Properties Of Dissolved Organic Matter Research Articles

Spatiotemporal dynamics of dissolved organic matter in subtropical karst cave waters identified by optical properties

Natural dissolved organic matter (DOM) is ubiquitous in aquatic environments and is an essential component in the carbon cycle in karst areas. To improve understanding of the carbon cycle in karst caves with heterogeneous hydrological processes, we examined the spatiotemporal variability of DOM composition and further uncovered its source and fate. Results may also provide insights into the feedbacks of organic carbon to carbon sinks in karst regions. In this study, concentrations and compositions of DOM, partial pressure of aqueous carbon dioxide (pCO2), dissolved inorganic carbon, and other physicochemical parameters were investigated in a karst cave at Mahuang, Southwest China. Ultraviolet-visible absorption spectroscopy was coupled with multiple statistical analyses to identify the compositional variations and potential fates of DOM in cave waters. The results showed that DOM dynamics were regulated by both terrigenous and biogenic drivers under the control of meteorological conditions. With higher air temperature, precipitation, and microbial activity, fulvic fractions were consumed to generate CO2, leading to the accumulation of refractory DOM in cave waters and changing the hydrochemical features. When temperature and precipitation decreased, DOM was dominated by lignin fractions, which served as an indicator of terrestrial inputs and vascular plants, suggesting variation in the preferential fraction of biological consumption. In addition, different hydrological path patterns influenced DOM properties in cave waters due to differences in recharging, the leaching process, and subsurface reworking. Thus, hydrology could serve as an important constraint on the coupling between dissolved organic and inorganic carbon.

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.

Differences in dissolved organic matter (DOM) composition of soils from native eucalypt forests and exotic pine plantations impacted by wildfire in Southwest Australia

In recent decades, plantations of exotic species have transformed large tracts of forested landscapes, where they often form a mosaic with the surrounding native forest. Wildfires have also increased in frequency and intensity globally, impacting both plantation and native forests and affecting soil organic matter (SOM), including dissolved organic matter (DOM). DOM of soils is considered highly labile and likely sensitive to effects of fire and changes in land use. However, it is unclear how wildfire influences key characteristics of DOM in exotic plantations compared to native forest. In this study, we investigated the impacts of wildfire on chemical properties of DOM in exotic Pinus radiata plantations and adjacent native jarrah (Eucalyptus marginata) forest in southwest Australia. We coupled fluorescence excitation-emission matrix parallel factor analysis (EEM-PARAFAC) and 1H‐NMR with soil nutrient analyses to characterise soil DOM (0–5 cm) from burned and unburned plots of each forest type, four months after the fire. Four fluorescent components (humic-, fulvic-, microbial and protein-like substances) were derived from soil DOM using the PARAFAC model. DOM from burnt forest soils was predominantly of lower molecular weight (E2:E3) and contained a greater microbial-derived component and a lower fulvic component than unburnt forest. As expected, soil pH and labile inorganic P increased in both forest types after fire. By contrast, an increase in the amount of DOC after fire was only recorded in native forest, while δ15N signatures were more enriched in pine plantation soils after fire but not in native forest soils. Differences in impacts of fire on DOC and δ15N between forest types suggest a different response to fire according forest type, even though these differences were not evident above ground. The 1H NMR spectra of DOM revealed all samples were largely dominated by aliphatic structures. However, there were no significant differences in the overall spectra between forest types or between burnt and unburnt forests. Overall, our findings suggest that transformations in soil DOM after fire were affected by feedbacks between forest type and fire (where soils from pine plantations appeared to be more affected by fire).

Using Stable Isotopes and Spectral Properties of Particulate and Dissolved Organic Matter to Quantify Typhoon‐Induced Organic Matter Decomposition

AbstractTyphoons exert a profound impact on marine biogeochemical processes, with organic matter (OM) decomposition serving as a key biogeochemical process in coastal waters after typhoons. Despite its significance, the specific contributions of particulate organic matter (POM) and dissolved organic matter (DOM) to OM decomposition triggered by typhoons remains unclear. In this study, stable isotopes of POM and spectral properties of DOM were investigated before and after Typhoon “Barija" in Zhanjiang Bay, northwestern South China Sea. The typhoon‐induced intrusion of high‐salinity seawater from the lower bay to the upper bay, driven by the external clockwise wind stress, led to the formation of an ocean front in the middle bay during the typhoon. Notably, the POM decomposition induced by the typhoon in the upper bay (72%) significantly surpassed that in the lower bay (5%), attributed to the barrier effect of the ocean front and increased vertical mixing in the upper bay. In contrast, the decomposition removed only 29% DOM in the upper bay, and a net addition of DOM occurred in the lower bay due to phytoplankton growth and POM decomposition. More importantly, despite the overall larger DOM in the water column, the POM inventory in the upper bay removed by typhoon‐induced decomposition (20.19 g m−2) exceeded that of DOM (16.08 g m−2). Overall, our study suggests that POM decomposition is more critical than DOM decomposition after typhoons, primarily influenced by the ocean front and vertical mixing. These findings contribute to a clearer understanding of the post‐typhoon biogeochemical dynamics in coastal waters.

Depth-dependent variations of physicochemical properties of sedimentary dissolved organic matter and the influence on the elimination of typical pharmaceuticals

Sedimentary dissolved organic matter (DOM) could exert a significant influence on the transformation of trace organic contaminants. However, the variations of sedimentary DOM properties with depth and their impact on trace organic contaminants biodegradation remain unclear. In this study, the qualitative changes in DOM properties with depth were assessed using spectral techniques. Specifically, within the sediment range of 0–30 cm, humic acid and fulvic acid fractions exhibited higher degrees of humification and aromatization at 10–20 cm, while hydrophilic fractions showed higher degrees of humification and aromatization at 20–30 cm. Furthermore, electrochemical methods were employed to quantitatively assess the electron transfer capacity of sedimentary DOM at different depths, which displayed consistent variation trend with humification and aromatization degree. The high degree of humification and aromatization, along with strong electron-accepting capability of DOM, significantly enhanced the biodegradation rates of tetracycline and ritonavir. To gain deeper insights into the influence of molecular composition of DOM on its properties, two-dimensional gas chromatography-quadrupole mass spectrometry analysis revealed that quinones and phenolic hydroxyl compounds govern the redox reactivity of DOM. Simulated experiment of DOM-mediated biodegradation of typical pharmaceuticals confirmed the role of quinones and phenolic hydroxyl groups in the redox reactivity of DOM.

Methods for molecular characterization of dissolved organic matter in the alpine water environment: an overview

The alpine area has become a sensitive indicator and amplifier of global climate change and human activities because of its unique geographical and climatic conditions. Being an essential part of biochemical cycling, dissolved organic matter (DOM) could effectively help understand the process, structure, and function of alpine aquatic ecosystems. Due to the low content and sampling difficulties, the analysis of DOM in alpine water demands high sensitivity with low sample volume, which has not been comprehensively reviewed. This review summarizes the DOM sampling, pretreatment, and analysis methods involving the characterization of concentration, spectroscopy, and molecular structure. Overall, conventional parameters are the basis of advanced characterization methods. Spectroscopic tests can reveal the optical properties of DOM in response to lights from ultraviolet to infrared wavelengths, to distinguish the chemical composition. Molecular structure characterizations can provide microscopic information such as functional groups, element ratios, and molecular weights. The combination of multiple methods can depict DOM composition from multiple perspectives. In sum, optimized sampling and pretreatment, high-sensitivity molecular characterization, and method integration are crucial for effectively analyzing DOM components in alpine waters. These perspectives help to standardize the DOM characterization process and to understand the correlation between DOM composition and its properties, as well as the migration and transformation of DOM.

In-reservoir transformation of dissolved organic matter as a function of hydrological flow

Reservoirs are vital to meet the ever-increasing demands for freshwater in a warming climate. Dissolved organic matter (DOM) represents an important pool of carbon and can be a major concern in drinking water sources. However, insights into DOM dynamics in temperate, semi-arid reservoirs remain limited. Therefore, we investigated the variations in DOM properties in Lake Diefenbaker, a large reservoir on the Canadian Prairies, by analyzing eight years of DOM concentrations and composition through linear mixed effect modeling. Contrary to expectations, reservoir dissolved organic carbon (DOC) concentration showed no correlation with inflow from the South Saskatchewan River (p = 0.12), while dissolved organic nitrogen (DON) increased with decreasing inflow (p = 0.002). DOM optical indices (SUVA254 and E4:E6 ratio) and DOC:DON ratio revealed a pronounced influence of inflow on reservoir DOM composition (p < 0.001), i.e., allochthonous characteristics increased with increasing flow, and autochthonous characteristics increased with declining flow. Travel time corrected comparison of approximately the same water parcel along the reservoir length revealed that increasing water residence time in downstream regions led to a significant transformation in DOM composition, favoring autochthonous characteristics (mean SUVA254 reduced by 0.52 L mg-C−1 m−1, and the E4:E6 and spectral slope ratio increased by 1.6 and 0.06, respectively). Autochthonous DOC inputs likely offset the allochthonous DOC losses, which resulted in a relatively stable DOC concentration throughout the reservoir (mean 3.7 mg L−1). Additionally, the effect of a large aquaculture operation on reservoir DOM properties was investigated, but no effect was detected. The results have significant implications for managing large river-reservoirs. Autochthonous DOM poses challenges to water processing, necessitating monitoring of DOM composition for reservoir drinking water quality. Insights on climate-induced changes in DOM properties will also assist with understanding changes to habitat conditions and contaminant transport.

Effects of production temperature on the molecular composition and seed-germination-promoting properties of sludge-based hydrochar-derived dissolved organic matter

Sludge hydrothermal carbonization demonstrates potential for converting sludge into multifunctional carbon materials for soil remediation. However, the influence of dissolved organic matter (DOM) with unclear molecular characteristics in sludge-based hydrothermal carbon on plant growth has not been sufficiently investigated. Herein, the effects of hydrothermal temperature on the molecular transformation pathways and plant-growth-promoting properties of DOM were investigated via FT-ICR MS–based molecular network analyses and seed germination experiments. Results indicated that the highest DOM yield was achieved at 220 °C. During low-temperature (180 °C) hydrothermal treatment, the hydrolysis of biopolymers, as well as the partial condensation and cyclization of small-molecule intermediates, occurred in the sludge. This process produced unsaturated CHNO compounds containing one or two N atoms, which promoted seed germination. Further, the toxicity of DOM to plants increased with rising hydrothermal temperature. This was accompanied by S doping and aromatization reactions, which mitigated the effects of plant growth hormones. This study provides theoretical support for the optimization of sludge hydrothermal treatment and production of plant growth hormones, enhancing the ecological value of sludge-based hydrochar.

The dependence in microcystin removal with powdered activated carbon on variant properties, carbon properties, and dissolved organic matter

Powdered activated carbon (PAC) is commonly used by water treatment plants to remove harmful cyanotoxins such as microcystins (MCs) produced during seasonal harmful algal blooms. MC removal by PAC depends upon the properties of the PAC, the properties of the MC variant, and the presence and properties of dissolved organic matter (DOM). To identify which of these factors has the greatest impact on the removal of MC by PAC, we evaluated the removal of four different MC variants (MC-LR, MC-LA, MC-RR and desmethylated MC-RR) by three different PAC types (wood-based, coal-blend and coal-based). The role of DOM properties was evaluated using DOM isolated from two different sources, a terrestrial source (Suwannee River Fulvic Acid, SRFA) and a microbial source (Grand Lake St Marys DOM, GLSM). The results of adsorption experiments conducted over a period of 72 h demonstrated the wood-based PAC, which had the highest surface area and mesopore volume of the PAC tested, had the highest adsorption rate and capacity for all four MC variants. Of the variants studied, neutrally charged MC-RR was adsorbed more rapidly and to a greater extent on all of the PAC types than were the other variants. Although MC-LA and MC-LR had the greatest hydrophobicity, their negative charges resulted in their being adsorbed the least. As expected, DOM inhibited microcystin adsorption to PAC. The degree of inhibition, however, did not significantly vary for the two DOM types evaluated, indicating the properties of the DOM on MC adsorption to PAC was less important than the PAC properties or MC variant properties. Overall, PAC properties were a more important factor in MC removal than were the MC properties or DOM conditions.

Emerging investigator series: impacts of land use on dissolved organic matter quality in agricultural watersheds: a molecular perspective.

In aquatic systems, dissolved organic matter (DOM) has important ecological and biogeochemical functions, where the molecular composition of DOM has larger-scale implications for climate change and global carbon cycles. However, there is limited information about the relationships between landscape characteristics and human disturbance that influence the molecular composition of DOM changes in watersheds. In this study, we collected water samples from 22 sites across a gradient of topographically characterized agricultural land coverage and community infrastructure development in the Kawartha region in Ontario, Canada. We employed a combination of Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) and absorbance spectroscopy to investigate changes in the molecular composition of DOM with increasing agricultural and community development disturbance on the optical and molecular characteristics of DOM. We found that dissolved organic carbon (DOC) concentrations in disturbed (>75%) watersheds ranged from 3.67-32.8 mg L-1 and were significantly higher than in watersheds with more abundant forest coverage (3.78-9.13 mg L-1). In addition, watersheds with higher phosphorus concentrations had more negative nominal oxygenation state of carbon (NOSC) values, suggesting biologically processed DOM correlating with increased phosphorus levels in aquatic systems. To relate the molecular properties of DOM to landscape metrics, we used Spearman's correlation analysis to reveal that agriculturally impacted and community developments enhanced the molecular signature of unsaturated hydrocarbon. In addition, we identified 65 dissolved organic phosphorus (DOP) molecules that significantly increased in abundance with disturbance, likely due to microbial mineralization of existing DOM with the addition of phosphorus to form larger, biologically inaccessible molecules. The overall recalcitrance of the identified molecules can serve as molecular signatures when evaluating the level of disturbance of a watershed.

Exploring the diversity of dissolved organic matter (DOM) properties and sources in different functional areas of a typical macrophyte - derived lake combined with optical spectroscopy and FT-ICR MS analysis

Lake Baiyangdian is one of China's largest macrophyte - derived lakes, facing severe challenges related to water quality maintenance and eutrophication prevention. Dissolved organic matter (DOM) was a huge carbon pool and its abundance, property, and transformation played important roles in the biogeochemical cycle and energy flow in lake ecosystems. In this study, Lake Baiyangdian was divided into four distinct areas: Unartificial Area (UA), Village Area (VA), Tourism Area (TA), and Breeding Area (BA). We examined the diversity of DOM properties and sources across these functional areas. Our findings reveal that DOM in this lake is predominantly composed of protein - like substances, as determined by excitation - emission matrix and parallel factor analysis (EEM - PARAFAC). Notably, the exogenous tyrosine-like component C1 showed a stronger presence in VA and BA compared to UA and TA. Ultrahigh - resolution mass spectrometry (FT - ICR MS) unveiled a similar DOM molecular composition pattern across different functional areas due to the high relative abundances of lignan compounds, suggesting that macrophytes significantly influence the material structure of DOM. DOM properties exhibited specific associations with water quality indicators in various functional areas, as indicated by the Mantel test. The connections between DOM properties and NO3N and NH3N were more pronounced in VA and BA than in UA and TA. Our results underscore the viability of using DOM as an indicator for more precise and scientific water quality management.

Evaluation of Freshwater Using Chromatographic Analyses of Dissolved Organic Matter Data from the Hypertrophic River Vääna, Estonia

This study aims to investigate if high-resolution dissolved organic matter (DOM) data, obtained from water by chromatographic analyses, enable us to assess whether seasonal climate change and anthropogenic activities in the catchment area have an impact on the ecosystem’s sustainability. More specifically, the changes in the molecular properties of heterogeneous DOM from the hypertrophic River Vääna, Estonia, during the winter and spring seasons have been examined. The chromatographic HPLC method (HPLC-SEC), combined with UV-Vis spectroscopic detection, was used to characterize and fractionate DOM. Changes in several chromatographic/molecular parameters were investigated. The microbial-derived low-molecular-mass aromatic and heterocyclic compounds, humic substances (HS), and protein-like (PL) components were identified in the DOM. The HS to PL fractions ratio has been found to reflect the seasonal climatic change and can be applied as a potential environmental indicator. The River Vääna’s water was evaluated as sustainable, and even an anthropogenic impact was revealed. The results point out the usefulness of HPLC-SEC together with UV-Vis spectroscopy detection for climate change-related DOM studies in real environmental conditions.

Urbanization influences dissolved organic matter characteristics but microbes affect greenhouse gas concentrations in lakes

Recognition and prediction of dissolved organic matter (DOM) properties and greenhouse gas (GHG) emissions is critical to understanding climate change and the fate of carbon in aquatic ecosystems, but related data is challenging to interpret due to covariance in multiple natural and anthropogenic variables with high spatial and temporal heterogeneity. Here, machine learning modeling combined with environmental analysis reveals that urbanization (e.g., population density and artificial surfaces) rather than geography determines DOM composition and properties in lakes. The structure of the bacterial community is the dominant factor determining GHG emissions from lakes. Urbanization increases DOM bioavailability and decreases the DOM degradation index (Ideg), increasing the potential for DOM conversion into inorganic carbon in lakes. The traditional fossil fuel-based path (SSP5) scenario increases carbon emission potential. Land conversion from water bodies into artificial surfaces causes organic carbon burial. It is predicted that increased urbanization will accelerate the carbon cycle in lake ecosystems in the future, which deserves attention in climate models and in the management of global warming.

Effect of experimental warming on dissolved organic matter and bacterial diversity in a forest swamp soil

Forest swamp ecosystems play an important role in the global carbon cycle, yet they are often overlooked. With global climate warming, it is inevitable that it will impact the relationship between soil microorganisms and organic matter. Dissolved organic matter (DOM) in the soil environment is extremely sensitive to environmental changes. Understanding the effects of climate change on DOM and microorganisms is crucial for assessing the stability of carbon (C) in forest swamp soils. Therefore, we conducted a 142-day laboratory warming incubation experiment (control: 10 °C, and warming: 20 °C) to investigate the response of forest swamp soil microbial and DOM properties to warming. Excitation-emission matrix (EEM) fluorescence spectroscopy was used to explore the changes in different material compositions of DOM over time at different incubation temperatures, and high-throughput sequencing was used to investigate the changes in soil bacteria. The DOM content (dissolved organic C, DOC) and the degree of humification index (HIX) increased with an increase in incubation time under warming conditions (p < 0.05). In contrast, microbial humic substances (C3) decreased with increasing time. Long-term warming has separated bacterial communities and gradually tightened the degree of connectivity between bacterial networks. The degree of soil humification (Mantel test r = 0.58, p < 0.01) and DOC (r = 0.21, p < 0.05) were the most critical indicators that changed the diversity of the bacterial community. Our findings suggest a degree of interaction between changes in forest swamp soil microbial communities and DOM under warming conditions. The results of this study contribute to our understanding of changes in DOM fluorescence indices in forest swamp soils under the influence of climate change and their associated microbial mechanisms.

Contrasting optical properties of dissolved organic matter between oceanic regions near the Getz and Dotson ice shelves in the Amundsen Sea, West Antarctica

The Amundsen Sea, located in West Antarctica, is experiencing rapid melting due to the intrusion of Circumpolar Deep Water, which is causing ice sheet thinning and basal melting. The resulting changes can affect the biogeochemical cycle of dissolved organic matter (DOM) by supplying iron from sea ice and/or glacier, thereby influencing primary production and ocean circulation. Therefore, it is crucial to understand the dynamics of the DOM in this region. In this study, our primary focus was to examine the optical properties of DOM in the oceanic regions adjacent to the West Getz Ice Shelf (WGIS) and the Dotson Ice Shelf (DIS). Significant differences in DOM optical properties, including the chromophoric DOM (CDOM) absorption coefficient at 350 nm (a350), spectral slope coefficient (S275–295), and specific UV absorbance at 254 nm (SUVA254), were observed between the WGIS and DIS regions (t-test, p < 0.05). Notably, the WGIS regions exhibited high a350 values. Additionally, the S275–295 and SUVA254 values, which serve as indices of molecular weight, indicated that the DOM pool in the WGIS regions was dominated by high molecular weight compounds with a substantial proportion of aromatic compounds. In contrast, the low values of a350 and SUVA254 along with the high S275–295 values in the DIS region suggested the dominance of low molecular weight CDOM associated with compounds of lower aromaticity. Furthermore, significant negative correlations were found between biomass of Phaeocystis antarctica (P. antarctica) and phosphate (PO4) in the WGIS regions (r2 = 0.82, p < 0.01 for WGIS 1 and r2 = 0.73, p < 0.01 for WGIS 2). However, no significant relationship was observed in the DIS region. These findings suggest that the high value and molecular weight of a350, extending from the surface layer to the deep layer, in the WGIS regions were associated with autochthonous sources, primarily driven by the colony-forming bloom of P. antarctica. These findings demonstrate that the quantity and quality of DOM in the Amundsen Sea are strongly affected by bloom conditions. The results emphasize that a combination of physical and biological processes interacts in complex ways to determine the characteristics of DOM in the Amundsen Sea.

Prediction of dissolved organic nitrogen via spectroscopic fingerprint in the shallow riverbed sediments of effluent-dominated rivers: A case study in Xi’an, northwest China

An evaluation on the impacts of effluent discharge to receiving rivers is in demand. Discharged dissolved organic nitrogen (DON) with high eutrophication potentials tends to accumulate in the shallow riverbed sediments especially for effluent-dominated rivers. Since DON characterization requires for advanced analytical tools, there is thereby a need but it is still a challenge to evaluate their distribution and transformation in the riverbed sediments. Considering the fact that DON is an important part of dissolved organic matter (DOM), which has plentiful spectral information, the authors hypothesize if it is feasible to predict DON variations in riverbed sediments via the DOM properties in surface water or sediment porewater? Here, the spatiotemporal spectral characteristics of DOM in river water and sediment porewater of two effluent-dominated rivers were investigated. Compared to surface water, more abundant relationships among fluorephores and spectra-derived parameters in porewater were observed. The relatively high DON contents in both surface water (0.16–0.47 mg-N/L) and sediment porewater (0.35–5.02 mg-N/L) compared to those reported in other natural waters highlighted the importance in predicting DON in effluent-dominated rivers. The DON in the oxic zone from winter samples carried more humus-like signals, while the fractions in summer samples recognized protein-like and humus-like signals. Two machine learning methods, i.e., random forest (RF) and extreme gradient boosting (XGBoost) were employed to provide evidence-based information on how DON contents and properties differed at variable depths. RF (R2: 0.920, 0.475) outperformed XGBoost (R2: 0.488, 0.316) in predicting DON in winter-oxic zone and winter-suboxic zone. While XGboot (R2: 0.515, 0.787) is superior than RF (R2: 0.317, 0.567) in summer-oxic zone and summer-suboxic zone. Fluorophore of C1 served as the most important variable for predicting DON in winter samples in both models. SUVA260 and SUVA280 were verified most impressive predictor for samples from suboxic zone in summer in both models. HIX represented the most important variable predicting DON in summer samples in XGBoost model. These findings are critical for understanding the transformation processes of DON in the shallow riverbed sediments in receiving rivers, and could provide important implications to improve wastewater discharge management and river health.

Effects of disturbance modes and carbon sources on the physiological traits and nutrient removal performance of microalgae (S. obliquus) for treating low C/N ratio wastewater

Wastewater treatment with microalgae is an ecologically sustainable process. In this study, the growth characteristics, nutrient removal, and spectral changes of dissolved organic matter (DOM) in microalgae bioreactors were investigated for treating low C/N ratio wastewater under different disturbance modes (agitation and aeration) and carbon sources (sucrose and humic acid). The results showed that the biomass and chlorophyll-a contents of Scenedesmus obliquus in the aeration condition (725.32–811.16 × 104 cells mL−1, 1.58–1.69 mg L−1) were higher than those in the agitation condition (426.06–465.14 × 104 cells mL−1, 1.48–1.61 mg L−1). The better removal of nutrients (TN, 29.62–36.39 mg L−1, TP, 1.84–2.30 mg L−1) by microalgae in sucrose-containing wastewater under agitation conditions occurred on the second day, with removal efficiencies of 21.33–30.67% and 44.84–58.51%, respectively; while it was on the fifth day both in sucrose and humic acid-containing wastewater under aeration conditions (TN, 19.56–31.20 mg L−1, TP, 0.26–0.30 mg L−1), with removal efficiencies of 13.92–46.75% and 88.36–90.50%, respectively. The wastewater DOM primarily consisted of humic-like substances under agitation and aeration conditions characterized by high levels of aromaticity, molecular weight and humification. Furthermore, the aromatization and humification properties of DOM in humic acid wastewater were higher than those in sucrose wastewater, which was corresponding with the lower removal and availability of pollutants by algae. Microalgae showed good biomass accumulation and nutrients removal at incubation time of 2 days (agitation condition) and 5 days (aeration condition), respectively. Consequently, a technical reference is provided for the microalgae coupled with other treatment processes.

DOM Associates with Greenhouse Gas Emissions in Chinese Rivers under Diverse Land Uses.

Growing evidence indicates that rivers are hotspots of greenhouse gas (GHG) emissions and play multiple roles in the global carbon budget. However, the roles of terrestrial carbon from land use in river GHG emissions remain largely unknown. We studied the microbial composition, dissolved organic matter (DOM) properties, and GHG emission responses to different landcovers in rivers (n = 100). The bacterial community was mainly constrained by land-use intensity, whereas the fungal community was mainly controlled by DOM chemical composition (e.g., terrestrial DOM with high photoreactivity). Anthropogenic stressors (e.g., land-use intensity, gross regional domestic product, and total population) were the main factors affecting chromophoric DOM (CDOM). DOM biodegradability exhibited a positive correlation with CDOM and contributed to microbial activity for DOM transformation. Variations in CO2 and CH4 emissions were governed by the biodegradation or photomineralization of dissolved organic carbon derived from autotrophic DOM and were indirectly affected by land use via changes in DOM properties and water chemistry. Because the GHG emissions of rivers offset some of the climatic benefits of terrestrial carbon (or ocean) sinks, intensified urban land use inevitably alters carbon cycling and changes the regional microclimate.

Patterns and drivers of prokaryotic communities in thermokarst lake water across Northern Hemisphere

AbstractAimThe formation of thermokarst lakes could make a large amount of carbon accessible to microbial degradation, potentially intensifying the permafrost carbon‐climate feedback via carbon dioxide/methane emissions. Because of their diverse functional roles, prokaryotes could strongly mediate biogeochemical cycles in thermokarst lakes. However, little is known about the large‐scale patterns and drivers of these communities.LocationPermafrost regions in the Northern Hemisphere.Time periodPresent day.Major taxa studiedProkaryotes.MethodsBased on a combination of large‐scale measurements on the Tibetan Plateau and data syntheses in pan‐Arctic regions, we constructed a comprehensive dataset of 16S rRNA sequences from 258 thermokarst lakes across Northern Hemisphere permafrost regions. We also used the local contributions to beta diversity (LCBD) to characterize the variance of prokaryotic species composition and screened underlying drivers by conducting a random forest modelling analysis.ResultsProkaryotes in thermokarst lake water were dominated by the orders Burkholderiales, Micrococcales, Flavobacteriales and Frankiales. The relative abundance of dominant taxa was positively associated with dissolved organic matter (DOM) properties, especially for the chromophoric/aromatic compounds. Microbial structure differed between high‐altitude and high‐latitude thermokarst lakes, with the dominance of Flavobacterium in high‐altitude lakes, and the enrichment of Polynucleobacter in high‐latitude lakes. More importantly, climatic variables were among the main drivers shaping the large‐scale variation of prokaryotic communities. Specifically, mean annual precipitation was the best predictor for prokaryotic beta diversity across the Northern Hemisphere, as well as in the high‐altitude permafrost regions, while mean annual air temperature played a key role in the high‐latitude thermokarst lakes.Main conclusionsOur findings demonstrate significant associations between dominant taxa and DOM properties, as well as the important role of climatic factors in affecting prokaryotic communities. These findings suggest that climatic change may alter DOM conditions and induce dynamics in prokaryotic communities of thermokarst lake water in the Northern Hemisphere.

An Effect-Based Method to Evaluate Spatio-Temporal Variations of Coastal Marine DOM Properties

Marine dissolved organic matter (DOM) is a key component of the global carbon cycle. While marine DOM properties are usually characterized by chemical approaches, an effort to integrate its variations in an ecological perspective is needed. Heterotrophic prokaryotes being the main consumers of marine DOM, targeting their responses to varying DOM properties provides an effect-based characterization of DOM properties. Dilution experiments of natural marine prokaryotic assemblages were used to assess the spatio-temporal heterogeneity of DOM properties in the coastal environment through its potential to stimulate heterotrophic prokaryotic growth. Reduced top-down pressures allowed to repeatedly evidence a stronger bottom-up stimulation potential of heterotrophic prokaryotes’ net growth with DOM collected in harbours under strong human and continental influence compared to DOM collected in more open coastal sites. Shedding the light on high spatial heterogeneity of DOM properties that corresponded only partly to dissolved organic carbon concentrations, this experimental approach represents a simple and reproducible method to improve our understanding of the marine DOM cycle.