Microbial Ecology, Biogeochemistry, and Signatures of Life at Low Temperature in Arctic Thermokarst Lake Sediments and High Sierra Snow Fields
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The subjects of biological methane (CH4) production and microbial community diversity and structure in extreme cold environments were at the core of this dissertation. CH4 is a potent greenhouse gas that contributes to the warming of the planet and has chemical properties that allow its detection by instruments that can go into space. In this sense, CH4 is also a biosignature, because its presence could be indicative of the presence of life. Mechanisms that lead to the formation of this gas on Earth include reactions mediated by biological enzymes that evolved early in life’s evolutionary time. Therefore, microbial communities composed of different metabolic guilds that act in synergy to decompose organic matter are key elements of understanding biological CH4 production. Two chapters of this dissertation were devoted to studying the potential for biological CH4 production from arctic thermokarst lake sediments. Thermokarst lakes are abundant in the area of continuous permafrost on the Coastal Plain on the North Slope of Alaska, and they are sensitive to climate warming. In Chapter 2 we detected biological CH4 production from sediments of two lakes by studying the isotopic composition of C in CH4 (δ13CCH4) and its concentration in porewaters, and by conducting sediment incubation experiments with no substrate additions. At CH4 concentrations no higher than ~ 2.2 µmoles CH4 g-1 dry sediment, an average δ13CCH4 signature ~ 72 0/00 in the surface sediment intervals of Siqlukaq Lake (Siq) was indicative of biogenic CH4, while a δ13CCH4 signature ~ 51 0/00 indicated that in Sukok Lake (Suk) there was a mixed signature in the range of thermogenic CH4. We also determined that the highest amount of CH4 produced in the lab at 10 °C (~ 7 µmoles CH4 g-1 dry sediment) occurred in Siq, the lake where we detected four times more organic carbon than in the other two sites (SukS and SukB). These findings were supported by high amounts in Siq sediments of archaeal lipid biomarkers and by copies of the mcrA gene, which is specific to the pathway of methanogenesis. Furthermore, we determined that the geochemical environment of these lakes was different, with higher concentrations of dissolved iron (Fe) in Siq (51.6 - 855 µM) than in SukB (0.1 - 482 µM) and SukS (33.8 - 141 µM); higher concentrations of sulfate (SO42-) in SukB (0 - 752 µM) and SukS (0 - 329 µM) than in Siq (0.0 - 23.9 µM); and similar concentrations of nitrate (NO3-) at all sites (0.0 - 213 µM) except for a couple of depths in SukB where NO3- was much higher (up to 2.3 mM).These differences in sediment geochemistry led us to investigate what the diversity and structure of the bacterial and archaeal assemblages was in the sediments of these two lakes. In Chapter 3 we sequenced the small subunit (SSU) ribosomal RNA (rRNA) gene and the SSU rRNA molecule of Bacteria and Archaea by Sanger and iTag sequencing (Illumina MiSeq). iTag sequence analyses showed that the microbial assemblage was highly diverse (maximum Simpson’s reciprocal index of 148) and that there was a difference in operational taxonomic units (OTUs) predicted for DNA and aRNA sequences between Siq, SukB, and SukS, as reflected by non-metric multidimensional scaling (NMDS). OTUs related to taxa that are capable of organic matter degradation were found among the OTUs comprising ≥ 1 % of the aRNA sequences, which were taken as a proxy for members of the community that may have been potentially active in the sediments at the time of sampling. These taxa included members of the phyla Actinobacteria and Bacteroidetes and represented up to 10 % of the sequences found in all the sites. There was an important presence (> 5 % of the sequences) of sulfate reducing bacteria (SRB) affiliated with the orders Syntrophobacterales and Desulfobacterales at SukB. OTUs at this site were also related to Gammaproteobacteria of the order Pseudomonadales, and to a few groups of unicellular Cyanobacteria. In Siq, OTUs related to the phylum Nitrospirae comprised ~ 5 % of the sequences, and the same group comprised ~ 2.5 % in SukB. Further investigation of the functional potential of the community using phylogenetic investigation of communities by reconstruction of unobserved state (PICRUSt) supported the potential for microbial mediated transformations of nitrogen and sulfur, and for CH4 cycling. One of the most noteworthy findings was that we observed distinct differences in the distribution of Archaea between the two lakes, in which methanogenic Archaea were mostly found in Siq. Through phylogenetic inference of all archaeal sequences we determined that archaea in these arctic thermokarst lakes are evolutionarily related to other archaea in the Arctic. As an appendix to this work, we also conducted a culturing effort to enrich for methanogenic Archaea adapted to this cold environment. This effort led to enrichment of several methanogens in co-culture with bacteria, augmenting the short list of cold-adapted methanogens in culture. The last chapter of this dissertation focused on an entirely different kind of cold tolerant, ice loving microbial community – those inhabiting high altitude snow packs. In these habitats, a variety of microorganisms including unicellular algae, bacteria, and fungi can be found under freezing conditions during the seasonal snowpack. Here, we aimed to uncover the diversity and structure of the microbial community in summer snowpacks where conspicuous red algae were present. We also aimed to understand the mechanisms that drive community assembly in this ephemeral habitat. Snow samples were collected from Mt. Conness (CNS) in the eastern Sierra Nevada of California (as our primary study site), and two other geographically distant sites: the Pacific Crest Trail (PCT) and Wheeler Peak (WP). Chemical analyses of inorganic and organic nutrients and Chlorophyll a (Chla) were conducted in CNS, and the snow microbiota in samples from all three sites was studied by iTag sequencing of the SSU rRNA gene of the three domains of life. Detailed analysis of the resampled data set of bacterial and archaeal iTag sequences indicated that the bacterial assemblage had a unique structure. In addition to commonly observed ‘singletons’ (OTUs with just 1 sequence) that dominate NGS datasets, we found OTUs that were only present at one site and comprised a high number of sequences (‘singleton OTUs’). Interestingly, these OTUs were mostly affiliated with two different families of Bacteroidetes in the genera Polaromonas and Herminiimonas. Furthermore, even though the bacterial assemblage was up to 80 % dissimilar between sites at the OTU level, there was a resemblance in taxonomic composition at the Order level among all three sites, and a few ‘core’ OTUs affiliated with the Phyla Bacteroidetes and Actinobacteria were found in all samples. Variation in structure of the abundant OTUs between sites (and depths) was reflected in the proportion of sequences related to two families of Betaproteobacteria (the Comamonadaceae and the Oxalobacteraceae) and groups of Bacteroidetes. To elucidate possible mechanisms for microbial community assembly and to explain bacterial assemblage structure we tested two hypotheses: (i) that the presence of snow algae determined bacterial assemblage structure, and (ii) that environmental factors affected the structure. A Spearman’s rank correlation between chlorophyll a (Chla; as a proxy to snow algae) and bacterial diversity, indicated that bacterial diversity decreased with Chla concentration. This finding was consistent with three different kinds of Speraman’s rank correlations (positive, logarithmic, and negative) between specific algal OTUs (from a parallel data set) and specific bacterial OTUs, including OTUs that were present in all three sites. A relationship between environmental factors and bacterial OTUs was found for only two of the environmental factors studied (DOC and PN). Further testing using multivariate statistical approaches may allow us to formulate possible explanations to how geographically distant regions share similar microbial community composition at this high taxonomic level, and determine what mechanisms drive community assembly in these snow packs.Together, the chapters of this dissertation allowed me to address first order questions concerning life in extremely cold environments that have yet to be well-studied, and that are particularly sensitive to climate warming. Gaining a better understanding of biogeochemical cycling in these ecosystems, may allow us to predict the impact of environmental change on the resident microbial communities, and ultimately how will that affect the cycling of carbon and other major nutrients. Furthermore, these ecosystems are of great interest, because they may be used as analogs to study biosignatures relevant to the search for life in other icy worlds in the Solar System.