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Studying competitive effects of microbial and light-induced degradation of dissolved organic matter (DOM) is crucially important for understanding the factors controlling aquatic carbon (C) transformation in boreal waters. However, studies addressing both DOM and trace element (TE) behavior are limited, which does not allow assessment of coupled C – TE (including macro- and micronutrients and toxicants) biogeochemical cycles in these environmentally important settings. Here we conducted a seasonally-resolved assessment on the degree of DOM and related major and TE transformation under biotic activity and sunlight using conventional incubations of humic surface waters from the European subarctic. We studied bio- and photodegradation over 2–3 weeks in an ombrotrophic peatbog continuum (subsurface water from piezometer – small peatland pool – outlet stream) during July, and in three horizons (0.5, 5 and 10 m) of deep (30 m), a stratified forest lake from the same region during June, August and September. Along the bog water continuum in July, biodegradation rate was the highest in subsurface waters collected via piezometer and the lowest in the acidic peatland pool (0.17 to 0.03 mg C L-1 d-1, respectively). Photodegradation was similar for piezometrically collected subsurface waters and the stream (about 0.3 mg C L-1 d-1), but was not detectable in the peatland pool. The waters of forest lake exhibited a strong seasonal effect of biodegradation, which was the highest in October and the lowest in June (0.04 and 0.02 mg C L-1 d-1, respectively). The photodegradation of DOM from the forest lake was observed only in June and August (0.19 and 0.07 mg C L-1 d-1, respectively). Biodegradation was capable of removing between 1 and 7 % of initial DOC, being the highest in the forest lake in October and in peatland pool in summer. The photolysis was capable of degrading a much higher proportion of the initial DOC (10–25 %), especially in the forest lake during June and the bog stream during July. The change of optical parameters confirmed the highest photodegradation occurs in June (Arctic summer) and demonstrates a decrease of chromophoric (aromatic) compounds during incubation, whereas biodegradation acted preferentially on aliphatic, low molecular weight compounds. Only a few trace metals were sizably affected by both photo- and biodegradation of DOM (Fe, Al, Ti, Nb and light REE), whereas V, Mn, Co, Cu and Ba were affected solely by biodegradation. Typical values of TE removal over a 2-week period of incubation ranged from 1 to 10 %. These effects were mostly pronounced in the less acidic forest lake compared to the bog waters. A likely mechanism of TE removal was their coprecipitation with coagulating Fe(III) hydroxides. When averaged across sites and seasons, DOM biodegradation and photodegradation processes could remove 5.3 and 10.8 mg C L-1 y-1, respectively. Compared to typical CO2 emissions from inland waters of the region, biodegradation of DOM can provide the totality of C-CO2 evasion from lake water surfaces whereas bio- and photodegradation are not sufficient to explain the observed fluxes in bog water continuum. Overall, these results demonstrated strong spatial and seasonal variability in DOM and TE complexes bio- and photodegradation, which was poorly accessed until now, and call for the need of a systematic assessment of both processes across seasons with high spatial resolution. [ABSTRACT FROM AUTHOR]

Permafrost thawing as a result of climate change has major consequences locally and globally for the biosphere as well as for human activities. The quantification of its extent and dynamics under different climate scenarios is needed to design local adaptation and mitigation measures and to better understand permafrost climate feedbacks. To this end, numerical simulation can be used to explore the response of soil thermo-hydric regimes to changes in climatic conditions. Mechanistic approaches minimize modelling assumptions by relying on the numerical resolution of continuum mechanics equations, but involve significant computational effort. In this work, the permaFoam solver is used along with high-performance computing resources to assess the impact of four climate scenarios of the Coupled Model Intercomparison Project – Phase 6 (CMIP6) on permafrost dynamics within a pristine, forest-dominated watershed in the continuous permafrost zone. Using these century time-scale simulations, changes in soil temperature, soil moisture, active layer thickness and water fluxes are quantified, assuming no change in vegetation cover. The most severe scenario (SSP5-8.5) suggests a dramatic increase in both active layer thickness and annual evapotranspiration, with maximum values on the watershed reached in 2100 of +46 % and +29 % respectively. For the active layer thickness, in current climatic conditions it would correspond to a 560 km southward shift. Moreover, in this scenario thermal equilibrium of near-surface permafrost with the new climatic conditions would not be reached in 2100, suggesting a further thawing of permafrost even in case of halting the climate change. [ABSTRACT FROM AUTHOR]

Despite the importance of structural control on metal stable isotope fractionation in inorganic and abiotic systems, the link between metal structural changes and related isotopic fractionation during reactions with organic surfaces and live cells remains poorly established. We conducted reversible adsorption of Fe(II) and Fe(III) on the surface of exopolysaccharide (EPS)-rich and EPS-poor Pseudomonas aureofaciens, and we allowed Fe intracellular uptake by growing cells. We analyzed the Fe isotopic composition of the remaining fluid and cell biomass, and compared the isotopic fractionation during adsorption and assimilation reaction with relative changes in Fe structural status between aqueous solution and bacterial cells, based on available and newly collected X-ray absorption spectroscopy (XAS) observations. Iron(III) adsorption onto P. aureofaciens at 2.8 ≤ pH ≤ 6.0 produced an enrichment of the cell surface in heavier isotopes with Δ57Fecell-solution ranging from +0.7 to +2.1‰, without a link to pH in EPS-rich cultures. In contrast, the magnitude of isotopic fractionation increased with pH in EPS-poor cultures. Iron(II) adsorption produced an even larger enrichment of the cell surface in heavier isotopes, by up to 3.2‰, tentatively linked to Fe(III) hydroxide precipitation. Intracellular assimilation of Fe(II) favored heavier isotopes and led to Δ57Fecell-solution of +0.8‰. In addition, Fe(III) cellular uptake produced an enrichment of the bacterial biomass in lighter isotopes with Δ57Fecell-solution of −1‰. The XAS analyses demonstrated the dominance of Fe(III)–phosphate complexes both at the cell surface and in the cell interior. We suggest that heavier isotope enrichment of the cell surface relative to the aqueous solution is due to strong Fe(III)–phosphoryl surface complexes and Fe complexation to ligands responsible for metal transfer from the surface to the inner cell. In case of Fe(II) adsorption or assimilation, its partial oxidation within the cell compartments may lead to cell enrichment in heavier isotopes. In contrast, loss of symmetry of assimilated Fe(III) relative to the aqueous Fe3+ ion and longer bonds of intracellular ions relative to aqueous Fe(III)–citrate or hydroxo-complexes could produce an enrichment of cells in lighter isotopes. The versatile nature of Fe(II) and Fe(III) fractionation without a distinct effect of pH and surface exopolysaccharide coverage suggests that, in natural soil and sedimentary environments, Fe isotope fractionation during interaction with heterotrophic bacteria will be primarily governed by Fe complexation with DOM and Fe redox status in the soil pore water. [ABSTRACT FROM AUTHOR]

Shallow thermokarst lakes are important sources of greenhouse gases (GHGs) such as methane (CH4) and carbon dioxide (CO2) resulting from continuous permafrost thawing due to global warming. Concentrations of GHGs dissolved in water typically increase with decreasing lake size due to coastal abrasion and organic matter delivery. We hypothesized that (i) CH4 oxidation depends on the natural oxygenation gradient in the lake water and sediments and increases with lake size because of stronger wind‐induced water mixing; (ii) CO2 production increases with decreasing lake size, following the dissolved organic matter gradient; and (iii) both processes are more intensive in the upper than deeper sediments due to the in situ gradients of oxygen (O2) and bioavailable carbon. We estimated aerobic CH4 oxidation potentials and CO2 production based on the injection of 13C‐labeled CH4 in the 0–10 cm and 10–20 cm sediment depths of small (~300 m2), medium (~3000 m2), and large (~106 m2) shallow thermokarst lakes in the West Siberian Lowland. The CO2 production was 1.4–3.5 times stronger in the upper sediments than in the 10–20 cm depth and increased from large (158 ± 18 nmol CO2 g−1 sediment d.w. h−1) to medium and small (192 ± 17 nmol CO2 g−1 h−1) lakes. Methane oxidation in the upper sediments was similar in all lakes, while at depth, large lakes had 14‐ and 74‐fold faster oxidation rates (5.1 ± 0.5 nmol CH4‐derived CO2 g−1 h−1) than small and medium lakes, respectively. This was attributed to the higher O2 concentration in large lakes due to the more intense wind‐induced water turbulence and mixing than in smaller lakes. From a global perspective, the CH4 oxidation potential confirms the key role of thermokarst lakes as an important hotspot for GHG emissions, which increase with the decreasing lake size. [ABSTRACT FROM AUTHOR]

Towards a better understanding of factors controlling carbon (C) exchange between inland waters and atmosphere, we addressed the inorganic carbon cycle in semiarid lakes of Central Eurasia, subjected to the strong impact of on-going climate change. As such, we assessed the hydrochemical variability and quantified its control on the formation of authigenic carbonate minerals, occurring within the upper layer of sediments in 43 semiarid lakes located in the southwest of Western Siberia (Central Eurasia). Based on measurements of pH, total dissolved solids (TDS), cationic and anionic composition, dissolved organic and inorganic C, as well as textural and mineralogical characterization of bottom sediments using X-ray diffraction and scanning electron microscopy, we demonstrate that lake water pH and TDS are primarily controlled by both the lithological and climatic context of the lake watershed. We have not revealed any direct relationships between lake morphology and water chemistry. The most common authigenic carbonates scavenging atmospheric CO2 in the form of insoluble minerals in lake sediments were calcite, aragonite, Mg-calcite, dolomite and hydromagnesite. The calcite was the most common component, aragonite mainly appears in lakes with sediments enriched in gastropod shells or artemia cysts, while hydromagnesite was most common in lakes with high Mg/Ca molar ratios, as well as at high DIC concentrations. The relationships between mineral formation and water chemistry established in this study can be generalized to a wide suite of arid and semiarid lakes in order to characterize the current status of the inorganic C cycle and predict its possible modification under on-going climate warming such as a rise water temperature and a change in hydrological connectivity, primary productivity and nutrient regime. [ABSTRACT FROM AUTHOR]