Ratio Of Dissolved Organic Matter Research Articles

Anthropogenically Driven Changes in the Carbon to Phosphorus Ratio of Marine Dissolved Organic Matter

AbstractMarine dissolved organic matter (DOM) cycles play a pivotal role in sustaining marine ecosystems and regulating the ocean's carbon sequestration from the atmosphere. However, the response of DOM cycles, including dissolved organic carbon (DOC) and dissolved organic phosphorus (DOP), to future climate change remains highly uncertain. Using the Community Earth System Model version 2 large ensemble simulations, we find that the C:P ratios in DOM are projected to increase by up to two‐fold in oligotrophic gyres by 2100. Increased upper ocean stratification reduces surface phosphate availability, thereby elevating phytoplankton C:P ratios and enhancing phytoplankton utilization of DOP, both acting to deprive DOM of P. Moreover, ocean stratification has a direct effect on exporting less DOC to the subsurface while accumulating more DOC at the sea surface. As a result of the strong sensitivity to ocean surface warming, the anthropogenically driven trends in upper ocean DOM concentration and its C:P ratios are estimated to emerge earlier from the simulated natural variability than upper ocean phosphate concentrations and net primary production—two key biogeochemical variables that are frequently monitored. This study suggests that changes in the C:P ratios of DOM could serve as a sensitive fingerprint of anthropogenic ocean warming, potentially exerting broad impacts on marine microbes. Our estimated 4% reduction in the globally integrated DOC export below 100 m is comparable to a 2% reduction in particulate organic carbon (POC) export by 2100, implying that global warming is likely to weaken the biological carbon pump through both DOC and POC.

Global patterns and drivers of dissolved organic matter across Earth systems: insights from H/C and O/C ratios

Dissolved organic matter (DOM) is ubiquitous and contains a complex pool of thousands of distinct molecules, and their chemical characteristics help us inform the fate of global carbon. However, a more holistic perspective of the molecular characteristics of DOM and underlying mechanisms across Earth systems and climates remains under study. Here, we present a comprehensive analysis of the molecular characteristics of DOM using two abundance-weighted average indices, i.e., H/C and O/C ratios, by compiling 2,995 samples from 317 studies covering waters, land, plant, petroleum, and atmosphere systems and climatic regions from the tropics to tundra. H/C ratios are lower on average in waters (H/C = 1.15 ± 0.005) and land (H/C = 1.20 ± 0.010) than in the other systems, while their O/C ratios rank between plant and atmosphere systems. In the waters and land systems, the H/C ratios of DOM vary from the highest to the lowest in the habitats of the land-to-ocean continuum generally as snow > glacier > marine ≥ freshwater/soil > groundwater. The H/C ratios show predictably U-shaped patterns along latitudinal gradients, indicating the lowest abundance of more hydrogen-saturated molecules at mid-latitudes of approximately 40°-50° in river water, lake water, and forest soil. The two ratios are primarily controlled by environmental factors such as pH, dissolved oxygen, and carbon and nitrogen contents. We further unveil additional and considerable links between the ratios and the extremes of climatic factors such as precipitation of warmest quarter and maximum temperature of warmest month. Our synthesis provides molecular-level perspectives to characterize the global distribution and underlying drivers of DOM, which is complementary for our understanding of global carbon cycle processes under future global change.

Effects of organic sulfur and arsenite/dissolved organic matter ratios on arsenite complexation with dissolved organic matter.

The speciation and fate of arsenic (As) in soil-water systems is a topic of great interest, in part due to growing awareness of As uptake into rice as an important human exposure pathway to As. Rice paddy and other wetland soils are rich in dissolved organic matter (DOM), leading to As/DOM ratios that are typically lower than those in groundwater aquifers or that have been used in many laboratory studies of As-DOM interactions. In this contribution, we evaluate arsenite (As(III)) binding to seven different DOM samples at As/DOM ratios relevant for wetland pore waters, and explore the chemical properties of the DOM samples associated with high levels of As(III)-DOM complexation. We integrate data from wet chemical analysis of DOM chemical properties, dialysis equilibrium experiments, and two-site ligand binding models to show that in some DOM samples, 15-60% of As(III) can be bound to DOM at environmentally-relevant As/DOM ratios of 0.0032-0.016μmol As/mmol C. Binding decreases as the As(III)/DOM ratio increases. The organic sulfur (Sorg) content of the DOM samples was strongly correlated with levels of As(III)-DOM complexation and "strong" binding site densities, consistent with theories that thiols are strong binding ligands for As(III) in natural organic matter. Finally, a whole-cell E. coli biosensor assay was used to show that DOM samples most effective at complexing As(III) also led to decreased microbial As(III) uptake at low As/DOC ratios. This work demonstrates that naturally-occurring variations in the Sorg content of DOM has a significant impact on As(III) binding to DOM, and has implications for As(III) availability to microorganisms.

Factors Controlling the Spatial Distribution of Dissolved Organic Matter With Changes in the C/N Ratio From the Upper to Lower Reaches of the Ishikari River, Japan

Dissolved organic matter (DOM), particularly dissolved organic nitrogen (DON), is an important source of energy and/or organic nutrients for heterotrophic microorganisms in rivers. Although various factors controlling the quantity and quality of stream and riverine DOM have been extensively studied, DON has been under-researched compared to dissolved organic carbon (DOC). The spatial distribution of DOC and DON concentrations with respect to the C/N ratio and DOM optical properties was investigated in the Ishikari River and its tributaries in Hokkaido, northern Japan. Here, the upper reaches are forested and the middle and lower reaches are encompassed by agricultural land, in particular paddy fields. Furthermore, dark incubation experiments were conducted using filtered riverine water (<0.7 µm) to determine the bioavailability of DOC and DON (particularly due to small microorganisms) considered as a factor controlling the spatial distribution. In the mainstream, DOC and DON concentrations increased with river flow in the upper and middle reaches and remained unchanged in the lower reaches. The C/N ratio of bulk DOM decreased continuously from the upper reaches to lower reaches. The optical properties exhibited changes in the DOM characteristics in terms of higher molecular weight and higher aromaticity from the upper to middle reaches, suggesting that flooded paddy fields largely altered the riverine DOM concentration and composition. In the lower reaches, the C/N ratio of the bulk DOM decreased with the river flow. However, according to principal component analysis, no changes were observed in the optical properties with river flow, suggesting that the C/N ratio of bulk DOM changed owing to in situ biological activity in the river. DOC biodegradation was observed at four sites in the upper and middle reaches but not at the two sites in the lower reaches. However, the DON concentration during the dark incubation experiments at all sites did not differ significantly, which implies that microbial degradation, particularly by small microorganisms, is a factor that decreased the C/N ratio of bulk DOM in the upper and middle reaches. In contrast, large microorganisms possibly degraded C-rich DOM to decrease the C/N ratio of bulk DOM in the lower reaches of the Ishikari River.

Variation in the content and fluorescent composition of dissolved organic matter in soil water during rainfall-induced wetting and extract of dried soil

Dissolved organic matter (DOM) is present in all soils, providing a readily available carbon source for microorganisms, which influences microbially mediated biogeochemical processes. Rainfall-induced wetting can alter the content and composition of soil DOM. However, conventional methods commonly used to extract DOM from soils involve air- or oven-drying followed by extraction with water, and the results vary considerably in terms of indexes used. Therefore, this study aimed to determine the variation in DOM content and composition of soil gravity water and capillary water during wetting, and establish a better method to obtain real soil DOM information. Following simulated rainfall (50 mm h−1, 0–24 h), gravity water and capillary water in fresh soil samples were separated using a high-speed refrigerated centrifuge. Additionally, DOM in dried soil samples was extracted using various soil/water ratios after drying by different methods. The DOM data obtained by conventional methods were compared with capillary water data. The results showed that DOM degradation occurred mainly in capillary water close to the surface of soil particles. Among the six fluorescent components of DOM identified, a tryptophan-like component (Ex/Em = 295/335 nm and 230/335 nm) was possibly derived from terrestrial plants, and a tyrosine-like component (Ex/Em = 265/305 nm) was likely derived from microbial secretion. Except for little variation in the fluorescence index, dissolved organic carbon concentrations in capillary water were double those in dried soil extracted by conventional methods. The humification index and spectral slope ratio of DOM extracted by conventional methods also markedly varied, and no clear patterns were observed for the variation in specific UV absorbance at 254 nm. These findings allow real information to be obtained regarding soil DOM during wetting, and better selection of the extraction method and indexes when studying soil DOM.

Nickel Binding Affinity with Size-Fractioned Sediment Dissolved and Particulate Organic Matter and Correlation with Optical Indicators

In rivers, the distribution and reactivity of heavy metals (HMs) are affected by their binding affinity with sediment dissolved organic matter (DOM) and particulate organic matter (POM). The HM-OM binding affinity affected by the interaction between DOM and POM is not well studied. This study investigated the Ni binding affinity to size-fractioned overlaying water DOM and alkaline extracted sediment POM solution (AEOM). The DOM/AEOM filtrates (<0.45 μm) were sequentially separated into five nominal molecular weight (MW) solutions. The AEOM optical indicators had lower autochthonous, higher terrestrial sources, and lower aromaticity than the DOM. The Ni mass (72.3 ± 6.4%) was primarily distributed in the low molecular weight DOM (<1 kDa), whereas the Ni (93.5 ± 0.4%) and organic carbon (OC) mass (85.3 ± 1.0%) were predominantly distributed in the high molecular weight AEOM. The Ni and DOM binding affinity, ([Ni]/[DOC])DOM ratio ranging from 0.76 to 27.32 μmol/g-C, was significantly higher than the ([Ni]/[DOC])AEOM ratios, which ranged from 0.64 to 2.64 μmol/g-C. The ([Ni]/[DOC])AEOM ratio correlated significantly with the selected optical indicators (r = 0.87–0.92, p < 0.001), but the ([Ni]/[DOC])DOM ratio correlated weakly with the optical indicators (r = 0.13–0.40, p > 0.05). In the present study, the Ni binding affinity with size-fractioned DOM/AEOM agrees with the hypothesis of the DOM and POM exchange conceptual model in sediment. The POM underwent a hydrolysis/oxidation process; hence, AEOM had a high molecular weight and stable chemical composition and structure. The Ni mainly attached to the high molecular weight AEOM and the ([Ni]/[DOC])AEOM ratios had a strong correlation with the AEOM optical indicators. In contrast, DOM had a high ([Ni]/[DOC])DOM ratio in low molecular weight DOM.

Origin and fate of dissolved organic matter in four shallow Baltic Sea estuaries

Coastal waters have strong gradients in dissolved organic matter (DOM) quantity and characteristics, originating from terrestrial inputs and autochthonous production. Enclosed seas with high freshwater input therefore experience high DOM concentrations and gradients from freshwater sources to more saline waters. The brackish Baltic Sea experiences such salinity gradients from east to west and from river mouths to the open sea. Furthermore, the catchment areas of the Baltic Sea are very diverse and vary from sparsely populated northern areas to densely populated southern zones. Coastal systems vary from enclosed or open bays, estuaries, fjords, archipelagos and lagoons where the residence time of DOM at these sites varies and may control the extent to which organic matter is biologically, chemically or physically modified or simply diluted with transport off-shore. Data of DOM with simultaneous measurements of dissolved organic (DO) nitrogen (N), carbon (C) and phosphorus (P) across a range of contrasting coastal systems are scarce. Here we present data from the Roskilde Fjord, Vistula and Öre estuaries and Curonian Lagoon; four coastal systems with large differences in salinity, nutrient concentrations, freshwater inflow and catchment characteristics. The C:N:P ratios of DOM of our data, despite high variability, show site specific significant differences resulting largely from differences residence time. Microbial processes seemed to have minor effects, and only in spring did uptake of DON in the Vistula and Öre estuaries take place and not at the other sites or seasons. Resuspension from sediments impacts bottom waters and the entire shallow water column in the Curonian Lagoon. Finally, our data combined with published data show that land use in the catchments seems to impact the DOC:DON and DOC:DOP ratios of the tributaries most.

Effects of natural dissolved organic matter on the complexation and biodegradation of 17α-ethinylestradiol in freshwater lakes

Natural dissolved organic matter (DOM) produced in algal blooms and overgrowths of macrophyte changes the elimination and ecotoxicity of estrogens in freshwater lakes. The complexation of 17α-ethinylestradiol (EE2) and various DOMs, including the water- and sediment-derived DOMs from the algal-dominant zone in Lake Taihu (TW and TS, respectively) and the macrophyte-dominant zone in Poyang Lake (PW and PS, respectively), and the humic acid (HA), was investigated along with the subsequent effects on EE2 biodegradation. Dialysis equilibrium experiments showed that binding to DOM significantly decreased the freely soluble concentrations of EE2. The binding capacity of the five DOMs followed the order of PW < TW < PS ≈ TS < HA. A negative correlation was found between the organic-carbon-normalized sorption coefficient (logKDOC) and the absorption ratio (E2/E3) of DOM, indicating that the large sized, aromatic molecules were involved in the complexation. The reduced freely soluble concentrations of EE2 did not inhibit its biodegradation by an EE2-degrading strain, Rhodobacter blasticus. Conversely, the autochthonous-dominated water-derived DOMs stimulated a more extensive biodegradation of EE2 than the sediment-derived DOMs, and the existence of HA resulted in the smallest increase in EE2 biodegradation. The promoting effect was associated with the increased concentration, activity, and transforming rate of R. blasticus by the bioavailable components in DOM. The present study suggests that the significant impact of natural DOM should be fully considered when assessing the fate and ecological risks of estrogens in eutrophic waters.

The Sources and Transformations of Dissolved Organic Matter in the Pearl River Estuary, China, as Revealed by Stable Isotopes

AbstractTo investigate the sources and transformation processes of bulk marine dissolved organic matter (DOM), seasonally collected water samples from the Pearl River Estuary (PRE), south China, were analyzed for concentration and stable isotopic ratios (δ13C and δ15N) of DOM. The δ13C values clearly indicated a shift of dissolved organic carbon (DOC) sources from terrigenous material in the river to predominantly plankton in the lower estuary. The distribution of δ13C during the estuarine mixing, in combination with the notable deviation of DOC concentration, provided strong evidence for a substantial DOC removal in the salinity range of 5 to 22. In contrast to δ13C, the δ15N of DOM exhibits a strong seasonality, implying a clear decoupling of estuarine DOC and dissolved organic nitrogen (DON) dynamics. During dry periods with low river discharge, sewage‐derived DON contributed significantly to the riverine loading. While freshwater‐saltwater mixing is important in regulating the distribution of DON, the distribution of δ15N along the salinity gradient provided strong evidence for active sediment‐water interaction and adsorption/desorption processes at the middle to high salinities. During wet periods, severe soil loss and erosion, and/or in situ biological production, adsorption onto suspended particles dominate the riverine DON fluxes, whereas the DON dynamics might be governed by different biogeochemical processes between spring and summer, as reflected by distinct δ15DON patterns in the estuarine zone. In spring, highly degraded DON originating from soil experienced little further biological alternation during estuarine mixing, whereas there was a tight coupling of DON production and consumption in summer.

The precipitation, growth and stability of mercury sulfide nanoparticles formed in the presence of marine dissolved organic matter.

The methylation of mercury is known to depend on the chemical forms of mercury (Hg) present in the environment and the methylating bacterial activity. In sulfidic sediments, under conditions of supersaturation with respect to metacinnabar, recent research has shown that mercury precipitates as β-HgS(s) nanoparticles (β-HgS(s)nano). Few studies have examined the precipitation of β-HgS(s)nano in the presence of marine dissolved organic matter (DOM). In this work, we used dynamic light scattering (DLS) coupled with UV-Vis spectroscopy and transmission electron microscopy (TEM) to investigate the formation and fate of β-HgS(s)nano formed in association with marine DOM extracted from the east and west of Long Island Sound, and at the shelf break of the North Atlantic Ocean, as well as with low molecular weight thiols. We found that while the β-HgS(s)nano formed in the presence of oceanic DOM doubled in size after 5 weeks, those forming in solutions with coastal DOM did not grow over time. In addition, when the HgII : DOM ratio was varied, β-HgS(s)nano only rapidly aggregated at high ratios (>41 μmol HgII per mg C) where the concentration of thiol groups was determined to be substantially low relative to HgII. This suggests that functional groups other than thiols could be involved in the stabilization of β-HgS(s)nano. Furthermore, we showed that β-HgS(s)nano forming under anoxic conditions remained stable and could therefore persist in the environment sufficiently to impact the methylation potential. Exposure of β-HgS(s)nano to sunlit and oxic environments, however, caused rapid aggregation and sedimentation of the nanoparticles, suggesting that photo-induced changes or oxidation of organic matter adsorbed on the surface of β-HgS(s)nano affected their stability in surface waters.

Photochemical reactions between mercury (Hg) and dissolved organic matter decrease Hg bioavailability and methylation

Atmospheric deposition of mercury (Hg) to surface water is one of the dominant sources of Hg in aquatic environments and ultimately drives methylmercury (MeHg) toxin accumulation in fish. It is known that freshly deposited Hg is more readily methylated by microorganisms than aged or preexisting Hg; however the underlying mechanism of this process is unclear. We report that Hg bioavailability is decreased by photochemical reactions between Hg and dissolved organic matter (DOM) in water. Photo-irradiation of Hg-DOM complexes results in loss of Sn(II)-reducible (i.e. reactive) Hg and up to an 80% decrease in MeHg production by the methylating bacterium Geobacter sulfurreducens PCA. Loss of reactive Hg proceeded at a faster rate with a decrease in the Hg to DOM ratio and is attributed to the possible formation of mercury sulfide (HgS). These results suggest a new pathway of abiotic photochemical formation of HgS in surface water and provide a mechanism whereby freshly deposited Hg is readily methylated but, over time, progressively becomes less available for microbial uptake and methylation.

Potential effects of biochar on the availability of phosphorus — mechanistic insights

Potential supply shortages of phosphorus (P) rock reserves call for a more efficient use of P fertilizer and exploring new ways of recycling the nutrient within agro-ecosystems. Conditioning arable land with biochar might contribute to achieving both goals. We examined three putative mechanisms governing P availability in biochar amended soils. Potential direct P inputs from eight biochars differing in feedstocks and production techniques were assessed using sequential P fractionation. Phosphorus sorption on one pyrochar and one hydrochar was also studied. Finally, competitive sorption between P and biochar-derived dissolved organic matter (DOM) on goethite was studied at three pH levels. Total P varied from 0.6 to 4.0mg P g−1 in biochars derived from phytomass, and was 47mg P g−1 in a biochar derived from sewage sludge. In two slightly acidic hydrochars most P was 0.1M NaOH extractable, Fe-associated, while in the remaining alkaline pyrochars most P was 1M HCl extractable, Ca-associated. Biochar intrinsic P was mostly of intermediate stability and might slowly replenish labile P in soils. Both the pyrochar and the hydrochar studied were weak sorbents for phosphate. We show that DOM released from biochar, however, can inhibit P sorption on goethite depending on the initial pH, P to DOM ratio and order of sorbate addition. Strong competition between biochar-DOM and P for sorption sites on goethite especially at low pH values suggests that biochar-DOM might enhance P fertilizer use efficiency especially in acidic, highly weathered and strongly P fixing soils.

Characterization of bulk and chromophoric dissolved organic matter in the Laurentian Great Lakes during summer 2013

The Laurentian Great Lakes contain ~ 21% of the Earth's surface freshwater and are experiencing dramatic decadal-scale changes to their water quality, biogeochemistry and ecosystem functions.We report here the first data set of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (DOM), including UV–visible absorbance and derived optical properties and fluorescence excitation–emission matrices (EEMs), for open-lakesurface waters from each of the Great Lakes. Concentrations of DOC ranged from 86 to 240 μmol/L and the absorption coefficient at 254 nm (a254) varied from 2.67 to 10.47 m− 1 in Great Lakes waters.UV–visible and fluorescence characterization indicated that the DOM in open-lake waters in all the Great Lakes was primarily autochthonous. Both DOC and a254 increased from Lake Superior to Lakes Erie and Ontario where aromaticity, molecular weight and humification degree of DOM were also higher.These variations are consistent with both the water transport pathway and the trend of increasing human population and relative watershed drainage area along the Great Lakes. Parallel factor analysis on fluorescence-EEM data revealed three terrestrial humic-likeDOM components (C1, C2 and C4) and one protein-likeDOM component (C3). Open Lake Huron had the highest protein-like/humic-like DOM ratio (C3/C1), while sites in Lakes Erie and Ontario had lower C3/C1 ratios, indicating a greater proportional influence of terrestrial DOM. Changes in DOM composition along the water transport pathway in the Great Lakes indicate varying impacts from terrestrial inputs and human activities in the different lake systems.

Benthic fluxes of dissolved organic nitrogen in the lower St. Lawrence estuary and implications for selective organic matter degradation

Abstract. The distribution of dissolved organic nitrogen (DON) and carbon (DOC) in sediment porewaters was determined at nine locations along the St. Lawrence estuary and in the gulf of St. Lawrence. In a previous manuscript (Alkhatib et al., 2012a), we have shown that this study area is characterized by gradients in the sedimentary particulate organic matter (POM) reactivity, bottom water oxygen concentrations, and benthic respiration rates. Based on the porewater profiles, we estimated the benthic diffusive fluxes of DON and DOC in the same area. Our results show that DON fluxed out of the sediments at significant rates (110 to 430 μmol m−2 d−1). DON fluxes were positively correlated with sedimentary POM reactivity and varied inversely with sediment oxygen exposure time (OET), suggesting direct links between POM quality, aerobic remineralization and the release of DON to the water column. DON fluxes were on the order of 30 to 64% of the total benthic inorganic fixed N loss due to denitrification, and often exceeded the diffusive nitrate fluxes into the sediments. Hence they represented a large fraction of the total benthic N exchange, a result that is particularly important in light of the fact that DON fluxes are usually not accounted for in estuarine and coastal zone nutrient budgets. In contrast to DON, DOC fluxes out of the sediments did not show any significant spatial variation along the Laurentian Channel (LC) between the estuary and the gulf (2100 ± 100 μmol m−2 d−1). The molar C / N ratio of dissolved organic matter (DOM) in porewater and the overlying bottom water varied significantly along the transect, with lowest C / N in the lower estuary (5–6) and highest C / N (&gt; 10) in the gulf. Large differences between the C / N ratios of porewater DOM and POM are mainly attributed to a combination of selective POM hydrolysis and elemental fractionation during subsequent DOM mineralization, but selective adsorption of DOM to mineral phases could not be excluded as a potential C / N fractionating process. The extent of this C- versus N- element partitioning seems to be linked to POM reactivity and redox conditions in the sediment porewaters. Our results thus highlight the variable effects selective organic matter (OM) preservation can have on bulk sedimentary C / N ratios, decoupling the primary source C / N signatures from those in sedimentary paleoenvironmental archives. Our study further underscores that the role of estuarine sediments as efficient sinks of bioavailable nitrogen is strongly influenced by the release of DON during early diagenetic reactions, and that DON fluxes from continental margin sediments represent an important internal source of N to the ocean.

Copper(II) Binding by Dissolved Organic Matter: Importance of the Copper-to-Dissolved Organic Matter Ratio and Implications for the Biotic Ligand Model

The ratio of copper to dissolved organic matter (DOM) is known to affect the strength of copper binding by DOM, but previous methods to determine the Cu(2+)-DOM binding strength have generally not measured binding constants over the same Cu:DOM ratios. In this study, we used a competitive ligand exchange-solid-phase extraction (CLE-SPE) method to determine conditional stability constants for Cu(2+)-DOM binding at pH 6.6 and 0.01 M ionic strength over a range of Cu:DOM ratios that bridge the detection windows of copper-ion-selective electrode and voltammetry measurements. As the Cu:DOM ratio increased from 0.0005 to 0.1 mg of Cu/mg of DOM, the measured conditional binding constant ((c)K(CuDOM)) decreased from 10(11.5) to 10(5.6) M(-1). A comparison of the binding constants measured by CLE-SPE with those measured by copper-ion-selective electrode and voltammetry demonstrates that the Cu:DOM ratio is an important factor controlling Cu(2+)-DOM binding strength even for DOM isolates of different types and different sources and for whole water samples. The results were modeled with Visual MINTEQ and compared to results from the biotic ligand model (BLM). The BLM was found to over-estimate Cu(2+) at low total copper concentrations and under-estimate Cu(2+) at high total copper concentrations.

Photoproduction of ammonium in the southeastern Beaufort Sea and its biogeochemical implications

Abstract. Photochemistry of dissolved organic matter (DOM) plays an important role in marine biogeochemical cycles, including the regeneration of inorganic nutrients. DOM photochemistry affects nitrogen cycling by converting bio-refractory dissolved organic nitrogen to labile inorganic nitrogen, mainly ammonium (NH4+). During the August 2009 Mackenzie Light and Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of NH4+ photoproduction (i.e. photoammonification) were determined using water samples from the SE Beaufort Sea, including the Mackenzie River estuary, shelf, and Canada Basin. The photoammonification efficiency decreased with increasing wavelength across the ultraviolet and visible regimes and was higher in offshore waters than in shelf and estuarine waters. The efficiency was positively correlated with the molar nitrogen:carbon ratio of DOM and negatively correlated with the absorption coefficient of chromophoric DOM (CDOM). Combined with collateral measurements of CO2 and CO photoproduction, this study revealed a stoichiometry of DOM photochemistry with a CO2 : CO : NH4+ molar ratio of 165 : 11 : 1 in the estuary, 60 : 3 : 1 on the shelf, and 18 : 2 : 1 in the Canada Basin. The NH4+ efficiency spectra, along with solar photon fluxes, CDOM absorption coefficients and sea ice concentrations, were used to model the monthly surface and depth-integrated photoammonification rates in 2009. The summertime (June–August) rates at the surface reached 6.6 nmol l−1 d−1 on the Mackenzie Shelf and 3.7 nmol l−1 d−1 further offshore; the depth-integrated rates were correspondingly 8.8 μmol m−2 d−1 and 11.3 μmol m−2 d−1. The offshore depth-integrated rate in August (8.0 μmol m−2 d−1) was comparable to the missing dissolved inorganic nitrogen (DIN) source required to support the observed primary production in the upper 10-m layer of that area. The yearly NH4+ photoproduction in the entire study area was estimated to be 1.4 × 108 moles, with 85% of it being generated in summer when riverine DIN input is low. Photoammonification could mineralize 4% of the annual dissolved organic nitrogen (DON) exported from the Mackenzie River and provide a DIN source corresponding to 7% of the riverine DIN discharge and 1400 times the riverine NH4+ flux. Under a climate warming-induced ice-free scenario, these quantities could increase correspondingly to 6%, 11%, and 2100 times. Photoammonification is thus a significant nitrogen cycling term and may fuel previously unrecognized autotrophic and heterotrophic production pathways in the surface SE Beaufort Sea.

Structural stability of coprecipitated natural organic matter and ferric iron under reducing conditions

The objective was to assess the interaction of Fe coprecipitated with dissolved organic matter (DOM) and its effect on Fe (hydr)oxide crystallinity and DOM retention under abiotic reducing conditions. A Fe-based coagulant was reacted with DOM from an agricultural drain and the resulting precipitate (floc) was exposed to S(-II) and Fe(II). Solution concentrations of Fe(II/III) and DOM were monitored, floc crystallinity was determined using X-ray diffraction, and the composition and distribution of functional groups were assessed using scanning transmission X-ray microscopy (STXM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Results indicate coprecipitation of Fe(III) with DOM forms a non-crystalline floc that withstands crystallization regardless of change in pH, Fe:DOM ratio and type of reductant added. There was no evidence that exposure to reducing conditions led to release of DOM from the floc, indicating that coprecipitation with complex natural DOM in aquatic environments may stabilize Fe (hydr)oxides against crystallization upon reaction with reduced species and lead to long term sequestration of the DOM. STXM analysis identified spatially distinct regions with remarkable functional group purity, contrary to the model of DOM as a relatively uniform complex polymer lacking identifiable organic compounds. Polysaccharide-like OM was strongly and directly correlated with the presence of Fe but showed different Fe binding strength depending on the presence of carboxylic acid functional groups, whereas amide and aromatic functional groups were inversely correlated with Fe content.

Characterization of dissolved organic matter extracted from fermentation effluent of swine manure slurry using spectroscopic techniques and parallel factor analysis (PARAFAC)

Dissolved organic matter (DOM) is the most active component in environmental system, and its chemical and structural characteristics are most likely to affect its biodegradation. DOM has several different fluorophores whose fluorescent characteristics contain a lot of information about structure and functional groups. Fluorescence spectroscopy and Fourier-transform infrared (FTIR) were used to examine the structural and biodegradable characteristics of DOM extracted from fermentation effluent of swine manure slurry. Humic-like acid, fulvic-like acid and protein-like fluorophores were observed in fluorescence spectra. Fluorescence index (FI) indicated that the fluorophores in the samples were representative of both terrestrial and microbially-derived organic matter. FTIR spectroscopy showed that DOM featured aliphatic CH, aromatic CC and carbohydrate CO functional groups. With PARAFAC model, four fluorescent components were identified: component 1 (protein-like), component 2 (soluble microbial byproduct-like), component 3 (non-humic like) and component 4 (fulvic-like). Component 1 was a dominant fluorescent component. Components 2, 3 and 4 were degraded by microorganisms as the anaerobic fermentation processes. The C/N ratio of DOM in F1 has a more significant decrease than that in F0, indicating the loss of carbon (CH4 and CO2) during this process. The C/N ratio of DOM in F2 obviously increased when additional swine manure slurry was added.

Formation of Nanocolloidal Metacinnabar in Mercury-DOM-Sulfide Systems

Direct determination of mercury (Hg) speciation in sulfide-containing environments is confounded by low mercury concentrations and poor analytical sensitivity. Here we report the results of experiments designed to assess mercury speciation at environmentally relevant ratios of mercury to dissolved organic matter (DOM) (i.e., <4 nmol Hg (mg DOM)(-1)) by combining solid phase extraction using C(18) resin with extended X-ray absorption fine structure (EXAFS) spectroscopy. Aqueous Hg(II) and a DOM isolate were equilibrated in the presence and absence of 100 μM total sulfide. In the absence of sulfide, mercury adsorption to the resin increased as the Hg:DOM ratio decreased and as the strength of Hg-DOM binding increased. EXAFS analysis indicated that in the absence of sulfide, mercury bonds with an average of 2.4 ± 0.2 sulfur atoms with a bond length typical of mercury-organic thiol ligands (2.35 Å). In the presence of sulfide, mercury showed greater affinity for the C(18) resin, and its chromatographic behavior was independent of Hg:DOM ratio. EXAFS analysis showed mercury-sulfur bonds with a longer interatomic distance (2.51-2.53 Å) similar to the mercury-sulfur bond distance in metacinnabar (2.53 Å) regardless of the Hg:DOM ratio. For all samples containing sulfide, the sulfur coordination number was below the ideal four-coordinate structure of metacinnabar. At a low Hg:DOM ratio where strong binding DOM sites may control mercury speciation (1.9 nmol mg(-1)) mercury was coordinated by 2.3 ± 0.2 sulfur atoms, and the coordination number rose with increasing Hg:DOM ratio. The less-than-ideal coordination numbers indicate metacinnabar-like species on the nanometer scale, and the positive correlation between Hg:DOM ratio and sulfur coordination number suggests progressively increasing particle size or crystalline order with increasing abundance of mercury with respect to DOM. In DOM-containing sulfidic systems nanocolloidal metacinnabar-like species may form, and these species need to be considered when addressing mercury biogeochemistry.

Binding Constants of Mercury and Dissolved Organic Matter Determined by a Modified Ion Exchange Technique

Ion-exchange techniques have been widely used for determining the conditional stability constants (logK) between dissolved organic matter (DOM) and various metal ions in aqueous solution. An exception is mercuric ion, Hg2+, whose exceedingly strong binding with reduced sulfur or thiol-like functional groups in DOM makes the ion exchange reactions difficult. Using a Hg-selective thiol resin, we have developed a modified ion-exchange technique which overcomes this limitation. This technique allows not only the determination of binding constants between Hg2+ and DOM of varying origins, but also the discrimination of complexes with varying coordination numbers [i.e., 1:1 and 1:2 Hg:thiol-ligand (HgL) complexes]. Measured logK values of four selected DOM isolates varied slightly from 21.9 to 23.6 for 1:1 HgL complexes, and from 30.1 to 31.6 for 1:2 HgL(2) complexes. These results suggest similar binding modes that are likely occurring between Hg2+ and key thiolate functional groups in DOM particularly at a relatively low Hg to DOM ratio. Future studies should further elucidate the nature and precise stoichiometries of binding between Hg2+ and DOM at environmentally relevant concentrations.