Molecular Determinants of Dissolved Organic Matter Reactivity in Lake Water

Abstract

Lakes in the boreal region have been recognized as the biogeochemical hotspots, yet many questions regarding the regulators of organic matter processing in these systems remain open. Molecular composition can be an important determinant of dissolved organic matter (DOM) fate in freshwater systems, but many aspects of this relationship remain unclear due to the complexity of DOM and its interactions in the natural environment. Here, we combine ultrahigh resolution mass spectrometry (FT-ICR-MS) with kinetic modeling of decay of >1,300 individual DOM molecular formulae identified by mass spectrometry, to evaluate the role of specific molecular characteristics in decomposition of lake water DOM. Our data is derived from a 4 months microbial decomposition experiment, carried out on water from three Swedish lakes, with the set-up including natural lake water, as well as the lake water pretreated with UV light. The relative decay rate of every molecular formula was estimated by fitting a single exponential model to the change in FT-ICR-MS signal intensities over decomposition time. We found a continuous range of exponential decay coefficients (k exp ) within different groups of compounds and show that for highly unsaturated and phenolic compounds the distribution of k exp was shifted toward the lowest values. Contrary to this general trend, plant-derived polyphenols and polycondensed aromatics were on average more reactive than compounds with an intermediate aromaticity. The decay rate of aromatic compounds increased with increasing nominal oxidation state of carbon, and molecular mass in some cases showed an inverse relationship with k exp in the UV-manipulated treatment. Further, we observe an increase in formulae-specific k exp as a result of the UV pretreatment. General trends in reactivity identified among major compound groups emphasize the importance of the intrinsic controllers of lake water DOM decay. However, we additionally indicate that each compound group contained a wide spectrum of reactivities, suggesting that high resolution is needed to further ascertain the complex reasons behind DOM reactivity in lake water.

Photochemically Induced Changes of Dissolved Organic Matter in a Humic-Rich and Forested Stream

“…In agreement with the loss of fluorescence the degree of aromaticity was lost on the first view (Table S7). The double bond equivalence (DBE) decreased as expected [9,80,81]. Nitrogen remains constant, carbon and oxygen decreased and sulphur increased, mostly from t = 10 (168 kW/m 2 cumRAD, second important drop).…”

“…In total, there was a decrease of 50% for peak B (ex: 320 to 400 nm; em: 380 to 500 nm) and of 75% for peak A (ex: 240 to 280 nm; em: 380 to 500 nm). These two peaks were also relevant in [81] or [85], where they also decreased under UV irradiation. This means that also the fDOM was reduced, more than the chromophoric DOM.…”

Greenhouse gas emissions from urban ponds are driven by nutrient status and hydrology

“…In support of this, recent work at the molecular level has highlighted that not all aromatic DOM is resistant to degradation (Mostovaya et al. ). Finally, positive relationships between aquatic CO 2 and NH 4 have been reported elsewhere (Schrier‐Uijl et al.…”

“…Positive correlations between dissolved CO 2 and SUVA have been observed for small peatland pools and ditches (Turner et al 2016, Peacock et al 2017, and could suggest that DOM with higher molecular weight or increasing aromaticity is more bioavailable to aquatic microbial communities (Tranvik 1990). In support of this, recent work at the molecular level has highlighted that not all aromatic DOM is resistant to degradation (Mostovaya et al 2017). Finally, positive relationships between aquatic CO 2 and NH 4 have been reported elsewhere (Schrier-Uijl et al 2011, Yu et al 2017.…”

Fundamental drivers of dissolved organic matter composition across an Arctic effective precipitation gradient

“…Aliphatic, N‐containing, and S‐containing compounds detected by FT‐ICR MS have been linked to chlorophyll a (Osterholz et al ), a proxy for primary production. Aliphatic compounds have also been found to be particularly susceptible to microbial degradation, although microbial degradation has been linked to the loss of a variety of molecular formulae (D'Andrilli et al ; Spencer et al ; Mostovaya et al ). The relative abundance comprised by aliphatic compounds in the current study was lower than previously observed in lakes and rivers (Wagner et al ; Kellerman et al ).…”