Photochemical alteration of organic carbon draining permafrost soils shifts microbial metabolic pathways and stimulates respiration

Abstract

In sunlit waters, photochemical alteration of dissolved organic carbon (DOC) impacts the microbial respiration of DOC to CO2. This coupled photochemical and biological degradation of DOC is especially critical for carbon budgets in the Arctic, where thawing permafrost soils increase opportunities for DOC oxidation to CO2 in surface waters, thereby reinforcing global warming. Here we show how and why sunlight exposure impacts microbial respiration of DOC draining permafrost soils. Sunlight significantly increases or decreases microbial respiration of DOC depending on whether photo-alteration produces or removes molecules that native microbial communities used prior to light exposure. Using high-resolution chemical and microbial approaches, we show that rates of DOC processing by microbes are likely governed by a combination of the abundance and lability of DOC exported from land to water and produced by photochemical processes, and the capacity and timescale that microbial communities have to adapt to metabolize photo-altered DOC.

Rapid CO2 Release From Eroding Permafrost in Seawater

“…Labile terrestrial DOC fractions can be quickly mineralized within the water column and have been shown to have a short residence time of approximately 2-5 years on the shelf (Alling et al, 2010;Letscher et al, 2011) during which approximately 30-50% of DOC is potentially being lost. Sunlight may further catalyze the turnover process in the upper water column by photooxidation (Cory et al, 2014;Ward et al, 2017). Eventually, resuspension and bioturbation also remobilize buried OC and slow carbon pools (Arndt et al, 2013;Rasmussen & Jorgensen, 1992), which decomposition can take up to decades (Schädel et al, 2014), postponing CO 2 production beyond one openwater season.…”

Oxygen Isotopes (δ18O) Trace Photochemical Hydrocarbon Oxidation at the Sea Surface

“…A similar observation was made with regard to photo-production of fatty acids during the IXTOC-I oil spill (Overton et al, 1980). This result is surprising because the initial sensitizers for crude oil (polycyclic aromatic hydrocarbons, resins, and asphaltenes; Payne & Phillips, 1985) are different than for DOC (tannins, lignin, pigments, and thermally altered organic matter; Ward & Cory, 2016;Ward et al, 2017), and thus the major photo-oxidation pathways are expected to differ considerably. This result is surprising because the initial sensitizers for crude oil (polycyclic aromatic hydrocarbons, resins, and asphaltenes; Payne & Phillips, 1985) are different than for DOC (tannins, lignin, pigments, and thermally altered organic matter; Ward & Cory, 2016;Ward et al, 2017), and thus the major photo-oxidation pathways are expected to differ considerably.…”

Photodegraded dissolved organic matter from peak freshet river discharge as a substrate for bacterial production in a lake-rich great Arctic delta

“…8b), where an initial sharp increase in SBP also suggested that the surviving bacteria were released from a relative shortage of growth substrate. Increased BP and growth have been observed in the presence of irradiated DOM in many freshwater systems (e.g., Anesio et al 2005;Paul et al 2012), including in the circumpolar Arctic (Cory et al 2013;Ward et al 2017), where photochemical oxidation of DOM accounts for the majority of C processing (Cory et al 2014). Prior work in the Mackenzie Delta has shown that lower molecular weight DOM derived from macrophytes supports higher rates of BGE in communities isolated from a variety of habitats (including TK and CON lakes;Tank 2009).…”

“…To some extent, therefore, irradiation of DOM by sunlight simultaneously stimulates and inhibits heterotrophic bacterial production (BP) (Scully et al 2003;Ruiz-Gonzalez et al 2013), leading to complex interactions that can result in enhanced, negative, mixed, or no effect on bacterial community metabolism (Lonborg et al 2016). In addition, the bacterial community composition may be altered either by exposure to ROS (Glaeser et al 2010;Glaeser et al 2014) or in response to the increased lability of the pool of DOM substrate (Judd et al 2007;Piccini et al 2009;Paul et al 2012;Ward et al 2017), giving rise to a species assemblage that is better suited to the ambient conditions.…”