Alfred Wegener Institute for Polar and Marine Research
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Genomic diversity and adaptation in Arctic marine bacteria.
Get PDFArctic marine bacteria experience seasonal changes in temperature, salinity, light, and sea ice cover. Time-series and metagenomic studies have identified spatiotemporal patterns in Arctic microbial communities, but a lack of complete genomes has limited efforts to identify the extent of genomic diversity in Arctic populations. We cultured and sequenced the complete genomes of 34 Arctic marine bacteria to identify patterns of gene gain, loss, and rearrangement that structure genomes and underlie adaptations to Arctic conditions. We found that the most abundant lineage in the Arctic (SAR11) is comprised of diverse species and subspecies, each encoding 50-150 unique genes. Half of the 16 SAR11 genomes harbor a genomic island with the potential to enhance survival in the Arctic by utilizing the osmoprotectant and potential methyl donor glycine betaine. We also cultured and sequenced four species representing an uncultured family of Pseudomonadales, four subspecies of Pseudothioglobus (SUP05), a genus of high GC Puniceispirillales (SAR116), and a family of low GC SAR116. Time-series 16S rRNA amplicon data indicate that this culture collection represents up to 60% of the marine bacterial community in Arctic waters. Their genomes provide insights into the evolutionary processes that underlie bacterial diversity and adaptation to Arctic waters.IMPORTANCEGenetic diversity has limited efforts to assemble and compare whole genomes from natural populations of marine bacteria. We developed a cultivation-based population genomics approach to culture and sequence the complete genomes of bacteria from the Arctic Ocean. Cultures and closed genomes obtained in this study represent previously uncultured families, genera, and species from the most abundant lineages of bacteria in the Arctic. We report patterns of gene gain, loss, rearrangement, and adaptation in the dominant lineage (SAR11), as well as the size, composition, and structure of genomes from several other groups of marine bacteria. This work demonstrates the potential for cultivation-based high-throughput genomics to enhance understanding of the processes underlying genomic diversity and adaptation
The Amazonian mangrove systems accumulate and release dissolved neodymium and hafnium to the oceans
Get PDFMangroves are essential tropical ecosystems nurturing a wide range of marine biodiversity and counteracting global warming by sequestering atmospheric carbon dioxide. Hence, the export mechanisms and fluxes of particulate and dissolved organic carbon and trace elements from mangroves directly influence coastal productivity, the global carbon cycle and thus global climate, which are, however, not well constrained. Here we find consistent radiogenic neodymium and hafnium isotopic compositions of porewater, sedimentary iron-manganese oxyhydroxides and coastal seawater, suggesting that the Amazonian mangrove belt supplies trace elements through porewater discharge, dissolution of iron-manganese oxyhydroxides and their interactions with seawater. Together, these processes supply 8.4 × 106 g yr-1 dissolved neodymium, equivalent to 64% of the total sources of neodymium to the Amazonian coastal seawater. Globally, mangrove systems along the continental margins contribute 6–9% of the net neodymium input to the ocean, which is similar to the contributions from atmospheric deposition. A contribution of this magnitude is potentially also the case for other trace elements, given the strong correlations between neodymium and iron (Pearson r = 0.92), and manganese (r = 0.75) concentrations across the entire river-ocean section, emphasizing the crucial role of mangrove system inputs in micro-nutrient cycling
Extraction Strategies for Profiling the Molecular Composition of Particulate Organic Matter on Glacier Surfaces
Get PDFPigmented microalgae thrive on supraglacial surfaces, producing "sticky" extracellular polymeric substances that combine into a mineral-organic matrix. Together, they enhance snow and ice melting by lowering the albedo. Understanding the chemical nature of particulate organic matter (POM) in this matrix is crucial in assessing its role in supraglacial carbon dynamics. We evaluated POM complexity in alga-rich snow and ice samples containing 0.3-6.4 wt % organic carbon (OC) via extractions with solvents of varying polarity, pH, and OM selectivity. Extraction yields were evaluated by OC analysis of the extracts, and the composition of extracted OM was analyzed using ultrahigh-resolution mass spectrometry. Individual hot water (HW), hydrochloric acid (HCl), and sodium hydroxide (NaOH) extractions achieved up to 87% efficiency, outperforming sequential, organic solvent-based extractions (<11%). OM extracted by HW, HCl, and NaOH combined had more molecular formulas (2827) than OM extracted with organic solvents (1926 formulas). Combined HW, NaOH, and HCl extractions yielded an OM composition with unsaturated, highly unsaturated, aromatic, and N-containing compounds, while unsaturated aliphatics and black carbon-derived polycyclic aromatics were enriched in the organic solvent extracts. This molecular profiling provides the first comprehensive insights into supraglacial POM composition, opening the window for understanding its role in the cryospheric carbon cycle
Arctic surface snow interactions with the atmosphere: Spatio-temporal isotopic variability during the MOSAiC expedition
Get PDFSnow on sea ice is crucial in moderating sea ice and atmosphere interactions, yet fully grasping snow’s isotopic composition and the processes shaping it presents substantial challenges, including sublimation and wind redistribution. This study utilizes a year of stable water isotope datasets from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019/2020 to explore the complex interactions between snow deposition processes and postdepositional changes affecting snow on Arctic sea ice including seasonal and spatial dynamics. We compare snow data with water vapor isotope measurements by examining 911 individual snow isotope measurements and integrating these discrete snow samples with continuous water vapor isotope data. Autumn shows a pronounced δ18O offset between snow and vapor. In winter, δ18O and d-excess in surface snow and water vapor diverge sharply, indicating kinetic fractionation under extremely cold temperatures as research vessel Polarstern drifted from the Siberian to the Atlantic Arctic. While water vapor δ18O responds rapidly to air temperature and humidity changes, surface snow δ18O values are modulated by postdepositional processes like sublimation and wind redistribution. We found that these 2 processes play a key role in isotopic enrichment that is intensified by the snow’s prolonged surface residence. Wind-driven snow redistribution, occurring during 67% of the winter period, leads to an average surface snow δ18O of −22‰ across the sea ice by redistributing and mixing fresh snow with more metamorphosed snow. This study provides new insights into how wind-driven redistribution and prolonged surface residence not only alter isotopic values in surface snow but also obscure seasonal isotopic patterns, complicating the interpretation of snow isotope records in the Arctic. Our research to understand the differences between the isotopic values of vapor and the isotopic values of snow provides insight into interactions between snow and the atmosphere, as well as the processes that alter isotopic values internally within the Arctic snowpack. Our study highlights the complexity of surface snow isotope geochemistry across the Arctic from the eastern to the central basin during the MOSAiC expedition window and how the underlying processes of water vapor transport, temperature–isotope relations, and the role of secondary processes, including wind redistribution and sea ice formation all contribute to the horizontal and vertical geochemistry patterns
MUSICA: Multi-scale Sea Ice Coupled Analysis: Anisotropic biophysical coupling of sea ice processes
Get PDFInfluence of snowpack characteristics on winter soil temperatures (Qeqertarsuaq, Kalaallit Nunaat)
Get PDFSeasonal snow cover plays a critical role in regulating Arctic soil temperatures, particularly in permafrost landscapes. Its insulating properties depend not only on snow depth but also on snow density and stratigraphy. However, these characteristics remain poorly understood in Arctic environments due to limited availability of high-resolution measurements. This thesis investigates how spatial variability in snow depth and snow density influences soil temperatures (6 cm) during late winter in a Low Arctic maritime tundra landscape on Qeqertarsuaq (Disko Island), Kalaallit Nunaat (Greenland). The study combines high-resolution snowpack data collected with the SnowMicroPen (SMP) in April 2024 with hourly soil temperature records from 14 temperature sensors. Snow depths at the sensor locations ranged from 0.16 m to 1.10 m (mean: 0.56 m), while snowpack densities varied between 247 kg m−3 and 409 kg m−3. Average soil temperatures from October 2023 to June 2024 spanned from −4.67 °C to −0.23 °C across the site. To capture the relationship between snow cover characteristics and soil temperatures, and to assess spatial variability and uncertainty, I used Bayesian hierarchical modeling. The results how that snow depths greater than approximately 0.5 m effectively insulate the soil by dampening the impact of air temperature fluctuations, reinforcing the snowpack’s role as a thermal buffer. I also observed a positive, though more uncertain, relationship between snow density and soil temperature, suggesting that denser snow may reduce insulating capacity. These findings indicate that snow density plays an important role in Arctic ground thermal regimes, but additional high-resolution observations and further model development are needed to better quantify its effects
Insight into origin and transformation of dissolved organic matter under contrasting nutrient conditions on the Northern Patagonian Continental Shelf, Southwestern Atlantic Ocean
No full textThis study analyzed the composition and optical properties of dissolved organic matter (DOM) in the Northern Patagonian Continental Shelf (Southwestern Atlantic Ocean), which embraces highly diverse hydrographic, biological and biogeochemical regimes. The main aim was to identify autochthonous and allochthonous sources of DOM, its bulk molecular characteristics and bioavailability, in relation to contrasting regional nutrient and circulation patterns. In the coastal sector at depths ≤ 50 m, only a moderate riverine input of DOM and nutrients was detected, yet high values of the humification index (HIX) pointed to a continental origin of aromatic DOM, with sediment resuspension significantly contributing dissolved humic carbon, silicate and phosphate. In the middle shelf, high dissolved organic carbon and chlorophyll levels coincided with low bacterial carbon production (BCP) and HIX values, indicating an autotrophic production of aliphatic DOM. This was supported at the outer shelf and shelf break by nitrate input from the Malvinas Current and in the middle shelf by potentially regenerated ammonium . In the shelf break, humic- and protein-like fluorescent DOM maxima were likely originated from phytoplankton lysates, as suggested by high tryptophan-like fluorescence signals. The variation of the biological activity index (BIX) was only partially explained by chlorophyll, while BCP patterns did not. There is evidence of a site-dependent compositional artifact in the BIX values due to a background of high-BIX, recalcitrant DOM, possibly aromatic low-molecular weight peptides. Wind-driven shifts in surface circulation influenced the distribution of DOM, nutrients, and temperature, highlighting their role in the region’s annual spring bloom and future DOM dynamics studies. Overall, the results indicate a transition from terrestrially influenced, humic-rich DOM in coastal waters to predominantly autotrophic, protein-like DOM offshore, driven by nutrient availability and wind-modulated circulation. This pattern underscores the key role of physical forcing in shaping DOM sources and transformation across the Northern Patagonian Shelf
Holocene Terrestrial Permafrost Contributes More Highly Reactive Organic Matter to the Laptev Sea Shelf Than Pleistocene Permafrost
Get PDFAbstract Warming can lead to mobilization of organic matter (OM) initially stored in circumarctic permafrost and subsequent greenhouse gas release to the atmosphere. Our understanding remains limited regarding how the extent of carbon release, that is, OM reactivity, varies across terrestrial permafrost types and how it changes during transport from land to marine shelves. In this study, we measured bulk organic (TOC, C/N), isotopic ( δ 13 C, Δ 14 C), and thermogravimetric properties (TGA) as proxies of OM reactivity on bulk and water‐soluble fractions (leachates) from terrestrial Holocene and Pleistocene permafrost, bulk surface sediments from the Laptev Sea, and sediment cores from the western Laptev Sea. Bulk OM from terrestrial Pleistocene permafrost exhibited lower reactivity compared to Holocene permafrost, as indicated by its lower thermoreactivity and more advanced degradation state, reflected in higher δ 13 C values and lower C/N ratios. Marine surface sediments showed relatively old radiocarbon ages and reduced OM thermoreactivity in the eastern Laptev Sea shelf compared to the central and western Laptev Sea shelf. This likely resulted from a higher contribution of Pleistocene permafrost‐derived OM. In the central and western Laptev Sea, a rapid decrease in OM thermoreactivity was observed near the coast, followed by a more gradual decline offshore. Downcore analyses revealed that the reduction in OM thermoreactivity primarily reflected degradation during cross‐shelf transport rather than after burial. Our results advance the understanding of OM reactivity differences between Pleistocene and Holocene permafrost, as well as changes in terrestrial permafrost OM thermoreactivity during transport and post‐burial.
Plain Language Summary Warming in the Arctic is releasing carbon from permafrost as microbes break down frozen soil, plants, and animal remains. To better understand this process, we studied how easily organic matter (OM) from both young and old terrestrial permafrost breaks down, as well as OM in surface and deeper sediments from the Laptev Sea shelf. We measured how easily the OM in these materials breaks down by examining the amount of the OM that decomposed at lower versus higher temperatures. On land, we found that old permafrost is more resistant to breakdown than young permafrost. In the eastern Laptev Sea shelf, surface sediments contained more OM from old, less degradable terrestrial permafrost. Moving westward, OM degradability decreases with increasing distance from the coast, suggesting rapid OM breakdown nearshore. Deeper sediment layers revealed that most OM degradation happened during transport from land to sea and across the shelf, with relatively little breakdown after burial. These findings improve our understanding of the differing vulnerabilities of young and old permafrost to degradation and emphasize the importance of transport processes in shaping OM reactivity distributions on Arctic Ocean marginal shelves.
Key Points Bulk organic matter and leachates from terrestrial Pleistocene permafrost exhibit lower thermoreactivity than Holocene permafrost Organic matter source and transport distance on shelf collectively control organic matter thermoreactivity in shelf surface sediment The majority of reactive organic matter from terrestrial permafrost is degraded during transport on land or in nearshore coastal area
Global, multi-scale standing deadwood segmentation in centimeter-scale aerial images
Get PDFWith tree mortality rates rising across many regions of the world, efficient methods to map dead trees are becoming increasingly important to monitor forest dieback, assess ecological impacts, and guide management strategies. Deep learning-based pattern recognition combined with the high spatial detail of aerial images from drones or airplanes provides an avenue for mapping dead tree crowns or partial canopy dieback, collectively referred to as standing deadwood. However, current methods for mapping standing deadwood are limited to specific biomes or image resolutions. Here, we present a transformer-based semantic segmentation model that generalizes across forest biomes and a wide range of image resolutions (1–28 cm) for mapping both dead tree crowns and partial canopy dieback. Our approach combines a SegFormer-based transformer architecture for image feature extraction and Focal Tversky Loss to mitigate class imbalance. We used a globally distributed crowd-sourced dataset of 434 high-resolution aerial images and manual delineations of standing deadwood of vastly varying quality. The orthophotos span all major forest biomes and cover 10,778 hectares. To further mitigate imbalances across biomes, resolutions, deadwood occurrence, and image sources, we developed a four-dimensional sampling scheme that ensures balanced representation during training. The models were trained and evaluated using heterogeneous crowd-sourced data, which, as expected, negatively affects the F1-scores. A visual inspection on independent data highlights the very precise quality of the segmentation. Our analysis revealed resolution-dependent performance variations across biomes, suggesting a relationship between optimal mapping resolution and biome-specific characteristics. We make both our model and a machine-learning-ready dataset publicly available on deadtrees.earth to support future research in tree mortality mapping