The role of microorganisms in greenhouse gas fluxes from aquatic ecosystems impacted by permafrost degradation in the Arctic

Thesis defense
 04/05/2026
 14:00:00
 Arthur Szylit, ESE
 Laboratory of Oceanology and Geosciences of the University du Littoral Côte d’Opale

This thesis was carried out under the co-supervision of Urania Christaki (Prof. ULCO) and Ludwig Jardillier (Prof. Université Paris-Saclay), with the guidance of Léa Cabrol (IRD) and Maïalen Barret (Université de Toulouse).

Abstract

Climate warming, particularly pronounced at high latitudes, is degrading Arctic and sub-Arctic permafrost. In the Yukon Territory (Western Canada), this ice- and carbon-rich permafrost is highly vulnerable. Its thaw forms thermokarst waterbodies and releases organic matter that microorganisms convert into greenhouse gases (GHG ; e.g., CH4, CO2). Although part of the CH4 is consumed by methanotrophs, these waterbodies constitute potential emission hotspots, thereby amplifying global warming through positive feedback. This thesis aimed to characterize prokaryotic communities, identify the microorganisms involved in CH4 emissions, and quantify these emissions. To achieve this, molecular, geochemical, and isotopic approaches were applied to water and sediment samples collected from various waterbodies in the Yukon Territory during two field campaigns. Our results show that small waterbodies impacted by thaw share similar prokaryotic communities, regardless of their geographical dispersion. Despite hydrological connectivity, pond communities remain distinct from those of large lakes, with low microbial connectivity between these two systems. A shift in dominant methanogens occurs along this continuum : Methanobacterium dominates in ponds, while Methanoregula and Methanosaeta dominate in lacustrine sediments. These ecosystems emitted CH4 (acetoclastic or methylotrophic methanogenesis). However, emission intensity varied significantly among small waterbodies, precluding any large-scale generalization. Methodologically, a DNA extraction protocol was optimized to overcome rapid filter clogging and low microbial biomass retention, a common challenge encountered in these aquatic ecosystems.

In conclusion, this work contributes to a better understanding of prokaryotic communities and methane dynamics in aquatic systems impacted by permafrost thaw. While permafrost thaw promotes the emergence of emission hotspots, the intensity of this response depends heavily on the internal and geomorphological characteristics of each waterbody. Integrating this high heterogeneity is essential for better predicting the evolution of Arctic GHG emissions.

Members of the jury

• Prof. Michaël HERMOSO, ULCO – President
• Prof. Julie LELOUP, Sorbonne Université – Reviewer
• Prof. Sébastien DUPERRON, MNHN – Reviewer
• Dr. Ingrid OBERNOSTERER, CNRS – Examiner
• Mr. Olivier CHAPLEUR, ICPEF, INRAE, Univ. Paris-Saclay – Examiner