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theses

Arctic tundra soils cover a vast portion of the planet, store massive amounts of carbon, and harbor microbial life throughout all seasons. Bacteria in Arctic tundra soils impact global carbon cycling, and their capabilities are becoming more consequential with climate change. This research aimed to understand metabolic capabilities of tundra bacteria and identify metabolically-active bacteria in frozen tundra soil. The Arctic tundra site of Kilpisjärvi, Finland served as a model landscape to explore the ecology and physiological potential of bacteria using bacterial isolates and soil incubations.

The effect of thaw on tundra soil bacteria is starting to be better understood, but very little is known about the impact of subzero temperature changes, when the ground is frozen. Soil respiration continues in the winter, though at slower rates. Identifying cryo-active bacterial communities is important since soil respiration is largely determined by microbial C mineralization through decomposition of complex soil organic matter. Although previous studies have examined the microbiomes of frozen soils, most have failed to detect which members of the bacterial communities are metabolically active. This detection is important as cold temperatures preserve commonly measured biomolecules such as DNA, RNA, etc., and may provide misleading information. To ascertain metabolically-active bacteria, stable-isotope probing of tundra soil incubations with 13C-cellobiose at subzero temperatures of 0, -4, and -16°C was carried out, and numerous active bacterial phyla including the Ignavibacteria, Candidatus Saccharibacteria, Verrucomicrobia were detected. Temperature was shown to impact which members of the bacterial community assimilated cellobiose, even within subzero ranges. Phylogenies of members of cryo-active bacterial phyla were further explored, and added new insights to known physiological capabilities of these groups. Implications of different bacterial communities active within subzero temperatures may suggest that nutrient cycling may be impacted by temperature shifts within frozen soils.

Another gap in tundra bacteriology is understanding the physiological abilities of tundra soil isolates. Arctic tundra soil isolates such as Mucilaginibacter mallensis, along with other members of the Mucilaginibacter genus are hypothesized to play an important role in processing carbon, but their genomic capabilities remain unexplored. Genomic analysis revealed that M. mallensis strain MP1X4 was adapted to process complex carbon, and had an abundance of loci associated with polysaccharide utilization and Carbohydrate Active Enzymes compared to other members of the genus. Other Arctic tundra isolates such as the Acidobacteria were found to have unusual membrane-bound isoprenoid structures such as hopanoids and carotenoids. These carotenoids and hopanoids are hypothesized to aid maintaining membrane fluidity. Arctic Acidobacteria such as Granulicella mallensis MP5ACTX8, Granulicella tundricola MP5ACTX9, Terriglobus saanensis SP1PR4, and new isolates A2288, M8UP23, M8UP39, MP8S11, MP8S7, and MP8S9 had their genomes sequenced, from which isoprenoid pathways were investigated. From genomic analysis of these Acidobacteria, biosynthetic pathways of carotenoids, such as phytoene, zeta-carotene, neurosporene, lycopene, and hopanoids, such such as diploptene, adenosylhopane, ribosylhopane, bacteriohopanetetrol (BHT), BHT acetylglucosamine, BHT glucosamine, and BHT cyclitol ether, were detected for some species.

Investigation into the ecology and genomes of Arctic bacteria provided insights into the bacterial communities assimilating carbon in subzero temperatures, and the possible genomic adaptations that allow these bacteria to live in the Arctic tundra soils.

ETD_9420

Ph.D.

Includes bibliographical references

by Preshita Gadkari

doi:10.7282/t3-7jmn-2697

ETD doctoral

The author owns the copyright to this work.