Rose M. Cory

Spatial and temporal variations in DOM composition in ecosystems: The importance of long‐term monitoring of optical properties

[1] Source, transformation, and preservation mechanisms of dissolved organic matter (DOM) remain elemental questions in contemporary marine and aquatic sciences and represent a missing link in models of global elemental cycles. Although the chemical character of DOM is central to its fate in the global carbon cycle, DOM characterizations in long-term ecological research programs are rarely performed. We analyzed the variability in the quality of 134 DOM samples collected from 12 Long Term Ecological Research stations by quantification of organic carbon and nitrogen concentration in addition to analysis of UV-visible absorbance and fluorescence spectra. The fluorescence spectra were further characterized by parallel factor analysis. There was a large range in both concentration and quality of the DOM, with the dissolved organic carbon (DOC) concentration ranging from less than 1 mgC/L to over 30 mgC/L. The ranges of specific UV absorbance and fluorescence parameters suggested significant variations in DOM composition within a specific study area, on both spatial and temporal scales. There was no correlation between DOC concentration and any DOM quality parameter, illustrating that comparing across biomes, large variations in DOM quality are not necessarily associated with corresponding large ranges in DOC concentrations. The data presented here emphasize that optical properties of DOM can be highly variable and controlled by different physical (e.g., hydrology), chemical (e.g., photoreactivity/redox conditions), and biological (e.g., primary productivity) processes, and as such can have important ecological consequences. This study demonstrates that relatively simple DOM absorbance and/or fluorescence measurements can be incorporated into long-term ecological research and monitoring programs, resulting in advanced understanding of organic matter dynamics in aquatic ecosystems.

Sunlight controls water column processing of carbon in arctic fresh waters

Illuminating the pathway to destruction Arctic lakes are an important source of atmospheric CO2 and therefore play a role in climate change. It is thus vital to know how the rapid Arctic warming will affect them. Cory et al. now show that light is the biggest culprit in the breakdown of carbon from thawing permafrost soils (see the Perspective by Tranvik). This carbon then moves out into Arctic lakes and streams. Contrary to previous expectations, these photochemical processes cause much more destruction of the organic molecules in fresh water than bacterial respiration does. Science, this issue p. 925; see also p. 870 Photochemical reactions cause most of the breakdown of carbon released from permafrost into arctic inland waters. [Also see Perspective by Tranvik] Carbon in thawing permafrost soils may have global impacts on climate change; however, the factors that control its processing and fate are poorly understood. The dominant fate of dissolved organic carbon (DOC) released from soils to inland waters is either complete oxidation to CO2 or partial oxidation and river export to oceans. Although both processes are most often attributed to bacterial respiration, we found that photochemical oxidation exceeds rates of respiration and accounts for 70 to 95% of total DOC processed in the water column of arctic lakes and rivers. At the basin scale, photochemical processing of DOC is about one-third of the total CO2 released from surface waters and is thus an important component of the arctic carbon budget.

Surface exposure to sunlight stimulates CO2 release from permafrost soil carbon in the Arctic

Recent climate change has increased arctic soil temperatures and thawed large areas of permafrost, allowing for microbial respiration of previously frozen C. Furthermore, soil destabilization from melting ice has caused an increase in thermokarst failures that expose buried C and release dissolved organic C (DOC) to surface waters. Once exposed, the fate of this C is unknown but will depend on its reactivity to sunlight and microbial attack, and the light available at the surface. In this study we manipulated water released from areas of thermokarst activity to show that newly exposed DOC is >40% more susceptible to microbial conversion to CO2 when exposed to UV light than when kept dark. When integrated over the water column of receiving rivers, this susceptibility translates to the light-stimulated bacterial activity being on average from 11% to 40% of the total areal activity in turbid versus DOC-colored rivers, respectively. The range of DOC lability to microbes seems to depend on prior light exposure, implying that sunlight may act as an amplification factor in the conversion of frozen C stores to C gases in the atmosphere.

Biotic and abiotic interactions in aquatic microcosms determine fate and toxicity of Ag nanoparticles: part 2-toxicity and Ag speciation.

To study the effects of complex environmental media on silver nanoparticle (AgNP) toxicity, AgNPs were added to microcosms with freshwater sediments and two species of aquatic plants (Potamogeton diversifolius and Egeria densa), followed by toxicity testing with microcosm surface water. Microcosms were designed with four environmental matrices in order to determine the contribution of each environmental compartment to changes in toxicity: water only (W), water + sediment (WS), water + plants (WP), and water + plants + sediment (WPS). Silver treatments included AgNPs with two different coatings, gum arabic (GA-AgNPs) or polyvinylpyrollidone (PVP-AgNPs), as well as AgNO(3). Water samples taken from the microcosms at 24 h postdosing were used in acute toxicity tests with two standard model organisms, early life stage zebrafish (Danio rerio) and Daphnia magna. Speciation of Ag in these samples was analyzed using Ag L3-edge X-ray absorption near edge spectroscopy (XANES). Silver speciation patterns for the nanoparticle treatments varied significantly by coating type. While PVP-AgNPs were quite stable and resisted transformation across all matrices (>92.4% Ag(0)), GA-AgNP speciation patterns suggest significantly higher transformation rates, especially in treatments with plants (<69.2% and <58.8% Ag(0) in WP and WPS, respectively) and moderately increased transformation with sediments (<85.6% Ag(0)). Additionally, the presence of plants in the microcosms (with and without sediments) reduced both the concentration of Ag in the water column and toxicity for all Ag treatments. Reductions in toxicity may have been related to decreased water column concentrations as well as changes in the surface chemistry of the particles induced by organic substances released from the plants.

Singlet oxygen in the coupled photochemical and biochemical oxidation of dissolved organic matter.

Dissolved organic matter (DOM) is a significant (>700 Pg) global C pool. Transport of terrestrial DOM to the inland waters and coastal zones represents the largest flux of reduced C from land to water (215 Tg yr(-1)) (Meybeck, M. Am. J. Sci. 1983, 282, 401-450). Oxidation of DOM by interdependent photochemical and biochemical processes largely controls the fate of DOM entering surface waters. Reactive oxygen species (ROS) have been hypothesized to play a significant role in the photooxidation of DOM, because they may oxidize the fraction of DOM that is inaccessible to direct photochemical degradation by sunlight. We followed the effects of photochemically produced singlet oxygen ((1)O(2)) on DOM by mass spectrometry with (18)O-labeled oxygen, to understand how (1)O(2)-mediated transformations of DOM may lead to altered DOM bioavailability. The photochemical oxygen uptake by DOM attributed to (1)O(2) increased with DOM concentration, yet it remained a minority contributor to photochemical oxygen uptake even at very high DOM concentrations. When DOM samples were exposed to (1)O(2)-generating conditions (Rose Bengal and visible light), increases were observed in DOM constituents with higher oxygen content and release of H(2)O(2) was detected. Differential effects of H(2)O(2) and (1)O(2)-treated DOM showed that (1)O(2)-treated DOM led to slower bacterial growth rates relative to unmodified DOM. Results of this study suggested that the net effect of the reactions between singlet oxygen and DOM may be production of partially oxidized substrates with correspondingly lower potential biological energy yield.

Changes in fulvic acid redox state through the oxycline of a permanently ice-covered Antarctic lake

Abstract.The McMurdo Dry Valleys of Antarctica contain many permanently ice-covered lakes that support populations of algae and bacteria in the water column. In these lakes the concentration of dissolved organic carbon (DOC) is typically greatest at depth. In Lake Fryxell, the DOC concentration is 25 mg C/L at 18 m and 5 mg C/L at 5 m, just below the ice-cover. Dissolved humic substances account for about 20–24% of the DOC in the lake water. The DOC sources to the photic zone of this lake are streamflow, extracellular release by phytoplankton and benthic algal mats and upward diffusion across the oxycline at 9.5 m. Experiments with fulvic acids isolated from four depths show that these humic substances have the capacity to act as electron acceptors in the anoxic degradation of acetate by an iron-and humic-reducing microorganism. We used fluorescence spectroscopy to characterize the abundance and redox state of the quinone functional groups in the oxidized and reduced fulvic acids in these experiments and in filtered whole water samples from the lake. The fluorescence intensity of fulvic acid was greater in the oxycline and bottomwaters than in the photic zone. This result suggests that incorporation of quinone functional groups into humic substances may be enhanced in zones of high bacterial activity. Statistical analysis of the excitation emission matrices (EEMs) was used to evaluate trends in the fulvic acid redox state with depth. The results indicate that fulvic acid in the upper photic zone was in an oxidized state and that fulvic acid in the bottomwaters was in a reduced state. The shift in the EEMs indicating a more reduced state occurred in the vicinity of the oxycline (8 to 11 m). The shift in the EEMs began in the zone from 8 and 9 m, where dissolved oxygen concentrations range from to 5 and 10 mg L–1, suggesting that fulvic acid was oxidized upon upward diffusion from the oxycline. This oxidation may be an abiotic process in which reduced humic substances interact with ferric iron generated in this zone.

A coupled geochemical and biogeochemical approach to characterize the bioreactivity of dissolved organic matter from a headwater stream

The bioreactivity or susceptibility of dissolved organic matter (DOM) to microbial degradation in streams and rivers is of critical importance to global change studies, but a comprehensive understanding of DOM bioreactivity has been elusive due, in part, to the stunningly diverse assemblages of organic molecules within DOM. We approach this problem by employing a range of techniques to characterize DOM as it flows through biofilm reactors: dissolved organic carbon (DOC) concentrations, excitation emission matrix spectroscopy (EEMs), and ultrahigh resolution mass spectrometry. The EEMs and mass spectral data were analyzed using a combination of multivariate statistical approaches. We found that 45% of stream water DOC was biodegraded by microorganisms, including 31–45% of the humic DOC. This bioreactive DOM separated into two different groups: (1) H/C centered at 1.5 with O/C 0.1–0.5 or (2) low H/C of 0.5–1.0 spanning O/C 0.2–0.7 that were positively correlated (Spearman ranking) with chromophoric and fluorescent DOM (CDOM and FDOM, respectively). DOM that was more recalcitrant and resistant to microbial degradation aligned tightly in the center of the van Krevelen space (H/C 1.0–1.5, O/C 0.25–0.6) and negatively correlated (Spearman ranking) with CDOM and FDOM. These findings were supported further by principal component analysis and 2-D correlation analysis of the relative magnitudes of the mass spectral peaks assigned to molecular formulas. This study demonstrates that our approach of processing stream water through bioreactors followed by EEMs and FTICR-MS analyses, in combination with multivariate statistical analysis, allows for precise, robust characterization of compound bioreactivity and associated molecular level composition.

Spatial and temporal distribution of singlet oxygen in Lake Superior.

A multiyear field study was undertaken on Lake Superior to investigate singlet oxygen ((1)O(2)) photoproduction. Specifically, trends within the lake were examined, along with an assessment of whether correlations existed between chromophoric dissolved organic matter (CDOM) characteristics and (1)O(2) production rates and quantum yields. Quantum yield values were determined and used to estimate noontime surface (1)O(2) steady-state concentrations ([(1)O(2)](ss)). Samples were subdivided into three categories based on their absorbance properties (a300): riverine, river-impacted, or open lake sites. Using calculated surface [(1)O(2)](ss), photochemical half-lives under continuous summer sunlight were calculated for cimetidine, a pharmaceutical whose reaction with (1)O(2) has been established, to be on the order of hours, days, and a week for the riverine, river-impacted, and open lake waters, respectively. Of the CDOM properties investigated, it was found that dissolved organic carbon (DOC) and a300 were the best parameters for predicting production rates of [(1)O(2)](ss). For example, given the correlations found, one could predict [(1)O(2)](ss) within a factor of 4 using a300 alone. Changes in the quantum efficiency of (1)O(2) production upon dilution of river water samples with lake water samples demonstrated that the CDOM found in the open lake is not simply diluted riverine organic matter. The open lake pool was characterized by low absorption coefficient, low fluorescence, and low DOC, but more highly efficient (1)O(2) production and predominates the Lake Superior system spatially. This study establishes that parameters that reflect the quantity of CDOM (e.g., a300 and DOC) correlate with (1)O(2) production rates, while parameters that characterize the absorbance spectrum (e.g., spectral slope coefficient and E2:E3) correlate with (1)O(2) production quantum yields.

Low molecular weight components in an aquatic humic substance as characterized by membrane dialysis and Orbitrap mass spectrometry

Suwannee River fulvic acid (SRFA) was dialyzed through a 100-500 molecular weight cutoff dialysis membrane, and the dialysate and retentate were analyzed by UV-visible absorption and high-resolution Orbitrap mass spectrometry (MS). A significant fraction (36% based on dissolved organic carbon) of SRFA passed through the dialysis membrane. The fraction of SRFA in the dialysate had a different UV-visible absorption spectrum and was enriched in low molecular weight molecules with a more aliphatic composition relative to the initial SRFA solution. Comparison of the SRFA spectra collected by Orbitrap MS and Fourier transform ion cyclotron resonance MS (FT-ICR MS) demonstrated that the mass accuracy of the Orbitrap MS is sufficient for determination of unique molecular formulas of compounds with masses <600 Da in a complex mixture, such as SRFA. The most intense masses detected by Orbitrap MS were found in the 100-200 Da mass range. Many of these low molecular masses corresponded to molecular formulas of previously identified compounds in organic matter, lignin, and plants, and the use of the standard addition method provided an upper concentration estimate of selected target compounds in SRFA. Collectively, these results provide evidence that SRFA contains low molecular weight components that are present individually or in loosely bound assemblies.

Chemical characterization of DOM in channels of a seasonal wetland

Abstract.Although wetlands are known to be important sources of dissolved organic matter (DOM) within watersheds, production of DOM within wetlands is not well understood. In the Okavango Delta, a large wetland located in Botswana, large amounts of DOM produced in the wetland are transported in the river network and to the subsurface. The purpose of this study was to gain insight into environmental processing of DOM in wetland surface waters by examining chemical characteristics of plant litter leachates and fulvic acids isolated from two surface water sites in the Panhandle (PHFA) and Seasonal Swamp (SSFA) of the Okavango Delta. Spectroscopic properties measured over the course of leaching experiments indicated a greater abundance of plant-derived DOM over time. Results of elemental and 13CNMR analyses showed that aromaticities and C:N ratios of PHFA and SSFA and a Cyperus papyrus leachate fulvic acid (CPLFA) were in the range typical for fulvic acids derived from vascular plants. Fluorescence analyses of fulvic acids using parallel factor analysis (PARAFAC) further indicated the importance of plant litter sources in surface water DOM. Environmental processing of DOM in downstream surface waters by bacterial and photodegradation was suggested by higher N and S content for SSFA compared to CPLFA and by differences in δ15N, δ34S, δ13C and fluorescence signatures among the 3 fulvic acid samples. These chemical characterization results suggest that a progressive enrichment of DOM by plant-derived material occurs with distance downstream and that this DOM undergoes some environmental processing within the surface water system.