Utilization of PARAFAC‐Modeled Excitation‐Emission Matrix (EEM) Fluorescence Spectroscopy to Identify Biogeochemical Processing of Dissolved Organic Matter in a Northern Peatland

Abstract

In this study, we contrast the fluorescent properties of dissolved organic matter (DOM) in fens and bogs in a Northern Minnesota peatland using excitation emission matrix fluorescence spectroscopy with parallel factor analysis (EEM-PARAFAC). EEM-PARAFAC identified four humic-like components and one protein-like component and the dynamics of each were evaluated based on their distribution with depth as well as across sites differing in hydrology and major biological species. The PARAFAC-EEM experiments were supported by dissolved organic carbon measurements (DOC), optical spectroscopy (UV-Vis), and compositional characterization by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS). The FT-ICR MS data indicate that metabolism in peatlands reduces the molecular weights of individual components of DOM, and oxygen-rich less aromatic molecules are selectively biodegraded. Our data suggest that different hydrologic and biological conditions within the larger peat ecosystem drive molecular changes in DOM, resulting in distinctly different chemical compositions and unique fluorescent fingerprints. PARAFAC modeling of EEM data coupled with ultrahigh resolution FT-ICR MS has the potential to provide significant molecular-based information on DOM composition that will support efforts to better understand the composition, sources, and diagenetic status of DOM from different terrestrial and aquatic systems.

Vertical Stratification of Peat Pore Water Dissolved Organic Matter Composition in a Peat Bog in Northern Minnesota

“…The most positive PC1 score was observed in deep pore water samples, followed by intermediate depth pore water samples that suggest evidence of microbial utilization of pore water DOM deeper in the peat column. The decrease in the ubiquitous high MW humic‐like component C4 with depth and the increase in C2 and C3 are in contrast to observations from a previous study in another northern peatland (Tfaily et al, ) where C3 was negatively correlated with C1, and C4 was relatively refractory. These data suggest that at the S1 bog, high molecular weight highly aromatic C4 appears to be more bioavailable for microbial communities than the C1 component and that DOM decomposition dynamics vary across different peatland sites.…”

“…Principal Component 1 (PC1) accounted for 74.4%, and Principal Component (PC2) accounted for another 21.3% of the variability. As illustrated in the loading plot of PCA (Figure a), the five PARAFAC components are distributed along PC1 such that the relative abundance of humic‐like terrestrial C4 and C1 (Tfaily et al, ) correlates most negatively with PC1, and the relative abundance of humic microbial‐like C3, C2, and protein‐like C5 (Tfaily et al, ) is positively correlated with PC1. The terrestrial and microbial/protein components have dissimilar distributions along PC1, suggesting that the DOM source controls PC1.…”

“…PARAFAC was used to statistically decompose EEMS into fluorescent groups/components (Ohno & Bro, ; Stedmon et al, ). Here PARAFAC modeling was applied by fitting the EEMS to a five‐component PARAFAC model (Tfaily et al, ), which was established for northern Minnesota peatlands. The analysis was carried out in MATLAB 7.0.4 (Mathworks, Natick, MA) with the DOM Fluor toolbox (Stedmon & Bro, ).…”

Hydrogenation of organic matter as a terminal electron sink sustains high CO2:CH4 production ratios during anaerobic decomposition

“…These groups which mediate fermentation, syntrophy, and utilize fumarate as the sole electron acceptor by direct interspecies electron transfer (Sieber et al, 2012;Wang et al, 2016) are also prevalent at Stordalen Mire (Mondav and Woodcroft et al 2014, supplemental data). Furthermore, Tfaily et al (2015) identified a unique fluorescent chromophore in pore waters from the nearby Glacial Lake Agassiz peatlands in Minnesota that appeared to be a microbial by-product originating from humic matter, consistent with the microbial transformation of unsaturated compounds. Increasing alkylation of dissolved organic matter with increasing depth was also observed in the Glacial Lake Agassiz peatlands which is consistent with our hypothesis of increasing hydrogenation of organic matter .…”

Dissolved organic matter characteristics of deciduous and coniferous forests with variable management: different at the source, aligned in the soil

“…Depending on the position in the van Krevelen diagram, all assigned formulae can roughly be grouped according to major classes of biopolymers found in natural organic matter like tannin, lignin, lipids, proteins, amino sugars, and hydrocarbons. We used the classification according to Sleighter and Hatcher 2007 The number of formulae in each class were then summed and normalized by the total number of assignable formulae for all functional groups to produce a relative abundance (as percent) for the six classes of biopolymers (Tfaily et al, 2015).…”