Properties of the humic-like material arising from the photo-transformation of L-tyrosine

Effects of Tryptophan and Tyrosine on the Transformation of Monophenols in Chromophoric Dissolved Organic Matter Solutions: Enhance the Forward Transformation and Reduce the Reverse Transformation

Tryptophan (Trp) and tyrosine (Tyr) are the primary precursors of protein-like components in dissolved organic matter. Phenolic compounds are ubiquitous in aquatic environments and are considered the main electron donor in chromophoric dissolved organic matter (CDOM). Our results showed that Trp and Tyr (50 μM) enhanced the transformation of six monophenols (20 μM) with varying numbers of -CH3 and -OCH3 substituent groups by a factor of 1.0-1.8. The enhancement factor increased with the ratio of Trp (Tyr) to monophenols. In four different CDOM solutions (5 mg C/L, pH 8.0), a maximum enhancement factor of 3.2-6.7 was observed at a Trp/monophenol concentration ratio of 50. Conversely, monophenols greatly inhibited the transformation of Trp or Tyr. The enhancement factor decreased as the initial pH increased from 3.0 to 10.0. Additionally, the enhancement factor was not directly proportional to the oxidation potential of monophenol. We propose that the promotion effects are generated through the direct oxidation of monophenols by Trp (Tyr) radicals as well as through the reaction between Trp (Tyr) radicals and the one-electron reductant of CDOM.

The Diversity and Community Composition of Three Plants' Rhizosphere Fungi in Kaolin Mining Areas

Mining activities in the kaolin mining area have led to the disruption of the ecological health of the mining area and nearby soils, but the effects on the fungal communities in the rhizosphere soils of the plants are not clear. Three common plants (Conyza bonariensis, Artemisia annua, and Dodonaea viscosa) in kaolin mining areas were selected and analyzed their rhizosphere soil fungal communities using ITS sequencing. The alpha diversity indices (Chao1, Shannon, Simpson, observed-species, pielou-e) of the fungal communities decreased to different extents in different plants compared to the non-kauri mining area. The β-diversity (PCoA, NMDS) analysis showed that the rhizosphere soil fungal communities of the three plants in the kaolin mine area were significantly differentiated from those of the control plants grown in the non-kaolin mine area, and the extent of this differentiation varied among the plants. The analysis of fungal community composition showed that the dominant fungi in the rhizosphere fungi of C. bonariensis and A. annua changed, with an increase in the proportion of Mycosphaerella (genus) by about 20% in C. bonariensis and A. annua. An increase in the proportion of Didymella (genus) by 40% in D. viscosa was observed. At the same time, three plant rhizosphere soils were affected by kaolin mining activities with the appearance of new fungal genera Ochrocladosporium and Plenodomus. Predictive functional potential analysis of the samples revealed that a significant decrease in the potential of functions such as biosynthesis and glycolysis occurred in the rhizosphere fungal communities of kaolin-mined plants compared to non-kaolin-mined areas. The results show that heavy metals and plant species are the key factors influencing these changes, which suggests that selecting plants that can bring more abundant fungi can adapt to heavy metal contamination to restore soil ecology in the kaolin mining area.

Photochemical behavior of dissolved organic matter derived from Alternanthera philoxeroides hydrochar: Insights from molecular transformation and photochemically reactive intermediates

Hydrochar-derived dissolved organic matter (HDOM) enters aquatic ecosystems through soil leaching and surface runoff following the application of hydrochar. However, the photochemical behavior of HDOM remains unclear. The photo-transformation of HDOM was analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), multiple spectroscopy methods, high-performance liquid chromatography, and combining synchronous fluorescence and Fourier-transform infrared spectroscopy with two-dimensional correlation spectroscopy. The results showed that with the increase of carbonization temperature, amide II in protein-like substances were observed to be preferentially photolyzed, and the protein-like substances were more sensitive to low irradiation time, while the duration time of the photochemical behavior of amide II and aliphatic C-H were more persistent. FT-ICR MS results showed that N and S-containing molecules, including lignins and lipids were more sensitive to ultraviolet irradiation. Furthermore, the photo-transformation of HDOMs was accompanied by the generation of triple excited state dissolved organic matter and singlet oxygen. Our findings will be beneficial for understanding the mechanisms of photo-transformation of HDOM and for predicting the possible behaviors of hydrochar produced at different temperatures before large-scale application.

Potential Linkage between Heavy Metal Pollution Risk Assessment and Dissolved Organic Matter Spectra in the WWTPs-River Integrated Area-Case Study from Ashi River

Direct sewage discharge can cause severe damage to the water environment of the river. However, the impacts of dissolved organic matter (DOM) in the discharge on the original pattern of DOM and the distribution of heavy metals (HMs) in the river are little known. How to monitor such areas in a long-term and systematic manner also needs to be urgently addressed. In this paper, we characterized the DOM of the sediments in the WWTPs (wastewater treatment plants)-river integrated zone by ultraviolet-visible absorption spectroscopy (UV-vis), three-dimensional excitation-emission matrix (3D-EEM) combined with parallel factor (PARAFAC) method. The effects of WWTP on receiving waters were investigated, and the potential link between DOM and HM pollution was explored. Hg (Igeo: 3.94 ± 0.65; EF: 44.83 ± 31.11), Cd (Igeo: 1.81 ± 0.69; EF: 8.02 ± 2.97), Cu (Igeo: 1.61 ± 0.83; EF: 6.85 ± 2.37), Zn (Igeo: 1.55 ± 0.54; EF: 7.24 ± 3.58), and Ni (Igeo: 1.46 ± 0.56; EF: 6.12 ± 1.99) in rivers were the primary risk sources of HM. The combined pollution risk indicates that the WWTPs-river integrated area is in a high pollution risk state. Moreover, α(254) has a significant correlation with pollution indicators and can be used as a proxy indicator. These results help to understand better the impact of WWTPs on receiving water bodies and the potential connection between DOM and HM pollution and provide new ideas for monitoring the water environment in highly polluted areas.

Photoinduced production of substances with humic-like fluorescence, upon irradiation of water samples from alpine lakes

Evidence is here provided that irradiation of some lake water samples can trigger the formation of fluorophores with humic-like properties, at the same time increasing water absorbance. This phenomenon is the opposite of photobleaching, which is often observed when natural waters are irradiated. The photoproduced humic-like fluorophores observed here would be of autochthonous rather than allochthonous origin, which marks a difference with the fraction of humic substances that derives from terrestrial sources. Photogeneration of humic-like compounds can be highlighted in water samples where the fluorescence signal of initially occurring humic substances is low, so that their photobleaching is minimised. Samples that are most likely to show photoinduced formation of humic-like fluorophores are in fact characterised by high values of protein-like vs. humic-like contribution ratios to fluorescence, as evidenced by parallel factor (PARAFAC) analysis. Mountain lakes in late summer appear to be suitable candidates to highlight the described phenomenon. In some cases, lake-water irradiation caused a decrease in the spectral slope of the absorbance that, together with increasing absorbance values, is consistent with an increase in molecular mass and aromaticity of organic matter. The absorbance increase triggered by irradiation might play a role in screening biologically harmful UV radiation, in mountain environments that would otherwise be characterised by very clear water that allows for easy transmission of UV light along the water column.

Photodegradation of organic micropollutants in aquatic environment: Importance, factors and processes

Photochemical reactions widely occur in the aquatic environment and play fundamental roles in aquatic ecosystems. In particular, solar-induced photodegradation is efficient for many organic micropollutants (OMPs), especially those that cannot undergo hydrolysis or biodegradation, and thus can mitigate chemical pollution. Recent reports indicate that photodegradation may play a more important role than biodegradation in many OMP transformations in the aquatic environment. Photodegradation can be influenced by the water matrix such as pH, inorganic ions, and dissolved organic matter (DOM). The effect of the water matrix such as DOM on photodegradation is complex, and new insights concerning the disparate effects of DOM have recently been reported. In addition, the photodegradation process is also influenced by physical factors such as latitude, water depth, and temporal variations in sunlight as these factors determine the light conditions. However, it remains challenging to gain an overview of the importance of photodegradation in the aquatic environment because the reactions involved are diverse and complex. Therefore, this review provides a concise summary of the importance of photodegradation and the major processes related to the photodegradation of OMPs, with particular attention given to recent progress on the major reactions of DOM. In addition, major knowledge gaps in this field of environmental photochemistry are highlighted.

Ultraviolet humic-like component contributes to riverine dissolved organic matter biodegradation

Biological degradation of dissolved organic matter (DOM) regulates its structure and fate in river ecosystems. Previous views suggested that labile components were dominantly consumed by microbial metabolism. Here we provide new observations that a part of recalcitrant compounds largely contribute to riverine DOM biodegradation. The excitation-emission matrix fluorescent spectroscopy combined with peak picking and parallel factor analysis are used to explore component variability during DOM incubation. Humic-like and tryptophan-like DOM are the primary components of riverine DOM, with proportion contributions of 39%-82% and 16%-61% for % of the maximum fluorescence intensity, respectively. After 56 days of aerobic incubation in the dark, large amounts of tyrosine-like DOM generation are observed. Elevated temperature enhances the decomposition of ultraviolet humic-like substance and further stimulates labile DOM bio-mineralization into carbon dioxide. Meanwhile, averaged proportions of amino acid compositions (peak B and T) markedly increase (p < 0.05) as the humic-like compositions (peak A, M and C) decrease after DOM incubation, suggesting incomplete degradation of refractory DOM from high-molecular to low-molecular weight compounds. The findings support the new notion of the continuous DOM biodegradation in a mode as "steps by steps", contributing to a new understanding of carbon cycling for the UN Sustainable Development Goal.

Photo-Reactivity and Photo-Transformation of Algal Dissolved Organic Matter Unraveled by Optical Spectroscopy and High-Resolution Mass Spectrometry Analysis

The rapid proliferation of planktonic algae induced by eutrophication and climate warming make algae dissolved organic matter (AOM) an important source of dissolved organic matter (DOM) in surface waters, but the understanding of the link between AOM composition and photo-reactivity/photo-transformation of DOM in aquatic systems is limited. Here, intracellular organic matter (IOM) from Microcystis aeruginosa was extracted and subjected to molecular weight (MW) fractionation. Results indicated that IOM had lower aromaticity and higher photosensitive activity compared to Suwannee River fulvic acid (SRFA). The photosensitive activity of IOM relied on both its molecular weight distribution and fluorescence components. The IOM fraction with the highest MW proteins had the lowest quantum yields of reactive intermediates (Φ_RIs), which increased with the decrease of MW, while the fractions with more low-excitation tyrosine-like components had relatively higher Φ_RIs. Parallel factor analysis and high-resolution mass spectrometry revealed that light radiation of IOM resulted in the composition transformation from tryptophan-like and tyrosine-like components to humic-like components, forming less aromatic and more saturated recalcitrant dissolved organic carbon. Our findings provide new insights into the photo-reactivity and photo-transformation of algae-derived organic matters and help to predict DOM formation involved in carbon cycling in water environment.

Fluorophores in surface freshwaters: importance, likely structures, and possible impacts of climate change

Fluorescence spectroscopy is one of the most useful techniques currently available for the characterisation of organic matter in natural water samples, because it combines easy availability of instrumentation, high sensitivity and limited requirements for sample treatment. The main fluorophores that can be found in natural waters are usually proteins (and/or free amino acids) and humic substances (humic and fulvic acids). The identification of these fluorescent compounds in water samples helps to obtain information about, among others, biological activity in the water body, possible transport of organic matter from soil, and the phenomenon of photobleaching that decreases both the absorbance and (usually) the fluorescence of natural organic matter. Interestingly, all these phenomena can be affected by climate change, which could alter to different extents the ratio between aquagenic and pedogenic fluorophores. Several events induced by warming in natural waters (and especially lake water) could enhance algal growth, thereby also enhancing the production of aquagenic organic matter. Intense precipitation events could increase the export of pedogenic material to surface waters, while photobleaching would be enhanced in the epilimnion of lakes when summer stratification becomes longer and more stable because of higher temperatures. Interestingly, photobleaching affects humic substances to a higher extent compared to protein-like material, thus protein fluorescence signals could be more preserved in stratified waters.

Photo-ammonification of low molecular weight dissolved organic nitrogen by direct and indirect photolysis

The photo-ammonification process plays a crucial role in the transformation of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN). However, previous studies have primarily focused on DON biotransformation than on abiotic processes. This study investigated the photo-ammonification process of nine model low molecular weight (LMW) DON molecules (e.g., amino acids, nucleotides, and urea) under the influence of different light sources. The results showed that photo-ammonification of model DON was mainly induced by UV light, while negligible contribution by visible light was found. Depending on their molecular structures, amino acids yielded different ammonia amounts, whereas negligible photo-ammonification was observed for nucleotides and urea. As for the reactive species, OH promoted ammonia yields of all the model amino acids; ³CDOM^⁎ contributed to the photo-ammonification of six amino acids; ¹O₂ only had a positive impact on ammonification of tryptophan, histidine, and tyrosine; and CO₃^- accelerated ammonia generation from histidine and methionine. In natural water samples, tryptophan, tyrosine, histidine, and methionine generated significant ammonia. OH and ¹O₂ were speculated as the contributing reactive species based on kinetic studies as well as significant fluorescent humic-like and tyrosine-like substances degradation in irradiated samples compared to the raw samples characterized by the EEM-PARAFAC analysis. The negative linear correlations between photo-ammonification rates and the E_LUMO-E_HOMO of the amino acids emphasized the importance of the role of the molecular structure. Overall, these results revealed the LMW DON photo-ammonification mechanism in sunlit surface waters and highlighted its significance in the nitrogen biogeochemical cycle as well as water quality management.

Elucidating the Role of Dissolved Organic Matter and Sunlight in Mediating the Formation of Ag-Au Bimetallic Alloy Nanoparticles in the Aquatic Environment

Elucidating the interactions between metal ions and dissolved organic matter and deciphering mechanisms for their mineralization in the aquatic environment are central to understanding the speciation, transport, and toxicity of nanoparticles (NPs). Herein, we examine the interactions between Ag⁺ and Au³⁺ ions in mixed solutions (χ_Ag = 0.2, 0.5, and 0.8) in the presence of humic acids (HAs) under simulated sunlight; these conditions result in the formation of bimetallic Ag-Au NPs. A key distinction is that the obtained alloy NPs are compositionally and morphologically rather different from NPs obtained from thermally activated dark processes. Photoillumination triggers a distinctive plasmon-mediated process for HA-assisted reductive mineralization of ions to bimetallic alloy NPs which is not observed in its dark thermal reduction counterpart. The initial nucleation of bimetallic NPs is dominated by differences in the cohesive energies of Ag and Au crystal lattices, whereas the growth mechanisms are governed by the strongly preferred incorporation of Ag ions, which stems from their greater photoreactivity. The bimetallic NPs crystallize in shapes governed by the countervailing influence of minimizing free energy through the adoption of Wulff constructions and the energetic penalties associated with twin faults. As such, assessments of the stability and the potential toxic effects of bimetallic NPs arising from their possible existence in aquatic environments will depend sensitively on the origins of their formation.

Tracking photodegradation products and bond-cleavage reaction pathways of triclosan using ultra-high resolution mass spectrometry and stable carbon isotope analysis

Triclosan (TCS) is an antimicrobial compound ubiquitously found in surface waters throughout the world. Although several studies have focused on the photochemical degradation of TCS, there is still limited knowledge about its environmental fate. In this study, we got molecular-level insights into the photochemical degradation of TCS. Significant stable carbon isotope fractionation was observed during photodegradation; different bond-cleavage reaction pathways under different photolytic conditions were characterized, using compound specific isotope analysis (CSIA). Photochemical modeling of TCS photodegradation showed that direct photolysis would be the main transformation pathway if pH > 7, even in presence of dissolved organic matter. Moreover, by use of ultrahigh resolution mass spectrometry, FT-ICR-MS, a broad and complex spectrum of organic by-products (some of which potentially toxic, as assessed by a quantitative structure-activity relationship approach) were identified. A detailed reaction mechanism was developed on the basis of the detected compounds. A possible sequence of steps leading to some of the detected product compounds in aqueous solution is suggested.

Transformation of amino acid tyrosine in chromophoric organic matter solutions: Generation of peroxide and change of bioavailability

Studying the photochemical transformation mechanism of dissolved free amino acids (DFAA) in chromophoric dissolved organic matter (CDOM) solution facilitates the understanding of DFAA's environmental fate and bioavailability change upon solar irradiation in natural surface waters. Tyrosine oxidation product (Tyr-OH) was synthesized to quantify the primary transformation product (tyrosine peroxide, Tyr-OOH) in CDOM solution. Both reactions between superoxide radical anion (O₂^-) and tyrosine radical (Tyr) and between singlet oxygen (¹O₂) and tyrosine (Tyr) yield Tyr-OOH, which is subsequently transformed into Tyr-OH. The reaction between O₂^- and Tyr not only generated Tyr-OOH but also caused the regeneration of Tyr. O₂^- and ¹O₂ contributed 30-44% to Tyr's transformation in CDOM solutions at pH 8.0, in which ¹O₂ oxidation accounted for 6-11%. The contribution of O₂^- to Tyr's phototransformation process was the difference between the total contribution of O₂^- and ¹O₂ and the individual contribution of ¹O₂. Compared with the fast assimilation of Tyr, Tyr-OH was stable in natural water under dark incubation, indicating that phototransformation decreased the bioavailability of Tyr.

Dissolved Metal(loid) Concentrations and Their Relations with Chromophoric and Fluorescent Dissolved Organic Matter in an Urban River in Shenzhen, South China

Urbanization is often accompanied by aquatic metal(loid) pollution, which is regulated by dissolved organic matter (DOM). However, the relationships between dissolved metal(loid) concentration and the bulk, chromophoric, and fluorescent DOM in black and odorous urban rivers are still poorly understood. Here, we investigated the dissolved metal(loid) concentrations of Zn, Cu, Cr, As, Pb, and Cd and their correlations with DOM-related parameters in water samples from a polluted urbanized watershed in Shenzhen, China. The results showed that the Zn and Cu concentrations in the mainstream and tributary exceeded the national standards, and the wastewater treatment plant (WWTP) was an important source, as indicated by the abrupt concentration increases downstream of the WWTP. The dissolved metal(loid) concentrations were not always significantly correlated with the dissolved organic carbon (DOC) concentration or the ultraviolet absorbance at 254 nm (UV254); however, they were more likely to be correlated with the maximum fluorescence intensity (Fmax) of protein-like fluorescent DOM components. A strong correlation between the Cu/DOC ratio and specific UV254 (SUVA254) previously reported did not exist in the present study. Instead, the Cu/DOC ratio was positively correlated with the Fmax/DOC ratios for protein-like fluorescent DOM components. Our study highlights that protein-like fluorescent DOM may be more important than humic-like fluorescence DOM and chromophoric DOM in terms of interacting with dissolved metal(loid)s in black and odorous urban rivers.

Contribution of the Excited Triplet State of Humic Acid and Superoxide Radical Anion to Generation and Elimination of Phenoxyl Radical

Contributions of excited triplet state of humic acid (³HA*) and superoxide radical anion (O₂^•-), which is mainly generated via the reaction of O₂ with HA-derived reducing intermediates (HA^•-), to phenol transformation were revealed using acetaminophen, 2,4,6-trimethylphenol and tyrosine as probe molecules. Phenol transformation was initiated by ³HA*, leading to the formation of the phenoxyl radical (PhO•), but the distribution of transformation intermediates was codetermined by ³HA* and HA^•-. The influence of HA^•- essentially resulted from the production of O₂^•-, which affected the fate of PhO•. PhO• could undergo dimerization, or react with O₂^•-, leading to either phenol peroxide formation (radical addition) or phenol regeneration (electron transfer). In addition, PhO• could bind to HA or react with HA radicals, particularly in the absence of O₂ and O₂^•-. These PhO• reactions were dependent on the reduction potential and structure of PhO•. This study also proved that the reaction of phenol with ¹O₂ and the reaction of PhO• with O₂^•- lead to the same oxidation product. The contributions of ³HA* and its generated ¹O₂, HA^•- and its generated O₂^•- to phenol transformation were pH-dependent.

Unexpected transformation of dissolved phenols to toxic dicarbonyls by hydroxyl radicals and UV light

Phenols are common anthropogenic and natural chemicals that contaminate drinking water sources. To reduce exposure to these compounds, hydroxyl radicals are often used as chemical oxidants during water treatment. Although this treatment process removes phenols, we have found that it unexpectedly produces toxic transformation products. We identify these products and simultaneously assess their toxicity with a technique that detects products formed when the transformation products react with amino acids and peptides. Our results highlight the potential risks of using oxidative treatment on alternative drinking water sources, such as contaminated groundwater and recycled municipal wastewater. They also suggest that these reactions produce these toxic transformation products in other situations, including in clouds and sunlit surface waters and within living cells.

Water treatment systems frequently use strong oxidants or UV light to degrade chemicals that pose human health risks. Unfortunately, these treatments can result in the unintended transformation of organic contaminants into toxic products. We report an unexpected reaction through which exposure of phenolic compounds to hydroxyl radicals (•OH) or UV light results in the formation of toxic α,β-unsaturated enedials and oxoenals. We show that these transformation products damage proteins by reacting with lysine and cysteine moieties. We demonstrate that phenolic compounds react with •OH produced by the increasingly popular UV/hydrogen peroxide (H₂O₂) water treatment process or UV light to form toxic enedials and oxoenals. In addition to raising concerns about potential health risks of oxidative water treatment, our findings suggest the potential for formation of these toxic compounds in sunlit surface waters, atmospheric water, and living cells. For the latter, our findings may be particularly relevant to efforts to understand cellular damage caused by in vivo production of reactive oxygen species. In particular, we demonstrate that exposure of the amino acid tyrosine to •OH yields an electrophilic enedial product that undergoes cross-linking reaction with both lysine and cysteine residues.

Correlations between aromaticity of dissolved organic matter and trace metal concentrations in natural and effluent waters: A case study in the Sagami River Basin, Japan

Chemical speciation, reactivity, and bioavailability of trace metals in aqueous systems arestrongly influenced by dissolved organic matter (DOM). DOM is a mixture of diverse components, so a range of organic molecules potentially participates in the occurrence of dissolved trace metals. In this study, we investigated water quality variables that influence dissolved trace metal concentrations in natural and effluent water systems with a particular attention given to the relationship between DOM optical properties and dissolved copper and iron concentrations. We found that specific UV absorbance (SUVA₂₅₄: an indicator of DOM aromaticity) has a significant correlation with dissolved trace metal to dissolved organic carbon concentration ratios ([Me]_T/[DOC]) for copper and iron in natural freshwaters and treated municipal wastewater in the Sagami River basin, Japan. This trend was also prevalent for other freshwaters in temperate climates except for Fe-rich waters. Our findings indicate that the concentrations of dissolved copper and iron in natural and effluent waters are significantly influenced not only by DOM concentration, but also by aromaticity of DOM, and that this DOM property can be inferred from spectrophotometric measurements.

Photomineralization of aqueous salicylic acids. Photoproducts characterization and formation of light induced secondary OH precursors (LIS-OH)

The photolysis of aqueous 5-chlorosalicylic acid (ClSA) and dihydroxybenzoic acid (DHBA), its main photoproduct, was studied to determine the extent of degradation caused by simulated solar light. Photoproducts identification was achieved using high resolution LC-MS and GC-MS. About 40 photoproducts from C₁₉ to C₁ were characterized, including a dihydroxycyclopentadienic acid, a ring contraction photoproduct, and numerous carbonyls and carboxylic acid derivatives that were detected thanks to derivatization. UV-visible spectral monitoring of the reactions revealed that ClSA and DHBA underwent photobleaching after developing a temporarily featureless absorbance between 300 and 500 nm. Measurement of OH radicals using terephtalic acid as a probe showed that OH radicals were generated with an average rate of 7 × 10^-9 M s^-1 and a total cumulated concentration of 10^-3 M, corresponding to ∼ 5-fold the initial concentration of DHBA. Furthermore, TOC analysis indicated that significant mineralization (51-90%) occurred. These findings are consistent with the formation of light induced secondary OH (LIS-OH) precursors such as featureless long wavelength absorbing compounds as well as non-absorbing hydroperoxides. The formation of LIS-OH illustrated here may also take place in the aqueous photodegradation of other substituted phenols likely present in dissolved organic matter and humic substructures, it deserves to be studied in more details.