@phdthesis{Bischoff2013,
  author    = {Bischoff, Juliane},
  title     = {Microbial communities and their response to Pleistocene and Holocene climate variabilities in the Russian Arctic},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-68895},
  school      = {Universit{\"a}t Potsdam},
  year      = {2013},
  abstract  = {The Arctic is considered as a focal region in the ongoing climate change debate. The currently observed and predicted climate warming is particularly pronounced in the high northern latitudes. Rising temperatures in the Arctic cause progressive deepening and duration of permafrost thawing during the arctic summer, creating an 'active layer' with high bioavailability of nutrients and labile carbon for microbial consumption. The microbial mineralization of permafrost carbon creates large amounts of greenhouse gases, including carbon dioxide and methane, which can be released to the atmosphere, creating a positive feedback to global warming. However, to date, the microbial communities that drive the overall carbon cycle and specifically methane production in the Arctic are poorly constrained. To assess how these microbial communities will respond to the predicted climate changes, such as an increase in atmospheric and soil temperatures causing increased bioavailability of organic carbon, it is necessary to investigate the current status of this environment, but also how these microbial communities reacted to climate changes in the past. This PhD thesis investigated three records from two different study sites in the Russian Arctic, including permafrost, lake shore and lake deposits from Siberia and Chukotka. A combined stratigraphic approach of microbial and molecular organic geochemical techniques were used to identify and quantify characteristic microbial gene and lipid biomarkers. Based on this data it was possible to characterize and identify the climate response of microbial communities involved in past carbon cycling during the Middle Pleistocene and the Late Pleistocene to Holocene. It is shown that previous warmer periods were associated with an expansion of bacterial and archaeal communities throughout the Russian Arctic, similar to present day conditions. Different from this situation, past glacial and stadial periods experienced a substantial decrease in the abundance of Bacteria and Archaea. This trend can also be confirmed for the community of methanogenic archaea that were highly abundant and diverse during warm and particularly wet conditions. For the terrestrial permafrost, a direct effect of the temperature on the microbial communities is likely. In contrast, it is suggested that the temperature rise in scope of the glacial-interglacial climate variations led to an increase of the primary production in the Arctic lake setting, as can be seen in the corresponding biogenic silica distribution. The availability of this algae-derived carbon is suggested to be a driver for the observed pattern in the microbial abundance. This work demonstrates the effect of climate changes on the community composition of methanogenic archae. Methanosarcina-related species were abundant throughout the Russian Arctic and were able to adapt to changing environmental conditions. In contrast, members of Methanocellales and Methanomicrobiales were not able to adapt to past climate changes. This PhD thesis provides first evidence that past climatic warming led to an increased abundance of microbial communities in the Arctic, closely linked to the cycling of carbon and methane production. With the predicted climate warming, it may, therefore, be anticipated that extensive amounts of microbial communities will develop. Increasing temperatures in the Arctic will affect the temperature sensitive parts of the current microbiological communities, possibly leading to a suppression of cold adapted species and the prevalence of methanogenic archaea that tolerate or adapt to increasing temperatures. These changes in the composition of methanogenic archaea will likely increase the methane production potential of high latitude terrestrial regions, changing the Arctic from a carbon sink to a source.},
  language  = {en}
}
@phdthesis{FrankFahle2013,
  author    = {Frank-Fahle, B{\´e}atrice A.},
  title     = {Methane-cycling microbial communities in permafrost affected soils on Herschel Island and the Yukon Coast, Western Canadian Arctic},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-65345},
  school      = {Universit{\"a}t Potsdam},
  year      = {2013},
  abstract  = {Permafrost-affected ecosystems including peat wetlands are among the most obvious regions in which current microbial controls on organic matter decomposition are likely to change as a result of global warming. Wet tundra ecosystems in particular are ideal sites for increased methane production because of the waterlogged, anoxic conditions that prevail in seasonally increasing thawed layers. The following doctoral research project focused on investigating the abundance and distribution of the methane-cycling microbial communities in four different polygons on Herschel Island and the Yukon Coast. Despite the relevance of the Canadian Western Arctic in the global methane budget, the permafrost microbial communities there have thus far remained insufficiently characterized. Through the study of methanogenic and methanotrophic microbial communities involved in the decomposition of permafrost organic matter and their potential reaction to rising environmental temperatures, the overarching goal of the ensuing thesis is to fill the current gap in understanding the fate of the organic carbon currently stored in Artic environments and its implications regarding the methane cycle in permafrost environments. To attain this goal, a multiproxy approach including community fingerprinting analysis, cloning, quantitative PCR and next generation sequencing was used to describe the bacterial and archaeal community present in the active layer of four polygons and to scrutinize the diversity and distribution of methane-cycling microorganisms at different depths. These methods were combined with soil properties analyses in order to identify the main physico-chemical variables shaping these communities. In addition a climate warming simulation experiment was carried-out on intact active layer cores retrieved from Herschel Island in order to investigate the changes in the methane-cycling communities associated with an increase in soil temperature and to help better predict future methane-fluxes from polygonal wet tundra environments in the context of climate change. Results showed that the microbial community found in the water-saturated and carbon-rich polygons on Herschel Island and the Yukon Coast was diverse and showed a similar distribution with depth in all four polygons sampled. Specifically, the methanogenic community identified resembled the communities found in other similar Arctic study sites and showed comparable potential methane production rates, whereas the methane oxidizing bacterial community differed from what has been found so far, being dominated by type-II rather than type-I methanotrophs. After being subjected to strong increases in soil temperature, the active-layer microbial community demonstrated the ability to quickly adapt and as a result shifts in community composition could be observed. These results contribute to the understanding of carbon dynamics in Arctic permafrost regions and allow an assessment of the potential impact of climate change on methane-cycling microbial communities. This thesis constitutes the first in-depth study of methane-cycling communities in the Canadian Western Arctic, striving to advance our understanding of these communities in degrading permafrost environments by establishing an important new observatory in the Circum-Arctic.},
  language  = {en}
}