensure full autonomy of lunar and Martian habitats. While cyanobacteria are recognized as one of the most promising components of bioregenerative life support systems, fungi remain fairly unexplored. Aspergillus niger has demonstrated resilience under space-relevant conditions and presents strong potential for applications in life support systems. This work provides a proof-of-concept evaluation of the feasibility of incorporating A. niger biotechnological potential into future life support systems by testing its growth on resources likely available on the Moon and Mars: regolith (lunar and Martian), and cyanobacterial biomass. A. niger growth was tested in media containing minerals leached from simulants of lunar (Lunar Highland Simulant LHS-1) or Martian regolith (Mars Global Simulant MGS-1), supplemented with organic nutrients derived from the biomass of diazotrophic, lithotrophic cyanobacteria. Two strains of Anabaena spp. (PCC 7120 and 7938) were tested, either untreated or after mechanical lysis. The best growth conditions were subsequently evaluated under simulated microgravity by incubating Fluorinated Ethylene Propylene (FEP) bag-bioreactors in a clinostat. Putative citric acid isomers levels in 7-day cultures were evaluated by LC-MS/MS based on the detection of m/z 191 and its characteristic collision-induced dissociation fragments (173 and 111 m/z). Results show that lysates of Anabaena sp. PCC 7938 effectively enabled growth of A. niger, supporting approximately 30% of fungal biomass accumulation when compared to the positive control (minimal medium). FEP bags were validated as a promising biocompatible bioreactor for A. niger liquid cultures under Earth's gravity and simulated microgravity, showing no statistically significant differences in biomass production between the tested gravity regimes. Putative citric acid isomers were detected in media supplemented with minerals from Martian regolith simulant, suggesting that its composition may support central metabolic pathways in A. niger. We demonstrated that A. niger can grow using resources likely available in future Moon and Mars outposts, namely using regolith-derived mineral solutions, and recycling resources from established bioprocesses in the space habitat, such as cyanobacterial biomass.

Cetaceans play a critical role in marine ecosystems and function as sentinel species for detecting environmental perturbations, underscoring the importance of assessing their health for effective marine conservation. This study employed 16S rRNA gene sequencing to characterize the prokaryotic communities present in exhaled breath condensate (EBC) samples from cetaceans, utilizing both short-read (Illumina) and long-read (PacBio) sequencing platforms. Putative pathogenic taxa were identified using the Multiple Bacterial Pathogen Detection (MBPD) database. Substantial differences in microbial community composition were observed between sequencing approaches. The PacBio platform yielded 2,373 amplicon sequence variants (ASVs) spanning 30 bacterial phyla, with 614 ASVs identified as potential pathogens. In contrast, the Illumina dataset generated 350 ASVs across 17 phyla, of which 46 were flagged as potentially pathogenic. Discrepancies were also evident in diversity metrics: PacBio-derived profiles exhibited higher alpha diversity and produced beta diversity clustering patterns that corresponded with sample metadata, while Illumina-based profiles did not reveal meaningful clustering. Distinct EBC microbial signatures were identified for Globicephala macrorhynchus and Delphinus delphis, with clear differences from the surrounding seawater microbiota. These findings support the use of EBC as a non-invasive and informative tool for respiratory microbiome analysis in marine mammals. Notably, this study provides the first characterization of the respiratory microbiota in D. delphis, offering a valuable methodological baseline for future research into host-microbiome interactions, health assessment and putative pathogen monitoring in free-ranging cetacean populations, using non-invasive approaches.

Genetic reference databases underpin a wide range of molecular approaches used to study cetacean biodiversity, including environmental DNA (eDNA), yet their reliability depends critically on data completeness, taxonomic accuracy, and metadata quality. Here, we present the first global assessment of mitochondrial sequence availability for cetaceans, evaluating taxonomic coverage, geographic representation, metadata completeness, and the distribution of five commonly targeted mitochondrial markers (12S rRNA, 16S rRNA, D-loop, cytochrome oxidase I, and cytochrome b). We retrieved 17,569 cetacean accessions from the NCBI Nucleotide database and an additional 259 COI-only records from BOLD Systems. Sequence availability was strongly biased toward Delphinidae and Balaenopteridae, whereas several families, notably Ziphiidae, were markedly underrepresented. We also identified discrepancies between database records and currently accepted cetacean taxonomy (e.g., outdated genera, non-accepted species, and collapsed higher-level taxa). Among markers, the D-loop dominated database representation, largely as standalone sequences (12,437 records), reflecting historical and current sequencing priorities and underscoring its continued relevance for population-level studies and eDNA marker development. Only 38% of accessions included geographic metadata, with georeferenced records concentrated primarily in the Americas and the Northwest Pacific, while large regions, including much of Africa, remained poorly represented. Although broad geographic patterns mirrored known family distributions, pronounced regional and taxonomic gaps persist. Our results highlight critical deficiencies in mitochondrial reference databases for cetaceans and emphasise the need for improved metadata standards, targeted sequencing of underrepresented taxa and regions, and open data sharing to enhance the effectiveness and global applicability of eDNA-based cetacean monitoring.

Marine biodiversity monitoring is critical for ecosystem-based management and marine spatial planning, yet conventional methods are often costly and logistically constrained. Environmental DNA (eDNA) has emerged as a non-invasive and efficient alternative, though current sampling approaches still rely on manual collection, increasing operational complexity and risks of contamination and sample degradation. This paper presents an autonomous eDNA sampling system integrated with an underwater robotic platform. The system enables programmable, in situ collection and preservation of multiple samples, supporting both standalone operation and deployment on an autonomous underwater vehicle (AUV). Field trials conducted off the northern Portuguese coast compared the performance of the proposed system with traditional manual sampling methods. Results demonstrate successful autonomous sample acquisition with DNA yields comparable to manual approaches, while reducing variability and improving consistency. The integration of robotic platforms with automated eDNA sampling provides a scalable and cost-effective solution for marine biodiversity assessment, supporting enhanced data acquisition for ecosystem monitoring and marine spatial planning.

The microbial rare biosphere, composed of low-abundance microorganisms in a community, lacks a standardized delineation method for its definition. Currently, most studies rely on arbitrary thresholds to define the microbial rare biosphere (e.g., 0.1% relative abundance per sample), hampering comparisons across studies. To address this challenge, we present ulrb (Unsupervised Learning based Definition of the Rare Biosphere), available as an R package. ulrb uses unsupervised machine learning to optimally classify taxa into abundance categories (e.g., rare, intermediate, or abundant) within microbial communities. We show that ulrb is more consistent than threshold-based approaches and can be applied to data derived from common microbial ecology protocols and non-microbial studies. ulrb can be used to identify different types of rarity and is statistically valid for the analysis of various dataset sizes. In conclusion, ulrb discerns rare from abundant organisms in a user-independent manner, finding applicability in selected ecological datasets.

The European Marine Omics Biodiversity Observation Network (EMO BON) is an initiative of the European Marine Biological Resource Centre (EMBRC) to establish a persistent genomic observatory amongst designated European coastal marine sites, sharing the same protocols for sampling and data curation. Environmental samples are collected from the water column and, at some sites, soft sediments and hard substrates (Autonomous Reef Monitoring Structures - ARMS), together with a set of mandatory and discretionary metadata (including Essential Ocean Variables - EOVs). Samples are collected following standardised protocols at regular and specified intervals and sequenced in large six-monthly batches at a centralised sequencing facility. The use of standard operating procedures (SOPs) during data collection, library preparation and sequencing aims to provide uniformity amongst the data collected from the sites. Coupled with strict adherence to open and FAIR (Findable, Accessible, Interoperable, Reusable) data principles, this ensures maximum comparability amongst samples and enhances reusability and interoperability of the data with other data sources. The observatory network was launched in June 2021, when the first sampling campaign took place.

Keywords: precipitation, aerosols, microorganisms, Antarctica, cloudsClouds and precipitation play an intrinsic role in the global climate, upholding the Earth's surface energy equilibrium and water cycle. Despite their significance, clouds and aerosols over Antarctica and the Southern Ocean remain poorly understood, primarily due to the extreme environment for observations and insufficient data. The Antarctic Peninsula (AP) has been exhibiting a significant warming trend over the last 60 years (Jones et al, 2019). Coupled with the rising temperatures, an increase in precipitation and surface melt is being observed across the AP, with major surface melts and precipitation events, both snowfall and rainfall, being associated with atmospheric rivers (ARs) (Gorodetskaya et al., 2023; Wille et al., 2021). ARs are long corridors of intense moisture and heat transport from subtropical and mid-latitude regions poleward, typically also carrying liquid-containing clouds to the AP. Moreover, ARs can impact the long-range transport of aerosols, as well as contribute to gas and aerosol exchange between the atmosphere and the ocean. Aerosols, which serve as cloud condensation and ice nuclei, determine cloud microphysical properties and influence cloud radiative forcing and precipitation formation. Given that a substantial percentage of aerosols are of biological origin, it is crucial to effectively identify and describe them.In this project, we aim to characterize bioaerosols, specifically microorganisms, present in the precipitation and surface snow in the AP. Rainfall and snowfall samples were collected during PROPOLAR campaigns on King George Island, northern AP, in the vicinity of Escudero and King Sejong stations. The precipitation samples were preserved and analysed using culturable and non-culturable methodologies. Bacterial strains were obtained and identified through 16S rRNA gene sequencing, which provided information about the diversity and phylogenetic relationships of the identified microorganisms. The identified organisms were categorized into six distinct genera, including those recognized for their ice nucleation capabilities, such as the Pseudomonas genus (Attard et al, 2012). The main phylum identified was Proteobacteria. We identified four strains among those analyzed as potentially novel species affiliated with the Spirosoma and Paenibacillus genera. These findings highlight the untapped potential of these regions in harbouring unique microbial biodiversity. Obtaining a comprehensive study of the microbial community in precipitation in Antarctica will pave the path to understanding the role these microorganisms have in cloud condensation processes and ice nucleation. More international efforts and campaigns are needed to gain information about aerosols, clouds and precipitation over the Southern Ocean. Acknowledgements: PROPOLAR (Portuguese Polar Program) projects APMAR/TULIP/ APMAR2 and FCT project MAPS (2022.09201.PTDC)References: Attard et al. 2012. Effects of atmospheric conditions on ice nucleation activity of Pseudomonas. Atmos. Chem. Phys. https://doi.org/10.5194/acp-12-10667-2012Gorodetskaya et al. (2023): Record-high Antarctic Peninsula temperatures and surface melt in February 2022: a compound event with an intense atmospheric river. npj Clim.Atmos.Sci. https://doi.org/10.1038/s41612-023-00529-6Jones et al. (2019). Sixty years of widespread warming in the Southern middle and high latitudes(1957–2016). J.Clim.https://doi.org/10.1175/JCLI-D-18-0565.1 Wille et al. (2021). Antarctic atmospheric river climatology and precipitation impacts. J.Geophys.Res.https://doi.org/10.1029/2020JD033788

ABSTRACTThe extensive microbial diversity found in the oceans is becoming to be uncovered despite limited knowledge and cultured representatives for many taxonomic groups. This study analysed the distribution and diversity of Planctomycetota at four water column profiles of the Eastern North Pacific subtropical front (ENPSF) using 16S rRNA gene sequencing. A dual approach, utilising PacBio long‐reads and Illumina short‐reads, was employed to enhance the accuracy of taxonomic assignment and compare sequencing methods. The diversity of Planctomycetota increased below the deep chlorophyll maximum level (175–200 m) and in the mesopelagic layer (500 m), with beta‐diversity clustering distinctly separating samples according to different depths, resulting in pronounced vertical stratification. This community structure mirrors nutrient availability, as Planctomycetota favour depths between 175 and 200 m, where high nitrate levels are present. More Planctomycetota amplicon sequence variants (ASVs) were identified with PacBio than with Illumina, improving detection of these bacteria. Phylogenetic analyses performed after manual curation of ASVs led to the discovery of several unknown genera of Planctomycetota, indicating that substantial diversity within this group remains to be discovered and studied in remote oligotrophic oceans.

Abandoned, lost or otherwise discarded fishing gear (ALDFG) represent a major source of marine plastic litter pollution. Similar to other plastic litter, these items can provide a new surface for the growth of biofilms harboring distinct microbial communities, containing potential opportunistic pathogens or pollutant-degrading microorganisms. While knowledge is increasing for marine plastic litter and microplastic-associated biofilms, there is a gap on the plastisphere research for fishing gear. This study aimed to comprehend the structure and dynamics of the microbial communities attached to plastic fishing nets, mimicking a scenario when lost at sea, but also to assess if polymer type can influence these communities. For that, a one-year in situ experiment was employed inside a recreational marina (port of Leixões, Portugal), using 3 types of plastic fishing nets (Braided Polyethylene (PE), Braided Nylon and Thin Nylon) submersed in the seawater. Seasonal samplings of nets and surrounding seawater were performed for microbial community analysis by 16 S rRNA metabarcoding. One month-old-nets samples were additionally collected for cultivation of bacterial strains in the laboratory. In general, microbial communities found in the biofilms attached to fishing nets were taxonomically distinct and more diverse, when compared to the surrounding seawater. Biofilm communities were not shaped by the polymer type, instead, they displayed a succession pattern over time. Biofilm communities were predominantly composed of the phyla Proteobacteria, Bacteroidetes and Verrucomicrobiota. Additionally, the families Sphingomonadaceae, Rubritaleaceae, Rhizobiaceae and Saprospiraceae were specifically associated with fishing net biofilms. From the 3 nets, a total of 123 bacterial strains from 46 bacterial genera were recovered. The genera Acinetobacter, Bacillus, Rhodococcus, Shewanella, Streptomyces and Vibrio were common to all nets. Commonly associated hydrocarbon and plastic - degrading taxa were highly abundant in the biofilm communities (> 2% abundance) and some were even possible to cultivate in laboratory. In addition, biofilm communities presented as well, potentially pathogenic genera, such as Clostridium and Mycobacterium, but in low abundances (< 1%). With this work, a deeper knowledge on the plastisphere associated with different plastic fishing gear was obtained, along with the isolation of bacterial strains with potential for future exploration of plastic biodegradation.

Permafrost soils are critical reservoirs for mercury (Hg), with the thawing process leading to the release of this element into the environment, posing significant environmental risks. Of particular concern is the methylated form of mercury, monomethylmercury (MMHg), known for its adverse effects on Human health. Microbial communities play a pivotal role in the formation of MMHg by facilitating Hg methylation and in the demethylation of MMHg, slowing the crossing of toxic threshold concentration in the environment. However, the specific microbes involved still need to be understood. This study aimed to identify the microbial drivers behind changes in Hg speciation (MMHg and Hg) in permafrost thaw lakes and assess the significance of the biotic component in Hg biogeochemistry. Sediment samples from two thermokarst lakes in the Canadian sub-Arctic were collected during the winter and summer of 2022. Gene-centric metagenomics using whole-genome sequencing (WGS) was employed to identify key genes involved in mercury methylation (hgcA and hgcB) and demethylation (merA and merB), supported by qPCR analyses. A seasonal decline in microbial diversity, involved in the Hg methylation, and hgcA gene coverage was observed from winter to summer, mirroring patterns in mercury methylation rates. Notably, hgcA sequences were significantly more abundant than merAB sequences, with contrasting seasonal trends. These results indicate a seasonal shift in the microbial community, transitioning from a dominance of mercury methylation in winter to a predominance of mercury demethylation in summer. Environmental drivers of these dynamics were integrated into a conceptual model. This study provide new insights on the microbial processes influencing the Hg cycle in Arctic permafrost undergoing degradation.

Mercury (Hg) is a natural occurring element but is often emitted from anthropogenic sources and reaches the Arctic via long-range atmospheric transport. Organic matter (OM)-rich thermokarst lakes are characteristic features of the permafrost landscape in this region, where monomethylmercury (MMHg) production can be enhanced, as this process is mainly carried out by prokaryotes. To better understand the complex Hg biogeochemical cycle, two distinct thermokarst lakes (SAS 1A and SAS 2A) in sporadic permafrost in the Sasapimakwananistikw (SAS) River Valley, Canadian Subarctic, were sampled during winter and summer of 2022. Water column analysis showed no seasonal variation in total Hg (THg) and MMHg concentrations in SAS 2A but significantly higher THg and MMHg in winter in SAS 1A. Biogeochemical parameters affecting the activity of known methylating communities drive both inter-lake and seasonal variations in the water column. Strong correlations between MMHg and dissolved organic carbon (DOC) were found, with SAS 1A showing almost seven times more MMHg variability with DOC than SAS 2A. This difference is potentially linked to variations in OM composition between the sites. The lakes showed high THg seasonality in sediments, with higher concentrations in winter. In contrast, no seasonal variation was observed in MMHg concentrations with SAS 1A exhibiting higher values. Different divalent mercury (Hg(II)) bioavailability might explain these differences. By conducting incubation experiments with isotope-enriched Hg in the sediment, the important role of sulfate-reducing bacteria in the methylation process was revealed. This study highlights the complexity of thermokarst lakes which are increasing in Northern landscapes and might be hotspots for MMHg formation.