In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Biology
In the
School of Biological Sciences

Katherine Duchesneau

Will defend her dissertation
Impacts of climate change on soil-microbe interactions and implications for carbon storage in northern peatlands
14, April, 2025
12 PM
The CHOA Seminar Room in EBB Krone 1005 or online
https://gatech.zoom.us/j/94786913234?pwd=frOxIqmDQuN1wuNKqm6BWRL1a3yEvz

Thesis Advisor:
Joel E. Kostka, Ph.D.
School of Biological Sciences
Georgia Institute of Technology

Committee Members:
Kostas T. Konstantinidis, Ph.D.
School of Civil and Environmental Engineering
Georgia Institute of Technology

Lauren Speare, Ph.D.
School of Biological Sciences
Georgia Institute of Technology

Kelly C. Wrighton, Ph.D.
College of Agricultural Sciences, Department of Soil and Crop Sciences
Colorado State University

Colleen M. Iversen, Ph.D.
Climate Change Science Institute and Environmental Sciences Division
Oak Ridge National Laboratory

ABSTRACT:
Northern peatlands store carbon as thick layers of peat comprised largely of decomposing Sphagnum moss biomass. The rate of decomposition in these cold, anoxic environments is regulated by heterotrophic microbial networks adapted to these extreme conditions. The enzyme latch hypothesis posits that oxygen limitation suppresses phenol oxidases, allowing phenolic compounds to accumulate and inhibit decomposition, but recent evidence suggests these plant-derived compounds fuel microbial growth under anoxia. Oxygen availability in peat is primarily determined by water table elevation which is itself influenced by warming and plant transpiration. Therefore, changes in water table elevation may have important interacting effects with warming on the microbial dynamics that limit peatland carbon storage. My dissertation research closely couples genome-resolved metagenomics and biogeochemistry approaches to interrogate the belowground heterotrophic microbial communities that mediate soil organic matter decomposition in peatlands. I leveraged the infrastructure of the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment, which includes 5 whole ecosystem warming treatments in a regression design with duplicates exposed to ambient and elevated CO2, to determine how environmental change drivers influence carbon turnover. An expansive drought occurred in 2021, allowing me to test the interaction of warming and water table drawdown on microbial activity. My results indicate that extended oxic periods in peat are exacerbated by warming, which impacts soil respiration and greenhouse gas production (CO2, CH4) under fluctuating redox conditions in surface peat. During water table drawdown from drought, aerobic respiration is favored, and I identify a group of warming-stimulated organisms whose metabolism is correlated with porewater CO2 concentrations, including active phenolic compound-degrading genomes in the phylum Proteobacteria and Actinobacteriota which incorporate phenolic compounds into their central carbon metabolism. In addition, I identify a core group of active methanogens and determine that their relative activity is inhibited by drought but increases with warming under typical water table conditions. Surprisingly, abundant and active methanogens (Candidatus Methanoflorens, Methanobacterium, and Methanoregula) show a stable relative abundance with warming, while displaying a versatile metabolism, including the potential for both acetoclastic and hydrogenotrophic methanogenesis. Using a metabolite-informed network analysis, my dissertation reveals that methanogen and warming-inhibited, facultatively-anaerobic carbon degraders are linked by specific classes of metabolites, including phenolic compounds, amino acids, and unsaturated lipids. Thus, my results at least partially support the enzyme latch hypothesis, and I conclude that the stimulating effect of temperature on methanogenesis is controlled by metabolic exchange between methanogens and temperature-sensitive, facultatively-anaerobic heterotrophs, which is in turn regulated by redox oscillations brought on by the combined influence of warming and drought.