Description of Dissolved Organic Matter Transformational Networks at the Molecular Level

Abstract

Dissolved Organic Matter (DOM) is an important component of the global carbon cycle. Unscrambling the structural footprint of DOM is key to understand its biogeochemical transformations at the mechanistic level. Although numerous studies have improved our knowledge of DOM chemical makeup, its three-dimensional picture remains largely unrevealed. In this work, we compare four solid phase extracted (SPE) DOM samples from three different freshwater ecosystems using high resolution mobility and ultrahigh-resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FT-ICR MS/MS). Structural families were identified based on neutral losses at the level of nominal mass using continuous accumulation of selected ions-collision induced dissociation (CASI-CID)FT-ICR MS/MS. Comparison of the structural families indicated dissimilarities in the structural footprint of this sample set. The structural family representation using Cytoscape software revealed characteristic clustering patterns among the DOM samples, thus confirming clear differences at the structural level (Only 10% is common across the four samples.). The analysis at the level of neutral loss-based functionalities suggests that hydration and carboxylation are ubiquitous transformational processes across the three ecosystems. In contrast, transformation mechanisms involving methoxy moieties may be constrained in estuarine systems due to extensive upstream lignin biodegradation. The inclusion of the isomeric content (mobility measurements at the level of chemical formula) in the structural family description suggests that additional transformation pathways and/or source variations are possible and account for the dissimilarities observed. While the structural character of more and diverse types of DOM samples needs to be assessed and added to this database, the results presented here demonstrate that Graph-DOM is a powerful tool capable of providing novel information on the DOM chemical footprint, based on structural interconnections of precursor molecules generated by fragmentation pathways and collisional cross sections.

Keywords: DOM; FT-ICR CASI-CID MS/MS; TIMS-FT-ICR MS; network; structural family.

Figure 1.

(−)ESI-FT-ICR MS spectra of the SRFA, Pantanal, HR-1, and HR-5 DOM samples (left panel), van Krevelen plots of the DOM samples highlighting the CHO, CHON, CHOS, and CHONS heteroatom classes (center panel), and 2D MS/MS plots obtained from the analysis of the DOM samples by the (−)ESI-FT-ICR CASI-CID MS/MS (right panel). Note that the number of chemical formulas indicated in the right panel for each DOM sample comprises the sum of precursors formulas obtained in both MS and MS/MS experiments, as well as fragment molecules assigned in more than 200 MS/MS spectra.

Figure 2.

Schematic representation of Graph-DOM models used for identifying structural families (top panel) in a DOM sample. Cytoscape networks of structurally interconnected precursors of the SRFA, Pantanal, HR-1, and HR-5 DOM samples (bottom panel).

Figure 3.

Venn diagram showing common structural families across DOM samples and unique families to SRFA, Pantanal, HR-1, and HR-5 determined by the comparison of families using Graph-DOM (top right panel). Cytoscape structural networks of common families found across the DOM samples (top left panel) and unique families to each sample (bottom).

Figure 4.

Representation of three common CHO families. The H₂O-CO₂-CO₂-H₂O (top left), CO₂-CO₂-CO₂ (top right), and H₂O-H₂O (bottom left) families are depicted with one homologous fragmentation pathways in the form of 2D MS/MS fragments m/z vs precursor m/z plots. A van Krevelen plot (bottom right) showcases the compositional pattern of the CHO class components from the SRFA, Pantanal, HR-1, and HR-5 samples, the CHO precursors from the structural families shared by all DOM samples, and the three selected structural families.

Figure 5.

Venn diagram describing common and unique isomers for the precursors of the families shared by all DOM samples (top). Section of the structural network of families common to all samples (bottom) highlighting the interconnected precursor formulas (nodes), the family IDs (color code), the neutral based functionalities (edge label), and the number of common isomers across DOM samples per precursor (numbers in brackets). Note that grey lines are associated with other not labeled shorter families.