Aromatic Dissolved Organic Matter Research Articles

Particles and Aeration at Mire‐Stream Interfaces Cause Selective Removal and Modification of Dissolved Organic Matter

AbstractPeatlands are dominant sources of dissolved organic matter (DOM) to boreal inland waters and play important roles in the aquatic carbon cycle. Yet before peat‐derived DOM enters aquatic networks, it needs to pass through peat‐stream interfaces that are often characterized by transitions from anoxic or hypoxic to oxic conditions. Aeration at these interfaces may trigger processes that impact the DOM pool, and its fate downstream. Here we experimentally assessed how the aeration of iron‐ and organic‐rich mire‐waters influences biodegradation, particle‐formation, and modification of DOM. In addition, we investigated how suspended peat‐derived particles from mires may influence these processes. We found that within 5 days of aeration, 20% of the DOM transformed into particulate organic matter (POM). This removal was likely due to combination of mechanisms including coprecipitation with oxidized iron, aggregation, and DOM‐adsorption onto peat‐derived particles. Peat‐derived particles promoted microbial activity, but biodegradation was a minor loss mechanism of DOM removal. Interestingly, microbial respiration accounted for only half of the oxygen loss, suggesting substantial nonrespiratory oxygen consumption. The differences observed in DOM characteristics between anoxic and aerated treatments suggest that hydrophilic, aromatic DOM coprecipitated with iron oxides in aerated samples, and the corresponding C:N analysis of generated POM revealed that these organic species were nitrogen‐poor. Meanwhile, POM formed via adsorption onto peat‐derived particles generated from nonaromatic DOM and more nitrogen‐rich species. Hence, selective removal of DOM, dissolved iron, and thus oxygen may be important and overlooked processes in mire‐dominated headwater systems.

Interactions of iron-based nanoparticles with soil dissolved organic matter: adsorption, aging, and effects on hexavalent chromium removal

The interactions and mechanisms between soil dissolved organic matter (DOM) and three types of iron-based nanoparticles (NPs), i.e., nanoscale zero-valent iron (nZVI) particles, Fe2O3 NPs, and Fe3O4 NPs, were investigated in short-term exposure experiments. The adsorption results showed that soil DOM was rapidly adsorbed on the surface of the iron-based NPs with the adsorption rate varying according to Fe3O4 > Fe2O3 > nZVI. Spectral analysis results revealed that aromatic DOM fractions with high-molecular-weights were preferentially adsorbed. The binding mechanism was determined as hydrogen bonding and ligand exchange via Fourier transform infrared spectroscopy (FT-IR) analysis. Scanning electron microscopy, FT-IR, X-ray photoelectron spectroscopy, and X-ray diffraction were used to identify the corrosion products of the three iron-based NPs at the adsorption equilibrium. The results suggest that Fe3O4 and/or γ-Fe2O3 and α-FeOOH were the main corrosion products of nZVIs and α-FeOOH was obtained as an aged product of Fe3O4 NPs. Results of Cr(VI) removal tests suggest that the aged nZVI achieved 79.87% of Cr(VI) removal and the Cr(VI) removal efficiency was significantly improved by coating DOM onto Fe2O3 NPs. The overall data indicate the fate and transformation of iron-based NPs and the enhancement for Cr(VI) removal after interactions between DOM and NPs.

Changes in Activity and Community Composition Shape Bacterial Responses to Size-Fractionated Marine DOM.

To study the response of bacteria to different size-fractions of naturally occurring dissolved organic matter (DOM), a natural prokaryotic community from North Atlantic mesopelagic waters (1000 m depth) was isolated and grown in (i) 0.1-μm filtered seawater (CONTROL), (ii) the low-molecular-weight (<1 kDa) DOM fraction (L-DOM), and (iii) the recombination of high- (>1 kDa) and low-molecular-weight DOM fractions (H + L-DOM), to test the potential effect of ultrafiltration on breaking the DOM size continuum. Prokaryotic abundance and leucine incorporation were consistently higher in the H + L-DOM niche than in the L-DOM and CONTROL treatments, suggesting a different interaction with each DOM fraction and the disruption of the structural DOM continuum by ultrafiltration, respectively. Rhodobacterales (Alphaproteobacteria) and Flavobacteriales (Bacteroidetes) were particularly enriched in L-DOM and closely related to the colored DOM (CDOM) fraction, indicating the tight link between these groups and changes in DOM aromaticity. Conversely, some other taxa that were rare or undetectable in the original bacterial community were enriched in the H + L-DOM treatment (e.g., Alteromonadales belonging to Gammaproteobacteria), highlighting the role of the rare biosphere as a seed bank of diversity against ecosystem disturbance. The relationship between the fluorescence of protein-like CDOM and community composition of populations in the H + L-DOM treatment suggested their preference for labile DOM. Conversely, the communities growing on the L-DOM niche were coupled to humic-like CDOM, which may indicate their ability to degrade more reworked DOM and/or the generation of refractory substrates (as by-products of the respiration processes). Most importantly, L- and/or H + L-DOM treatments stimulated the growth of unique bacterial amplicon sequence variants (ASVs), suggesting the potential of environmental selection (i.e., changes in DOM composition and availability), particularly in the light of climate change scenarios. Taken together, our results suggest that different size-fractions of DOM induced niche-specialization and differentiation of mesopelagic bacterial communities.

Rainstorm events shift the molecular composition and export of dissolved organic matter in a large drinking water reservoir in China: High frequency buoys and field observations

Rainstorm events can flush large amounts of terrestrial organic-rich material into lakes that are used for drinking water. To date, few studies have been carried out to investigate how rainstorm events change the molecular composition, bio-lability, and flux of upstream-imported dissolved organic matter (DOM), which can impact the odor and taste of drinking water as well as the efficiency of wastewater treatment. We undertook high-frequency buoy monitoring and point sample collection (n = 495), during high, moderate, and low inflow discharge, in Lake Qiandao, a key drinking water source for about 10 million people. Data from two online fluorescent DOM sensors deployed and field samples collected at the river site, Jiekou, and the lake site, Xiaojinshan, showed that rainstorm events increased the specific UV absorbance (SUVA254), humification index (HIX), humic-like components (C1-C2), and FT-ICR MS derived condensed aromatic and polyphenolic compounds (p < 0.001) and decreased the spectral slope of DOM (S275–295), spectral slope ratio (SR), biological index (BIX), and highly bio-degradable peptide-like and aliphatic substances (p < 0.001). Our results suggest that rainstorm events enhanced the export to the lake of colored, hydrophobic, and aromatic DOM. Upstream-derived dissolved organic carbon (DOC) concentrations decreased (p < 0.001), while DOC bio-availability (BDOC) increased only slightly (p < 0.05) during rainstorm events. The loss rate of DOC in Lake Qiandao is 0.82 × 104 t C yr−1, of which 0.30 × 104 t C yr−1 is highly bio-labile, and higher occurrences of both ≥ 25 mm d − 1 and ≥ 50 mm d − 1 rainfall events are anticipated by linear fittings for this region in the future. The application of in situ fluorescence sensors provides an early warning of DOC surge incidents caused by rainstorm events and may be useful in advising drinking water treatment plant managers of changes in raw water DOM quality and treatability.

Effects of digestate on biomass of a selected energy crop and soil properties.

A large number of digestates have not been fully utilized due to a lack of scientific, reasonable guidance, as well as imperfect technology. Hybrid giant Napier has great potential for use as a type of energy plant. As such, this study investigated the effects of digestate on the growth of a candidate energy crop and examined whether digestate was an ecologically viable means for soil restoration. The results showed that the total yields of all treatment groups receiving irrigation of digestate were higher (5.19-26.00%) than those of the control. The total phosphorus, total potassium, available nitrogen, available phosphorus, and available potassium content of the soil had also increased after digestate application, compared with the control. Urease activities for all treatments increased 15.28 to 69.44% more than that of the corresponding control. Soil dissolved organic matter (DOM) mainly contained humic-like and fulvic-like components through the application of digestate. More fluorescent components were also identified by two-dimensional correlation spectroscopy (2D-COS). These fluorescent components can improve the aromaticity and molecular weight of soil DOM so as to improve soil quality. Digestate improved not only the aboveground biomass accumulation, but also the chemical properties of the soil, which was an appropriate strategy for restoring soil quality and contributing to the sustainable development of marginal. The long-term impact of digestate application on soil quality will require additional long-term experiments. © 2020 Society of Chemical Industry.

Long-term biochar addition alters the characteristics but not the chlorine reactivity of soil-derived dissolved organic matter

Biochar is widely and increasingly applied to farmlands. However, it remains unclear how long-term biochar addition alters the characteristics and chlorine reactivity of soil-derived dissolved organic matter (DOM), an important terrestrial disinfection byproduct (DBP) precursor in watersheds. Here, we analyzed the spectroscopic and molecular-level characteristics of soil-derived DOM and the formation and toxicity of DBP mixtures from DOM chlorination for two long-term (5 and 11 years) biochar addition experimental farmlands. As indicated by spectroscopic indices and Fourier transform ion cyclotron resonance mass spectrometry analyses, 11 years of biochar addition could increase the humic-like and aromatic and condensed aromatic DOM and decrease the microbial-derived DOM, while 5 years of biochar addition at the other site did not. The response of condensed aromatic dissolved black carbon did not increase with increasing cumulative biochar dose but appeared to be affected by biochar aging time. Despite the possible increase in aromatic DOM, biochar addition neither increased the reactivity of DOM in forming trihalomethanes, haloacetonitriles, chloral hydrates, or haloketones nor significantly increased the microtoxicity or genotoxicity of the DBP mixture. This study indicates that biochar addition in watersheds may not deteriorate the drinking water quality via the export of terrestrial DBP precursors like wildfire events.

Urbanization alters coastal plain stream carbon export and dissolved oxygen dynamics

Coastal plain streams in the southeastern United States supply carbon that supports important coastal ecosystems, but the effects of urbanization on carbon export from these streams have not been extensively studied. This study aimed to determine how urbanization changes coastal plain stream organic matter quality, rates of carbon export, and dissolved oxygen dynamics that have implications for stream ecosystem function. Organic matter quality, organic carbon export, and dissolved oxygen concentrations were measured for multiple years (2009 & 2013–2015) in North Carolina coastal plain streams that spanned a gradient of urbanization. Based on spectral characteristics, dissolved organic matter (DOM) quality appeared to shift from characteristic blackwater in minimally impacted streams to clear streamwater in urban streams due to large reductions in chromophoric DOM concentrations, aromaticity, and molecular weight. Differences in spectral indices and characteristics of dissolved organic carbon export suggest that urbanization reduced natural sources of DOM and provided various urban sources of DOM that were likely more bioavailable. Particulate organic matter in the urban streams was indicative of more labile autochthonous sources than that of the less impacted streams, and rates of particulate carbon export increased and shifted to higher flows with watershed impervious surface cover. Diel variation of dissolved oxygen increased with watershed impervious surface cover, indicating that urbanization and associated changes in carbon and nutrient cycling altered stream function. While the effects of urbanization on carbon export were similar to previous studies in other regions, the unique blackwater state of natural streams and receiving waters in the study area make them especially susceptible to negative ecological impacts from altered carbon and nutrient export. Management actions that conserve or restore natural carbon sources to the stream may help mitigate multiple negative effects of urbanization in southeastern US coastal plain streams.

Effect of straw-derived dissolved organic matter on the adsorption of sulfamethoxazole to purple paddy soils

The presence of sulfamethoxazole (SMX) in croplands has become an international concern. The environmental behavior and fate of SMX in agricultural soils are not well understood, especially when the adsorption behavior is disturbed by the dissolved organic matter (DOM) released by crop straw. As canola straw is one of the biomasses widely returned to farmlands, we characterized DOM derived from pristine and decomposed canola straw, and explored the effects and mechanisms of the DOMs on regulating SMX adsorption to purple paddy soils. The spectral analysis showed that the molecular weight, aromaticity, and hydrophobicity of canola straw-derived DOM increased as decomposition proceeded. These physicochemical properties collectively determined the effects of the DOM on SMX adsorption. The DOM derived from pristine canola straw increased SMX maximum adsorption capacity of the soils by approximately 2.6 times, but this positive effect gradually decreased to a steady state by day 90 in the straw decomposition period. Nevertheless, the SMX adsorption behavior in the soils was invariably determined by the DOM extracts. These adsorption processes of SMX were well fitted by the double-chamber kinetics model and the Langmuir and Freundlich thermodynamic models. Thermodynamic parameters indicated that SMX adsorption onto the soils was spontaneous and endothermic, and this adsorption characteristics was not significantly (p > 0.05) changed by the DOM extracts. However, the adsorption kinetics were altered by those DOMs, i.e., the fast and slow adsorption processes were both diminished. Correspondingly, co-adsorption and cumulative adsorption were identified as the main mechanisms determining SMX adsorption to the purple paddy soils in the presence of the straw-derived DOMs. These results collectively indicated that the DOMs released by straw in croplands may decrease the ecological risks of organic pollutants by inhibiting their migration processes.

Dissolved organic matter released from rice straw and straw biochar: Contrasting molecular composition and lead binding behaviors

It remains debatable whether carbonized straw reapplying is a better solution than direct straw reapplying. Comparison of the characteristics and complexation behaviors of dissolved organic matter (DOM) derived from straw (ST) and biochar (BC) may offer new insights, but little current information exists. Herein, DOM samples were characterized by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), revealing that the molecular weight and condensed aromatic components of BCDOM (457.70 Da and 71.16%, respectively) were higher than those of STDOM (433.48 Da and 3.13%, respectively). In particular, the N-containing compounds of BCDOM was more aromatic than STDOM. By combining spectroscopic techniques, complexation modeling, and chemometric analysis, BCDOM was shown to exhibit higher binding parameters (log KM) and more binding sites for Pb than STDOM. Noteworthily, the two binding sites, aromatic NO and aromatic NO2, existed only in the interaction of BCDOM with Pb. Furthermore, while phenol-OH displayed the fastest response to Pb in both STDOM and BCDOM, the binding sequences were not exactly the same. These differences may be related to the variations in the aromaticity and N-containing structures of DOM detected by FTICR-MS. These findings have implications on the stewardship of straw- and biochar-amended soil.

Variation in spectral characteristics of dissolved organic matter derived from rape straw of plants grown in Se-amended soil

Straw return is an effective management practice. It not only utilizes agricultural waste but also introduces dissolved organic matter (DOM) into the soil. Selenium (Se) is an essential trace element in the human diet and contributes to the popularity of Se-enriched agricultural products in the Chinese market. Moreover, there are still some Se-enriched agricultural products that have yet to be utilized. This study investigated whether Se addition in soil caused component changes in the DOM extracted from rape straw. DOM extracted from rape straw grown in soil with four Se levels (0, 0.1, 0.5 and 1.0 mg Se kg−1 soil) was characterized by UV-Visible spectroscopy, fluorescence spectroscopy and FTIR spectroscopy. The UV-visible spectra revealed that 0.1 mg Se kg−1 soil reduced the molecular weight of DOM and caused the presence of more irreplaceable aromatic structures in the substituent groups of the DOM, while 0.5 and 1.0 mg Se kg−1 soil only reduced the DOM molecular weight. Fluorescence spectroscopy indicated that Se improved the humification degree but reduced the aromaticity of DOM. FTIR spectra proved that Se altered the contents of carboxylic acids, amino acids, alcohols and aromatic heterocycles in DOM, which were maximized in the 0.5 and 1.0 mg Se kg−1 treatment groups. We concluded that Se application could change the composition of DOM extracted from rape straw, potentially impacting the nutrient bioavailability in soil. This study provides basic data on Se-enriched rape straw utilization for eco-agriculture.

Measuring the influence of environmental conditions on dissolved organic matter biodegradability and optical properties: a combined field and laboratory study

Fluorescence spectroscopy is a common tool to assess optical dissolved organic matter (DOM) and a number of characteristics, including DOM biodegradability, have been inferred from these analyses. However, recent findings on soil and DOM dynamics emphasize the importance of ecosystem-scale factors, such as physical separation of substrate from soil microbial communities and soil physiochemical cycles driving DOM stability. We apply this principle to soil derived DOM and hypothesize that optical properties can only supply information on biodegradability when evaluated in the larger ecosystem because substrate composition and the activity/abundance of the microbial community ultimately drive DOM degradation. To evaluate biodegradability in this context, we assessed aqueous soil extracts for water extractable organic carbon (WEOC) content, biodegradability, microbial biomass and DOM characteristics using fluorescence spectroscopy across a range of environmental conditions (covariant with season and land use) in northern Vermont, USA. Our results indicate that changes in environmental conditions affect composition, quantity, and biodegradability of DOM. WEOC concentrations were highest in the fall and lowest in the summer, while no significant differences were found between land covers; however, DOM biodegradability was significantly higher in the agricultural site across seasons. Despite a shift in utilized substrate from less aromatic DOM in summer to more aromatic DOM in winter, biodegradability was similar for all seasons. The only exception was cold temperature incubations where microbial activity was depressed, and processing was slowed. These results provide examples on how fluorescence based metrics can be combined with context relevant environmental parameters to evaluate bioavailability in the context of the larger ecosystem.

Spatiotemporal variations of DOM components in the Kushiro River impacted by a wetland.

Dissolved organic matter (DOM) has been recognized as a serious water quality problem in natural water bodies receiving pollution loads from point and nonpoint sources. The present study investigates the spatiotemporal variability of DOM composition in the Kushiro River and its tributaries (Eastern Hokkaido, Japan) impacted by the Kushiro wetland. Water samples were collected in the wet and dry seasons from several locations of the river and analyzed for DOM characteristics by UV-visible and excitation-emission matrix fluorescence spectroscopy techniques and by developing water quality index. Rather than the spatial effect, significant seasonal impacts on DOM pollution in the Kushiro River were observed. Overall concentrations of DOM decreased during the dry season. The increase of specific ultraviolet absorbance in the dry season indicated an increasing trend of humification, aromaticity and molecular weight of DOM. Five fluorescent peaks, including peaks A, C, M, B, and T were predicted by EEM spectra. Peaks A and C were found to be the most dominating peaks in both the seasons and indicated enrichment of humic-like matters in river water. The intensities of poly-aromatic humic substances as well as DOM components of microbial origin increase in the wet season and proteins like autochthonous DOM increase during the dry season. The study recognized the contribution of freshly produced DOM component by the decomposition of wetland plants in wet season and effect of snowfall in the dry season. Analysis of three fluorescence indices revealed that the river water primarily contains terrestrially dominated DOM. A significant impact of the adjacent WWTPs and wetland to the river water DOM were also observed. The water quality index of river water DOM showed low to medium levels of DOM pollution in the Kushiro River.

Dynamics of dissolved organic matter in headwaters: comparison of headwater streams with contrasting DOM and nutrient composition

Dissolved organic matter (DOM) in freshwaters is recognised as a significant and active component of the global carbon budget. DOM exported from terrestrial ecosystems may be compositionally and functionally altered by chemical and biological reactions as it is transported downstream. The processes affecting DOM in headwater streams remain uncertain but are potentially highly variable depending on DOM composition and nutrient availability as a function of soils, land-use, and human pressures. To investigate variability of DOM reactivity we took water samples from two contrasting headwater catchments, considered functional ‘end-members’ for DOM export: one rich in aromatic DOM and low in inorganic nutrients (peatland), and another (nearby) catchment characterized by less aromatic DOM and high nutrient loadings (agricultural grassland). Under controlled laboratory conditions, we evaluated the effects of light, presence/absence of aquatic biota and nutrient enrichment on short-term changes in DOM quantity and quality in these samples. For the peat stream, exposure to sunlight (with UV) resulted in net abiotic DOM removal, whereas in the agricultural stream it led to net biological DOM production. Nutrient addition accelerated DOM production in both streams. We conclude that in-stream changes in DOM quantity and quality represent the net effect of multiple consumption and production processes whose relative importance is strongly influenced by source-dependent DOM composition and environmental factors such as inorganic nutrient content and sunlight exposure. Our findings suggest that headwater streams may be more active processors of carbon and nutrients than presumed hitherto.

Chemical Alterations of Dissolved Organic Matter by Permanganate Oxidation.

Dissolved organic matter (DOM) is ubiquitous in raw drinking water and can efficiently scavenge oxidants, such as permanganate. Here, changes to DOM induced by permanganate oxidation under typical drinking water treatment conditions (6 μM, 1 h) to bulk DOM properties, DOM functional groups, and DOM chemical formulae were examined for two DOM isolate types (terrestrial and microbial). Permanganate oxidation did not mineralize DOM, rather changes were compositional in nature. Optical properties suggest that permanganate oxidation decreased DOM aromaticity (decreased SUVA-254), decreased DOM electron-donating capacity, and decreased DOM average molecular weight (increased E2/E3 ratios). Fourier-transform-infrared spectroscopy second derivative analyses revealed that permanganate does not oxidize DOM alkene groups, suggesting permanganate access to functional groups may be important. Four ionization techniques were used with ultrahigh-resolution mass spectrometry: negative and positive ion mode electrospray ionization and negative and positive ion mode laser/desorption ionization. The results from all four techniques were combined to understand changes in DOM chemical formulae. It was concluded that nitrogen-containing aromatic compounds and alkylbenzenes were oxidized by permanganate to form nitrogen-containing aliphatic compounds and benzoic acid-containing compounds. This work highlights how multiple ionization techniques coupled with UHR-MS can enable a more detailed characterization of DOM.

Characterisation of shallow groundwater dissolved organic matter in aeolian, alluvial and fractured rock aquifers

Groundwater organic matter is processed within aquifers through transformations such as the adsorption of dissolved organic matter (DOM) to minerals and biodegradation. The molecular character of DOM varies according to its source and this can impact its bioavailability and reactivity. Whilst the character of DOM in riverine and oceanic environments is increasingly well understood, the sources, character and ultimately the fate of groundwater DOM remains unclear. Here we examine groundwater DOM from contrasting hydrogeological settings in New South Wales, Australia. For the first time, we identify the distinct molecular composition of three groundwater DOM end-members including a modern terrestrial input, an aged sedimentary peat source, and an aged stable by-product pool. We also identify and characterise the processing pathway of DOM in semi-arid, low sedimentary organic carbon (OC) environments. Based on size exclusion chromatography, ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), isotopic analyses (13C, 14C and 3H) and principle component analysis (PCA), we show that in higher rainfall temperate coastal peatland environments, large amounts of aged sedimentary organic carbon can leach into groundwater resulting in higher molecular weight (500 g mol−1 < molecular weight > 1000 g mol−1) and highly aromatic groundwater DOM with high O/C ratios and low H/C ratios. We show that in semi-arid environments with low rainfall rates and high groundwater residence times, groundwater dissolved organic carbon (DOC) is processed into increasingly low molecular weight (<350 g mol−1), low aromaticity DOM with low O/C ratios and high H/C ratios by subsurface processing mechanisms such as biodegradation and adsorption. We provide the first comprehensive study of groundwater DOM characterisation based on multiple analytical techniques, and highlight the impact of source inputs and processing on groundwater DOM composition at a molecular level.

The importance of aromaticity to describe the interactions of organic matter with carbonaceous materials depends on molecular weight and sorbent geometry.

Dissolved organic matter (DOM) is ubiquitous in aquatic environments where it interacts with a variety of particles including carbonaceous materials (CMs). The complexity of both DOM and the CMs makes DOM-CM interactions difficult to predict. In this study we have identified the preferential sorption of specific DOM fractions as being dependent on their aromaticity and molecular weight, as well as on the surface properties of the CMs. This was achieved by conducting sorption batch experiments with three types of DOM (humic acid, Suwannee River natural organic matter, and a compost extract) and three types of CMs (graphite, carbon nanotubes, and biochar) with different geometries and surface complexities. The non-adsorbed DOM fraction was analyzed by size exclusion chromatography and preferentially sorbed molecular weight fractions were analyzed by UV/vis and fluorescence spectroscopy. All three sorbent types were found to preferentially sorb aromatic DOM fractions, but DOM fractionation depended on the particular combination of sorbent and sorbate characteristics. Single-walled carbon nanotubes only sorbed the smaller molecular weight fractions (<1 kDa). The sorption of smaller DOM fractions was not accompanied by a preference for less aromatic compounds, contrary to what was suggested in previous studies. While graphite preferentially sorbed the most aromatic DOM fraction (1-3 kDa), the structural heterogeneity of biochar resulted in reduced selectivity, sorbing all DOM > 1 kDa. The results explain the lack of correlation found in previous studies between the amount of aromatic carbon in a bulk DOM and its sorption coefficient. DOM sorption by CMs was generally controlled by DOM aromaticity but complex sorbent surfaces with high porosity, curvatures and functional groups strongly reduced the importance of aromaticity.

Spectroscopic and molecular-level characteristics of dissolved organic matter in the Pearl River Estuary, South China

Coastal populations are expanding globally, resulting in great anthropogenic impacts on the organic matter in estuaries and regional carbon cycles. However, the molecular-level characteristics of dissolved organic matter (DOM) within highly disturbed estuaries are still not well understood. Here, water samples collected during two seasons (wet and dry) from the subtropical Pearl River Estuary of China were analyzed using absorption and fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to determine the spatiotemporal variations in the DOM characteristics. In the seaward direction, the abundances of chromophoric and fluorescent DOM decreased by greater percentages than the bulk dissolved organic carbon concentration. The spectroscopy and FT-ICR MS analyses collectively indicated seaward declines in the aromaticity of DOM and terrestrial DOM contributions from natural terrestrial markers and anthropogenic synthetic surfactants. In particular, the S content in DOM was much higher here than in previously reported estuaries, suggesting a strong anthropogenic impact on the estuarine DOM. Greater terrestrial and anthropogenic signatures in DOM were observed in the wet season than in the dry season. Importantly, this study implies that the terrestrial and anthropogenic contributions to DOM were strongly driven by season in the anthropogenically disturbed subtropical estuary.

Dissolved organic matter as P source for biofilms in two contrasting low-order streams

Forest streams receive large amounts of leaves whose leachates are an important source of dissolved organic matter (DOM), providing not only carbon but also organic nutrients to the microbial communities in streams. I carried out a field study to evaluate the effect of different DOM concentrations on the biofilm structure and functional traits in two similar forest streams belonging to the same catchment. I compared biofilm biomass and nutri- ent content throughout one year, algal species composition, and biofilm community-level physiological profiles in two streams with different DOM concentration and aromaticity. Dissolved nutrient concentrations were higher in the stream with higher DOM concentration, with a concomitant higher biofilm biomass, and there was also a temporal pattern, with higher values during the autumn. Phosphorus content in biofilms was also higher in the high DOM stream, coincidently with a higher capacity of the community to utilize organic P source (glucose-1-P) as a substrate. In contrast, the biofilms from the stream with lower DOM concentrations preferentially used N-organic substrates (amino acids and amines). These results reveal that the biofilms of forest streams make use of organic matter nutrients, so that streams with different DOM loads may differ in biofilm biomass due to changes in both bacterial and autotrophic biomass. In addition, biofilm dynamics may be related to forest phenology, as the highest OM input in this deciduous forest is represented by tree leaves, which supply DOM through leachates, and in particular, with P-rich leachates. In conclusion, different DOM concentrations in two nearby streams led to differences in the community-level physiological profile, as has been previously demonstrated at larger spatial scales in oceans, lakes and along larger rivers.

Linking stream microbial community functional genes to dissolved organic matter and inorganic nutrients

AbstractThere is now increasing evidence for the importance of microbial regulation of biogeochemical cycling in streams. Resource availability shapes microbial community structure, but less is known about how landscape‐mediated availability of nutrients and carbon can control microbial functions in streams. Using comparative metagenomics, we examined the relationship between microbial functional genes and composition of dissolved organic matter (DOM), nutrients, and suspended microbial communities in 11 streams, divided into three groups based on the predominant land cover category (agriculture, forested, or wetland). Using weighted gene co‐occurrence network analysis, we identified clusters of functions related to DOM composition, agricultural land use, and/or wetland and forest land cover. Wetland‐dominated streams were characterized by functions related to nitrogen metabolism and processing of aromatic carbon compounds, with strong positive correlations with dissolved organic carbon concentration and DOM aromaticity. Forested streams were characterized by metabolic functions related to monomer uptake and carbohydrates, such as mannose and fructose metabolism. In agricultural streams, microbial functions were correlated with more labile, protein‐like DOM, PO4, and NO3, likely reflecting functional adaptation to labile DOM and higher nutrient concentrations. Distinct changes in the functional composition and loss of functional diversity of microorganisms became evident when comparing natural to agricultural catchments. Although all streams showed signs of functional redundancy, loss of species richness per function in agricultural catchments suggests that microbial functions in natural catchments may be more resilient to disturbance. Our results provide new insight into microbial community functions involved in nutrient and carbon biogeochemical cycles and their dependence on specific environmental settings.

Absorption and Fluorescence Spectra of Dissolved Organic Matter in Rivers of the Three Gorges Reservoir Area Under the Background of Urbanization

Studying the influences of urbanization on the composition, chemical structure, source and migration of dissolved organic matter (DOM) in rivers is of great significance for aquatic ecosystem protection and biogeochemical carbon (C) cycling. The DOM spectroscopic characteristics of the three typical rivers (Taohuaxi, Nanhe, and Puli) with different urbanization levels in the Three Gorges Reservoir area were investigated using the ultraviolet-visible (UV-Vis) absorption spectrum and three-dimensional fluorescence spectroscopy. The results demonstrated that the higher the degree of urbanization in rivers, the greater the concentrations of DOM and chromophoric dissolved organic matter (CDOM), the lower the humification degree and aromaticity of DOM, the smaller the proportion of hydrophobic components, the higher the relative level of protein-like substance concentration in DOM, and the greater the contribution of DOM from freshly autochthonous origins. The DOM of the three rivers was dominated by fulvic acids (E3/E4>3.5), and CDOM concentration was positively correlated with the concentration of DOM (P<0.01). There were significant positive correlations between the humification degree (SUVA254), aromaticity (SUVA280), and hydrophobic component (SUVA260) (P<0.01). The sources of humic substances in the DOM were mixed by terrestrial input and microbial or algae production (reflected by the fluorescence index averages of 1.7157-1.7571), and mainly from microbial sources.