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Title: "Wavelength-dependent controls on the coupled photochemical and microbial degradation of dissolved organic matter in arctic surface waters"

Abstract:
Surface waters emit nearly as much carbon dioxide (CO2) to the atmosphere as is transferred from the atmosphere to the land surface. This large flux of CO2 from surface waters is driven by the export of dissolved organic matter (DOM) from land to streams and lakes, where it can be oxidized to CO2. Sunlight-driven degradation (photodegradation) of DOM contributes to CO2 emissions from surface waters by both completely oxidizing DOM to CO2 and by chemically altering DOM into forms that are more or less labile for microbes to respire to CO2. However, the amount of DOM that is oxidized to CO2 in surface waters as a result of photodegradation is uncertain because the relative importance of ultraviolet (UV) and visible wavelengths of sunlight for DOM photodegradation is poorly characterized in any surface waters.
Here, a light-emitting diode (LED)-based approach was used to directly quantify the wavelength-dependent photodegradation of DOM in arctic surface waters to produce both CO2 and chemically altered DOM that increases or decreases microbial respiration. Effects of photodegradation by UV and visible wavelengths of LED light were tested on both modern DOM in arctic surface waters and ancient DOM from permafrost soils that represent a future source of DOM as the arctic warms. Controls on the coupled photochemical and microbial degradation of DOM by UV and visible light were identified, and the impacts of wavelength-dependent photodegradation on rates of CO2 production in surface waters were assessed.

The oxidation of modern DOM to CO2 by both UV and visible light was greatest in waters with high concentrations of dissolved iron and aromatic DOM. Additionally, the efficiency of CO2 production at all wavelengths decreased with increasing light exposure history of DOM due to the depletion of fractions of DOM most susceptible to photodegradation. These findings demonstrate that spatial and temporal variability in the composition and sunlight exposure history of DOM in surface waters can result in substantial variability in rates of photochemical CO2 production.

The net effect of photodegradation on the amount of modern DOM that was labile to microbial respiration also depended on the wavelength-dependent light exposure history of DOM. Microbial respiration increased substantially with increasing oxidation of DOM by UV light, relative to dark controls. In contrast, photodegradation of DOM by visible light increased respiration at low amounts of DOM oxidized but had diminishing or negative effects on respiration with increasing DOM oxidized by light. Rates of respiration of modern, photodegraded DOM in surface waters depended most strongly on the effects of visible light, ranging from 70% higher to 120% lower than rates predicted from prior work.

Ancient DOM draining from permafrost soils was also shown to be labile to microbial respiration to CO2 after photodegradation by UV and visible light. Similar to modern DOM, the effects of permafrost DOM photodegradation on microbial respiration depended on the extent of DOM oxidation by light. Additionally, the radiocarbon age and stable carbon (13C) isotopic composition of carbon respired to CO2 indicated that ancient, lipid and lignin-like fractions of permafrost DOM were most labile to respiration, both in the dark and after photodegradation. Together, results of this dissertation demonstrate that CO2 production rates may vary substantially over space and time in surface waters, both now and in the future, in response to wavelength-dependent controls on the coupled photochemical and microbial degradation of DOM.

Link to abstract: https://drive.google.com/file/d/1Mix5U83XX_YcqYoDUA7_KEQGRy53Eu-N/view