Decoding DOM Degradation with Metatranscriptomics : How Do Sunlight and Microbial Communities Interact to Degrade Dissolved Organic Matter in Arctic Freshwaters?

• Main Author: Nalven, Sarah G.
• Other Authors: Crump, Byron C., Sharpton, Thomas J., Myrold, David D., Thurber, Andrew R., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University. Graduate School
• Format: Master Thesis
• Language: English; unknown
• Published: Oregon State University
• Subjects: Soil microbiology > Arctic regions; Microbial metabolism; Water > Organic compound content > Arctic regions; Freshwater microbiology > Arctic regions; Carbon cycle (Biogeochemistry) > Arctic regions; Microorganisms > Effect of light on > Arctic regions; Gene expression; Arctic; Global warming; Tundra
• Online Access: https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8910jx591

Arctic soils are warming, making vast stores of organic carbon available for conversion to CO₂. This could create a positive feedback loop and accelerate global warming, but the processes that convert this carbon into CO₂ are not well understood. We investigated how the combined activities of sunlight and microbes degrade soil dissolved organic matter (DOM), an important component of the carbon processed in arctic freshwaters. DOM leached from the organic layer of moist acidic tundra was exposed to natural sunlight (24 h) or kept in the dark, inoculated and incubated with a soil microbial community, and analyzed for DOM composition (FT-ICR MS) and microbial gene expression (metatranscriptomics). We found that DOM degraded by sunlight was similar in composition to DOM degraded by microbes, and consequently, microbial activity was lower when incubated with sunlight-exposed DOM. We also found sunlight-exposed DOM caused global shifts in both microbial gene expression and the taxonomic groups conducting this expression. Greater expression of transcription and translation genes suggested growth, while lower expression of metabolism, motility, and transport genes suggested reduced investment in scavenging. Photo-exposure of DOM also caused reduced expression of enzymes involved in aromatic degradation, oxygenases, and decarboxylases, suggesting sunlight degraded aromatics, oxidized DOM, and decarboxylated DOM. Shifts in expression of transporters for small, labile compounds and nutrient-containing compounds suggested photo-exposure may have altered bioavailability of these compounds in the DOM pool. These findings demonstrate that even small amounts of sunlight can alter DOM in ways that evoke profound changes in microbial functioning, supporting the idea that sunlight plays a key role in determining the microbial processing of DOM in arctic freshwaters.