ABSTRACT
Redox-active functional groups in dissolved organic matter (DOM) are crucial for microbial electron transfer and methane emissions. However, the extent of aquatic DOM redox properties across northern high-latitude lakes and their relationships with DOM composition have not been thoroughly described. We quantified electron donating capacity (EDC) and electron accepting capacity (EAC) in lake DOM from Canada to Alaska and assessed their relationships with parameters from absorbance, fluorescence, and ultrahigh resolution mass spectrometry (FT-ICR MS) analyses. EDC and EAC are strongly tied to aromaticity and negatively related to aliphaticity and protein-like content. Redox-active formulae spanned a range of aromaticity, including highly unsaturated phenolic formulae, and correlated negatively with many aliphatic N and S-containing formulae. This distribution illustrates the compositional diversity of redox-sensitive functional groups and their sensitivity to ecosystem properties such as local hydrology and residence time. Finally, we developed a reducing index (RI) to predict EDC in aquatic DOM from FT-ICR MS spectra and assessed its robustness using riverine DOM. As the hydrology of the northern high-latitudes continues to change, we expect differences in the quantity and partitioning of EDC and EAC within these lakes, which have implications for local water quality and methane emissions.
Subject(s)
Dissolved Organic Matter , Lakes , Ecosystem , Oxidation-Reduction , MethaneABSTRACT
Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) has been increasingly employed to characterize dissolved organic matter (DOM) across a range of aquatic environments highlighting the role of DOM in global carbon cycling. DOM analysis commonly utilizes electrospray ionization (ESI), while some have implemented other techniques, including dopant-assisted atmospheric pressure photoionization (APPI). We compared various extracted DOM compositions analyzed by negative ESI and positive APPI doped with both toluene and tetrahydrofuran (THF), including a fragmentation study of THF-doped riverine DOM using infrared multiple photon dissociation (IRMPD). DOM compositions followed the same trends in ESI and dopant-assisted APPI with the latter presenting saturated, less oxygenated, and more N-containing compounds than ESI. Between the APPI dopants, THF-doping yielded spectra with more aliphatic-like and N-containing compounds than toluene-doping. We further demonstrate how fragmentation of THF-doped DOM in APPI resolved subtle differences between riverine DOM that was absent from ESI. In both ionization methods, we describe a linear relationship between atomic and formulaic N-compositions from a range of DOM extracts. This study highlights that THF-doped APPI is useful for uncovering low-intensity aliphatic and peptide-like components in autochthonous DOM, which could aid environmental assessments of DOM across biolability gradients.
