Abstract
Developing a microbial ecological understanding of Arctic thermokarst lake sediments in a geochemical context is an essential first step toward comprehending the contributions of these systems to greenhouse gas emissions, and understanding how they may shift as a result of long term changes in climate. In light of this, we set out to study microbial diversity and structure in sediments from four shallow thermokarst lakes in the Arctic Coastal Plain of Alaska. Sediments from one of these lakes (Sukok) emit methane (CH4) of thermogenic origin, as expected for an area with natural gas reserves. However, sediments from a lake 10 km to the North West (Siqlukaq) produce CH4 of biogenic origin. Sukok and Siqlukaq were chosen among the four lakes surveyed to test the hypothesis that active CH4-producing organisms (methanogens) would reflect the distribution of CH4 gas levels in the sediments. We first examined the structure of the little known microbial community inhabiting the thaw bulb of arctic thermokarst lakes near Barrow, AK. Molecular approaches (PCR-DGGE and iTag sequencing) targeting the SSU rRNA gene and rRNA molecule were used to profile diversity, assemblage structure, and identify potentially active members of the microbial assemblages. Overall, the potentially active (rRNA dominant) fraction included taxa that have also been detected in other permafrost environments (e.g., Bacteroidetes, Actinobacteria, Nitrospirae, Chloroflexi, and others). In addition, Siqlukaq sediments were unique compared to the other sites, in that they harbored CH4-cycling organisms (i.e., methanogenic Archaea and methanotrophic Bacteria), as well as bacteria potentially involved in N cycling (e.g., Nitrospirae) whereas Sukok sediments were dominated by taxa typically involved in photosynthesis and biogeochemical sulfur (S) transformations. This study revealed a high degree of archaeal phylogenetic diversity in addition to CH4-producing archaea, which spanned nearly the phylogenetic extent of currently recognized Archaea phyla (e.g., Euryarchaeota, Bathyarchaeota, Thaumarchaeota, Woesearchaeota, Pacearchaeota, and others). Together these results shed light on expansive bacterial and archaeal diversity in Arctic thermokarst lakes and suggest important differences in biogeochemical potential in contrasting Arctic thermokarst lake sediment ecosystems.
Highlights
The thawing front of permafrost underneath shallow (∼2–3 m deep) arctic thermokarst lakes is an active area of carbon (C) cycling, where carbon dioxide (CO2) and methane (CH4) are produced and consumed by microbially driven processes (Bretz and Whalen, 2014; Lofton et al, 2014; Heslop et al, 2015)
The majority of arctic thermokarst lake Archaea cloned sequences and iTag sequences were related to members of the phylum Bathyarchaeota (Meng et al, 2014; Evans et al, 2015), those originating from Sukok Lake
We set out to study the microbial assemblages that may be actively involved in geochemical processes relevant to thermokarst lake sediments
Summary
The thawing front (talik or thaw bulb) of permafrost underneath shallow (∼2–3 m deep) arctic thermokarst lakes is an active area of carbon (C) cycling, where carbon dioxide (CO2) and methane (CH4) are produced and consumed by microbially driven processes (Bretz and Whalen, 2014; Lofton et al, 2014; Heslop et al, 2015). The Coastal Plain of northern Alaska is a large (i.e., 22,000 square kilometers; Wang et al, 2012) matrix of permafrost peppered with thermokarst lakes, some of which produce either ebullient CH4 generated by present-day biogenic processes (e.g., Siqlukaq Lake) or as a result of release by thermogenic processes (e.g., Sukok Lake, which overlies a portion of the national petroleum reserve; Matheus Carnevali et al, 2015). As part of an interdisciplinary study on CH4-related process in this region, we hypothesized that the microbial assemblages in the sediments of the ebullient thermogenic CH4 lakes would be distinct from the biogenic CH4 producing lakes. We took a deeper look into the phylogenetic diversity of the Archaea by sequencing cloned archaeal rRNA genes (partially covered)
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