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    Microbial lipid signatures and substrate potential of organic matter in permafrost deposits - implications for future greenhouse gas production

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    A terrestrial permafrost core from Buor Khaya in northern Siberia comprising deposits of Late Pleistocene to Early Holocene age has been investigated to characterize living and past microbial communities with respect to modern and paleoclimate environmental conditions, and to evaluate the potential of the organic matter (OM) for greenhouse gas generation. Microbial life markers - intact phospholipids and phospholipid fatty acids - are found throughout the entire core and indicate the presence of living microorganisms also in older permafrost deposits. Biomarkers for past microbial communities (branched and isoprenoid GDGT as well as archaeol) reveal links between increased past microbial activity and intervals of high OM accumulation accompanied by increased OM quality presumably caused by local periods of moister and warmer environmental conditions. Concentrations of acetate as an excellent substrate for methanogenesis are used to assess the OM quality with respect to microbial degradability for greenhouse gas production. For this purpose two acetate pools are determined: the pore-water acetate and OM bound acetate. Both depth profiles reveal similarities to the OM content and quality indicating a link between the amount of the stored OM and the potential to provide substrates for microbial greenhouse gas production. The data suggest that OM stored in the permafrost deposits is not much different in terms of OM quality than the fresh surface organic material. Considering the expected increase of permafrost thaw due to climate warming, this implies a potentially strong impact on greenhouse gas generation from permafrost areas in future with positive feedback on climate variation

    Mikrobielle Signaturen und Qualitätsbewertung des organischen Materials in sibirischen Permafrostablagerungen für zukünftige biogene Treibhausgasproduktion

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    The Earth´s high latitude regions, where recent warming has been amplified, are of particular concern in the light of future climate change. Annual average temperatures in the Arctic have risen by 0.6 °C per decade over the last 30 years, and this warming trend resulted into the thaw of perennially frozen ground (permafrost), which exposes substantial amounts of previously frozen organic carbon to decomposition by microorganisms. Permafrost is a widespread phenomenon in the Arctic, which constitutes a historical carbon sink of global importance storing twice as much carbon as is currently present in in the atmosphere. Despite having functioned as a carbon sink in the past, it is predicted that significant permafrost thaw will enhance microbial decomposition of the organic carbon and increase microbial greenhouse gas (GHG) emission from the soils into the atmosphere; accelerating warming and promoting further permafrost thaw via a positive feedback. Although permafrost thaw has a strong climate feedback potential, the majority of coupled climate earth system models do not currently include this carbon-climate feedback, and estimates of its size are accompanied by large uncertainties. This doctoral thesis therefore aims to evaluate the future impact of microbial GHG generation from thawing permafrost deposits of different depositional ages for the global carbon cycle. Thus, a combination of detailed bio- and geochemical analyses was conducted on permafrost deposits from two glacial-interglacial cycles (Late Saalian - Eemian and Weichselian - Holocene) on two study sites within the zone of continuous permafrost in north-eastern Siberia to address the research objectives of this doctoral thesis: (1) Characterization of the stored OM of different permafrost units of contrasting ages and regional settings to assess its substrate potential for microbial GHG production and reveal differences between the individual permafrost units. (2) Classification of the distribution of present and past microbial communities in the active layer and in the individual permafrost units to reveal the role of microorganisms within the past, present and future dynamics of microbial GHG production. (3) Verification of the assessed substrate potential from OM characterization via experimental stimulation of in-situ microbial GHG generation from thawed permafrost OM of different depositional ages. Firstly, although proxy analyses indicate a terrestrial source for all investigated samples, the results reveal differences in the amount and quality of the organic matter (OM) between the individual permafrost units. Curve progressions of the hydrogen index (quality proxy of OM) and TOC (total amount of organic carbon) indicate a positive relation between the OM quality and amount. In addition, permafrost deposits of elevated OM quality and amount are found to possess increased concentrations of free and bound organic acetate, which are used as indicators for the substrate potential of the individual permafrost deposits in terms of microbially produced GHG. Secondly, in order to examine the environmental factors controlling the differences in the quality of the OM, glycerol dialkyl glycerol tetraethers (GDGTs) are used as biomarkers for a past microbial community and therefore are applied as proxies for paleo-environmental living conditions. Analyses reveal that permafrost deposits which accumulated under anoxic and/or wetter environmental conditions are characterized by elevated OM quality and have a stronger potential to provide substrates for microbial greenhouse gas production. Based on this findings, glacial permafrost deposits (Weichselian and Saalian), especially those which accumulated during the interstadial period MIS 3 (Marine Isotope Stage 3), possess a higher carbon-climate feedback potential than interglacial permafrost deposits (Holocene and Eemian). The consistency of the results across the two study sites suggests that this varying potential of the individual permafrost units to serve as substrate providers is not only a local, but a regional phenomenon in the north-eastern Siberian Arctic. This information is very important for improving the accuracy of projections regarding the size of the carbon-climate feedback expected from Siberian permafrost thaw. Finally, in this doctoral project biomarkers representing a living microbial community (phospholipids (PLs) and their fatty acid side-chain inventory (PLFAs)) are applied in combination with incubation experiments and geochemical investigations to identify living microbial cells in permafrost systems and reveal the role of microorganisms in GHG emission from thawing permafrost. Both living and inactive microbial cells are found in all permafrost deposits, which are well adapted to the cold ambient temperatures and can be reactivated by thawing processes. As a consequence of permafrost thaw, a positive correlation between the increase of microbial cells, the decrease in the freely available substrate concentrations, and the amount of microbially produced carbon dioxide can be observed, which is also seen within the active layer. Moreover, detailed analyses of the molecular composition of the OM using FT-ICR-MS reveal that microbes consume organic molecules such as carboxylic acids in thawed permafrost deposits, which are part of the stored and old OM. Additionally the results show, that the amount of microbial GHG production not only depends on the concentrations of living cells in the permafrost deposits, but also on the amount and quality of the OM, and therefore also on the availability of organic substrates. In summary, the research in this thesis found that permafrost deposits of different OM quality possess different carbon-cycle feedback potentials. The elevated OM quality of permafrost deposits, which accumulated during the interstadial of the Weichselian glacial period (Yedoma), means that these deposits have a higher carbon-climate feedback potential than others. The extensive distribution of those deposits in north-eastern Siberia makes this a key region of global significance to consider in projections of the climate-carbon feedback from thawing permafrost and further geochemical analysis concentrated on Arctic permafrost deposits can help to constrain the size of this feedback.Während der letzten 30 Jahre sind die Temperaturen in den hohen Breitengraden der Erde (Arktis) um 0.6 °C pro Jahrzehnt angestiegen. Als Folge der Erwärmung tauen die über lange Zeiträume gefrorenen sibirischen Böden (Permafrost) auf, was zur Freisetzung von großen Mengen an organischem Kohlenstoff und zu dessen Zersetzung durch Mikroorganismen führt. Permafrostablagerungen sind sehr weit verbreitet in der sibirischen Arktis und bilden einen großen Kohlenstoffspeicher, in dem etwa zweimal so viel organischer Kohlenstoff gespeichert ist als sich derzeitig in der Atmosphäre befindet. Die fortschreitende Klimaerwärmung bewirkt, dass diese Permafrostablagerungen immer tiefer auftauen und der gespeicherte organische Kohlenstoff frei zugänglich für mikrobielle Abbauprozesse wird. Als Folge dessen ist ein erhöhter Ausstoß von mikrobiell gebildeten Treibhausgasen aus dem Permafrost in die Atmosphäre zu erwarten, was den globalen Treibhauseffekt begünstigt und zu weiteren Auftauprozessen führt (Klima-Rückkopplungseffekt). Derzeitig berücksichtigen die globalen Klimasystemmodelle noch nicht die Klima-Rückkopplungseffekte aus den auftauenden Permafrostablagerungen, da ihr Einfluss auf das globale Klimasystem noch immer nicht eindeutig ist. Hauptziel der vorliegenden Doktorarbeit ist es daher die Bedeutung der mikrobiellen Treibhausgasbildung aus unterschiedlichen Permafrostablagerungen für den globalen Kohlenstoffkreislauf zu bewerten. Durch die Kombination von detaillierten bio- und geochemischen Analysen an Permafrostablagerungen, die während verschiedener glazialen und interglazialen Perioden (Saale-Eiszeit, Eem-Warmzeit, Weichsel-Eiszeit, Holozän) an unterschiedlichen Standorten innerhalb der kontinuierlichen Permafrostzone im Nordosten Sibiriens abgelagert worden sind, ergeben sich die einzelnen Ziele dieser Arbeit: (1) Charakterisierung des eingelagerten organischen Materials in verschiedenen Permafrostablagerungen unterschiedlichen Alters und Ablagerungsortes zur Bewertung des jeweiligen Substratpotentials für mikrobielle Treibhausgasbildung. (2) Einordnungen der Verteilung der gegenwärtigen und vergangenen mikrobiellen Gemeinschaften in der aufgetauten Bodenschicht (active layer) und in verschiedenen Permafrostablagerungen, um die Rolle der Mikroorganismen innerhalb der vergangenen, gegenwärtigen und zukünftigen mikrobiellen Treibhausgasdynamik zu verstehen. (3) Überprüfung des abgeschätzten Substratpotentials durch die Charakterisierungen des organischen Materials via experimenteller Stimulation von mikrobieller Treibhausgasbildung aus verschiedenen Permafrostablagerungen. Die Untersuchungen zeigen, dass auch wenn alle Proxyanalysen auf ein eindeutigen terrestrischen Ursprung des organischen Material hindeuten, es jedoch Unterschiede in der Menge und Qualität des organischen Materials zwischen den unterschiedlichen Permafrostablagerungen gibt. Die Tiefenprofile des Hydrogenindexes (Proxy für die Qualität des organischen Materials) und des TOC (Gesamtmenge an organischem Material) deuten auf einen positiven Zusammenhang zwischen der Qualität und der Menge an organischem Material hin. Zusätzlich besitzen Permafrostablagerungen mit erhöhter Qualität und Menge an organischem Material auch höhere Konzentrationen an freiem und gebundenem Acetat, welche als Indikatoren für das Substratpotential zur mikrobiellen Treibhausgasbildung in den einzelnen Permafrostablagerungen dienen. Zur Identifizierungen der Umweltfaktoren, welche die Unterschiede in der Qualität des organischen Material beeinflussen können, wurden in dieser Doktorarbeit Glycerol Dialkyl Glycerol Tetraether (GDGTs) als Biomarker für eine vergangene mikrobielle Gemeinschaft verwendet und darüber hinaus auch als Proxy für vergangene Umweltbedingungen angewandt. Die Analysen zeigen, dass Permafrost, der unter sauerstoffarmen und/oder feuchten Umwelt- und Bodenbedingungen gebildet wurde eine bessere Qualität des organischen Materials und ein höheres Potential zur Bereitstellung von Substraten für die mikrobielle Produktion von Treibhausgasen besitzt. Daher kann abgeleitet werden, dass Permafrostablagerungen, die während einer glazialen Perioden (Weichsel und Saale), insbesondere während des Interstadials MIS 3 (Marines Isotopenstadium 3) abgelagert worden sind, über ein höheres Klima-Rückkopplungspotential verfügen im Vergleich zu Permafrostablagerungen aus interglazialen Perioden (Eem und Holozän). Die bio- und geochemischen Ergebnisse von beiden Untersuchungsstandorten zeigen an, dass die Unterschiede in den verschiedenen Permafrostablagerungen in Bezug auf die Bereitstellung von Substraten für die mikrobielle Treibhausgasproduktion nicht nur ein lokales, sondern ein regionales Phänomen in der nordöstlichen sibirischen Arktis sind. Daher liefert diese Doktorarbeit wichtige grundlegende Informationen, die zur Erstellung von zukünftigen detaillierten Klimamodellen des Kohlestoff-Klima-Kreislaufes in Zusammenhang mit dem Auftauen von Permafrost notwendig sind. Des Weiteren werden in dieser Doktorarbeit Biomarker, die eine lebende mikrobielle Gemeinschaft beschreiben (Phosphor-Lipide (PLs) und deren Fettsäuren (PLFAs)), in Kombination mit Inkubationsexperimenten und geochemischen Untersuchungen angewandt, um lebende mikrobielle Zellen in Permafrostablagerungen und deren Bedeutung im Zusammenhang mit dem Auftauen von Permafrost zu identifizieren. Lebende, inaktive und an die kalten Umweltbedingungen perfekt angepasste mikrobielle Zellen wurden in allen Permafrostablagerungen gefunden und können durch Auftauprozesse re-aktiviert werden. Als Folge des Auftauens von Permafrost ist ein positiver Zusammenhang zwischen der Zunahme an mikrobiellen Zellen und der Abnahme der Konzentration an freiverfügbaren organischen Substraten sowie der Menge an mikrobielle gebildetem Kohlenstoffdioxids zu sehen, welcher auch in der heutigen Auftauschicht (active layer) zu beobachten ist. Weitere detaillierte Analysen der molekularen Zusammensetzung mit FT-ICR-MS zeigen, dass Mikroorganismen in auftauenden Permafrostablagerungen organische Moleküle abbauen, die Teil des abgelagerten organischen Materials sind (zum Beispiel Carboxylsäuren). Zusätzlich zeigen die Ergebnisse, dass die Menge an mikrobiell produzierten Treibhausgasen nicht nur von der Konzentration an lebenden Zellen, sondern auch von der Menge und Qualität des organischen Materials und damit auch von der Verfügbarkeit an organischen Substraten abhängt. Zusammengefasst zeigen die Ergebnisse dieser Doktorarbeit, dass Permafrostablagerungen mit unterschiedlicher Qualität des organischen Materials ein unterschiedliches Kohlenstoffkreislauf-Rückkopplungspotential besitzen. Die erhöhte Qualität des organischen Materials in den Permafrostablagerungen aus der Weichsel-Eiszeit (Yedoma-Ablagerungen) lassen im Vergleich zu anderen Permafrostablagerungen auf ein stärkeres Klima-Rückkopplungspotential dieser Permafrostablagerungen schließen. Die großflächige Verbreitung dieser Yedoma-Ablagerungen in der sibirischen Arktis macht dies zu einer Region von globaler Bedeutung in Anbetracht ihres Klima-Rückkopplungspotenzials. Weitere geochemische Analysen an unterschiedlichen arktischen Permafrostablagerungen können demnach helfen die zukünftige Rolle der Arktis im globalen Klimasystem besser einzuordnen und ihr Klima-Rückkopplungspotenzial besser abzuschätzen.BMBF, 03G0836B, CARBOPERM - Kohlenstoff im Permafros

    Bulk parameter data, formate and acetate data and microbial life and past biomarkers from Eemian to Holocene permafrost core material drilled on Bol'shoy Lyakhovsky Island (Siberia) within the CarboPerm project

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    Within the Russian-German research project CARBOPERM the organic matter (OM) in several permafrost cores from Bol'shoy Lyakhovsky Island in NE Siberia was investigated. In the context of the observed global warming the aim was to evaluate the potential of freeze-locked OM from different depositional ages (Eemian to Holocene) to act as a substrate provider for microbial production of greenhouse gases from thawing permafrost. To assess this potential bulk elemental parameters (total carbon (TC), total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), TOC/TN ratios, Hydrogen Index and Oxygen Index data), the concentrations of free (directly extractable part) and bound (after ester cleavage) formate and acetate, the concentration of phospholipid life markers and the concentration of branched and isoprenoid glycerol dialkyl glycerol tetraethers (GDGT) were determined

    Microbial lipid distribution and substrate potential of the organic matter in terrestrial Siberian permafrost deposits from NE Siberia

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    The investigation of microbial ecosystems in permafrost sediments is an important approach to understand the role of microbial organic matter transformation in permafrost sediments for past and future climate changes, and is of high relevance in today’s geoscience research (Wagner, 2008) due to the current debate on the temperature vulnerability of permafrost deposits. Especially, the interplay between the organic substrate and the distribution of the living and past microbial communities in Late Pleistocene (Yedoma) and Holocene permafrost deposits, as well as the substrate potential of the organic matter stored in potentially thawing permafrost deposits are in the focus of the current study. Our investigation is part of the BMBF CarboPerm project an interdisciplinary Russian-German cooperation on the formation, turnover and release of carbon from Siberian permafrost landscapes. Sample material derived from terrestrial permafrost cores drilled at the coast of Bour Khaya in the North-Eastern Siberian Arctic. The gathered core material comprises Late Pleistocene to early Holocene deposits separated by an ice wedge. The microbial life markers (intact phospholipids, PLs) prove the presence of currently living microorganisms in the entire permafrost sequence and show the highest concentration in the uppermost sample indicating an abundant microbial life in the active layer. In comparison, the PL profile is strongly decreased in the underlying permafrost deposits. Nevertheless, the inventory of the Phospholipid fatty acids (PLFAs) suggests that the cell membrane temperature adaptation to cold environmental conditions is mainly regulated via the ratio between iso- and anteiso-fatty acids (FAs) as well as the ratio between saturated and unsaturated FAs. The surface samples show higher proportions of anteiso and unsaturated FAs (adaptation to cooler conditions), which might derive from the fact that surface layers are more affected from the harsh Siberian winter conditions than the deeper constantly cold permafrost deposits, where the above-ground temperature extremes are buffered due to the overlying deposits. Indeed within the deeper permafrost sequence the variations of the ratios are rather small, indicating adaptation to similar constantly cold temperature conditions. Other microbial markers (GDGTs), already partly degraded and, therefore, not indicating microbial life, reveal similarities with the TOC content and an increase especially in Late Pleistocene deposits. This suggests increased microbial life during intervals in the Late Pleistocene presumably caused by periods of moisture and temperature increased environments. Pore water analysis reveals the presence of low molecular weight organic acids (LMWOA) such as acetate, being excellent substrates for microbial metabolism. In the Late Pleistocene deposits below the ice wedge the substrate depth profiles show significant similarities to the TOC content. These points to a link between the organic matter and the LMWOA concentrations solved in the pore water and to the potential of those permafrost layers to provide substrates for microbial greenhouse gas production. In contrast, in the active layer the LMWOA concentrations are low, reflecting an active microbial turnover in the surface layers. Ester cleavage experiments on the residual organic matter resulted in the release of ester linked LMWOAs forming a potential substrate pool when released in future. These bound LMWOA profiles are even better correlated to the TOC content suggesting that the deeper permafrost deposits (older organic material)are not significantly different from those in the surface sediment (fresh organic material). Overall this indicates that the organic matter stored in the permafrost deposits and, therefore, removed from the surface carbon cycle is not much different in terms of organic matter quality than the fresh surface organic material. Considering the discussed increase of permafrost thawing, this might imply a strong impact on the generation of greenhouse gases from permafrost areas in future with its feedback on climate evolution. In a second and ongoing study, four terrestrial permafrost cores spanning from the Eemian interglacial into the Holocene form Bol’shoy Lyakhovsky Island are investigated with the focus on the differences and potential of the organic matter by comparing Eemian, Late Pleistocene and Holocene deposits. First results already reveal similar relations between the living and dead microbial communities with respect to the availability of free substrates, and the quality and amount of the total organic carbon. The results on the future potential of these deposits will also be presented

    Geochemical and microbial biomarker parameters from permafrost deposits (Buor Khaya, Siberia)

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    A terrestrial permafrost core from Buor Khaya in northern Siberia comprising deposits of Late Pleistocene to Early Holocene age has been investigated to characterize living and past microbial communities with respect to modern and paleoclimate environmental conditions and to evaluate the potential of the organic matter (OM) for greenhouse gas generation. Microbial life markers?intact phospholipids and phospholipid fatty acids?are found throughout the entire core and indicate the presence of living microorganisms also in older permafrost deposits. Biomarkers for past microbial communities (branched and isoprenoid glycerol dialkyl glycerol tetraether as well as archaeol) reveal links between increased past microbial activity and intervals of high OM accumulation accompanied by increased OM quality presumably caused by local periods of moister and warmer environmental conditions. Concentrations of acetate as an excellent substrate for methanogenesis are used to assess the OM quality with respect to microbial degradability for greenhouse gas production. For this purpose two acetate pools are determined: the pore water acetate and OM bound acetate. Both depth profiles reveal similarities to the OM content and quality indicating a link between the amount of the stored OM and the potential to provide substrates for microbial greenhouse gas production. The data suggest that OM stored in the permafrost deposits is not much different in terms of OM quality than the fresh surface organic material. Considering the expected increase of permafrost thaw due to climate warming, this implies a potentially strong impact on greenhouse gas generation from permafrost areas in future with positive feedback on climate variation
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