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.

To address the link between the composition and decomposition of freshwater dissolved organic matter (DOM), we manipulated the DOM from three boreal lakes using preincubations with UV light to cleave large aromatic molecules and polyvinylpyrrolidone (PVP) to remove colored phenolic compounds. Subsequently, we monitored the dissolved organic carbon (DOC) loss over 4 months of microbial degradation in the dark to assess how compositional changes in DOM affected different aspects of the reactivity continuum, including the distribution of the apparent decay coefficients. We observed profound effects on decomposition kinetics, with pronounced shifts in the relative share of rapidly and more slowly decomposing fractions of the DOM. In the UV‐exposed treatment initial apparent decay coefficient k0 was almost threefold higher than in the control. Significantly higher relative DOC loss in the UV‐exposed treatment was sustained for 2 months of incubation, after which decay coefficients converged with those in the control. The PVP removed compounds with absorbance and fluorescence characteristics representative of aromatic compounds, which led to slower decomposition, compared to that in the control. Our results demonstrate the reactivity continuum underlying the decomposition of DOM in freshwaters and highlight the importance of intrinsic properties of DOM in determining its decomposition kinetics.

The reactivity continuum (RC) model is a powerful statistical approach for describing the apparent kinetics of bulk organic matter (OM) decomposition. Here, we used ultrahigh resolution mass spectrometry data to evaluate the main premise of the RC model, namely that there is a continuous spectrum of reactivity within bulk OM, where each individual reactive type undergoes exponential decay. We performed a 120 day OM decomposition experiment on lake water, with an untreated control and a treatment preexposed to UV light, and described the loss of bulk dissolved organic carbon with RC modeling. The behavior of individual molecular formulas was described by fitting the single exponential model to the change in peak intensities over time. The range of the empirically derived apparent exponential decay coefficients (k) was indeed continuous. The character of the corresponding distribution, however, differed from the conceptual expectations, due to the effects of intrinsic averaging, overlaps in formula-specific loss and formation rates, and the limitation of the RC model to include apparently accumulating compounds in the analysis. Despite these limitations, both the RC model-simulated and empirical (mass spectrometry-derived) distributions of k captured the effects of preexposure to UV light. Overall, we present experimental evidence that the reactivity continuum within bulk OM emerges from a range of reactivity of numerous individual components. This constitutes direct empirical support for the major assumption behind the RC model of the natural OM decomposition.

Anaerobic oxidation of methane (AOM) is a potentially important methane sink in lake sediments, but the biogeochemistry and microbial ecology of this process are understudied. Potential electron acceptors for AOM include Fe(III) and sulfate; however, it is not clear to which extent low sulfate concentrations constrain the coupling of AOM to sulfate reduction, nor if Fe(III) reduction drives AOM directly or via a cryptic sulfur cycle. We investigated AOM pathways in the sediment of iron-rich Danish Lake Ørn through anoxic sediment slurry incubations with additions of 13 C-labeled methane as a substrate, sulfate and Fe(III) as potential electron acceptors, and molybdate as an inhibitor of sulfate reduction. The experiments demonstrated the co-occurrence of sulfateand iron-dependent modes of AOM, with the former supported by recycling of sulfate coupled to iron reduction. Quantitative PCR analysis demonstrated the abundance of archaea of the ANME-2d clade (Ca. Methanoperedenaceae) as likely drivers of AOM. Our study demonstrates that sulfate-dependent AOM can consume methane at sulfate concentrations typical for freshwater systems and emphasizes the importance of sulfur and iron cycling in the regulation of methane emission from freshwater sediments.