Composition and Biodegradation of DOM Leached from Permafrost End-members across the Western Canadian Arctic

Organic matter, upon dissolution into the aqueous state as dissolved organic matter (DOM), can undergo mineralization by microbes (biodegradation). There has been increasing effort to characterize DOM released from thawing permafrost because it may perpetuate a permafrost carbon feedback. Permafrost-derived DOM has a composition that can be highly susceptible to biodegradation (biolabile), but studies to date have been limited in scope. Importantly, diversity in deposit type and thaw modification processes have led to spatial and stratigraphic variability in permafrost, but our understanding of how the composition and biolability of DOM derived from differing permafrost types (end-members) is poor. Furthermore, few studies couple biolability measurements with assessing the microbial community structure, despite the important role that these microbes play in degrading DOM. This project aims to investigate how the composition of DOM leached from diverse permafrost end-members may vary, how compositional differences may relate to biodegradation rates, and how microbial communities sourced from contrasting thaw-affected areas may differ in their structure, or enable differences in biodegradation rates. Using Fourier transform ion cyclotron resonance mass spectrometry, we identified marked variation in DOM composition among permafrost end-member types. Permafrost leachates were generally higher in aliphatics, lower in aromatics, and were less oxygenated than active layer leachates. Tills were compositionally dissimilar to all other permafrost end-members. Compounds unique to Yedoma were predominantly aliphatic, while compounds unique to peat, lacustrine, and diamicton spanned saturation and oxygenation. DOM compositional differences were linked to varying carbon-normalized biodegradation rates in the incubation experiment. Though there was variation by site, DOM compositional shifts demonstrated that microbially-produced/protein-like components were preferentially consumed over terrestrially-derived/humic-like components. Biodegradation rates were slightly elevated in samples treated with microbial communities from aqueous environments, and the source of the inocula influenced the direction and extent of structural divergence of the communities over time. In all cases, compositional differences appear to reflect not only variation in permafrost parent materials, but also a strong effect from thaw-driven modification processes. Constraining DOM composition, biolability, and assessing its stratigraphic variability, linked with assessing microbial communities, will become more pressing as the spatial and stratigraphic extent of thaw increases with future warming.