High Molecular Weight Dissolved Organic Matter Research Articles

A comparative study of optical and size properties of dissolved organic matter in the lower Mississippi River and Pearl River

Monthly water samples were collected from the lower Mississippi and Pearl Rivers between January 2009 and August 2011 to investigate the heterogeneity in the dynamic variations of dissolved organic carbon (DOC), colloidal organic carbon, chromophoric and fluorescence dissolved organic matter (CDOM and FDOM), PARAFAC-derived fluorescent components, and other optical properties including spectral slope, specific UV absorbance (SUVA), and fluorescence indices between the two contrasting river systems. The lower Mississippi River exhibits relatively lower concentrations of DOC (306 ± 41 μM C) and CDOM (27.9 ± 5.7 m−1 at 254 nm), featuring lower aromaticity (indicated by SUVA254) and apparent molecular weight (or higher spectral slope) with weak seasonal variability. The Pearl River, in contrast, has elevated levels of DOC (537 ± 212 μM C) and CDOM (66.4 ± 31.4 m−1), characterized by higher aromaticity, higher molecular weight, and significant seasonality, primarily originating from soil-derived allochthonous sources. The abundance of the >1 kDa colloidal DOC was, on average, 58 ± 3 % of the bulk DOC in the lower Mississippi River and 68 ± 6 % in the lower Pearl River. The >1 kDa high-molecular weight DOM (HMW-DOM) consistently had lower spectral slope and biological index (BIX) values, but higher humification index (HIX) values compared to both bulk DOM and low-molecular-weight DOM (LMW-DOM) counterparts. These trends could be representative of other similar large and small rivers. Four PARAFAC-derived fluorescent components (three humic-like and one protein-like) were identified for both rivers. A positive correlation between discharge and terrestrial humic-like fluorescent components indicated their dominant allochthonous sources, while the protein-like component decreased with increasing discharge, consistent with its autochthonic source and a dilution effect during high flow seasons. The occurrence of large flood events during the sampling period contributed to large DOC pulses, with DOM of higher aromaticity and HMW-DOM. This has important implications for coastal ocean ecosystems, which are increasingly impacted by river flooding events under changing climate conditions. Our results also provide an improved understanding of DOM dynamics in two representative rivers and establish a baseline dataset for future studies to assess changes in sources and composition of DOM and their impacts on the coastal ocean in response to climate, hydrological, and anthropogenic influences.

Improved solid-state 13C and 15N NMR reveals fundamental compositional divide between refractory dissolved organic carbon and nitrogen in the sea

Marine dissolved organic matter (DOM) is one of the largest reservoirs of organic carbon and nitrogen reservoirs in the world. Yet, despite its global importance, most DOM remains molecularly uncharacterized. Solid-state nuclear magnetic resonance (NMR) spectroscopy of isolated DOM fractions represents one of the most powerful techniques to understand overall structural composition. However, it is well known that standard cross polarization magic angle spinning (CP/MAS) NMR, the technique used for almost all past solid-state NMR studies of DOM, is at best “semi-quantitative,” and underestimates fully substituted NMR active nuclei. Additionally, almost all past solid-state NMR work analyzed high molecular weight (HMW) material isolated by ultrafiltration, which is now understood to represent mostly 14C-young, “semi-labile” compounds. In contrast, there is far less information regarding the composition of older, low molecular weight (LMW) DOM, which represents the vast majority of the ocean’s accumulated refractory DOM pool.Here, we applied 13C and 15N solid-state multiCP/MAS NMR, improved NMR methods optimized to more quantitatively resolve fully substituted NMR nuclei, to both HMW and LMW DOM isolated from the surface and deep North Pacific Subtropical Gyre. These methods confirm past work indicating most nitrogen containing HMW DOM as amide compounds, but also demonstrate a modest heterocyclic N component not previously identified. In contrast, we found that LMW DON is almost entirely aromatic heterocyclic N, consistent with the hypothesis that heterocyclic N structures may be largely responsible for the accumulation of the ocean’s refractory DON pool. Surprisingly, however, we find DOC aromatic functionalities still represent only a very minor portion of either the HMW or the LMW refractory carbon pools, in marked contrast to refractory DON composition. Together, these more quantitative solid-state NMR techniques likely represent the most accurate picture of DON and DOC functional and compound-class makeup to date, and so have broad implications for our understanding of marine DOM structure and cycling. Specifically, our new data suggests that while chemical composition likely acts as a key control on DOM lability, the most refractory components of DOC and DON have very different compositions, sources, and cycling, supporting the idea that DOC and DON cycling in the ocean may be largely decoupled.

What Is the Molecular Weight of "High" Molecular Weight Dissolved Organic Matter?

The use of ultrafiltration to isolate high molecular weight dissolved organic matter (HMWDOM) from seawater is a fundamental tool in the environmental organic chemist's toolbox. Yet, important characteristics of HMWDOM relevant to its origin and cycling, such as its molecular weight distribution, remain poorly defined. We used diffusion-ordered NMR spectroscopy coupled with mixed-mode chromatography to separate and characterize two major components of marine HMWDOM: acylpolysaccharides (APS) and high molecular weight humic substances (HS). The molecular weights (MWs) of APS and HS both fell within distinct, narrow envelopes; 2.0-16 kDa for APS and 0.9-6.5 kDa for HS. In water samples from the North Pacific Ocean the average MW of both components decreased with depth through the mesopelagic. However, the minimum MW of APS was >2 kDa, well above the molecular weight cutoff of the ultrafilter, suggesting APS removal processes below 2 kDa are highly efficient. The MW distribution of APS shows only small variations with depth, while the MW distribution of HS narrowed due to removal of HMW components. Despite the narrowing of the MW distribution, the concentration of HS did not decrease with depth between 15 and 915 m. This suggests that HMW HS produced in surface waters was either degraded into lower MW compounds without significant remineralization, or that HMW HS was remineralized but replaced by an additional source of HS in the mesopelagic ocean. Based on these results, we propose potential pathways for the production and removal of these major components of HMWDOM.

High-molecular-weight dissolved organic matter enhanced phosphorus availability in paddy soils: Evidence from field and microcosm experiments

Dissolved organic matter (DOM) derived from organic fertilizers may increase soil phosphorus (P) availability. However, the frequently observed correlation between soil P availability and dissolved organic carbon (DOC) content has led to an excessive focus on DOC content at the expense of DOM properties. The present study investigated the influence of DOM characteristics on soil P availability by using a 6-year field experiment and microcosms of P sorption in paddy soil. Our results showed that partial replacement of chemical P fertilizer with manure or crop straw increased P fertilizer-use efficiency, even when decreasing chemical P input by 34%, compared to normal chemical fertilization. The microcosm experiment demonstrated that DOM properties, rather than total DOC content, determine soil P sorption capacity, despite the significant correlation between DOC content and P availability observed in the field experiment. Manure-DOM exerted stronger inhibitory effects on P sorption than straw-DOM, and high molecular weight (HMW)-fractionated DOM exerted stronger inhibitory effects on P sorption than low-molecular-weight-fractionated DOM by 16–20%. The mechanism is primarily attributed to high molecular weight (HMW) DOM, which possesses unique characteristics such as strong aromaticity. These attributes lead to competition for soil P sorption sites and a decrease in soil surface charge, collectively suppressing P sorption in paddy soil. Our study points out a promising avenue for regulating organic matter properties with organic fertilization to improve P use efficiency in agricultural soils.

Mechanism of methylmercury photodegradation in the yellow sea and East China Sea: Dominant pathways, and role of sunlight spectrum and dissolved organic matter

Mercury (Hg) is among the most concerned contaminants in the world due to its high toxicity, prevalent existence in the environments, and bioaccumulation via food chain. Methylmercury (MeHg) is the major form of Hg that accumulates along the food chain and poses threat to humans and wild life. Photodegradation is the dominant process that MeHg is eliminated from freshwater system and upper ocean. The formation of MeHg-dissolved organic matter (DOM) complexes and a variety of free radicals (FR)/reactive oxygen species (ROS) have been previously proposed to be involved in MeHg photodegradation. However, most of these studies were conducted in freshwater, and the mechanism of MeHg photodegradation in seawater remains unclear. In this study, the main pathways of MeHg photodegradation in the seawater of Yellow Sea (YS) and East China Sea (ECS) were investigated using FR/ ROS scavenger addition and DOM competing-ligand addition techniques. The results showed that direct photodegradation of MeHg-DOM complexes is the major pathway of MeHg photodegradation in the YS and ECS, while indirect photolysis of MeHg by hydroxyl radical (·OH) also plays a certain role at some sites. MeHg photodegradation was found to be mainly induced by ultraviolet (UV) light rather than visible light in YS and ECS seawater, and the contribution of UV-B was higher than UV-A which was opposite to that previously reported in freshwater. The energy for breaking the bond of CHg in MeHg-Cl complexes formed in seawater is higher than that in MeHg-DOM complexes and this may cause the relatively greater contribution of UV-B with higher energy to MeHg photodegradation in seawater. In addition, MeHg photodegradation in various fractions of natural DOM with different molecular weights, hydrophilicity/hydrophobicity and acid-base was tested. MeHg photodegradation rates (kd) varied in these fractions and kd in high molecular weight DOM and hydrophobic Acid (HOA) fractions were faster than that in the other fractions. A significantly positive correlation was observed between kd and thiol concentrations while there was no significant correlation between kd and other measured parameters representing the composition of DOM (specific UV absorbance at 254 nm (SUVA254), spectral slope (SR), chromophoric dissolved organic matter (CDOM), humification index (HIX), biological index (BIX) and fluorescent components). These results indicate that thiol may be the key functional group in DOM affecting the photodegradation of MeHg in the YS and ECS.

Controls on the composition of dissolved organic matter in treatment wetland source waters of South Florida, USA

Dissolved organic matter (DOM) plays an important role in the phosphorus (P) and nitrogen (N) cycle, but the examination of coupled biogeochemical cycling in treatment wetlands, like the Everglades Stormwater Treatment Areas (STAs), has been limited. Here, we studied eight STA inflow waters to gain insights into the production and potential photo- and biolability of DOM in surface waters flowing into these treatment wetlands. A suite of optical and fluorometric indices was coupled with amino acid and bulk elemental analyses to evaluate differences in inflow water quality for particulate organic matter (POM), DOM (<0.2 μm), and high and low molecular weight DOM (HMW-DOM and LMW-DOM). A photochemical experiment was conducted with these inflow waters to assess how solar exposure alters DOM turnover and its relationship with nutrient concentrations, and to evaluate the influence of photochemistry on potential STA performance. The DOM of STA inflow water was predominantly composed of allochthonous terrestrial sources, and the LMW-DOM fraction represented 70 to 94% of the total DOM pool. Inflow waters separated along the biophysical dimension of our PCA based on their photolability and bioavailability, driven by differences in the inflow water's DOM composition. UV exposure decreased humic and aromaticity proxies with concomitant photo-production of LMW compounds and increases in bioavailability proxies. Photo-exposure significantly decreased nitrate and dissolved organic carbon (DOC) concentrations for most of the sites and dissolved organic N (DON) for three of the eight sites, but no significant changes were noted for ammonia. One site exhibited an increase in nitrate with UV exposure and had DOM primarily from terrestrial origins. Spectroscopic indices, PARAFAC components, and amino acids provided corroborating evidence that further supported the differences in photo- and biolability at each inflow site, which is likely driven by their original DOM characteristics. PARAFAC components were correlated with dissolved organic P (DOP) and suggest that there may be adsorption of DOP with humic components in the LMW fraction. This work highlights the importance of photochemistry in the STAs, the complexity of DOM turnover in STA source waters, and the potential interactions between dissolved P and other DOM components, which may further influence the treatment efficiency of the STAs.

Role of lake dissolved organic matter in cyanobacteria removal by cationic polyacrylamide flocculation and screen filtration

Prompt removal of densely accumulated cyanobacteria is a straightforward method to alleviate the harm of cyanobacterial blooms to lakes. In this study, a cyanobacteria harvesting ship based on flocculation and filtration was used for the removal of cyanobacteria in a bay of Dianchi Lake (Yunnan Province, China), in which the fate and influence of lake dissolved organic matter (DOM) were investigated. After flocculation with 1–3 mg/L cationic polyacrylamide (CPAM) and filtration with the 100-µm-pore-sized screens of a rotary drum filter (RDF), >90 % of cyanobacteria was removed and the DOM was slightly reduced from 7.83 mg/L to 4.61 ∼ 6.49 mg/L. Tryptophan protein-like and high molecular weight (MW) DOM was selectively removed, increasing the humification index and decreasing the biological index in the treated water. Lake DOM, especially the high MW fractions of lake DOM, played an important role in CPAM flocculation. Cyanobacteria could not be effectively flocculated in Milli-Q water while the removal efficiency increased to over 90 % when 1–3 mg/L high MW DOM of Dianchi Lake was added. Three lakes in Yunnan Province were also plagued by cyanobacterial blooms, including Erhai Lake, Qilu Lake, and Xingyun Lake. The high MW DOM (1.7–2.7 mg/L) in the lakes could also deduce the effective flocculation of cyanobacteria with CPAM. The flocculation mechanism could function by the interactions of quaternary ammonium groups of CPAM and carboxylate groups of high MW DOM that form net-like polyelectrolyte complexes (PECs), and coexisting cyanobacteria cells were encapsulated into the PECs, leading to the formation of large flocs under stirring conditions. In the lakes with high densities of cyanobacteria, there might be enough high-MW DOM to promote the flocculation of CPAM, and after flocculation-filtration treatment, both cyanobacteria and cyanobacteria-deprived DOM were effectively removed.

A Study on the Behavior of Cadmium in the Soil Solution-Plant System by the Lysimeter Method Using the 109Cd Radioactive Tracer.

In soils, cadmium (Cd) and its compounds, originating from industrial activities, differ both in mobility as well as in their ability to permeate the soil solution from naturally occurring cadmium compounds (native Cd). Therefore, the determination of the parameters of cadmium mobility in soils and its accumulation by plants in the soil-soil solution-plant system is very important from both scientific and practical viewpoints. 109Cd was used as a radioactive tracer to study the processes of the transition of Cd into the aqueous phase and its uptake by plants over the course of a vegetative lysimeter experiment. Using sequential extraction according to the Tessier-Förstner procedure and modified BCR schemes, certain patterns were determined in the distribution of Cd/109Cd among their forms in various compounds in the soil, along with the coefficients of the enrichment of native stable Cd with radioactive 109Cd. It was shown that the labile pool of stable Cd compounds (29%) was significantly smaller than that of radioactive 109Cd (69%). The key parameters characterizing the migration capacity of Cd in the soil-soil solution-plant system were determined. It was found that the distribution coefficient of native Cd between the soil and the quasi-equilibrium lysimeter solution exceeded the similar value for the 109Cd radionuclide by 2.2 times, and the concentration coefficients of Cd and 109Cd in the barley roots were 9 times higher than in its vegetative parts. During the experiment, the average removal of Cd (109Cd) from the soil by each barley plant was insignificant: 0.002 (0.004)%. Based on the results of 13C nuclear magnetic resonance (NMR) spectroscopy of a lyophilized sample of the high-molecular-weight dissolved organic matter (HMWDOM) of the soil solution, its components were determined. It transpired that the isolated lyophilized samples of HMWDOM with different molecular weights had an identical structural and functional composition. The selective sorption parameters of the HMWDOM and humic acid (HA) with respect to Cd2+ ions were determined by the isotope dilution method.

The desorption mechanism of dissolved organic matter on pollutants and the change of biodiversity during sediment dredging

Sediment dredging is an effective means to control the endogenous pollution of lakes, which could significantly change the concentration and composition of organic matter, especially dissolved organic matter (DOM) in the lake. DOM is particularly important for the release of endogenous pollutants, which will inevitably bring an impact on aquatic biodiversity. Nevertheless, in recent research little attention has been paid to the desorption mechanism of DOM on pollutants and the change of biodiversity during dredging. This study investigated the physicochemical properties of DOM in the sediment by taking a sediment dredging project in Dianchi Lake in China for example. The correlations of DOM properties with the desorption behavior of nitrogen (N), phosphorus (P), cadmium (Cd), lead (Pb) and the biodiversity of aquatic organisms were analyzed. The results show that the aromaticity and humification of DOM were improved after dredging, and the high molecular weight DOM was degraded into low molecular weight substance. The desorption amount of N, P and heavy metals (Cd, Pb) were decreased as the pH values increased. Moreover, NH4+-N promoted the release of Pb2+ from DOM, while the release of PO43--P was inhibited. Correlation analysis shows that the physicochemical properties of DOM exactly affected the release of N, P, Cd and Pb. It was easier to desorb pollutants with low aromaticity and humification of DOM, leading to a decrease in the diversity of aquatic organisms. This study identified the desorption mechanism of endogenous pollutants in DOM and the ecological risk to aquatic organisms, providing a theoretical basis for the prevention and control of water pollution.

Characterization of the vertical size distribution, composition and chemical properties of dissolved organic matter in the (ultra)oligotrophic Pacific Ocean through a multi-detection approach

This work presents a multi-analytical approach for the characterization of marine dissolved organic matter (DOM). The determination of marine dissolved organic carbon (DOC) was performed by size exclusion chromatography (SEC) and validated using a certified reference material (CRM) as well as through an intercomparison exercise. Multi-detection SEC, fluorescence and electrochemical methods were used in order to describe the size distribution spectra, the composition and chemical properties of marine DOM, in the (ultra)oligotrophic West Tropical South Pacific Ocean (WTSP). In this work, we defined the state of degradation of DOC in the different size fractions, operationally defined by SEC. We estimated that on average 62% of DOC was of humic nature (0.5–10 kDa), of which ~9% was able to complex trace elements, such as iron (Fe). Our results tend to support that non-refractory DOC is of high molecular weight (HMW), nitrogen (N)-rich, aliphatic, and has a weak fluorescence quantum yield and an enhanced binding capacity for Fe. The ageing of marine DOM occurring within mesopelagic waters is mainly driven by microbial respiration and alters these chemical properties. Although our results are in agreement with a paradigm describing oceanic DOM biogeochemistry known as the size-reactivity continuum, 3 μmolC L−1 of very HMW (> 10 kDa) were still observed in a water mass mainly composed of Pacific Deep Waters. This persistence could be explained by a significant content (5%) of aromatic carbon that may protect HMW DOM from long term biodegradation.

Interaction between polychlorinated biphenyls and dissolved organic matter of different molecular weights from natural and anthropic sources

Polychlorinated biphenyls (PCBs) are compounds of significant interest due to high toxicity, persistence, long-range atmospheric transport, and bioaccumulation. These compounds can interact with components present in the environment, including dissolved organic matter (DOM) in soils and waters, thereby modifying its availability and movement. In this study, DOM was fractionated by ultrafiltration and characterized according to its hydrophobicity and hydrophilicity, then the interaction of a series of PCBs and different DOM fractions was evaluated. The DOM was collected from the surface waters of three sectors located along a river in the southern part of America. These sectors are subject to different anthropic activities, thus the DOM of sector 1, with the least anthropic influence, was mainly hydrophobic and with a high content of aromatic structures. In contrast, the DOM collected from sectors 2 and 3, where anthropic activity is highest, was slightly hydrophobic and hydrophilic, respectively. The DOM of these two sectors was mainly composed of low molecular weight macromolecules. These results revealed that more hydrophobic PCBs (i.e., 101, 118, 138, and 180) have a greater affinity to DOM with a higher molecular weight (i.e., >1 kDa). In turn, PCBs with lesser chlorination and hydrophobicity presented a greater affinity to DOM with a lower molecular weight. In conclusion, our study shows that the high molecular weight DOM is responsible for mobilizing PCBs with a high degree of chlorination.

Delineating Source Contributions to Stream Dissolved Organic Matter Composition Under Baseflow Conditions in Forested Headwater Catchments

AbstractDissolved organic matter (DOM) composition in streams reflects the dynamic interplay between DOM sources, mobilization mechanisms, and biogeochemical transformations within soils and receiving water bodies. The information regarding DOM sources being mobilized during baseflow can improve our ability to predict hydrological and biogeochemical responses to environmental changes, with implications for catchment management strategies. The objective of this study was to characterize the spatial changes in DOM composition along a headwater stream in a small mountainous catchment during baseflow and to link the findings in‐stream with possible DOM sources in the catchment. DOM was monitored over 1.5 years at three sites in the Große Ohe catchment (Bavarian Forest National Park, Southeast Germany) which strongly varied in slope between upper and lower part of the catchment, using UV‐Vis absorption indicators of aromaticity (SUVA) and molecular weight (E2:E3) from high‐frequency probe measurements. Additionally, discrete samples were collected and analyzed by ultrahigh‐resolution Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). At baseflow conditions, dissolved organic carbon (DOC) concentrations, a proxy of DOM amount, ranged from 1.5 to 4.7 mg L−1, but were similar along the stream. However, DOM quality exhibited clear spatial patterns, with overall high aromatic and low molecular weight DOM in the lower part of the catchment, which has a higher proportion of hydromorphic soils. Moreover, molecular data revealed that oxygen‐rich, aromatic compounds increased in their abundance at high DOC concentrations in both the steep upper and the flatter lower part of the catchment, with also additional input of oxygen‐depleted aromatic compounds identified in the lower part. In contrast, nitrogen‐rich aliphatic compounds were negatively correlated with DOC concentration, indicating a higher contribution of deep groundwater at low DOC concentrations.

Combined coagulation/decantation-ozone/hydrogen peroxide-biological aerated filter process for advanced treatment of bio-treated leachate from Chinese herbal medicine residue

To reach discharge limit, further treatment of bio-treated leachate from Chinese herbal medicine residue (BLCHMR) was very imperative. In this study, performance of combined coagulation/decantation-ozone/hydrogen peroxide (O3/H2O2)-biological aerated filter (BAF) technology used to further treat BLCHMR was investigated with pilot-scale experiment. Under optimal conditions, the COD (40-44 mg/L) and color (13-17 time) in treated BLCHMR indicated that combined process can efficiently treat BLCHMR. O3/H2O2 had good performance not only in mineralization and decomposing of organic matter but also in decolorization of BLCHMR. For dissolved organic matter (DOM), O3/H2O2 reacted with non-biodegradable fraction preferentially and oxidized different molecular weight (MW) fractions equally. O3/H2O2-BAF generated more higher MW DOM (namely F2 fraction) than BAF alone. Meanwhile, O3/H2O2 mainly influenced the amount and biodegradability on DOM but not the removal rate on DOM. Accordingly, the helpful influence on O3/H2O2 perhaps can be owed to the generalized influence on DOM but not the specific influence on improved biodegradability. Finally, inspiring (namely very low) cost (respectively, 0.3419 $/per ton bio-treated leachate and 0.5766 $/Kg COD removed) was achieved in the combined process.

Lake browning may fuel phytoplankton biomass and trigger shifts in phytoplankton communities in temperate lakes

Lake browning—the increase in catchment-derived (allochthonous) dissolved organic matter (DOM) to lakes—is altering lake physicochemical environments, with consequences for phytoplankton biomass and community composition. We hypothesized that as lakes brown, there will be an increase in phytoplankton biomass and a shift to cyanobacteria-dominated phytoplankton communities as a result of the reduced light availability and increased DOM-bound nutrients (e.g., nitrogen, phosphorus, iron). We tested this hypothesis by sampling temperate lakes in central Ontario (Canada) spanning DOM quantity and quality gradients. We found that lake browning results in larger concentrations of more refractory (i.e., aromatic, high molecular weight) DOM and greater concentrations of nutrients; however, internal nutrient loading was also an important nutrient source in these lakes. We also found that these changes were related to the predominant species in the phytoplankton community. Diatoms dominated in clear oligotrophic lakes. Low levels of lake browning, with concentrations of dissolved organic carbon (DOC) between 4 and 8 mg L− 1, resulted in a shift from diatoms to cyanobacteria. Higher levels of lake browning, with concentrations of DOC between 8 and 12 mg L− 1, resulted in a replacement of cyanobacteria with mixotrophic species. Lake browning appears to fuel phytoplankton chlorophyll-a concentrations while triggering shifts to phytoplankton able to survive if not thrive in progressively browner waters. Lake browning may therefore have consequences on energy transfer through the lower food web.

Sorption Constant of Bisphenol A and Octylphenol Onto Size-Fractioned Dissolved Organic Matter Using a Fluorescence Method.

Dissolved organic matter (DOM) is a complex and heterogeneous mixture ubiquitously present in aquatic systems. DOM affects octylphenol (OP) and bisphenol A (BPA) distribution, transport, bioavailability, and toxicity. This study investigated OP and BPA sorption constants, log KCOC, with three size-fractioned DOM. The molecular weights of the sized fractions were low molecular weight DOM (LDOM, <1 kDa), middle molecular weight DOM (MDOM, 1–10 kDa), and high molecular weight DOM (HDOM, 10 kDa–0.45 μm). The log KCOC ranged from 5.34 to 6.14 L/kg-C for OP and from 5.59 to 6.04 L/kg-C for BPA. The OP and BPA log KCOC values were insignificantly different (p = 0.37) and had a strong positive correlation (r = 0.85, p < 0.001). The OP and BPA LDOM log KCOC was significantly higher than the HDOM and MDOM log KCOC (p = 0.012 for BPA, p = 0.023 for OP). The average specific ultraviolet absorption (SUVA254) values were 32.0 ± 5.4, 13.8 ± 1.0, and 17.9 ± 2.8 L/mg-C/m for LDOM, MDOM, and HDOM, respectively. The log KCOC values for both OP and BPA had a moderately positive correlation with the SUVA254 values (r = 0.79–0.84, p < 0.002), which suggested the aromatic group content in the DOM had a positive impact on sorption behavior.

Metal binding by dissolved organic matter in hypersaline water: A size fractionation study using different isolation methods

The influence of dissolved organic matter (DOM) on mineral extraction from salt lake brines depend on DOM quality. This study contributes to our knowledge of DOM’s metal binding behavior in hypersaline environments by characterization of DOM from lakes in the Qaidam Basin, i.e., Qarhan Lake (LQDOM), Da Qaidam (DQDOM) and West Ginair Salt Lake (WGDOM). The DOM was fractionated based on solid phase extraction (SPE) and ultrafiltration (UF), and the spectral and metal binding behavior of these fractions were studied by absorption spectroscopy, Pb(II) titration techniques and fluorescence parallel factor (PARAFAC) analysis. The results showed that bulk DOM generally contained more dissolved organic carbon (DOC), lower specific UV absorbance (SUVA254), higher fluorescence and biological indices, comparable humification index, and lower condition stability constants compared to the other nature waters. Compared with UF, SPE-derived DOM exhibited higher DOC recovery and aromaticity and lower carbohydrate yield. It appeared that the SPE procedure used affects the spectral composition of bulk DOM to a larger extent than UF. Source and molecular weight (MW)-dependent differences in abundance and quality of brine DOM was indicated by higher SUVA254 in high MW DOM, for LQDOM and DQDOM, and humic-like fluorophores were mainly in high MW-DOM in each lake. Moreover, the high MW humic-like component exhibited higher metal binding potential than the bulk and low MW counterparts for LQDOM and DQDOM, while the inverse was observed for WGDOM. This study revealed the effects of isolation techniques on interpretation of DOM characteristics, and meanwhile highlighted the importance of origin- and MW-dependent DOM in manipulating the behavior, fate, and bioavailability of heavy metals in salt lake brine.

Sorption of dissolved organic matter by freshwater bacterioplankton

Here we describe the study of the sorption of high-molecular components of dissolved organic matter (DOM) of natural waters - humic acids - by eight bacterial strains isolated from freshwater. Isotherms of DOM sorption by cells are described by the Langmuir equation. Gram-positive bacteria adsorb more DOM than gram-negative bacteria, with high molecular weight DOM fractions binding in a relatively selective manner. A high positive correlation (r = 0.91) was found between the decrease in the molecular weight of DOM during the sorption of these compounds by cells and the maximum sorption value. The formation of a polysaccharide capsule increases the binding of DOM by the cells of Pseudomonas sp. to the level of gram-positive bacteria.

Spectroscopic indices trace spatiotemporal variability of dissolved organic matter in a river system with Karst characteristic

River acts as a continuum of dissolved organic matter (DOM) transport and transformation. However, underlying controls of fluvial DOM spatiotemporal variability has yet to be revealed. Here, we explored the DOM component, molecular weight and source in the Longchuan River, a headwater tributary of the Yangtze with Karst characteristic. Concentrated rainfall, severe soil erosion and in-situ metabolism primarily controlled DOM component and molecular weight in the Karst river, resulting in accumulation of recalcitrant (high molecular weight) DOMs from tributaries to the main stem in the post-wet period. We found a robust relationship between DOM absorption coefficients (a280 and a350) and dissolved organic carbon (DOC) concentrations from a multiple linear regression model: [DOC] = 0.098a285 + 0.591a350 + 1.337 (R2 = 0.83, p < 0.001). High fluorescence index (FI) values (>1.9) indicated a dominant origin of autochthonous DOM. Biological index (BIX) values exhibited an increasing trend along the flow direction, suggesting a large amount of recently produced DOM from phytoplankton downstream. Our findings demonstrated that both allochthonous and autochthonous inputs regulated DOM dynamics in a river continuum with Karst characteristic, highlighting spatiotemporal variability of DOM response to River Continuum Concept (RCC).

Composition and lability of riverine dissolved organic matter: Insights from thermal slicing ramped pyrolysis GC–MS, amino acid, and stable isotope analyses

Dissolved organic matter (DOM) is one of the key carbon substrates transported by rivers from land to sea, yet only a small fraction of it has been characterized at the molecular level. Here we characterize high molecular weight DOM (HMW-DOM, >1 kDa) isolated by ultrafiltration from nine North American rivers using thermal slicing ramped pyrolysis gas chromatography mass spectrometry (TSRP-GC–MS), carbon and nitrogen stable isotopes, elemental composition and total hydrolyzable amino acids (THAA). Samples from the more anthropogenically altered watersheds carried more enriched δ15N-HMW-DOM signatures than observed in samples from the more pristine watersheds. The TSRP-GC–MS results, coupled with THAA data, constrained DOM degradation process. Specifically, the diversity of pyrolyzates of HMW-DOM samples correlated well with the degradation index derived from the THAA data, with more degraded samples being more thermally stable and less molecularly diverse. The relationship between degradation index and pyrolyzate diversity observed in the HMW-DOM samples from across such a broad spatial scale suggests that degradative processes reduce riverine DOM complexity through the loss of more reactive moieties. Overall, TSRP-GC–MS, in combination with specific compound or isotope analyses, illuminates processes controlling DOM composition in different North American river systems with varying degrees of bio-lability and anthropogenic influence.

Phosphonate cycling supports methane and ethylene supersaturation in the phosphate‐depleted western North Atlantic Ocean

AbstractIn oligotrophic ocean regions, dissolved organic phosphorus (DOP) plays a prominent role as a source of phosphorus (P) to microorganisms. An important bioavailable component of DOP is phosphonates, organophosphorus compounds with a carbon‐phosphorus (C‐P) bond, which are ubiquitous in high molecular weight dissolved organic matter (HMWDOM). In addition to being a source of P, the degradation of phosphonates by the bacterial C‐P lyase enzymatic pathway causes the release of trace hydrocarbon gases relevant to climate and atmospheric chemistry. In this study, we investigated the roles of phosphate and phosphonate cycling in the production of methane (CH4) and ethylene (C2H4) in the western North Atlantic Ocean, a region that features a transition in phosphate concentrations from coastal to open ocean waters. We observed an inverse relationship between phosphate and the saturation state of CH4 and C2H4 in the water column, and between phosphate and the relative abundance of the C‐P lyase marker gene phnJ. In phosphate‐depleted waters, methylphosphonate and 2‐hydroxyethylphosphonate, the C‐P lyase substrates that yield CH4 and C2H4, respectively, were readily degraded in proportions consistent with their abundance and bioavailability in HMWDOM and with the concentrations of CH4 and C2H4 in the water column. We conclude that phosphonate degradation through the C‐P lyase pathway is an important source and a common production pathway of CH4 and C2H4 in the phosphate‐depleted surface waters of the western North Atlantic Ocean and that phosphate concentration can be an important control on the saturation state of these gases in the upper ocean.