Consumption of CH3Cl, CH3Br, and CH3I and emission of CHCl3, CHBr3, and CH2Br2 from the forefield of a retreating Arctic glacier

The Arctic is one of the most rapidly warming regions of the Earth, with predicted temperature increases of 5–7 ∘C and the accompanying extensive retreat of Arctic glacial systems by 2100. Retreating glaciers will reveal new land surfaces for microbial colonisation, ultimately succeeding to tundra over decades to centuries. An unexplored dimension to these changes is the impact upon the emission and consumption of halogenated organic compounds (halocarbons). Halocarbons are involved in several important atmospheric processes, including ozone destruction, and despite considerable research, uncertainties remain in the natural cycles of some of these compounds. Using flux chambers, we measured halocarbon fluxes across the glacier forefield (the area between the present-day position of a glacier's ice-front and that at the last glacial maximum) of a high-Arctic glacier in Svalbard, spanning recently exposed sediments (<10 years) to approximately 1950-year-old tundra. Forefield land surfaces were found to consume methyl chloride (CH3Cl) and methyl bromide (CH3Br), with both consumption and emission of methyl iodide (CH3I) observed. Bromoform (CHBr3) and dibromomethane (CH2Br2) have rarely been measured from terrestrial sources but were here found to be emitted across the forefield. Novel measurements conducted on terrestrial cyanobacterial mats covering relatively young surfaces showed similar measured fluxes to the oldest, vegetated tundra sites for CH3Cl, CH3Br, and CH3I (which were consumed) and for CHCl3 and CHBr3 (which were emitted). Consumption rates of CH3Cl and CH3Br and emission rates of CHCl3 from tundra and cyanobacterial mat sites were within the ranges reported from older and more established Arctic tundra elsewhere. Rough calculations showed total emissions and consumptions of these gases across the Arctic were small relative to other sources and sinks due to the small surface area represented by glacier forefields. We have demonstrated that glacier forefields can consume and emit halocarbons despite their young age and low soil development, particularly when cyanobacterial mats are present

Patterns in Microbial Assemblages Exported From the Meltwater of Arctic and Sub-Arctic Glaciers

Meltwater streams connect the glacial cryosphere with downstream ecosystems. Dissolved and particulate matter exported from glacial ecosystems originates from contrasting supraglacial and subglacial environments, and exported microbial cells have the potential to serve as ecological and hydrological indicators for glacial ecosystem processes. Here, we compare exported microbial assemblages from the meltwater of 24 glaciers from six (sub)Arctic regions - the southwestern Greenland Ice Sheet, Qeqertarsuaq (Disko Island) in west Greenland, Iceland, Svalbard, western Norway, and southeast Alaska - differing in their lithology, catchment size, and climatic characteristics, to investigate spatial and environmental factors structuring exported meltwater assemblages. We found that 16S rRNA gene sequences of all samples were dominated by the phyla Proteobacteria, Bacteroidetes, and Actinobacteria, with Verrucomicrobia also common in Greenland localities. Clustered OTUs were largely composed of aerobic and anaerobic heterotrophs capable of degrading a wide variety of carbon substrates. A small number of OTUs dominated all assemblages, with the most abundant being from the genera Polaromonas, Methylophilus, and Nitrotoga. However, 16-32% of a region's OTUs were unique to that region, and rare taxa revealed unique metabolic potentials and reflected differences between regions, such as the elevated relative abundances of sulfur oxidizers Sulfuricurvum sp. and Thiobacillus sp. at Svalbard sites. Meltwater alpha diversity showed a pronounced decrease with increasing latitude, and multivariate analyses of assemblages revealed significant regional clusters. Distance-based redundancy and correlation analyses further resolved associations between whole assemblages and individual OTUs with variables primarily corresponding with the sampled regions. Interestingly, some OTUs indicating specific metabolic processes were not strongly associated with corresponding meltwater characteristics (e.g., nitrification and inorganic nitrogen concentrations). Thus, while exported assemblage structure appears regionally specific, and probably reflects differences in dominant hydrological flowpaths, OTUs can also serve as indicators for more localized microbially mediated processes not captured by the traditional characterization of bulk meltwater hydrochemistry. These results collectively promote a better understanding of microbial distributions across the Arctic, as well as linkages between the terrestrial cryosphere habitats and downstream ecosystems

Catchment characteristics and seasonality control the composition of microbial assemblages exported from three outlet glaciers of the Greenland Ice Sheet

Glacial meltwater drains into proglacial rivers where it interacts with the surrounding landscape, collecting microbial cells as it travels downstream. Characterizing the composition of the resulting microbial assemblages in transport can inform us about intra-annual changes in meltwater flowpaths beneath the glacier as well as hydrological connectivity with proglacial areas. Here, we investigated how the structure of suspended microbial assemblages evolves over the course of a melt season for three proglacial catchments of the Greenland Ice Sheet (GrIS), reasoning that differences in glacier size and the proportion of glacierized versus non-glacierized catchment areas will influence both the identity and relative abundance of microbial taxa in transport. Streamwater samples were taken at the same time each day over a period of 3 weeks (summer 2018) to identify temporal patterns in microbial assemblages for three outlet glaciers of the GrIS, which differed in glacier size (smallest to largest; Russell, Leverett, and Isunnguata Sermia [IS]) and their glacierized: proglacial catchment area ratio (Leverett, 76; Isunnguata Sermia, 25; Russell, 2). DNA was extracted from samples, and 16S rRNA gene amplicons sequenced to characterize the structure of assemblages. We found that microbial diversity was significantly greater in Isunnguata Sermia and Russell Glacier rivers compared to Leverett Glacier, the latter of which having the smallest relative proglacial catchment area. Furthermore, the microbial diversity of the former two catchments continued to increase over monitored period, presumably due to increasing hydrologic connectivity with proglacial habitats. Meanwhile, diversity decreased over the monitored period in Leverett, which may have resulted from the evolution of an efficient subglacial drainage system. Linear discriminant analysis further revealed that bacteria characteristic to soils were disproportionately represented in the Isunnguata Sermia river, while putative methylotrophs were disproportionately abundant in Russell Glacier. Meanwhile, taxa typical for glacierized habitats (i.e., Rhodoferax and Polaromonas) dominated in the Leverett Glacier river. Our findings suggest that the proportion of deglaciated catchment area is more influential to suspended microbial assemblage structure than absolute glacier size, and improve our understanding of hydrological flowpaths, particulate entrainment, and transport

Greenland melt drives continuous export of methane from the ice-sheet bed

Ice sheets are currently ignored in global methane budgets1,2. Although ice sheets have been proposed to contain large reserves of methane that may contribute to a rise in atmospheric methane concentration if released during periods of rapid ice retreat3,4, no data exist on the current methane footprint of ice sheets. Here we find that subglacially produced methane is rapidly driven to the ice margin by the efficient drainage system of a subglacial catchment of the Greenland ice sheet. We report the continuous export of methane-supersaturated waters (CH4(aq)) from the ice-sheet bed during the melt season. Pulses of high CH4(aq) concentration coincide with supraglacially forced subglacial flushing events, confirming a subglacial source and highlighting the influence of melt on methane export. Sustained methane fluxes over the melt season are indicative of subglacial methane reserves that exceed methane export, with an estimated 6.3 tonnes (discharge-weighted mean; range from 2.4 to 11 tonnes) of CH4(aq) transported laterally from the ice-sheet bed. Stable-isotope analyses reveal a microbial origin for methane, probably from a mixture of inorganic and ancient organic carbon buried beneath the ice. We show that subglacial hydrology is crucial for controlling methane fluxes from the ice sheet, with efficient drainage limiting the extent of methane oxidation5 to about 17 per cent of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive fluxes (4.4 to 28 millimoles of CH4 per square metre per day) rivalling that of major world rivers6. Overall, our results indicate that ice sheets overlie extensive, biologically active methanogenic wetlands and that high rates of methane export to the atmosphere can occur via efficient subglacial drainage pathways. Our findings suggest that such environments have been previously underappreciated and should be considered in Earth’s methane budget

Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet

The Greenland Ice Sheet is currently not accounted for in Arctic mercury budgets, despite large and increasing annual runoff to the ocean and the socio-economic concerns of high mercury levels in Arctic organisms. Here we present concentrations of mercury in meltwaters from three glacial catchments on the southwestern margin of the Greenland Ice Sheet and evaluate the export of mercury to downstream fjords based on samples collected during summer ablation seasons. We show that concentrations of dissolved mercury are among the highest recorded in natural waters and mercury yields from these glacial catchments (521–3,300 mmol km−2 year−1) are two orders of magnitude higher than from Arctic rivers (4–20 mmol km−2 year−1). Fluxes of dissolved mercury from the southwestern region of Greenland are estimated to be globally significant (15.4–212 kmol year−1), accounting for about 10% of the estimated global riverine flux, and include export of bioaccumulating methylmercury (0.31–1.97 kmol year−1). High dissolved mercury concentrations (~20 pM inorganic mercury and ~2 pM methylmercury) were found to persist across salinity gradients of fjords. Mean particulate mercury concentrations were among the highest recorded in the literature (~51,000 pM), and dissolved mercury concentrations in runoff exceed reported surface snow and ice values. These results suggest a geological source of mercury at the ice sheet bed. The high concentrations of mercury and its large export to the downstream fjords have important implications for Arctic ecosystems, highlighting an urgent need to better understand mercury dynamics in ice sheet runoff under global warming

Assessing anaerobic activity in perennial subzero hypersaline spring of the high Arctic: focus on methanogenesis, anaerobic oxidation of methane and sulphur reduction

The Lost Hammer (LH) spring in the Canadian high Arctic perennially discharges subzero (-5°C) hypersaline (24% salt) brines through thick layers of permafrost (> 500 m), and so far accounts for the only described terrestrial methane seep in frozen settings on Earth. The present thesis aimed to ascertain that actively metabolising, indigenous, microbial communities do populate the sediments of the LH spring outlet despite the extreme conditions in situ. Incubation experiments with LH sediments could not confirm that microbial consortia undergo anaerobic methane metabolism but revealed that the reduction of sulphur compounds (SR) with hydrogen, most likely hydrogenotrophic sulphate reduction, was potentially carried out by some cryophilic populations under combined hypersaline and subzero (down to -20°C) conditions. Unusual H2S releases from LH sediments were also detected at high temperatures (80°C); the biogenicity of these releases could however not be confirmed and could alternatively reflect abiotic processes. Pyrosequencing analyses of both 16S rRNA (i.e. cDNA) and 16S rRNA genes (i.e. DNA) on 30 cm layers of LH sediments retrieved in April 2012 and July 2012 indicated fairly stable bacterial and archaeal communities at the phylum level, but a greater bacterial diversity at the species level (> 97% sequence similarities). The composition of the LH communities however differed significantly from previous surveys of the site, either reflecting site's heterogeneity and/or differences in sequencing coverage. Potentially active bacterial and archaeal communities were respectively dominated by clades related to the T78 Chloroflexi group and Halobacteria species, as indicated by 16S rRNA results; no sequence related to ANME-1 archaea were detected unlike in previous investigations of the site. The present study indicated that SR, hydrogenotrophy (possibly coupled to autotrophy), and hydrocarbon degradation (other than methane), most likely account for important metabolic processes carried out by LH microbial communities. Overall, the obtained findings provided additional evidence that the LH system host active communities of anaerobic, halophilic, and cryophilic microorganisms despite the extreme conditions in situ.La source d'eau Lost Hammer (LH), située dans l'extrême arctique canadien, déverse des eaux hypersalines (salinité de 24 %) et froides, ayant une température constante avoisinant les 5°C, à travers d'épaisses couches de pergélisol (> 500 m). LH est considérée comme le seul suintement terrestre de méthane documenté à ce jour se situant en milieu continuellement gelé sur Terre. Cette thèse visait à déterminer si les communautés microbiennes indigènes aux sédiments de la source LH sont métaboliquement actives in situ, malgré les conditions extrêmes de la source. Des expériences d'incubations de sédiments de LH n'ont pu confirmer que les consortia microbiens métabolisent du méthane de façon anaérobique, mais ont révélé que des populations cryophiles sont probablement capables de réduire des composés de soufre, probablement la réduction de sulfate, sous des conditions hypersalines et jusqu'à 20°C. Des échappements de H2S des sédiments ont aussi été détectés à haute température (80°C); l'authenticité biologique de ces échappements nécessite d'être confirmée et pourrait alternativement refléter des processus chimiques abiotiques. Des analyses de pyroséquençage du 16S ARNr (ADNc) et du gène du 16S ARNr (ADN) sur des couches de 30 cm de sédiments collectés en avril 2012 et juillet 2012 ont indiqué que les communautés d'archées et de bactéries de LH sont assez stables au niveau du phylum, mais que la diversité entre les communautés de bactéries est plus variable au niveau de l'espèce (similarité des séquences > 97 %). La composition des communautés de LH différait par contre significativement de celle décrite lors d'études antérieures du site, reflétant possiblement une hétérogénéité du site, ou des différences de couverture de séquençage. Les résultats de pyroséquençage du 16S ARNr ont indiqué que les communautés de LH de bactéries et d'archées potentiellement actives étaient dominées respectivement par des clades reliés au groupe T78 des Chloroflexi et à des espèces de Halobacteria; aucune séquence reliée aux archées ANME-1 ne fut détectée contrairement à ce qui fut observé lors d'investigations précédentes du site. La présente recherche a indiqué que la réduction de composés de soufre, l'hydrogénotrophie (possiblement couplée à l'autotrophie), et la dégradation d'hydrocarbures (autres que le méthane) sont probablement d'importants processus métaboliques chez les communautés microbiennes de LH. Dans l'ensemble, les résultats obtenus ont fourni des évidences additionnelles que la source LH abrite des microorganismes anaérobiques, halophiles, et cryophiles actifs, malgré les conditions in situ extrêmes

Evidence of in situ microbial activity and sulphidogenesis in perennially sub-0 \ub0C and hypersaline sediments of a high Arctic permafrost spring

The lost hammer (LH) spring perennially discharges subzero hypersaline reducing brines through thick layers of permafrost and is the only known terrestrial methane seep in frozen settings on Earth. The present study aimed to identify active microbial communities that populate the sediments of the spring outlet, and verify whether such communities vary seasonally and spatially. Microcosm experiments revealed that the biological reduction of sulfur compounds (SR) with hydrogen (e.g., sulfate reduction) was potentially carried out under combined hypersaline and subzero conditions, down to 1220 \ub0C, the coldest temperature ever recorded for SR. Pyrosequencing analyses of both 16S rRNA (i.e., cDNA) and 16S rRNA genes (i.e., DNA) of sediments retrieved in late winter and summer indicated fairly stable bacterial and archaeal communities at the phylum level. Potentially active bacterial and archaeal communities were dominated by clades related to the T78 Chloroflexi group and Halobacteria species, respectively. The present study indicated that SR, hydrogenotrophy (possibly coupled to autotrophy), and short-chain alkane degradation (other than methane), most likely represent important, previously unaccounted for, metabolic processes carried out by LH microbial communities. Overall, the obtained findings provided additional evidence that the LH system hosts active communities of anaerobic, halophilic, and cryophilic microorganisms despite the extreme conditions in situ.Peer reviewed: YesNRC publication: Ye