An ultrahigh-resolution mass spectrometry index to estimate natural organic matter lability.

Natural organic matter (NOM) is a significant component of marine, freshwater, and glacial ecosystems, affecting multiple biogeochemical processes in the environment.1–7 The combination of allochthonous and autochthonous inputs in aquatic ecosystems contribute greatly to the chemical complexity of organic matter (OM), including an assortment of biochemically identifiable compound classes.8,9 Microorganisms utilize and produce autochthonous OM during microbial metabolism, a relationship that links the microbes to the quality and quantity of OM.10 OM quality is determined by the mixture of different types of chemical species, including labile and recalcitrant compounds. Therefore, the OM quality/chemical character is important to investigate, as it plays a role in microbial metabolism and carbon turnover rates. Within ecosystems, OM is introduced and/or produced, transformed, stored, and transported to downstream ecosystems; yet its origin, chemical characteristics, and overall contribution to the global carbon cycle are not well defined. Determining the molecular composition and fate of OM in aquatic environments is essential to better understand the global carbon cycle.10–12 Until recently, the ability to identify OM components, sources, and chemical processes was very limited. However, determining the chemical constituents of OM at the molecular level remains the ultimate measure for probing OM source material, evolution, and transport, and is used to trace lipid-, protein-, amino sugar-, cellulose-, lignin-, and black carbon-like character, along with potential structural information such as aliphatic and phenolic moieties.13–15 NOM from marine, freshwater, and glacial ecosystems is comprised of chemical constituents that are polyfunctional, heterogeneous, polyelectrolytic, vary in molecular weights, and is present at high and low concentrations. High magnetic field (>7 tesla) ultrahigh-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICRMS)16 is currently the only mass spectrometry technique capable of achieving the resolution and accuracy required to directly determine molecular formulae of NOM constituents. High field strength and field homogeneity are the keys to produce both ultrahigh mass resolving power (m/Δm50% >500,000, in which Δm50% is the mass spectral peak full width at half-maximum peak height) and mass accuracy (rms error <1.0 ppm) for compositionally complex NOM. Within the last two decades, great strides have been made with FTICRMS in chemically characterizing NOM samples from environments worldwide that were never before probed at the molecular level. FTICRMS advances mainly affect which molecular species are observed, not the mass accuracy on which the elemental composition assignments are based. Confirming specific naturally occurring isotopic mass spacing patterns for complex NOM elemental compositions, such as CcHhNnOoSs, thus extended our analytical understanding of NOM beyond just CcHhOo components. Because NOM constituents represent various chemical classes, it has been, and continues to be, effective to visualize and interpret FTICRMS molecular data with van Krevelen diagrams, namely, plots of atomic H/C versus O/C ratios.9 NOM typically exhibits hydrogen saturation and oxygenation as 0 < H/C <2.5 and 0 < O/C <1.2. In a van Krevelen plot, NOM species typically cluster into regions corresponding to compound classes and aromatic nature in various references.9,10,13,17–19 Extending those observations further, Kim et al.9 also proposed identification of possible chemical reaction pathways visualized from a van Krevelen diagram, for particular ranges of H/C and O/C ratios. That information can then be used to determine OM sources, evolution, microbial influence, degree of hydrogen, carbon, and oxygen saturation, and potential structural families to assess the role of organic carbon in an environment. Taken together, a great deal of information of NOM characterization by FTICRMS has been gained by graphical interpretation of van Krevelen diagrams and continues to evolve today.15 Recent ultrahigh-resolution FTICRMS studies of microbially derived NOM reinforce the value of van Krevelen diagrams as a reflection of the chemical character of the NOM that is similar to the labeled class regions.18 For example, microbially derived, bioavailable fractions, therefore classified as more labile NOM, have been linked to high H/C and low O/C ratios in the lipid-, protein-, and amino sugar-like regions of the van Krevelen digram.18,20–23 NOM constituents grouped in such regions also contain a high percentage of heteroatoms: CcHhNnOoSs species that function as electron donors for microbial metabolism and/or are metabolic intermediates and products. It should be noted that OM from Pony Lake, Antarctica, contains no higher plant (lignin), allochthonous inputs and is therefore completely microbially derived. Nevertheless, OM produced in microbial metabolism at Pony Lake exhibits compositional classes similar to those from terrestrially derived lignin-like material, as seen from the van Krevelen diagram, and thus shares the chemical reactivity in that region in common with Suwannee River fulvic acid.18 The difference between autochthonous, microbially derived OM and allochthonous freshwater, terrestrially derived and more recalcitrant OM, was addressed in that work, touching on the concept of more versus less labile OM nature from each source. The question then remains; can we use FTICRMS-derived elemental compositions to determine the extent of labile versus recalcitrant nature of NOM in marine, freshwater, and glacial environments? Determining the fate of more and less labile OM in the environment remains a chemical and biological controversy; however, in this study, we address this continuing dialogue and aim to identify chemical molecular information to define NOM labile nature that can be used to provide insight into physical, chemical, and biological processing in the environment. Battin et al.12 define labile NOM constituents as bioavailable carbon that is utilized in heterotrophic activity. Therefore, environmental processing of labile NOM contributes to the global carbon cycle; production and release of OM can increase CO2 concentration in the atmosphere.12 Direct measurements on the labile nature of NOM in marine, freshwater, and glacial environments is achieved by studying in situ microbial processing of NOM constituents over time, which can be labor intensive and costly. As multiple complementary techniques are commonly employed to extensively characterize various NOM fractions in different ecosystems with or without in situ experimentation, the present objective is to focus solely on a new interpretation of molecular data generated by ultrahigh-resolution FTICRMS to extend our current understanding of the labile nature of NOM in various environments, when other biological information is not available. A single ultrahigh-resolution FTICR mass spectrum of NOM could serve as a surrogate for time-consuming bioavailability studies; biological assays on NOM bioavailability based on respiration or other processing by bacteria can extend to multiple weeks and months.6 Here, we introduce a specific molecular lability index by use of many ultrahigh-resolution FTICRMS datasets, both new and previously published, to incorporate this novel analysis based on the chemical composition of NOM samples to distinguish between more and less labile molecular constituents. The degree of lability would therefore link to more or less bioavailable species of OM detectable by FTICRMS, the microbial biological processing of carbon within ecosystems, and could also be used to compare across ecosystems. The proposed molecular lability boundary (MLB) divides the data into two groups encompassing more or less labile material, derived from calculated hydrogen saturation molecular values that group together within three different chemical regions (lipid-, protein-, and amino sugar-like) in van Krevelen diagrams, previously classified with stronger microbial influence and linked to more labile nature.23–25 We have selected molecular data determined by FTICRMS over the last decade from multiple environments to validate the MLB, including OM studies investigating microbial abundance, microbial community, and respiration rates of specific microbes. We considered marine, freshwater, and glacial environments for this work, to span a range of ecosystems investigated by FTICRMS.