Inferring Ecosystem Function from Dissolved Organic Matter Optical Properties: A Critical Review.

Over the last 30 years, the optical property community has shifted from conducting dissolved organic matter (DOM) measurements on new complex mixtures in natural and engineered systems to furthering ecosystem understanding in the context of past, present, and future carbon (C) cycling regimes. However, the appropriate use of optical properties to understand DOM behavior in complex biogeochemical systems is of recent debate. This critical review provides an extensive survey of DOM optical property literature across atmospheric, marine, and terrestrial biospheres using a categorical approach that probes each biosphere and its subdivisions. Using this approach, a rubric of ecosystem variables, such as productive nature, C cycling rate, C inputs, and water quality, sets the foundation for interpreting commonly used optical property DOM metrics such as fluorescence index (FI), humification index (HIX), and specific ultraviolet absorbance at 254 nm (SUVA254). Case studies and a meta-analysis of each biosphere and subdivision found substantial overlap and characteristic distributions corresponding to ecosystem context for FI, HIX, and SUVA254, signifying chromophores and fluorophores from different ecosystems may be more similar than originally thought. This review challenges researchers to consider ecosystem connectivity when applying optical property results rather than making traditional "if this, then that" results-style conclusions.

A History of Molecular Level Analysis of Natural Organic Matter by FTICR Mass Spectrometry and The Paradigm Shift in Organic Geochemistry.

Natural organic matter (NOM) is a complex mixture of biogenic molecules resulting from the deposition and transformation of plant and animal matter. It has long been recognized that NOM plays an important role in many geological, geochemical, and environmental processes. Of particular concern is the fate of NOM in response to a warming climate in environments that have historically sequestered carbon (e.g., peatlands and swamps) but may transition to net carbon emitters. In this review, we will highlight developments in the application of high-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) in identifying the individual components of complex NOM mixtures, focusing primarily on the fraction that is dissolved in natural waters (dissolved organic matter or DOM). We will first provide some historical perspective on developments in FTICR technology that made molecular-level characterizations of DOM possible. A variety of applications of the technique will then be described, followed by our view of the future of high-field FTICR MS in carbon cycling research, including a particularly exciting metabolomic approach.

Subsurface In Situ Detection of Microbes and Diverse Organic Matter Hotspots in the Greenland Ice Sheet.

We used a deep-ultraviolet fluorescence mapping spectrometer, coupled to a drill system, to scan from the surface to 105 m depth into the Greenland ice sheet. The scan included firn and glacial ice and demonstrated that the instrument is able to determine small (mm) and large (cm) scale regions of organic matter concentration and discriminate spectral types of organic matter at high resolution. Both a linear point cloud scanning mode and a raster mapping mode were used to detect and localize microbial and organic matter "hotspots" embedded in the ice. Our instrument revealed diverse spectral signatures. Most hotspots were <20 mm in diameter, clearly isolated from other hotspots, and distributed stochastically; there was no evidence of layering in the ice at the fine scales examined (100 µm per pixel). The spectral signatures were consistent with organic matter fluorescence from microbes, lignins, fused-ring aromatic molecules, including polycyclic aromatic hydrocarbons, and biologically derived materials such as fulvic acids. In situ detection of organic matter hotspots in ice prevents loss of spatial information and signal dilution when compared with traditional bulk analysis of ice core meltwaters. Our methodology could be useful for detecting microbial and organic hotspots in terrestrial icy environments and on future missions to the Ocean Worlds of our Solar System.

Biological Ice-Nucleating Particles Deposited Year-Round in Subtropical Precipitation.

Airborne bacteria that nucleate ice at relatively warm temperatures (>-10°C) can interact with cloud water droplets, affecting the formation of ice in clouds and the residency time of the cells in the atmosphere. We sampled 65 precipitation events in southeastern Louisiana over 2 years to examine the effect of season, meteorological conditions, storm type, and ecoregion source on the concentration and type of ice-nucleating particles (INPs) deposited. INPs sensitive to heat treatment were inferred to be biological in origin, and the highest concentrations of biological INPs (∼16,000 INPs liter-1 active at ≥-10°C) were observed in snow and sleet samples from wintertime nimbostratus clouds with cloud top temperatures as warm as -7°C. Statistical analysis revealed three temperature classes of biological INPs (INPs active from -5 to -10°C, -11 to -12°C, and -13 to -14°C) and one temperature class of INPs that were sensitive to lysozyme (i.e., bacterial INPs, active from -5 to -10°C). Significant correlations between the INP data and abundances of taxa in the Bacteroidetes, Firmicutes, and unclassified bacterial divisions implied that certain members of these phyla may possess the ice nucleation phenotype. The interrelation between the INP classes and fluorescent dissolved organic matter, major ion concentrations (Na+, Cl-, SO42-, and NO3-), and backward air mass trajectories indicated that the highest concentrations of INPs were sourced from high-latitude North American and Asian continental environments, whereas the lowest values were observed when air was sourced from marine ecoregions. The intra- and extracontinental regions identified as sources of biological INPs in precipitation deposited in the southeastern United States suggests that these bioaerosols can disperse and affect meteorological conditions thousands of kilometers from their terrestrial points of origin.IMPORTANCE The particles most effective at inducing the freezing of water in the atmosphere are microbiological in origin; however, information on the species harboring this phenotype, their environmental distribution, and ecological sources are very limited. Analysis of precipitation collected over 2 years in Louisiana showed that INPs active at the warmest temperatures were sourced from terrestrial ecosystems and displayed behaviors that implicated specific bacterial taxa as the source of the ice nucleation activity. The abundance of biological INPs was highest in precipitation from winter storms and implied that their in-cloud concentrations were sufficient to affect the formation of ice and precipitation in nimbostratus clouds.

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

RATIONALE: Determining the chemical constituents of natural organic matter (NOM) by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICRMS) remains the ultimate measure for probing its source material, evolution, and transport; however, lability and the fate of organic matter (OM) in the environment remain controversial. FTICRMS-derived elemental compositions are presented in this study to validate a new interpretative method to determine the extent of NOM lability from various environments. METHODS: FTICRMS data collected over the last decade from the same 9.4 tesla instrument using negative electrospray ionization at the National High Magnetic Field Laboratory in Tallahassee, Florida, was used to validate the application of a NOM lability index. Solid-phase extraction cartridges were used to isolate the NOM prior to FTICRMS; mass spectral peaks were calibrated internally by commonly identified NOM homologous series, and molecular formulae were determined for NOM composition and lability analysis. RESULTS: A molecular lability boundary (MLB) was developed from the FTICRMS molecular data, visualized from van Krevelen diagrams, dividing the data into more and less labile constituents. NOM constituents above the MLB at H/C ≥1.5 correspond to more labile material, whereas NOM constituents below the MLB, H/C <1.5, exhibit less labile, more recalcitrant character. Of all marine, freshwater, and glacial environments considered for this study, glacial ecosystems were calculated to contain the most labile OM. CONCLUSIONS: The MLB extends our interpretation of FTICRMS NOM molecular data to include a metric of lability, and generally ranked the OM environments from most to least labile as glacial > marine > freshwater. Applying the MLB is useful not only for individual NOM FTICRMS studies, but also provides a lability threshold to compare and contrast molecular data with other FTICRMS instruments that survey NOM from around the world.

Comprehensive characterization of marine dissolved organic matter by Fourier transform ion cyclotron resonance mass spectrometry with electrospray and atmospheric pressure photoionization.

We compare the ultrahigh resolution 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectra of marine dissolved organic matter (DOM) isolated from two sites in the Weddell Sea (Antarctica) obtained by complementary electrospray ionization (ESI) and atmospheric pressure photoionization (APPI). Ions produced by APPI extend to higher carbon unsaturation than those produced by ESI, indicated by higher double-bond equivalents (rings plus double bonds) minus oxygen (DBE-O) values, whereas ESI-generated ions are more oxygenated. Moreover, many sulfur-containing compounds were efficiently ionized by ESI but not detected by APPI. Because the mass spectra obtained by ESI and APPI are significantly different, both are necessary to obtain a more complete description of the molecular composition of marine DOM.

Photochemically induced changes in dissolved organic matter identified by ultrahigh resolution fourier transform ion cyclotron resonance mass spectrometry.

Sunlight-induced molecular changes have been observed in two samples of dissolved organic matter (DOM) collected in the Cape Fear River system, North Carolina, USA. The molecular composition of a water sample collected in the Black River (sample B210, salinity 0) and another water sample collected within the Cape Fear River estuary (sample M61, salinity 13.7) were analyzed using an ultrahigh resolution 9.4 Tesla (T) electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer. Additionally, the Ultraviolet/Visible (UV/vis) absorbance as well as the excitation emission matrix (EEM) fluorescence spectra were determined to identify changes in the optical properties associated with photochemical reactions of the chromophoric DOM (CDOM). The molecular formulas forthe Cape Fear River Estuary (M61) sample before the irradiation experiments indicated the presence of highly aromatic compounds which were not present in the unirradiated Black River sample (B210). These aromatic compounds, with oxygen-subtracted double bond equivalents (DBE-O) values greater than nine, are more photoreactive and readily photodegraded relative to saturated compounds. Compounds with DBE-O values below nine are less photoreactive. The UV/vis absorbance and EEM fluorescence results supported this different photodegradation behavior, suggesting that the photoreactivity of CDOM is highly dependent on the molecular composition of the CDOM.