Tryptophan-like Component Research Articles

Unraveling the role of anthropogenic and hydrologic drivers with respect to the optical and molecular properties of dissolved organic matter and organic phosphorus in a P-contaminated river

Anthropogenic and hydrological drivers are key factors influencing the fate of dissolved organic matter (DOM) and dissolved organic phosphorus (DOP) in river runoff. However, how anthropogenic disturbances and hydrological conditions jointly affect the composition and characteristics of DOM and DOP in river runoff remains unclear. This study used fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and the stable water isotopes to interpret the chemical composition and properties of DOM and DOP as well as their linkages to anthropogenic disturbances and hydrological conditions in a typical P-contaminated tributary to the central Yangtze River. The results show in the wet season, the average abundance of humic-like components in DOM exceeded 60 %, while the average abundance of tryptophan-like components in DOM exceeded 50 % in the dry season. During the dry season, hydrological conditions had a greater impact on highly unsaturated DOM compounds compared to anthropogenic disturbances because a decrease in precipitation reduced the transport of terrestrial DOM into aquatic systems and increased water retention time in the river, promoting the production of unsaturated compounds from photochemistry. The effects of the two factors were similar in the wet season because active agricultural activities and intense precipitation jointly facilitated the entry of exogenous humics into the runoff, leading to the similar relative abundance of highly unsaturated DOM compounds associated with both factors. Anthropogenic disturbances had a greater impact on aliphatic DOM and DOP than hydrological conditions, which was associated with intense human activities in the watershed, such as phosphate mining, agricultural cultivation, and domestic sewage discharge. This study provides new knowledge about the composition, properties and underlying mechanisms of DOM and DOP in the P-contaminated watershed runoff.

Comprehensive insights into the impact of magnetic biochar on protein hydrolysis in sludge anaerobic digestion: Protein structures, microbial activities and syntrophic metabolisms

The addition of composite conductive materials is being increasingly recognized as a promising strategy to enhance anaerobic digestion (AD) performance. However, the influence of these materials on protein hydrolysis has been poorly documented. Here, a novel magnetic biochar derived from oil sludge and straw was synthesized using different iron sources and successfully applied in sludge AD. Experimental results revealed that magnetic biochar modified by Fe2+ exhibited excellent electron transfer capacity, moderate magnetization, diverse functional groups (e.g. C=O, C-O=O-), and abundant iron distribution. These characteristics significantly enhanced the hydrolysis of tryptophan-like components, leading to increased methane production (144.44 mL gVS−1vs 79.72 mL gVS−1 in the control test). Molecular docking analysis revealed that the binding of magnetic biochar related Fe2+ and Fe3+, onto sludge proteins via hydrogen bond played a key role in promoting subsequent protein hydrolysis. Additionally, the noteworthy conservation of protein structures from α-helix and β-sheet to random coil, along with the breakdown of the amide I-associated C=O group and amide III-related CN and NH bonds following the addition of magnetic biochar, accelerated the degradation of sludge protein. Observation of variations in protease activity, coenzyme F420, electron transfer system (ETS), and conductivity within the AD systems, particularly the enrichment of Methanospirillum and Methanosaeta archaea, as well as the Petrimonas, Comamonas, and Syntrophomonas bacteria, suggested that magnetic biochar facilitated a conducive environment by improving hydrolysis-acidification and the direct interspecies electron transfer (DIET) process for acetoclastic methanogens. Moreover, metabolic pathways further proved that tryptophan metobalism and acetoclastic methanogenesis were both facilitated by magnetic biochar. This study provides an in-depth understanding of the impact of magnetic biochar on protein hydrolysis in sewage sludge AD.

Leaching behaviors of dissolved organic matter from face masks revealed by fluorescence EEM combined with FRI and PARAFAC

Despite numerous studies investigating the occurrence and fate of microplastics, no effort has been devoted toward exploring the characteristics of dissolved organic matter (DOM) leached from face masks mainly made of plastics and additives used in large quantities during the COVID-19 pandemic. By using FTIR, UV–vis, fluorescence EEM coupling with FRI and PARAFAC, and kinetic models of leaching experiments, we explored the leaching behaviors of face mask-derived DOM (FM-DOM) from commonly used face masks including N95, KN95, medical surgical masks, etc. The concentration of FM-DOM increased quickly at early 0–48 h and reached equilibrium at about 48 h measured in terms of dissolved organic carbon and fluorescence intensity. The protein-like materials ranged from 80.32 % to 89.40 % of percentage fluorescence response (Pi,n) were dominant in four types of FM-DOM analyzed by fluorescence EEM-FRI during the leaching experiments from 1 to 360 h. Four fluorescent components were identified, which included tryptophan-like components, tyrosine-like components, microbial protein-like components, and fulvic-like components with fluorescence EEM-PARAFAC models. The multi-order kinetic model (Radj2 0.975–0.999) fitted better than the zero-order and first-order kinetic model (Radj2 0.936–0.982) for all PARAFAC components of FM-DOM based on equations derived by pseudo kinetic models. The leaching rate constants (kn) ranged from 0.058 to 30.938 and the half-life times (T1/2) ranged from 2.73 to 24.87 h for four FM-DOM samples, following the solubility order of fulvic-like components (C4) > microbial protein-like components (C3) > tryptophan-like components (C1) > tyrosine-like components (C2) for FM-DOM from four types of face masks during the leaching experiment from 0 to 360 h. These novel findings will contribute to the understanding of the underappreciated environment impact of face masks in aquatic ecosystems.

Spatial and seasonal controls on dissolved organic matter composition in shallow aquifers under the rapidly developing city of Patna, India

The distribution and composition of dissolved organic matter (DOM) affects numerous (bio)geochemical processes in environmental matrices including groundwater. This study reports the spatial and seasonal controls on the distribution of groundwater DOM under the rapidly developing city of Patna, Bihar (India). Major DOM constituents were determined from river and groundwater samples taken in both pre- and post-monsoon seasons in 2019, using excitation-emission matrix (EEM) fluorescence spectroscopy. We compared aqueous fluorescent DOM (fDOM) composition to satellite-derived land use data across the field area, testing the hypothesis that the composition of groundwater DOM, and particularly the components associated with surface-derived ingress, may be controlled, in part, by land use. In the pre-monsoon season, the prominence of tryptophan-like components likely generated from recent biological activity overwhelmed the humic-like and tyrosine-like fluorescence signals. Evidence from fluorescence data suggest groundwater in the post-monsoon season is composed of predominantly i) plant-derived matter and ii) anthropogenically influenced DOM (e.g. tryptophan-like components). Organic tracers, as well as Eh and Cl−, suggest monsoonal events mobilise surface-derived material from the unsaturated zone, causing dissolved organic carbon (DOC) of more microbial nature to infiltrate to >100 m depth. A correlation between higher protein:humic-like fluorescence and lower vegetation index (NDVI), determined from satellite-based land use data, in the post-monsoon season, indicates the ingression of wastewater-derived OM in groundwater under the urban area. Attenuated protein:humic-like fluorescence in groundwater close to the river points towards the mixing of groundwater and river water. This ingress of surface-derived OM is plausibly exacerbated by intensive groundwater pumping under these areas. Our approach to link the composition of aqueous organics with land use could easily be adapted for similar hydrogeochemical settings to determine the factors controlling groundwater DOM composition in various contexts.

Elevated nano-α-Fe2O3 enhances arsenic metabolism and dissolved organic carbon release of Microcystis aeruginosa under a phytate environment.

Little information is available on the effects of nano-α-Fe2O3 on arsenic (As) metabolism of algae and potential associated carbon (C) storage in As-contaminated water with dissolved organic phosphorus (DOP) as a phosphorus (P) source. In this study, Microcystis aeruginosa (M. aeruginosa) was used to investigate impacts of nano-α-Fe2O3 on cell growth and As metabolism of algae under a phytate (PA) environment as well as potential associated C storage. Results showed that nano-α-Fe2O3 had a subtle influence on algal cell growth in a PA environment. Herein, algal cell density (OD680) and chlorophyll a (Chla) were inhibited at elevated nano-α-Fe2O3 levels, which simultaneously limited the decrease of Yield. As suggested, the complexation of PA with nano-α-Fe2O3 could alleviate the negative influence on algal cell growth. Furthermore, the elevated nano-α-Fe2O3 increased As methylation in the PA environment due to higher monomethylarsenic (MMA) and dimethylarsenic (DMA) concentrations in the test media. Additionally, microcystins (MCs) in the media changed consistently with UV254, both of which were relatively lower at 10.0 mg·L-1 nano-α-Fe2O3. Enhanced As(V) methylation of algal cells was found to simultaneously reduce the release risk of As(III) and MC while increasing dissolved organic carbon (DOC) content in media, suggesting unfavorable C storage. Three-dimensional fluorescence analysis revealed that the main DOC constituent was the tryptophan-like component in aromatic proteins. Correlation analysis showed that decreases in pH and the zeta potential and an increase in Chla may lead to metabolic As improvements in M. aeruginosa. The obtained findings highlight the need for greater focus on the potential risks of DOP combined with nano-α-Fe2O3 on algal blooms as well as the biogeochemical cycling processes of As and C storage in As-contaminated water with DOP as the P source.

Eutrophication and salinization elevate the dissolved organic matter content in arid lakes

Dissolved organic matter (DOM) plays an essential role in the global lake carbon cycle. Understanding DOM composition and monitoring its spatiotemporal dynamics are of great significance for understanding the lake carbon cycle, controlling water pollution, and protecting water resources. However, previous studies have focused mainly on eutrophic freshwater lakes, with limited attention given to saline lakes. Based on in situ data collected in ten lakes in northwestern China, this study reported the changes in DOM components in different lake types. Parallel factor analysis (PARAFAC) was used to analyze the three-dimensional excitation emission matrix (EEMs) to obtain the DOM fluorescence components. The contributions of different environmental factors to the changes in DOM components were quantified by the generalized linear model (GLM). The results showed that the eutrophication index was significantly positively related to dissolved organic carbon (DOC) (R2 = 0.95, p < 0.01) and colored DOM (CDOM) (R2 = 0.96, p < 0.01) concentrations. Terrestrial humic-like and tryptophan-like components, which are highly correlated with human activities, explained 62% and 64% of the variations in DOC and CDOM, respectively. In sum, the contributions of human activities to the DOC and CDOM variations were 61% and 57%, respectively. Salinity also showed significant positive correlations with both DOC (R2 = 0.88, p < 0.01) and CDOM (R2 = 0.87, p < 0.01). Lake salinization led to increases in DOM concentration, and salinity contributed 20% and 16% to the DOC and CDOM variations, respectively. Therefore, human activities and salinity codetermined the DOM concentration and its composition in the western arid lakes.Based on these findings, this study proposed a feasible flowchart for remotely estimating DOM in saline lakes using satellite data. This study is significant for the long-term monitoring of the carbon cycle and the effective protection of lake water resources in saline lakes.

Characteristics, sources and driving factors of riverine CDOM in a severe erosion basin on the Loess Plateau, China

Chromophoric dissolved organic matter (CDOM) plays a crucial role in aquatic ecosystems and biogeochemical processes. The characteristics, sources and influencing factors of riverine CDOM were determined with 56 samples collected in the Wuding River (WDR) Basin in spring and summer. The CDOM optical characteristics varied greatly during different seasons. The mean S275-295 was higher in spring than in summer, while SUVA254 exhibited the opposite result. The mean biological source index (BIX) was lower in summer than in spring, while the mean humification index (HIX) was higher in summer than in spring (p < 0.01). C1 and C3 (humic-like components) and C2 (tryptophan-like component) were extracted by parallel factor analysis (PARAFAC). C1 and C3 were prominent in summer, while C2 was prominent in spring. The effects of environmental factors on CDOM showed great differences. Chlorophyll a (Chla), dissolved organic carbon (DOC), electrical conductivity (EC) and dissolved oxygen (DO) had significant effects on the CDOM concentration (p < 0.05) in spring, while only DOC exerted significant effects on the CDOM concentration in summer. However, water quality parameters played important roles in evaluating the CDOM fluorescence components in summer. Total suspended matter (TSM) and turbidity (Tur) exerted significant effects on the humic-like fluorescence peaks (p < 0.05), while pH, Chla, DO, and EC had significant influences on tryptophan-like substances during this period. Based on redundancy analysis, precipitation, water erosion area and human activity intensity greatly affected the CDOM concentration, C1 and C3, while human activity intensity significantly affected C2 in the sub-basins in summer. CDOM was not significantly influenced by the surrounding environmental factors in spring except for the human activity intensity, which greatly influenced C1 and C3.

Transformation of algal-dissolved organic matter via sunlight-induced photochemical and microbial processes: interactions between two processes.

Algal-dissolved organic matter (ADOM) is an important fraction of dissolved organic carbon (DOC) in eutrophic water. Although ADOM is known to be readily transformed by microbes, the role of sunlight-induced photochemical process and the interactions between two processes on ADOM transformation remains unclear. In this study, three types of treatments for ADOM, including photochemical process under natural solar light (L treatment), microbial process (M treatment), and the simultaneous photochemical plus microbial process (L&M), were performed for 18days. Our results showed that M treatment was more effective for the loss of DOC, chromophoric DOM (CDOM) at short wavelengths (a254 and a280), than L treatment, while L treatment was more effective for the transformation of a350 and the fluorescent components of the ubiquitous humic-like component and the tryptophan-like component. Comparison in the decay kinetics of DOC and CDOM in the three treatments showed that the simultaneous photochemical and biological processes exhibited an inhibitory effect on DOC decay rate but not the percentage of labile DOC fraction. Higher relative abundance of protein-like substances was found after L&M treatment, while the relative abundance of humic-like substance and aromaticity increased after M treatment, and the low molecular-weight compounds were produced after L treatment. Our results emphasized the importance of photochemistry in processing ADOM to mediate the chemodiversity in natural water.

Heterogeneity of molecular-level and photochemical of dissolved organic matter derived from decomposing submerged macrophyte and algae

Aquatic macrophytes and algae are the most important sources of autochthonous dissolved organic matter (DOM), and their transformation and reuse significantly affect aquatic ecosystem health. In this study, Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was used to identify the molecular features between submerged macrophyte-derived DOM (SMDOM) and algae-derived DOM (ADOM). The photochemical heterogeneity between SMDOM and ADOM by UV254-irradiation and their molecular mechanism were also discussed. The results showed that the molecular abundance of SMDOM was dominated by lignin/CRAM-like structures, tannins, and concentrated aromatic structures (sum of 91.79%), while that of ADOM was dominated by lipids, proteins, and unsaturated hydrocarbons (sum of 60.30%). UV254-radiation resulted in a net reduction of tyrosine-like, tryptophan-like and terrestrial humic-like, and conversely a net production of marine humic-like. The light decay rate constants obtained by the multiple exponential function model fitting revealed that both tyrosine-like and tryptophan-like components of SMDOM could be rapidly and directly photodegraded, while the photodegradation of tryptophan-like in ADOM depended on the production of photosensitizers. The photo-refractory fractions of both SMDOM and ADOM were as follows: humic-like > tyrosine-like > tryptophan-like. Our results provide new insights into the fate of autochthonous DOM in aquatic ecosystems where “grass-algae” coexist or evolve.

Mechanisms of antimony release from lacustrine sediments with increasing temperature

Antimony (Sb) is more mobile in lacustrine sediments with seasonal warming. However, the mechanisms of Sb mobility in sediments are still unclear, especially considering the interactions among Sb, iron (Fe), manganese (Mn), and dissolved organic matter (DOM). In this study, high-resolution dialysis (HR-Peeper) and multi-spectral techniques simultaneously investigated changes in Sb, Fe, Mn, and DOM in two different ecological types (algal and grass) sediments with increasing temperature. We found that the dissolved Sb rapidly increased with the increase in temperature. The oxidation of Sb(III) to Sb(V) by Fe/Mn oxides in oxygen (O2) rich overlying water and surface sediment layers was one of the reasons for Sb concentration enhancement in pore water. Further, using excitation-emission matrix and parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectroscopy, fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy (2D-COS) revealed that complexation with DOM was the other reasons for Sb concentration increasing in sediments. This was demonstrated by the similar distribution pattern and significant correlation between Sb and tryptophan-like components. Titration experiments further revealed that Sb was more stably bound to tryptophan-like components in the aromatic C–H (660 cm−1), alcoholic C–O (1115 cm−1), alkene CC (1615 cm−1), and carboxylic acid OH (3390 cm−1) groups. The tryptophan-like components from the algae region had a higher binding force than that from the macrophytes region. Our study effectively promotes an understanding of Sb mobilization in lacustrine sediments.

Isotopic and spectral signatures unravel the sources, preservation and degradation of sedimentary organic matter in the Dongzhai Harbor mangrove estuary, southern China

Sedimentary organic matter (SOM) production and transformation processes have a prominent role in regulating biogenic element cycling in mangrove ecosystems. However, aquaculture biodeposits can substantially affect source apportionments and biogeochemical processes of SOM within mangroves. In this study, natural stable isotopes (13C and 15N), spectral techniques, and hydrogeochemical analysis were used to elucidate the sources, preservation, and degradation of SOM across the Dongzhai Harbor (DZH) estuary, southern China, where aquaculture systems are extensive. Along the estuarine salinity gradient, sediment and bottom water samples were collected from the upper fluvial mangrove zones to the lower marine endmember. Results indicated that source contributions to SOM along the estuarine gradient were spatially heterogeneous. Aquaculture-derived organic matter was the largest contributor to the Sanjiang River (44.29 %) in the upper estuary, and terrestrial sources from C3 mangrove forests were highest in the Yanfeng East River (62.87 %) near the middle estuary. Farther seaward, marine plankton sources were elevated in the Dongzhai Harbor channel (64.15 %) and Yanfeng West River (48.83 %) of the lower estuary. The terrestrial/microbial humic-like component (C1), tryptophan-like component (C2), and soil-derived fulvic acid (C3) were identified from the samples using fluorescence excitation emission matrices and parallel factor analysis (EEM-PARAFAC). Results show that in the upper and middle estuary, SOM was preserved by sedimentation due to the abundance of fine-grained aquaculture-derived biodeposits, weak hydrodynamic conditions, and the formation of Fe/Mn-humus complexes. In contrast, in the lower estuary, SOM originated from marine plankton and was rapidly degraded via aerobic respiration. Moreover, correlations between δ13C and δ15N values vs C/N ratios further suggest progressive SOM degradation from initial diagenesis to post-depositional geochemical processes. This study provides new insights into aquacultural and environmental factors controlling SOM preservation and degradation within estuaries. The findings have implications for shoreline stability and the protection and restoration of estuarine mangrove wetlands.

Response of dissimilatory perchlorate reducing granular sludge (DPR-GS) system to high-strength perchlorate and starvation stress in UASB reactor: Performance, kinetics and recovery mechanism

Under high-strength perchlorate concentration ranging from 300 to 1300 mg/L, the dissimilatory perchlorate reducing granular sludge (DPR-GS) was successfully cultivated in an up-flow anaerobic sludge blanket (UASB) reactor to investigate performance and starvation recovery for treating perchlorate wastewater. The mature DPR-GS had excellent perchlorate removal efficiency (98.47%) at 1200 mg/L. Microbial community analysis suggested that the dominant genera in mature DPR-GS were Sulfurospirillum and Acinetobacter. The DPR-GS system conformed well to the modified Stover-Kincannon model with the maximum substrate utilization (Umax) of 90.01 kg/(m3·d) and the saturation value constant (KB) of 83.93 kg/(m3·d). Additionally, the starvation recovery of DPR-GS system was first investigated. The perchlorate removal efficiency recovered to 91.39% after 8 days under the 30-day starvation stress, and the microbial activity also recovered to normal condition. The three-dimensional excitation-emission matrix (3D-EEM) proved that the tryptophan-like component in extracellular polymeric substance (EPS) increased during the starvation period, which was associated with immune functions and played an important role in maintaining the microbial activity. Meanwhile, the secretion of humic acid-like component in EPS promoted electron transfer efficiency, thus improving microbial metabolic activity during the recovery period. Further analysis by the parallel factor (PARAFAC) of 3D-EEM confirmed that the decrease of immune substances and the increase of metabolic products in EPS during the starvation recovery period, which enhanced the recovery of microbial activity and the improvement of perchlorate removal performance. This study revealed the response of the DPR-GS system to high-strength perchlorate and starvation stress, thus provided a further basis of perchlorate tolerance and starvation recovery for the application of DPR-GS system to remove high-strength concentrated perchlorate from contaminated water.

Characteristics of chromophoric dissolved organic matter in the Northern Andaman Sea

The dissolved organic matter (DOM) in the ocean is a large carbon pool that plays an important role in the global carbon cycle. Investigation of the characteristics and behaviors of DOM in some areas facilitates a better understanding of biochemical processes in the water column. In February 2020, water samples were collected to characterize the distribution and optical properties of DOM from the Northern Andaman Sea and the Northeastern Bay of Bengal. A high dissolved organic carbon (DOC) concentration appeared in the Northern Andaman Sea, which is strongly affected by the DOM contents of the Irrawaddy and Salween Rivers. A barrier layer that resulted from the freshwater input was observed above the thermocline. This layer was believed to have had a consistent effect on the concentration of DOM in the surface water and contributed to the differences in DOC concentration on each side of the Preparis Channel. Based on the fluorescence excitation emission matrix and parallel factor analysis, four fluorescent components were identified, including three humic-like components (C1, C2, and C4 with their maximal Ex/Em at ≤240/418 nm, 315/384 nm, and 270 (360)/442 nm and represented as peaks A, M, and A+C, respectively) and one protein-like component (C3 with maximal Ex/Em at 275/334 nm and represented as peak T). The humic-like components were mainly derived from terrestrial inputs. Low levels of humic-like components were confined in the upper water, with strong photodegradation in the euphotic zone. C3 was a typical tryptophan-like component that represented freshly produced autochthonous DOM. The correlation between C3 and biological index and apparent oxygen utilization suggested that it was highly bioavailable and not easy to be preserved in the deep sea. Overall, our results showed the distributions of DOM in the Northern Andaman Sea and the Northeastern Bay of Bengal and revealed the behaviors and controlling factors for DOM in the upper water.

Geochemical impact of dissolved organic matter on antimony mobilization in shallow groundwater of the Xikuangshan antimony mine, Hunan Province, China

Dissolved organic matter (DOM) is widely used in aquatic systems to control the environmental fate of As. However, similar to the behavior of As, Sb mobilization driven by DOM is poorly understood. A total of 25 samples were collected from shallow groundwater in the Xikuangshan mine to compare the spectroscopic characteristics and chemical properties of DOM between high- and low-Sb groundwater and to determine the roles of DOM in Sb mobility. The concentrations of Sb and DOM varied from 0.003 to 18.402 mg/L (mean: 3.407 mg/L) and 0.38 to 9.90 mg/L (mean: 2.49 mg/L), respectively. The DOM of the D3x4 water was primarily dominated by terrestrial and microbial humic-like and fulvic acid substances, with a relatively small contribution of tryptophan-like components. Complexing agents, competitive adsorption, and photopromoted oxidation under sunlight were considered as the formation mechanisms for DOM-controlled Sb(V)-dominated Sb species in D3x4 water. The weakly alkaline and oxidizing conditions, and the presence of Fe hydroxides facilitated the promotion of Sb(V) concentration. The findings of this study further enhance our understanding of the Sb migration mechanism in oxic groundwater.

Spatio-temporal distribution, photoreactivity and environmental control of dissolved organic matter in the sea-surface microlayer of the eastern marginal seas of China

Abstract. As the boundary interface between the atmosphere and ocean, the sea-surface microlayer (SML) plays a significant role in the biogeochemical cycles of dissolved organic matter (DOM) and macronutrients in marine environments. In our study, the optical properties of DOM were compared between the sub-surface water (SSW) and the SML during spring, summer and winter in the East China Sea (ECS) and the Yellow Sea (YS). In addition, photoexposure experiments were designed to compare photochemical degradation processes of DOM between the SML and the SSW. Chromophoric DOM (CDOM), fluorescent DOM, dissolved organic carbon, chlorophyll a (Chl a), picoplankton, nutrients and bacteria were frequently enriched in the SML. The enrichment factors (EFs) of tryptophan-like component 4 were significantly higher than other fluorescence components; the longer wavelength absorption values of CDOM showed higher EFs in the SML, and a more significant relationship between CDOM and Chl a in the SML, indicating that autochthonous DOM was more strongly enriched in the SML than the terrestrial DOM. Higher EFs were generally observed in the SML in the off-shore regions than in the coastal regions, and CDOM in the SML was photobleached more after relatively strong irradiation, as also indicated by the lower percentages of humic-like DOM and lower specific UV absorbance values (SUVA254) in the SML than the SSW. Compared to the SSW, the elevated nutrients may stimulate phytoplankton growth, biological activity and then production of abundant fresh autochthonous DOM in the SML. Our results revealed a new enrichment model for exploring the air–sea interface environment, which can explain the more autochthonous properties of DOM in the SML than the SSW.

Dynamics and internal links of dissolved carbon in a karst river system: Implications for composition, origin and fate

Dissolved carbon (DC) deciphers biotic and abiotic processes in aquatic ecosystems, representing a critical component of global carbon cycling. However, underlying drivers of riverine DC dynamics and internal links have yet to be studied. Here, we investigated fluvial physicochemical characteristics, dissolved inorganic carbon (DIC) species, carbon dioxide (CO2) exchange, dissolved organic carbon (DOC) compositions and properties in a karst river system Qijiang, Southwest China. Carbonate dissolution combined with photosynthetic uptake could explain dynamics of DIC species. Carbon sequestration caused low-magnitude of partial pressure of aqueous CO2 (pCO2, 620.3 ± 1028.7 μatm) and water-air CO2 flux (F, 154.3 ± 772.6 mmol/m2/d), yielding an annual CO2 emission of 0.079 Tg CO2/y. Relatively high biological index (BIX, 0.77–0.96 on average) but low humification index (HIX, 0.67–0.78 on average) indicated notable autochthonous processes. Humic-like component was the predominant DOC, accounting for 39.0%-75.2% with a mean of 57.2% ± 6.17%. Meanwhile, tryptophan-like component (5.84% ± 2.31%) was also identified as collective DOC by parallel factor analysis (PARAFAC) across samples. Biological metabolism established internal linkages between DIC and DOC in the karst river system. Our findings highlighted biological process as a determinant for DC cycling in karst aquatic ecosystems.

Characterization of Dissolved Organic Matter of Sediments in Urban Streams Using EEMs–PARAFAC and Absorption Spectroscopy: A Case Study in Wuhan, China

Urbanization has notably changed the characteristics and functions of watershed ecosystems worldwide, influencing the characteristics of chromophoric dissolved organic matter (CDOM) and dissolved organic matter (DOM) of sediments in urban streams. In this study, the biogeochemical characteristics of 42 water samples and the optical absorption and excitation–emission matrix spectra (EEMs) of 14 sediment samples collected from 14 urban streams in Wuhan were systematically examined. In addition, five water samples and one sediment sample were collected in Mulan Lake as a reference for non-urban areas. The a254 values of sediments in urban streams ranged widely (25.7–197.6 m−1), and the mean (116.32 ± 60.5 m−1) was significantly higher than the reference (51.52 m−1), indicating clear individual differences and a higher concentration of CDOM. Two humus-like components and one tryptophan-like component were effectively identified by parallel factor analysis (PARAFAC). The fluorescence index (FI)/biological index (BIX) of DOM of sediments in urban streams was mostly within 1.4–1.7/0.8–1.0, indicating a compound of both allochthonous and autochthonous sources. Compared with the reference, lower FI and BIX and higher humification index (HIX) revealed a higher allochthonous input and humification degree of DOM of sediments in urban streams. Spearman’s correlation analysis and redundancy analysis demonstrated that heavy metals and other water quality parameters had a considerable impact on CDOM concentrations and DOM components. This study could support the use of DOM as an effective tool to monitor the water environment and provide insights into future water pollution management strategies.

Photodegradation, bacterial metabolism, and photosynthesis drive the dissolved organic matter cycle in the Heilongjiang River

Dissolved organic matter (DOM) plays a vital role in the biogeochemistry of aquatic ecosystems. However, the mechanisms of DOM cycling in the water column during different seasons have not been fully elucidated to date. The differences in DOM degradation in summer, autumn, and winter water columns were evaluated in this study. The results showed that bacteria played an essential role in the degradation of DOM in the summer water column. Photochemical degradation was the primary degradation pathway of DOM in the autumn and winter water columns. Notably, while DOM is degraded, photosynthetic bacteria produce organic matter through photosynthesis to replenish the water column. EEM-PARAFAC analysis indicated more tryptophan component C1 in summer, but the contents of humic substance component C2 and terrestrial substance C3 were higher in autumn and winter. In summer, more tryptophan-like components were consumed by bacteria, and Cyanobacteria produced more organic matter through photosynthesis to replenish the water column. Moreover, a similar bacterial community structure and a more active tryptophan biosynthesis pathway were found in autumn and winter. Random forest models identified representative bacteria involved in the DOM transformation process in different seasons. The above findings may be helpful to explore the degradation characteristics of DOM in different seasons and predict the fate of DOM in the water column in the future.

Multi-spectroscopic investigation of the molecular weight distribution and copper binding ability of dissolved organic matter in Dongping Lake, China

The properties and metal-binding abilities of dissolved organic matter (DOM) rely on its molecular weight (MW) structure. In this study, the spatial differences of DOM in compositions, MW structures, and binding mechanisms with copper (Cu2+) in Dongping Lake were investigated by applying excitation-emission matrix combining parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectra, two-dimensional correlation spectra (2D-COS), and Fourier transform infrared (FTIR) spectra. The EDOM for the entrance of the Dawen River and PDOM for the macrophyte-dominated region were divided from DOM of Dongping Lake based on hierarchical clustering analysis (HCA) and principal component analysis (PCA) and were size-fractioned into MW < 500 kDa and <100 kDa fractions. According to EEM-PARAFAC, Dongping Lake was dominated by tryptophan-like substances with MW < 500 kDa. The concentration of PDOM was higher than that of EDOM (p < 0.05). 2D-COS showed that protein-like components preceded humic-like components binding to Cu2+ regardless of sample type (215 nm > 285 nm > 310–360 nm). The Cu2+ binding capacity of DOM exhibited specific differences in space, components, and molecular weights. The humic-like component 1 (C1) and tryptophan-like component 4 (C4) of PDOM showed stronger binding abilities than those of EDOM. Endogenous tryptophan-like component 4 (C4) had a higher binding affinity for Cu2+ than humic-like components (logKa: C4 > C1 > C2) in PDOM irrespective of MW. Humic-like components with MW < 500 kDa displayed higher binding potentials for Cu2+. FTIR spectra showed that the main participants of DOM-Cu complexation included aromatic hydrocarbons, aliphatic groups, amide Ⅰ bands, and carboxyl functional groups. This study provides spatial-scale insights into the molecular weight structure of DOM in influencing the behavior, fate, and bioavailability of heavy metals in lakes.

Dissimilar evolution of soil dissolved organic matter chemical properties during revegetation with arbor and shrub in desertified land of the Mu Us Desert

The chemistry of dissolved organic matter (DOM) in soil determines its bioavailability and is crucial to soil nutrient cycling and vegetation restoration. However, the response of DOM to revegetation in degraded ecosystems is not well understood. This study analyzed soil chronosequences from semifixed (SF) sand in the Mu Us Desert, China that was afforested with arbor and shrub 23–54 years ago. The DOM organic composition and chemical properties were investigated by fluorescence excitation-emission matrix spectrophotometry and UV–visible spectroscopy. As the revegetation progressed, DOM concentration at 0–20 cm continually increased. In arbor land, the DOM was mainly influenced by dissolved organic carbon, and the average concentration significantly increased from 67.5 to 203.2 mg C•kg−1. In the shrubland, changes in DOM were mainly influenced by dissolved organic nitrogen, with the average concentration significantly increasing from 3.7 to 8.9 mg N•kg−1. Arbor afforestation significantly increased the proportion of hydrophobic humic-like components (C1), with a decline in bioactive tryptophan-like components (C3) and higher DOM molecular complexity (i.e., higher molecular weight, aromaticity, and degree of humification). However, in shrubland, the proportions of C1 and C3 were significantly lower than those in arbor forest soil, with DOM molecular complexity slightly increased, and DOM tended to become more bioavailable over time. Linear regression analysis indicated that DOM rich in carbon had a high molecular complexity, while DOM rich in nitrogen is more bioavailable. These findings reveal a dissimilar evolution in the DOM composition and chemistry in the two forest types throughout the SF sand revegetation period. The DOM under arbor forest was more stable and conducive to soil organic matter sequestration, while the DOM under shrub forest contained more bioactive substances and was more favorable for nutrient cycling.