Photochemical release of biologically available nitrogen from aquatic dissolved organic matter

High-Resolution Mass Spectrometry Combined with Reactive Oxygen Species Reveals Differences in Photoreactivity of Dissolved Organic Matter from Microplastic Sources in Aqueous Environments

Microplastics (MPs)-derived dissolved organic matter (MPs-DOM) is becoming a non-negligible source of DOM pools in aquatic systems, but there is limited understanding about the photoreactivity of different MPs-DOM. Herein, MPs-DOM from polystyrene (PS), polyethylene terephthalate (PET), poly(butylene adipate-co-terephthalate) (PBAT), PE, and polypropylene (PP), representing aromatic, biodegradable, and aliphatic plastics, were prepared to examine their photoreactivity. Spectral and high-resolution mass spectrometry analyses revealed that PS/PET/PBAT-DOM contained more unsaturated aromatic components, whereas PE/PP-DOM was richer in saturated aliphatic components. Photodegradation experiments observed that unsaturated aromatic molecules were prone to be degraded compared to saturated aliphatic molecules, leading to a higher degradation of PS/PET/PBAT-DOM than PE/PP-DOM. PS/PET/PBAT-DOM was mainly degraded by hydroxyl (•OH) via attacking unsaturated aromatic structures, whereas PE/PP-DOM by singlet oxygen (1O2) through oxidizing aliphatic side chains. The [•OH]ss was 1.21-1.60 × 10-4 M in PS/PET/PBAT-DOM and 0.97-1.14 × 10-4 M in PE/PP-DOM, while the [1O2]ss was 0.90-1.35 × 10-12 and 0.33-0.44 × 10-12 M, respectively. This contributes to the stronger photoreactivity of PS/PET/PBAT-DOM with a higher unsaturated aromatic degree than PE/PP-DOM. The photodegradation of MPs-DOM reflected a decreasing tendency from aromatic-unsaturated molecules to aliphatic-saturated molecules. Special attention should be paid to the photoreactivity and environmental impacts associated with MPs-DOM containing highly unsaturated aromatic compounds.

Inferring the Molecular Basis for Dissolved Organic Matter Photochemical and Optical Properties

Despite the widespread use of photochemical and optical properties to characterize dissolved organic matter (DOM), a significant gap persists in our understanding of the relationship among these properties. This study infers the molecular basis for the optical and photochemical properties of DOM using a comprehensive framework and known structural moieties within DOM. Utilizing Suwannee River Fulvic Acid (SRFA) as a model DOM, carboxylated aromatics, phenols, and quinones were identified as dominant contributors to the absorbance spectra, and phenols, quinones, aldehydes, and ketones were identified as major contributors to radiative energy pathways. It was estimated that chromophores constitute ∼63% w/w of dissolved organic carbon in SRFA and ∼47% w/w of overall SRFA. Notably, estimations indicate the pool of fluorescent compounds and photosensitizing compounds in SRFA are likely distinct from each other at wavelengths below 400 nm. This perspective offers a practical tool to aid in the identification of probable chemical groups when interpreting optical and photochemical data and challenges the current "black box" thinking. Instead, DOM photochemical and optical properties can be closely estimated by assuming the DOM is composed of a mixture of individual compounds.

Short-term photodegradation of autochthonous and allochthonous dissolved organic matter in Lake Taihu, China

Photochemistry is one of the key processes that shape the quality of dissolved organic matter (DOM) in aquatic systems, yet the photoreactivity of DOM from different sources remains largely unclear. In this study, DOM from 10 typical autochthonous and allochthonous sources in Lake Taihu basin were exposed to simulated sunlight, and quantitative and compositional changes of the DOM were explored by measuring its UV-Visable absorption and fluorescence spectroscopy. Photochemical release of nutrients was also explored during the incubations. Results showed that, although DOM from most sources experienced photobleaching effects with decreased absorption coefficients at 254 nm (a(254)) and fluorescence component intensities after light exposure, photochemical alterations of DOM linked to their original composition. Macrophyte-derived (Potamogeton malaianus) DOM, with the largest molecular size, showed increased a(254), humic- and protein-like fluorescence component (C1 and C2) abundances, and inorganic nutrient concentrations relative to dark controls, indicating photo-release of labile components. However, DOM with relatively higher aromaticity, e.g., from agricultural water and the lake, showed photobleaching effects and increased humification degree, probably due to the loss of aromatic components. Allochthonous anthropogenic DOM, e.g., from sewage, showed stronger photo-ammonification, likely relating to the fresh labile N-containing compositions. The form of inorganic nutrient releases during the DOM photolysis also varied with the original DOM sources. Macrophyte-derived DOM incubations showed larger photo-releases of NO3- and PO43-, while NO2- dominated inorganic nutrient releases during groundwater DOM light incubations. Thus, this study concludes that the photoreactivity of DOM closely relates to its original composition and sources.

The Transformation and Export of Organic Carbon Across an Arctic River-Delta-Ocean Continuum

The Arctic Ocean is surrounded by land that feeds highly seasonal rivers with water enriched in high concentrations of dissolved and particulate organic carbon (DOC and POC). Explicit estimates of the flux of organic carbon across the land-ocean interface are difficult to quantify and many interdependent processes makes source attribution difficult. A high-resolution 3-D biogeochemical model was built for the lower Yukon River and coastal ocean to estimate biogeochemical cycling across the land-ocean continuum. The model solves for complex reactions related to organic carbon transformation, including mechanistic photodegradation and multi-reactivity microbial processing, DOC-POC flocculation, and phytoplankton dynamics. The baseline DOC and POC flux out of the delta from April to September 2019, was 977 and 536 Gg C (∼80% of the annual total), but only 50% of the DOC and 25% of the POC exited the plume across the 10 m isobath. Microbial breakdown of DOC accounted for a net loss of 168 Gg C (17% of delta export) within the plume and photodegradation accounted for a net loss of 46.6 Gg C DOC (5% of delta export) in 2019. Flocculation decreased the total organic carbon flux by only 6.4 Gg C (∼1%), while POC sinking accounted for 63.3 Gg C (10%) settling in the plume. The loss of chromophoric dissolved organic matter due to photodegradation increased the light available for phytoplankton growth throughout the coastal ocean, demonstrating the secondary effects that organic carbon reactions can have on biological processes and the net coastal carbon flux.

Photodissolution of submillimeter-sized microplastics and its dependences on temperature and light composition

Photodissolution has the potential to efficiently remove microplastics from the surface ocean. Here, we examined the effects of temperature and incident sunlight composition on the photodissolution of submillimeter-sized microplastics of polypropylene (PP), polystyrene (PS), and thermoplastic polyurethane (TPU) in seawater. The photoproduction of dissolved organic carbon (DOC), chromophoric dissolved organic matter, and dissolved nitrogen (TPU only) was observed to increase exponentially within 7 days of full-spectrum irradiation. The temperature dependence of photodissolution increased with irradiation time for PP and PS but remained relatively constant for TPU. A 20 °C increase in temperature enhanced DOC photoproduction by 10 times for PP, three times for PS, and four times for TPU at 7-d irradiation, giving activation energies of 59.4-84.8 kJ mol^-1. Photodissolution of all three polymers was exclusively driven by ultraviolet-B (UVB) radiation. PS-derived DOC was photomineralizable, while PP- and TPU-derived DOC appeared photo-resistant. Extrapolating the lab-based DOC photoproduction rates to warm surface oceans yields lifetimes of 6.5 years for PP, 3.6 years for PS, and 3.7 years for TPU. This study demonstrates that photodissolution of the tested microplastics is restricted to the thin UVB-penetrable surface ocean and that water temperature plays a critical role in controlling the photodissolution of these microplastics.

Molecular and spectroscopic changes of peat-derived organic matter following photo-exposure: Effects on heteroatom composition of DOM

The temporal evolution of molecular compositions and changes in structural features of Hillsboro Canal (Florida, USA) dissolved organic matter (DOM) was studied with an emphasis on nitrogen and sulfur containing molecules, after a 13 day time-series exposure to simulated sunlight. The Hillsboro Canal drains from the ridge and slough wetland environment underlain by peat soils from the northern extent of the Greater Everglades Ecosystem. The Hillsboro Canal-DOM was characterized by combining ultrahigh-resolution mass spectrometry (FT-ICR-MS), high-field nuclear magnetic resonance spectroscopy (¹H NMR), size exclusion chromatography (SEC) with UV detection, and ultraviolet/visible (UV/vis) absorbance and excitation emission matrix (EEM) fluorescence spectroscopy. Size exclusion chromatography (SEC) demonstrated progressive depletion of higher mass molecules and a concomitant decrease of absorbance during photo-irradiation. NMR and FT-ICR-MS revealed nonlinear temporal evolution of DOM. In fact, FT-ICR-MS showed an initial depletion of supposedly chromophoric molecules often carrying major unsaturation accompanied by an uneven evolution of numbers of CHO, CHOS and CHNO compounds. While CHNO compounds continually increased throughout the entire photo-exposure time, CHO and CHOS compounds temporarily increased but declined after further light exposure. Progressive loss of highly unsaturated compounds was accompanied by production of low mass CHO and CHNO compounds with high O/C ratios. Area-normalized ¹H NMR spectra of DOM in water and of the water insoluble fraction (~5%) in methanol revealed clear distinctions between irradiated and non-irradiated samples and congruent evolution of DOM structural features during irradiation, with more uniform trends in methanolic-DOM. Photoirradiation caused initial photoproduction of oxygenated aliphatic compounds, continued depletion of phenols and oxygenated aromatics, substantial change from initial natural product derived olefins to photoproduced olefins, and uneven evolution of carboxylated and alkylated benzene derivatives. This study demonstrates longer-term heteroatom-dependent photochemistry of DOM, which will affect the speciation of N and S heteroatoms, their connections to inorganic nutrients, and potentially their bioavailability.

Impacts of haze on the photobleaching of chromophoric dissolved organic matter in surface water

Sunlight plays an important role in the photochemical processes of chromophoric dissolved organic matter (CDOM), which is closely related to water self-purification and primary productivity of healthy aquatic ecosystem health. The fine particles of haze, a widespread air pollutant, absorb natural ultraviolet (UV) irradiation and have an unknown degree of influence on the photochemical transformation of CDOM. Here, an in-situ experiment investigating how the amount and composition of CDOM changes under hazy conditions was conducted in Ningbo, southeastern China, a city that frequently suffers from seasonal haze pollution. The results indicated that haze attenuated UV light under different weather conditions. The UV intensities were reduced from 1124.90 ± 91.58 to 510.26 ± 40.26 μW cm^-2 and 748.54 ± 101.68 to 316.32 ± 40.48 μW cm^-2 on sunny and cloudy days, respectively; these values approached those on rainy days (186.97 ± 28.58 μW cm^-2). Consequently, the loss of dissolved organic carbon during the irradiation test was reduced on hazy days (e.g., from 5.63% to 2.59% on sunny/hazy days). The impact of haze on CDOM photobleaching was further assessed by an excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis. On hazy days, the EEM-PARAFAC components were saved from photobleaching to different degrees; and humic-like substances showed a stronger protective effect from haze than protein-like substances because of their higher photosensitivity. Consequently, haze could cause more terrestrial CDOM to remain in surface water. UV intensity played a critical role in the composition characteristics of CDOM. This study identifies the linkage between atmospheric pollution and water quality and demonstrates that long-term and large-scale haze may adversely influence aquatic ecology through pollutant/nutrient accumulation.

Nonpolar Side Chains Affect the Photochemical Redox Reactions of Copper(II)-Amino Acid Complexes in Aqueous Solutions

Photochemical redox reactions of Cu(II) complexes of eight amino acid ligands (L) with nonpolar side chains have been systematically investigated in deaerated aqueous solutions. Under irradiation at 313 nm, the intramolecular carboxylate-to-Cu(II) charge transfer within Cu(II)-amino acid complexes leads to Cu(I) formation and the concomitant decomposition of amino acids. All amino acid systems studied here can produce ammonia and aldehydes except proline. For the 1:1 Cu(II) complex species (CuL), the Cu(I) quantum yields at 313 nm (Φ_Cu(I),CuL) vary by fivefold and in the sequence (0.10 M ionic strength at 25 °C) alanine (0.094) > valine (0.059), leucine (0.059), isoleucine (0.056), phenylalanine (0.057) > glycine (0.052) > methionine (0.032) > proline (0.019). This trend can be rationalized by considering the stability of the carbon-centered radicals and the efficient depopulation of the photoexcited state, both of which are dependent on the side-chain structure. For the 1:2 Cu(II) complex species (CuL₂), the Cu(I) quantum yields exhibit a similar trend and are always less than those for CuL. The photoformation rates of ammonia, Cu(I), and aldehydes are in the ratio of 1:2.0 ± 0.2:0.7 ± 0.2, which supports the proposed mechanism. This study suggests that the direct phototransformation of Cu(II)-amino acid complexes may contribute to the bioavailable nitrogen for aquatic microorganisms and cause biological damage on cell surfaces in sunlit waters.

One- and Two-Band Sensors and Algorithms to Derive aCDOM(440) from Global Above- and In-Water Optical Observations

The colored (or chromophoric, depending on the literature) dissolved organic matter (CDOM) spectral absorption coefficient, aCDOM(λ), is a variable of global interest that has broad application in the study of biogeochemical processes. Within the funding for scientific research, there is an overarching trend towards increasing the scale of observations both temporally and spatially, while simultaneously reducing the cost per sample, driving a systemic shift towards autonomous sensors and observations. Legacy aCDOM(λ) measurement techniques can be cost-prohibitive and do not lend themselves toward autonomous systems. Spectrally rich datasets carefully collected with advanced optical systems in diverse locations that span a global range of water bodies, in conjunction with appropriate quality assurance and processing, allow for the analysis of methods and algorithms to estimate aCDOM(440) from spectrally constrained one- and two-band subsets of the data. The resulting algorithms were evaluated with respect to established fit-for-purpose criteria as well as quality assured archival data. Existing and proposed optical sensors capable of exploiting the algorithms and intended for autonomous platforms are identified and discussed. One-band in-water algorithms and two-band above-water algorithms showed the most promise for practical use (accuracy of 3.0% and 6.5%, respectively), with the latter demonstrated for an airborne dataset.

The impacts of reservoirs on the sources and transport of riverine organic carbon in the karst area: A multi-tracer study

Reservoirs have been constructed as clean energy sources in recent decades with various environmental impacts. Karst rivers typically exhibit high dissolved inorganic carbon (DIC) concentrations, whether and how reservoirs affect carbon cycling, especially organic carbon (OC)-related biogeochemical processes in karst rivers, are unclear. To fill this knowledge gap, multiple tracer methods (including fluorescence excitation-emission matrix (EEM), ultraviolet (UV) absorption, and stable carbon (δ¹³C) and radiocarbon (Δ¹⁴C) isotopes) were utilized to track composition and property changes of both particulate OC (POC) and dissolved OC (DOC) along river-transition-reservoir transects in the Southwest China karst area. The changes in chemical properties indicated that from the river to the reservoir, terrestrial POC is largely replaced by phytoplankton-derived OC, while gradual coloured dissolved organic matter (CDOM) removal and addition of phytoplankton-derived OC to the DOC pool occurred as water flowed to the reservoir. Higher primary production in the transition area than that in the reservoir area was observed, which may be caused by nutrient released from suspended particles. Within the reservoir, the production surpassed degradation in the upper 5 m, resulting in a net DIC transformation into DOC and POC and terrestrial DOM degradation. The primary production was then gradually weakened and microbial degradation became more important down the profile. It is estimated that ~3.1-6.3 mg L^-1 (~15.5-31.5 mg-C m^-2 (~10-21%)) DIC was integrated into the OC pool through the biological carbon pump (BCP) process in the upper 5 m in the transition and reservoir areas. Our results emphasize the reservoir impact on riverine OC transport, and due to their characteristics, karst areas exhibit a higher BCP potential which is sensitive to human activities (more nutrient are provided) than non-karst areas.

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.

Photo-ammonification in surface water samples: Mechanism and influencing factors

Dissolved organic nitrogen (DON) accounts for a large proportion of the total aquatic nitrogen. Compared with dissolved inorganic nitrogen (DIN), the reactivity of DON has received limited attention. Photo-ammonification contributes significantly to the transformation of DON to DIN. However, information on the mechanism of this process is limited. This study investigated the photo-ammonification process of different natural surface water samples. The effects of seasons and rainfall on this process were explored, and the contributing factors were identified. Results showed that the seasonal effect on photo-ammonification differed for different water samples, whereas rainfall increased the rates of photo-ammonification for most of the lakes. The concentrations of reactive species, including triplet states of chromophoric dissolved organic matter (³CDOM*) and singlet oxygen (¹O₂), were found to be significantly correlated with water optical-parameters. Multivariable linear regression analysis (R² = 0.617) revealed that the photo-ammonification of DON was mainly facilitated by ³CDOM* whereas ¹O₂ competed with ³CDOM* and showed an inhibiting effect. The components of dissolved organic matter (DOM) were identified by fluorescence excitation emission matrices coupled with parallel factor analysis and were found to be greatly influenced by the location. Allochthonous humic-like components were found to promote the production of reactive species while tryptophan-like component was found to be a reactive species consumer. This study revealed that the composition of DOM and the reactive species governed the rates of photo-ammonification.

Molecular composition and biodegradation of loggerhead sponge Spheciospongia vesparium exhalent dissolved organic matter

Sponges are critical components of marine reefs due to their high filtering capacity, wide abundance, and alteration of biogeochemical cycling. Here, we characterized dissolved organic matter (DOM) composition in the sponge holobiont exhalent seawater of a loggerhead sponge (Spheciospongia vesparium) and in ambient seawater in Florida Bay (USA), as well as the microbial responses to each DOM pool through dark incubations. The sponge holobiont removed 6% of the seawater dissolved organic carbon (DOC), utilizing compounds that were low in carbon and oxygen, yet high in nitrogen content relative to the ambient seawater. The microbial community accessed 7% of DOC from the ambient seawater during a 5-day incubation but only 1% of DOC from the sponge exhalent seawater, suggesting a decrease in lability possibly due to holobiont removal of nitrogen-rich compounds. If this holds true for other sponges, it may have important implications for DOM lability and cycling in coastal environments.

Role of clay-associated humic substances in catalyzing bioreduction of structural Fe(III) in nontronite by Shewanella putrefaciens CN32

Previous studies have shown that humic substances can serve as electron shuttle to catalyze bioreduction of structural Fe(III) in clay minerals, but it is unclear if clay-sorbed humic substances can serve the same function. It is unknown if the electron shuttling function is dependent on electron donor type and if humic substances undergo change as a result. In this study, humic acid (HA) and fulvic acid (FA) were sorbed onto nontronite (NAu-2) surface. Structural Fe(III) in HA- and FA-coated NAu-2 samples was bioreduced by Shewanella putrefaciens CN32 using H₂ and lactate as electron donors. The results showed a contrasting effect of humic substances on bioreduction of structural Fe(III), depending on the electron donor type. With H₂ as electron donor, humic substances had little effect on bioreduction of Fe(III) (the reduction extent: 26.2%, 27.4%, 29.3% for HA-coated, FA-coated, and uncoated NAu-2, respectively). In contrast, these substances significantly enhanced bioreduction of Fe(III) with lactate as electron donor (the reduction extent: 20.2%, 20.7%, 11.5% for HA-coated, FA-coated, and uncoated NAu-2, respectively). This contrasting behavior is likely caused by the difference in reaction free energy and electron transport process between H₂ and lactate. When H₂ served as electron donor, more energy was released than when lactate served as electron donor. In addition, because of different cellular locations of lactate dehydrogenase (inner membrane) and H₂ hydrogenase (the periplasm), electrons generated by H₂ hydrogenase may pass through the electron transport chain more rapidly than those generated from lactate dehydrogenase. Through their functions as electron shuttle and/or carbon source, clay-sorbed HA/FA underwent partial transformation to amino acids and other compounds. The availability of external carbon source played an important role in the amount and type of secondary product generation. These results have important implications for coupled iron and carbon biogeochemical cycles in clay- and humic substance-rich environments.

Using ESI FT-ICR MS to Characterize Dissolved Organic Matter in Salt Lakes with Different Salinity

Dissolved organic matter (DOM) composition in salt lakes is critical for water quality and aquatic ecology, and the salinization of salt lakes affects the DOM composition. To the best of our knowledge, no study has explored the effects of salinity on salt lake DOM composition at the molecular level. In this work, we selected Qinghai Lake (QHL) and Daihai Lake (DHL) as typical saline lakes. The two lakes have similar geographical and climatic conditions, and the salinity of QHL is higher than that of DHL. Fourier transform ion cyclotron resonance mass spectrometry coupled with electrospray ionization was applied to compare the DOM molecular composition in the two lakes. At higher salinity, the DOM showed larger average molecular weight, higher oxidation degree, and lower aromaticity. Moreover, the proportion of DOM that is vulnerable to microbial degradation (e.g., lipids), photo-degradation (e.g., aromatic structures), or both processes (e.g., carbohydrates and unsaturated hydrocarbons) reduced at higher salinity. On the contrary, compounds that are refractory to microbial degradation (e.g., lignins/CRAM-like structures and tannins) or photo-degradation (e.g., aliphatic compounds) accumulated. Our study provides a useful and unique method to study DOM molecular composition in salt lakes with different salinity and is helpful to understand DOM transformation during the salinization of salt lakes.

Mechanistic Insights into Dissolved Organic Sulfur Photomineralization through the Study of Cysteine Sulfinic Acid

Photochemical reactions convert dissolved organic matter (DOM) into inorganic and low-molecular-weight organic products, contributing to its cycling across environmental compartments. However, knowledge on the formation mechanisms of these products is still scarce. In this work, we investigate the triplet-sensitized photodegradation of cysteine sulfinic acid, a (photo)degradation product of cysteine, to sulfate (SO₄^2-). We use kinetic analysis, targeted experiments, and previous literature from several fields of chemistry to explain the elementary steps that lead to the release of sulfate. Our analysis indicates that triplet sensitizers act as one-electron oxidants on the sulfinate S lone pair. The resulting radical undergoes C-S fragmentation to form SO₂, which becomes hydrated to sulfite/bisulfite (S(IV)). S(IV) is further oxidized to SO₄^2- in the presence of triplet sensitizers and oxygen. We point out that the reaction sequence SO₂ ⇌ S(IV) → SO₄^2- is valid independently of the chemical structure of the model compound and might represent a sulfate photoproduction mechanism with general validity for DOS. Our mechanistic investigation revealed that amino acids in general might also be photochemical precursors of CO₂, ammonia, acetaldehyde, and H₂O₂ and that reaction byproducts can influence the rate and mechanism of S(IV) (photo)oxidation.

Compositions and spatio-temporal distributions of different nitrogen species and lability of dissolved organic nitrogen from the Dafengjiang River to the Sanniang Bay, China

Field investigations and incubation experiment were carried out in the Dafengjiang River (DFJR) and Sanniang Bay (SNB) surface water from September 2017 to July 2018 to study the composition and spatio-temporal distributions of different nitrogen (N) species, lability of dissolved organic N (DON), and contributions of different N species from the DFJR to the dissolved inorganic N (DIN) level in the SNB. The spatio-temporal distributions of different N species exhibited significant seasonal variation (p < 0.05). The average contribution of DIN, DON, and particulate nitrogen (PN) to TN was 31.91%, 46.57%, and 21.52%, respectively. Comparatively being the dominant form of N in the study area, the average lability of DON across the incubation sites D1, D5 and D7 ranged from 72%-79%. DIN, DON, and PN from the DFJR respectively contributed to 55.95%, 7.03%, and 5.63% of water quality in the SNB.

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.

Photo-induced phosphate release during sediment resuspension in shallow lakes: A potential positive feedback mechanism of eutrophication

Dissolved phosphate (P_i) can be released during resuspended sediments exposed to sunlight. However, the significance of this phenomenon in the process of eutrophication is not clear. In this study, the behavior of photo-induced P_i release during sediment resuspension in shallow lakes with the different trophic states was investigated. The amount of photo-induced P_i release in the sediment resuspension from Lake Liangzi, Lake Dong, Lake Tangxun and Lake Longyang in China was 0.013, 0.019, 0.032, and 0.048 mg/L, respectively, and increased as the trophic states of the lakes increased. The results of phosphorus speciation analysis showed that the phosphate monoester in the particulate phosphorus is the organic phosphorus species participated in the photochemical reaction. The steady-state concentration of hydroxyl radical (OH) in the sediment resuspension also increased along with the trophic states of lakes increased and dissolved organic matter (DOM), nitrate, and Fe³⁺ presented in sediment resuspension were the main photosensitizers for OH production. All these results indicate that the increase of trophic states of lakes leads to the accumulation of organic phosphorus and OH, resulting in more dissolved phosphate photo-released, which accelerate the eutrophication process in a form of positive feedback.

Rapid photodegradation of terrestrial soil dissolved organic matter (DOM) with abundant humic-like substances under simulated ultraviolet radiation

Solar ultraviolet (UV) radiation exhibits a significant degradation for dissolved organic matter (DOM) in natural water ecosystems. However, research on photodegradation process of terrestrial components (e.g., humic-like substances) of DOM are limited due to drastic water dilution and rapid degradation. Here, photochemical degradation of terrestrial soil DOM with abundant humic-like substances from different land use were investigated by utilizing spectral technologies. Simulated UV radiation caused obvious losses on concentration, component structures, and fluorescence characteristic of soil DOM samples. The correlations between absorption specific parameters (a₂₈₀, SUVA₂₅₄, and S_R) and dissolved organic carbon (DOC) were especially pronounced (p < 0.05), which could be used as valid indicators to determine changes in DOM composition and molecular size during photobleaching process. The decreases of DOM fluorescence intensity were corresponded to first-order kinetic and half-life reactions. The greatest reduction on fluorescence intensity (31.56-81.97%) belonged to peak C (i.e., humic-like substances). Overall, DOM from forest and grass soil ecosystems was more easily photochemical degraded than anthropogenic soil DOM. Enhancive contribution of fresh DOM formed by photodegradation increased autochthonous characteristic and bioavailable nutrition by increasing biological index (BIX) values and ammonia nitrogen (NH₄⁺-N) concentration. The slight microbial decomposition effects on DOM happened in unsterilized dark condition. Our findings provided insights for understanding the rapid photodegradation processes of composition and structure of terrestrial DOM. Graphical abstract.

Photochemical Production of Sulfate and Methanesulfonic Acid from Dissolved Organic Sulfur

Photodegradation processes play an important role in releasing elements tied up in biologically refractory forms in the environment, and are increasingly being recognized as important contributors to biogeochemical cycles. While complete photo-oxidation of dissolved organic carbon (to CO₂) and dissolved organic phosphorous (to PO₄^3-) has been documented, the analogous photoproduction of sulfate from dissolved organic sulfur (DOS) has not yet been reported. Recent high-resolution mass spectrometry studies showed a selective loss of organic sulfur during photodegradation of dissolved organic matter, which was hypothesized to result in the production of sulfate. Here, we provide evidence of ubiquitous production of sulfate, methanesulfonic acid (MSA), and methanesulfinic acid (MSIA) during photodegradation of DOM samples from a wide range of natural terrestrial environments. We show that photochemical production of sulfate is generally more efficient than the production of MSA and MSIA, as well as volatile S-containing compounds such as CS₂ and COS. We also identify possible molecular precursors for sulfate and MSA, and we demonstrate that a wide range of relevant classes of DOS compounds (in terms of S oxidation state and molecular structure) can liberate sulfate upon photosensitized degradation. This work suggests that photochemistry may play a more significant role in the aquatic and atmospheric fate of DOS than currently believed.

Indirect link between riverine dissolved organic matter and bacterioplankton respiration in a boreal estuary

Increasing loading of terrestrially derived dissolved organic matter tends to enhance bacterioplankton respiration (BR) in boreal estuaries, but knowledge on the mechanisms behind this effect is not complete. We determined the stable isotopic signature of the reactive estuarine dissolved organic carbon (DOC) in the Öre estuary (Baltic Sea) by using the Keeling plot method. The δ¹³C ratio of the estuarine labile DOC varied from -26.0‰ to -18.7‰ with most values resembling those typical for DOC of coastal phytoplanktonic origin (-18 to -24‰), while being distinctly higher than those of DOC from ter-res-trial sources (-28‰ to -27‰). Furthermore, the δ¹³C of the respired carbon was positively correlated to DOC concentrations, indicating that carbon of marine origin increasingly dominated the reactive substrates when input of organic matter into the estuary became higher. This suggests that riverine organic matter mainly affects BR indirectly, by providing nutrients that stimulate the production of phytoplankton-derived reactive DOC in the estuary. Thus, riverine derived DOC per se may not be as important for coastal CO₂ emissions as previously thought.

Photoinduced disinfection in sunlit natural waters: Measurement of the second order inactivation rate constants between E. coli and photogenerated transient species

This work uncovers the implications of the estimation of exogenous inactivation rates for E. coli after the initial lag phase, and presents a strategy for the determination of the second-order inactivation rate constants (k^2nd) of these bacteria with relevant transient species promoted by solar light in natural waters. For this purpose, specific precursors were considered (nitrate ion, rose bengal, anthraquinone-2-sulfonate) as well as the respective photo-generated transient species (i.e., hydroxyl radical (•OH), singlet oxygen (¹O₂) and triplet excited states). Under these conditions and by using suitable reference compounds (acesulfame K and 2,4,6-trimethylphenol in different series of experiments), the k^2nd values were obtained after developing a proper competition kinetics methodology. The k^2nd values were (2.5 ± 0.9) × 10¹¹, (3.8 ± 1.6) × 10⁷ and (1.8 ± 0.7) × 10¹⁰ M^-1 s^-1 for the inactivation of E. coli by •OH, ¹O₂ and the triplet state of anthraquinone-2-sulfonate (³AQ2S*), respectively. The measurement of a reaction rate constant that is higher than the diffusion-control limit for small molecules in aqueous solution implies that bacteria behave differently from molecules, e.g., because of the large size difference between bacteria and the transients. The obtained k^2nd values were used for the modeling of the bacteria inactivation kinetics in outdoor systems (both water bodies and SODIS bottles), limited to the exponential decay phase that follows the initial lag time. Afterwards, the role of dissolved organic matter (DOM) as precursor of transient species for bacterial elimination was systematically studied. The interaction of different sunlight wavelength regions (UVB, UV-A, blue, green and yellow light) with Suwannee river (SW) and Nordic Lake organic matter (ND) was tested, and the photoinduced disinfection exerted by DOM isolates (SW DOM, Suwannee River Humic Acid, Suwannee River Fulvic Acid or Pony Lake Fulvic Acid) was compared. It was not possible to achieve a complete differentiation of the individual contributions of DOM triplet states (³DOM*) and ¹O₂ to bacterial inactivation. However, the application of competition kinetics to E. coli under solar irradiation in the presence of SW led to a k^2nd value of (2.17 ± 0.40) × 10¹⁰ M^-1 s^-1, which is very near the value for inactivation by ³AQ2S* and suggests that the latter behaved very similar to SW-³DOM* and was a good ³DOM* proxy in the present case. The determination of the second-order inactivation rate constants of E. coli with •OH, ³DOM* and ¹O₂ represents a significant progress in the understanding of the external inactivation pathways of bacteria. It also allows predicting that, after the lag phase, ¹O₂ would contribute to photoinactivation to a far lesser extent than •OH and ³DOM*.

Transcriptome Changes of Escherichia coli, Enterococcus faecalis, and Escherichia coli O157:H7 Laboratory Strains in Response to Photo-Degraded DOM

In this study, we investigated gene expression changes in three bacterial strains (Escherichia coli C3000, Escherichia coli O157:H7 B6914, and Enterococcus faecalis ATCC 29212), commonly used as indicators of water quality and as control strains in clinical, food, and water microbiology laboratories. Bacterial transcriptome responses from pure cultures were monitored in microcosms containing water amended with manure-derived dissolved organic matter (DOM), previously exposed to simulated sunlight for 12 h. We used RNA sequencing (RNA-seq) and quantitative real-time reverse transcriptase (qRT-PCR) to compare differentially expressed temporal transcripts between bacteria incubated in microcosms containing sunlight irradiated and non-irradiated DOM, for up to 24 h. In addition, we used whole genome sequencing simultaneously with RNA-seq to identify single nucleotide variants (SNV) acquired in bacterial populations during incubation. These results indicate that E. coli and E. faecalis have different mechanisms for removal of reactive oxygen species (ROS) produced from irradiated DOM. They are also able to produce micromolar concentrations of H₂O₂ from non-irradiated DOM, that should be detrimental to other bacteria present in the environment. Notably, this study provides an assessment of the role of two conjugative plasmids carried by the E. faecalis and highlights the differences in the overall survival dynamics of environmentally-relevant bacteria in the presence of naturally-produced ROS.

Photodegradation of river dissolved organic matter and trace metals in the largest European Arctic estuary

Photo-induced degradation of dissolved organic matter (DOM) and organo-mineral colloids is one of the major factor responsible for transformation of DOM and dissolved metals in boreal and subarctic waters. In contrast to fairly good understanding of this process in inland waters of high latitude zone, the transformation of riverine DOM and associated trace element (TE) colloids in the Arctic estuaries remains virtually unknown. We incubated, under sunlight in outdoor pools, quartz reactors filled with mixtures of sterile filtered riverine and estuarine water. The water samples were collected in the estuarine zone of the largest European Arctic river, Severnaya Dvina. After 1month of exposure to sunlight, ≤5% change of dissolved organic carbon (DOC) concentration and specific ultraviolet (254nm) absorption occurred. This decrease was within the experimental uncertainty and it implies quite high resistance of river dissolved organic matter to photo-degradation in this estuary. Moreover, very low photodegradability of DOM in the freshwater point of the Severnaya Dvina River may require revisiting the current paradigm of the importance of DOC photolysis in large Arctic rivers. A novel finding was that the percentages of overall removal of Fe and some insoluble elements were quite similar across the full range of studied salinities, whereas the apparent rate of metal removal decreased with the increase of salinity. Overall, the salinity weakly impacted the removal of riverine DOC and metals in the estuarine water via photolysis and coagulation under sunlight. As a result, photoreactivity of DOM and dissolved metals in riverine end members corrected for estuarine dilution can be used to approximate the photolytic transformation of riverine material in the Arctic coastal zone.

Impacts of global changes on the biogeochemistry and environmental effects of dissolved organic matter at the land-ocean interface: a review

Dissolved organic matter (DOM) is an important component in the biogeochemistry and ecosystem function of aquatic environments at the highly populated land-ocean interface. The mobilization and transformation of DOM at this critical interface are increasingly affected by a series of notable global changes such as the increasing storm events, intense human activities, and accelerating glacier loss. This review provides an overview of the changes in the quantity and quality of DOM under the influences of multiple global changes. The profound implications of changing DOM for aquatic ecosystem and human society are further discussed, and future research needs are suggested for filling current knowledge gaps. The fluvial export of DOM is strongly intensified during storm events, which is accompanied with notable changes in the chemical composition and reactivity of DOM. Land use not only changes the mobilization of natural DOM source pools within watersheds but also adds DOM of distinct chemical composition and reactivity from anthropogenic sources. Glacier loss brings highly biolabile DOM to downstream water bodies. The changing DOM leads to significant changes in heterotrophic activity, CO₂ out gassing, nutrient and pollutant biogeochemistry, and disinfection by-product formation. Further studies on the source, transformations, and downstream effects of storm DOM, temporal variations of DOM and its interactions with other pollutants in human-modified watersheds, photo-degradability of glacier DOM, and potential priming effects, are essential for better understanding the responses and feedbacks of DOM at the land-ocean interface under the impacts of global changes.

Photochemical behavior of dissolved and colloidal organic matter in estuarine and oceanic waters

Chromophoric dissolved organic matter (CDOM), carbohydrates, and amino acids were analyzed to investigate the photochemistry of total dissolved (<0.22μm) organic matter (DOM), high-molecular-weight (HMW, 1kDa-0.22μm) DOM and low-molecular-weight (LMW, <1kDa) DOM at stations in the Yangtze River and its coastal area, and in the Western Pacific Ocean. Results revealed that the humic-like and tryptophan-like CDOM fluorescent components in riverine, coastal, and oceanic surface waters were photodegraded during irradiation. However, the photochemical behavior of tyrosine-like component was obscured by the excessive fluorescence intensities of humic- and tryptophan-like fluorescent components. Light sensitivity varied depending on the source material; terrestrially derived DOM was more susceptible to irradiation than autochthonous DOM. In contrast to the expected photodegradation of CDOM, photo-induced synthetic reaction transformed the LMW matters to polysaccharides (PCHO) and degradation reaction decomposed the HMW DOM to Monosaccharides. Colloidal DOM preferentially underwent photodegradation, whereas permeate DOM mainly photosynthesized PCHO. The total hydrolysable amino acid (THAA) pool changed because of the additional input by the photodegradation of DOM or THAA itself. The compositions of THAA changed during the irradiation experiments, indicating that the different photochemical behavior of individual amino acids were related to their different original photoreactivities; the relatively stable amino acids (e.g., Ser and Gly) significantly accumulated during irradiation, whereas photo-active aromatic amino acids (e.g. Tyr and His) were prone to photodegradation. The data presented here demonstrated that irradiation significantly influence the conversion between dissolved and colloid organic matter. These results can promote the understanding of irradiation effect on the carbon and nitrogen cycle in riverine, estuarine and oceanic ecosystems.

Evaluation of leachate dissolved organic nitrogen discharge effect on wastewater effluent quality

Nitrogen is limited more and more frequently in wastewater treatment plant (WWTP) effluents because of the concern of causing eutrophication in discharge waters. Twelve leachates from eight landfills in Florida and California were characterized for total nitrogen (TN) and dissolved organic nitrogen (DON). The average concentration of TN and DON in leachate was approximately 1146mg/L and 40mg/L, respectively. Solid-phase extraction was used to fractionate the DON based on hydrophobic (recalcitrant fraction) and hydrophilic (bioavailable fraction) chemical properties. The average leachate concentrations of bioavailable (bDON) and recalcitrant (rDON) DON were 16.5mg/L and 18.4mg/L, respectively. The rDON fraction was positively correlated, but with a low R², with total leachate apparent color dissolved UV₂₅₄, chemical oxygen demand (COD), and humic acid (R² equals 0.38, 0.49, and 0.40, respectively). The hydrophobic fraction of DON (rDON) was highly colored. This fraction was also associated with over 60% of the total leachate COD. Multiple leachate and wastewater co-treatment simulations were carried out to assess the effects of leachate on total nitrogen wastewater effluent quality using removals for four WWTPs under different scenarios. The calculated pass through of DON suggests that leachate could contribute to significant amounts of nitrogen discharged to aquatic systems.

Photobleaching of chromophoric dissolved organic matter (CDOM) in the Yangtze River estuary: kinetics and effects of temperature, pH, and salinity

The kinetics and temperature-, pH- and salinity-dependences of photobleaching of chromophoric dissolved organic matter (CDOM) in the Yangtze River estuary (YRE) were evaluated using laboratory solar-simulated irradiation and compared to those of Suwannee River humic substances (SRHSs). Nearly all CDOM in water at the head of the estuary (headwater herein) was photobleachable in both summer and winter, while significant fractions of CDOM (13-29%) were resistant to photobleaching in saltier waters. The photobleaching rate constant in the headwater was 25% higher in summer than that in winter. The absorbed photon-based photobleaching efficiency (PE) increased with temperature following the linear Arrhenius equation. For a 20 °C increase in temperature, PE increased by ∼45% in the headwater and by 70-81% in the saltier waters. PE for YRE samples exhibited minima at pH from 6 to 7 and increased with both lower and higher pH values, contrasting the consistent increase in PE with pH shown by SRHSs. No consistent effect of salinity on PE was observed for both SRHSs and YRE samples. Photobleaching increased the spectral slope coefficient between 275 nm and 295 nm in summer, consistent with the behavior of SRHSs, but decreased it in winter, implying a difference in the molecular composition of chromophores between the two seasons. Temperature, salinity, and pH modified the photoalteration of the spectral shape but their effects varied spatially and seasonally. This study demonstrates that CDOM quality, temperature, and pH should be incorporated into models involving quantification of photobleaching.

Influence of land use and land cover on the spatial variability of dissolved organic matter in multiple aquatic environments

Water quality of lakes, estuaries, and coastal areas serves as an indicator of the overall health of aquatic ecosystems as well as the health of the terrestrial ecosystem that drains to the water body. Land use and land cover plays not only a significant role in controlling the quantity of the exported dissolved organic matter (DOM) but also influences the quality of DOM via various biogeochemical and biodegradation processes. We examined the characteristics and spatial distribution of DOM in five major lakes, in an estuary, and in the coastal waters of the Mississippi, USA, and investigated the influence of the land use and land cover of their watersheds on the DOM composition. We employed absorption and fluorescence spectroscopy including excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis modeling techniques to determine optical properties of DOM and its characteristics in this study. We developed a site-specific PARAFAC model to evaluate DOM characteristics resulting in five diverse DOM compositions that included two terrestrial humic-like (C1 and C3), two microbial humic-like (C2 and C5), and one protein-like (C4) DOM. Our results showed elevated fluorescence levels of microbial humic-like or protein-like DOM in the lakes and coastal waters, while the estuarine waters showed relatively high fluorescence levels of terrestrial humic-like DOM. The results also showed that percent forest and wetland coverage explained 68 and 82% variability, respectively, in terrestrial humic-like DOM exports, while 87% variability in microbially derived humiclike DOM was explained by percent agricultural lands. Strong correlations between microbial humic-like DOM and fluorescence-derived DOM indices such as biological index (BIX) and fluorescence index (FI) indicated autochthonous characteristics in the lakes, while the estuary showed largely allochthonous DOM of terrestrial origin. We also observed higher concentrations of total dissolved phosphorous (TDP) and ammonium nitrogen (NH₄-N) in coastal waters potentially due to photodegradation of refractory DOM derived from the sediment-bound organic matter in the coastal wetlands. This study highlights the relationships between the DOM compositions in the water and the land use and land cover in the watershed. The spatial variability of DOM in three different types of aquatic environments enhances the understanding of the role of land use and land cover in carbon cycling through export of organic matter to the aquatic ecosystems..

A survey of photogeochemistry

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

Linking optical properties of dissolved organic matter to multiple processes at the coastal plume zone in the East China Sea

Because of the significance in photosynthesis, nutrient dynamics, trophodynamics and biological activity, dissolved organic matter (DOM) is important to the microbial community in the coastal plume zone. In this study, we investigated the hydrodynamic processes, photodegradation and biodegradation of DOM at the Yangtze River plume in the East China Sea through analyzing water quality and optical properties of DOM. Surface water samples were collected to examine water quality and fluorescence properties of fluorescent dissolved organic matter (FDOM). The results indicated that dilution was the key factor in the multiple processes, and the mixing process gradually increased from nearshore to offshore in coastal water. Four components of FDOM representing humic-like substances (C1 & C4) and protein-like substances (C2 & C3) were identified, and all components showed nearly conservative behaviors. Protein-like substances were more mutable compared to humic-like substances. The photodegradation of humic-like substances caused brown algae blooms to some extent. The molecular weight of humic substances gradually decreased along the mixing process. FDOM in the plume zone was both of terrigenous and autochthonous origins, and the characteristic of terrigenous origin was obvious compared to that of autochthonous origin.

Effect of algal flocculation on dissolved organic matters using cationic starch modified soils

Modified soils (MSs) are being increasingly used as geo-engineering materials for the sedimentation removal of cyanobacterial blooms. Cationic starch (CS) has been tested as an effective soil modifier, but little is known about its potential impacts on the treated water. This study investigated dissolved organic matters in the bloom water after algal removal using cationic starch modified soils (CS-MSs). Results showed that the dissolved organic carbon (DOC) could be decreased by CS-MS flocculation and the use of higher charge density CS yielded a greater DOC reduction. When CS with the charge density of 0.052, 0.102 and 0.293meq/g were used, DOC was decreased from 3.4 to 3.0, 2.3 and 1.7mg/L, respectively. The excitation-emission matrix fluorescence spectroscopy and UV254 analysis indicated that CS-MS exhibits an ability to remove some soluble organics, which contributed to the DOC reduction. However, the use of low charge density CS posed a potential risk of DOC increase due to the high CS loading for effective algal removal. When CS with the charge density of 0.044meq/g was used, DOC was increased from 3.4 to 3.9mg/L. This study suggested, when CS-MS is used for cyanobacterial bloom removal, the content of dissolved organic matters in the treated water can be controlled by optimizing the charge density of CS. For the settled organic matters, other measures (e.g., capping treatments using oxygen loaded materials) should be jointly applied after algal flocculation.

Development of an in Situ NMR Photoreactor To Study Environmental Photochemistry

Photochemistry is a key environmental process directly linked to the fate, source, and toxicity of pollutants in the environment. This study explores two approaches for integrating light sources with nuclear magnetic resonance (NMR) spectroscopy: sample irradiation using a "sunlight simulator" outside the magnet versus direct irradiation of the sample inside the magnet. To assess their applicability, the in situ NMR photoreactors were applied to a series of environmental systems: an atmospheric pollutant (p-nitrophenol), crude oil extracts, and groundwater. The study successfully illustrates that environmentally relevant aqueous photochemical processes can be monitored in situ and in real time using NMR spectroscopy. A range of intermediates and degradation products were identified and matched to the literature. Preliminary measurements of half-lives were also obtained from kinetic curves. The sunlight simulator was shown to be the most suitable model to explore environmental photolytic processes in situ. Other light sources with more intense UV output hold potential for evaluating UV as a remediation alternative in areas such as wastewater treatment plants or oil spills. Finally, the ability to analyze the photolytic fate of trace chemicals at natural abundance in groundwater, using a cryogenic probe, demonstrates the viability of NMR spectroscopy as a powerful and complementary technique for environmental applications in general.

Dynamic exchanges between DOM and POM pools in coastal and inland aquatic ecosystems: A review

Dynamic exchanges between dissolved organic matter (DOM) and particulate organic matter (POM) plays a critical role in organic carbon cycling in coastal and inland aquatic ecosystems, interactions with aquatic organisms, mobility and bioavailability of pollutants, among many other ecological and geochemical phenomena. Although DOM-POM exchange processes have been widely studied from different aspects, little to no effort has been made to date to provide a comprehensive, mechanistic, and micro-spatial schema for understanding various exchange processes occurring in different aquatic ecosystems in a unified way. The phenomena occurring between DOM and POM were explained here with the homogeneous and heterogeneous mechanisms. In the homogeneous mechanism, the participating components are only organic matter (OM) constituents themselves with aggregation and dissolution involved, whereas OM is associated with other components such as minerals and particulate colloids in the heterogeneous counterpart. Besides the generally concerned processes of aggregation/dissolution and adsorption/desorption, other ecological factors such as sunlight and organisms can also participate in DOM-POM exchanges through altering the chemical nature of OM. Despite the limitation of current analytical technologies, many unknown and/or unquantified processes need to be identified to unravel the complicated exchanges of OM between its dissolved and particulate states. Based on the review of several previous mathematical models, we proposed a unified conceptual model to describe all major dynamic exchange mechanisms on the basis of exergy theory. More knowledge of dynamic DOM-POM exchanges is warranted to overcome the potential problems arising from a simple division of OM into dissolved versus particulate states and to further develop more sophisticated mathematic models.

Does river restoration affect diurnal and seasonal changes to surface water quality? A study along the Thur River, Switzerland

Changes in river water quality were investigated along the lower reach of the Thur River, Switzerland, following river restoration and a summer storm event. River restoration and hydrological storm events can each cause dramatic changes to water quality by affecting various bio-geochemical processes in the river, but have to date not been well documented, especially in combination. Evaluating the success of river restoration is often restricted in large catchments due to a lack of high frequency water quality data, which are needed for process understanding. These challenges were addressed in this study by measuring water quality parameters including dissolved oxygen (DO), temperature, pH, electrical conductivity (EC), nitrate and dissolved organic carbon (DOC) with a high temporal frequency (15 min-1h) over selected time scales. In addition, the stable isotopes of water (δD and δ(18)O-H2O) as well as those of nitrate (δ(15)N-NO3(-) and δ(18)O-NO3(-)) were measured to follow changes in water quality in response to the hydrological changes in the river. To compare the spatial distribution of pre- and post-restoration water quality, the sampling stations were chosen upstream and downstream of the restored section. The diurnal and seasonal changes were monitored by conducting 24-hour campaigns in three seasons (winter, summer and autumn) in 2012 and 2013. The amplitude of the diurnal changes of the various observed parameters showed significant seasonal and spatial variability. Biological processes--mainly photosynthesis and respiration--were found to be the major drivers of these diurnal cycles. During low flow in autumn, a reduction of nitrate (attributed to assimilation by autotrophs) in the pre-dawn period and a production of DOC during the daytime (attributed to photosynthesis) were observed downstream of the restored site. Further, a summer storm event was found to override the influence of these biological processes that control the diurnal changes. High frequency daily monitoring of key water quality parameters over different seasons is shown to be essential in evaluating river restoration success.

Enhanced Indirect Photochemical Transformation of Histidine and Histamine through Association with Chromophoric Dissolved Organic Matter

Photochemical transformations greatly affect the stability and fate of amino acids (AAs) in sunlit aquatic ecosystems. Whereas the direct phototransformation of dissolved AAs is well investigated, their indirect photolysis in the presence of chromophoric dissolved organic matter (CDOM) is poorly understood. In aquatic systems, CDOM may act both as sorbent for AAs and as photosensitizer, creating microenvironments with high concentrations of photochemically produced reactive intermediates, such as singlet oxygen (1O2). This study provides a systematic investigation of the indirect photochemical transformation of histidine (His) and histamine by 1O2 in solutions containing CDOM as a function of solution pH. Both His and histamine showed pH-dependent enhanced phototransformation in the CDOM systems as compared to systems in which model, low-molecular-weight 1O2 sensitizers were used. Enhanced reactivity resulted from sorption of His and histamine to CDOM and thus exposure to elevated 1O2 concentrations in the CDOM microenvironment. The extent of reactivity enhancement depended on solution pH via its effects on the protonation state of His, histamine, and CDOM. Sorption-enhanced reactivity was independently supported by depressed rate enhancements in the presence of a cosorbate that competitively displaced His and histamine from CDOM. Incorporating sorption and photochemical transformation processes into a reaction rate prediction model improved the description of the abiotic photochemical transformation rates of His in the presence of CDOM.

Response of a stoichiometrically imbalanced ecosystem to manipulation of nutrient supplies and ratios

Cuatro Ciénegas Basin (CCB) is a desert ecosystem that hosts a large diversity of water bodies. Many surface waters in this basin have imbalanced nitrogen (N) to phosphorus (P) stoichiometry (total N:P > 100 by atoms), where P is likely to be a limiting nutrient. To investigate the effects of nutrient stoichiometry on planktonic and sediment ecosystem components and processes, we conducted a replicated in situ mesocosm experiment in Lagunita, a shallow pond located in the southwest region of the basin. Inorganic N and P were periodically added to mesocosms under three different N:P regimes (P only, N:P = 16 and N:P = 75) while the control mesocosms were left unamended. After three weeks of fertilization, more than two thirds of the applied P was immobilized into seston or sediment. The rapid uptake of P significantly decreased biomass C:P and N:P ratios, supporting the hypothesis that Lagunita is P-limited. Meanwhile, simultaneous N and P enrichment significantly enhanced planktonic growth, increasing total planktonic biomass by more than 2-fold compared to the unenriched control. With up to 76% of added N sequestered into the seston, it is suspected that the Lagunita microbial community also experienced strong N-limitation. However, when N and P were applied at N:P = 75, the microbes remained in a P-limitation state as in the untreated control. Two weeks after the last fertilizer application, seston C:P and N:P ratios returned to initial levels but chlorophyll a and seston C concentrations remained elevated. Additionally, no P release from the sediment was observed in the fertilized mesocosms. Overall, this study provides evidence that Lagunita is highly sensitive to nutrient perturbation because the biota is primarily P-limited and experiences a secondary N-limitation despite its high TN:TP ratio. This study serves as a strong basis to justify the need for protection of CCB ecosystems and other low-nutrient microbe-dominated systems from anthropogenic inputs of both N and P.

Assessing biophysical controls on Gulf of Mexico hypoxia through probabilistic modeling

A mechanistic model was developed to predict midsummer bottom-water dissolved oxygen (BWDO) concentration and hypoxic area on the Louisiana shelf of the northern Gulf of Mexico, USA (1985-2011). Because of its parsimonious formulation, the model possesses many of the benefits of simpler, more empirical models, in that it is computationally efficient and can rigorously account for uncertainty through Bayesian inference. At the same time, the model incorporates important biophysical processes such that its parameterization can be informed by field-measured biological and physical rates. The model is used to explore how freshwater flow, nutrient load, benthic oxygen demand, and wind velocity affect hypoxia on the western and eastern sections of the shelf, delineated by the Atchafalaya River outfall. The model explains over 70% of the variability in BWDO on both shelf sections, and outperforms linear regression models developed from the same input variables. Model results suggest that physical factors (i.e., wind and flow) control a larger portion of the year-to-year variability in hypoxia than previously thought, especially on the western shelf, though seasonal nutrient loads remain an important driver of hypoxia, as well. Unlike several previous Gulf hypoxia modeling studies, results do not indicate a temporal shift in the system's propensity for hypoxia formation (i.e., no regime change). Results do indicate that benthic oxygen demand is a substantial BWDO sink, and a better understanding of the long-term dynamics of this sink is required to better predict how the size of the hypoxic zone will respond to proposed reductions in nutrient loading.

Seasonal photochemical transformations of nitrogen species in a forest stream and lake

The photochemical release of inorganic nitrogen from dissolved organic matter is an important source of bio-available nitrogen (N) in N-limited aquatic ecosystems. We conducted photochemical experiments and used mathematical models based on pseudo-first-order reaction kinetics to quantify the photochemical transformations of individual N species and their seasonal effects on N cycling in a mountain forest stream and lake (Plešné Lake, Czech Republic). Results from laboratory experiments on photochemical changes in N speciation were compared to measured lake N budgets. Concentrations of organic nitrogen (N_org; 40–58 µmol L⁻¹) decreased from 3 to 26% during 48-hour laboratory irradiation (an equivalent of 4–5 days of natural solar insolation) due to photochemical mineralization to ammonium (NH₄⁺) and other N forms (N_x; possibly N oxides and N₂). In addition to N_org mineralization, N_x also originated from photochemical nitrate (NO₃⁻) reduction. Laboratory exposure of a first-order forest stream water samples showed a high amount of seasonality, with the maximum rates of N_org mineralization and NH₄⁺ production in winter and spring, and the maximum NO₃⁻ reduction occurring in summer. These photochemical changes could have an ecologically significant effect on NH₄⁺ concentrations in streams (doubling their terrestrial fluxes from soils) and on concentrations of dissolved N_org in the lake. In contrast, photochemical reactions reduced NO₃⁻ fluxes by a negligible (<1%) amount and had a negligible effect on the aquatic cycle of this N form.

Spatial distributions of and diurnal variations in low molecular weight carbonyl compounds in coastal seawater, and the controlling factors

We studied the spatial distributions of and the diurnal variations in four low molecular weight (LMW) carbonyl compounds, formaldehyde, acetaldehyde, propionaldehyde, and glyoxal, in coastal seawater. The samples were taken from the coastal areas of Hiroshima Bay, the Iyo Nada, and the Bungo Channel, western Japan. The formaldehyde, acetaldehyde, and glyoxal concentrations were higher in the northern part of Hiroshima Bay than at offshore sampling points in the Iyo Nada and the Bungo Channel. These three compounds were found at much higher concentrations in the surface water than in deeper water layers in Hiroshima Bay. It is noteworthy that propionaldehyde was not detected in any of the seawater samples, the concentrations present being lower than the detection limit (1 nanomole per liter (nM)) of the high performance liquid chromatography (HPLC) system we used. Photochemical and biological experiments were performed in the laboratory to help understand the characteristic distributions and fates of the LMW carbonyl compounds. The primary process controlling their fate in the coastal environment appears to be their biological consumption. The direct photo degradation of propionaldehyde, initiated by ultraviolet (UV) absorption, was observed, although formaldehyde and acetaldehyde were not degraded by UV irradiation. Our results suggest that the degradation of the LMW carbonyl compounds by photochemically formed hydroxyl radicals is relatively insignificant in the study area. Atmospheric deposition is a possible source of soluble carbonyl compounds in coastal surface seawater, but it may not influence the carbonyl concentrations in offshore waters.

Ammonium production off central Chile (36°S) by photodegradation of phytoplankton-derived and marine dissolved organic matter

We investigated the production of ammonium by the photodegradation of dissolved organic matter (DOM) in the coastal upwelling system off central Chile (36°S). The mean penetration of solar radiation (Z1%) between April 2011 and February 2012 was 9.4 m, 4.4 m and 3.2 m for Photosynthetically Active Radiation (PAR; 400-700 nm), UV-A (320-400 nm) and UV-B (280-320 nm), respectively. Ammonium photoproduction experiments were carried out using exudates of DOM obtained from cultured diatom species (Chaetoceros muelleri and Thalassiosira minuscule) as well as natural marine DOM. Diatom exudates showed net photoproduction of ammonium under exposure to UVR with a mean rate of 0.56±0.4 µmol L(-1) h(-1) and a maximum rate of 1.49 µmol L(-1) h(-1). Results from natural marine DOM showed net photoproduction of ammonium under exposure to PAR+UVR ranging between 0.06 and 0.2 µmol L(-1) h(-1). We estimated the potential contribution of photochemical ammonium production for phytoplankton ammonium demand. Photoammonification of diatom exudates could support between 117 and 453% of spring-summer NH4(+) assimilation, while rates obtained from natural samples could contribute to 50-178% of spring-summer phytoplankton NH4(+) requirements. These results have implications for local N budgets, as photochemical ammonium production can occur year-round in the first meters of the euphotic zone that are impacted by full sunlight.

Dual roles of dissolved organic matter as sensitizer and quencher in the photooxidation of tryptophan

The photooxidation processes of tryptophan (Trp) in the presence of dissolved organic matter (DOM) were identified and quantified by steady-state photolysis experiments, laser spectroscopy and kinetic modeling. In sunlight, Trp photooxidation is dominated by the reaction with excited triplet DOM ((3)DOM), accounting for approximately 50-70% of the total degradation, depending on the DOM concentration and source. Reaction with singlet oxygen and direct photolysis are secondary processes that are both still more important than the reaction with hydroxyl radical. Both direct photolysis and reaction with (3)DOM form Trp radical cation (Trp(•+)) via Trp photoionization and direct oxidation, respectively. The Trp(•+) can be converted back to Trp by suitable electron or hydrogen atom donors. Transient absorption spectroscopy shows that DOM itself and low-molecular-weight analogues of redox-active moieties can reduce the lifetime of photochemically produced Trp(•+) and thus quench Trp degradation. This study demonstrates that DOM plays dual roles in the photodegradation of Trp acting as a sensitizer and quencher. The photochemistry of Trp and the participation of DOM have direct implications for photochemical reactions in extracellular proteins as well as for organic compounds in aquatic systems with similar photoionization processes.

Photochemical and microbial alterations of DOM spectroscopic properties in the estuarine system Ria de Aveiro

The chromophoric dissolved organic matter (CDOM) of marine (MZ) and brackish water (BZ) zones of Ria de Aveiro showed different spectral characteristics and susceptibility to photochemical alterations, reflecting the different amounts and prevailing sources of organic matter.

Reactivity differences of combined and free amino acids: quantifying the relationship between three-dimensional protein structure and singlet oxygen reaction rates

It has long been appreciated that the photooxidation kinetics of amino acid (AA) residues in an intact protein differ from those of free AAs due to differences in the local steric microenvironment, such as its location in the three-dimensional structure. Yet there are only a few studies that have quantified the effect of protein structure on the photochemical reactivity of its residues. This is important for predicting phototransformation rates of AAs in aquatic environments where AAs in combined forms (e.g., oligopeptides and proteins) are more abundant than free AAs. In this work, the photochemical reactivity differences between free and combined AAs were assessed. Singlet oxygen ((1)O2) reaction kinetics of individual photooxidizable residues in the protein glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were examined. The results suggest that the (1)O2 accessibility of residues in intact GAPDH has a profound effect on their photodegradation kinetics and for histidine residues can explain most of the variation in (1)O2 reactivity. Additionally, (1)O2-accessibile surface area values of residues calculated from protein crystal structure data are useful in predicting their reaction rates in GAPDH. This work illustrates a new approach to assess the differential photochemical reactivity of AA-based biomolecules in natural environments or engineered applications.

Sources, factors, mechanisms and possible solutions to pollutants in marine ecosystems

Algal toxins or red-tide toxins produced during algal blooms are naturally-derived toxic emerging contaminants (ECs) that may kill organisms, including humans, through contaminated fish or seafood. Other ECs produced either naturally or anthropogenically ultimately flow into marine waters. Pharmaceuticals are also an important pollution source, mostly due to overproduction and incorrect disposal. Ship breaking and recycle industries (SBRIs) can also release various pollutants and substantially deteriorate habitats and marine biodiversity. Overfishing is significantly increasing due to the global food crisis, caused by an increasing world population. Organic matter (OM) pollution and global warming (GW) are key factors that exacerbate these challenges (e.g. algal blooms), to which acidification in marine waters should be added as well. Sources, factors, mechanisms and possible remedial measures of these challenges to marine ecosystems are discussed, including their eventual impact on all forms of life including humans.

UV-induced photochemical heterogeneity of dissolved and attached organic matter associated with cyanobacterial blooms in a eutrophic freshwater lake

Cyanobacterial blooms represent a significant ecological and human health problem worldwide. In aquatic environments, cyanobacterial blooms are actually surrounded by dissolved organic matter (DOM) and attached organic matter (AOM) that bind with algal cells. In this study, DOM and AOM fractionated from blooming cyanobacteria in a eutrophic freshwater lake (Lake Taihu, China) were irradiated with a polychromatic UV lamp, and the photochemical heterogeneity was investigated using fluorescence excitation-emission matrix (EEM)-parallel factor (PARAFAC) analysis and synchronous fluorescence (SF)-two dimensional correlation spectroscopy (2DCOS). It was shown that a 6-day UV irradiation caused more pronounced mineralization for DOM than AOM (59.7% vs. 41.9%). The EEM-PARAFAC analysis identified one tyrosine-, one humic-, and two tryptophan-like components in both DOM and AOM, and high component photodegradation rates were observed for DOM versus AOM (k > 0.554 vs. <0.519). Moreover, SF-2DCOS found that the photodegradation of organic matters followed the sequence of tyrosine-like > humic-like > tryptophan-like substances. Humic-like substances promoted the indirect photochemical reactions, and were responsible for the higher photochemical rate for DOM. The lower photodegradation of AOM benefited the integrality of cells in cyanobacterial blooms against the negative impact of UV irradiation. Therefore, the photochemical behavior of organic matter was related to the adaptation of enhanced-duration cyanobacterial blooms in aquatic environments.

Contribution of chemical water properties to the differential responses of bacterioneuston and bacterioplankton to ultraviolet-B radiation

The surface microlayer (SML) is characterized by different physicochemical properties from underlying waters (UW). However, whether these differences in abiotic factors underlie the distinct sensitivity of bacterioneuston (i.e. SML bacteria) and bacterioplankton to environmental stressors remains to be addressed. We investigated the contribution of abiotic factors to the UV-B sensitivity of bacterioneuston and bacterioplankton. Nutrients (especially nitrogen and phosphate) emerged as important determinants of bacterial UV-B sensitivity. The role of particles, nutrients, and dissolved organic components on bacterial UV-B sensitivity was further evaluated using dilution cultures. Filtered samples were twofold more UV sensitive than unfiltered samples, suggesting a UV-protective effect of particles. High nutrient concentrations attenuated bacterial UV-B sensitivity (up to 40%), compared with unamended conditions, by influencing bacterial physiology and/or community composition. Suspending cells in natural water, particularly from the SML, also attenuated UV-B sensitivity (up to 23%), compared with suspension in an artificial mineral solution. Bioassays using Pseudomonas sp. strain NT5I1.2B revealed that chemical water properties influence UV-induced oxidative damage. UV-B sensitivity was associated with high cell-specific activities. The chemical environment of the SML and UW influences UV-B effects on the corresponding bacterial communities. Maintaining low cell activities might be advantageous in stressful environments, like the SML.

Overlapping photodegradable and biodegradable organic nitrogen in wastewater effluents

Photochemical degradation of dissolved organic nitrogen (DON) in final effluent of trickling filter and activated sludge wastewater treatment plants (WWTPs) was studied. Inorganic N, mostly nitrite, was produced from the photodegradation of DON for samples from both WWTPs. Photodegradable DON (PDON), biodegradable DON (BDON), and overlapping photodegradable-biodegradable DON (OPBDON) were determined. BDON was associated with PDON as well as non-PDON. BDON and PDON concentrations in the final effluent samples were 4.71 and 4.62 mg N/L for the trickling filter plant and 3.95 and 3.73 mg N/L for the activated sludge plant, indicating that photodegradation is as important as biodegradation in the mineralization of effluent DON in receiving waters. OPBDON, which is more problematic in the water environment because it can be mineralized by light or bacteria or both, was 3.68 and 2.64 mg N/L (57% and 43% of total DON) in the final effluent samples from the trickling filter and activated sludge plants, respectively. The DON fraction that is resistant to biodegradation and photodegradation was 10% to 20% of total DON.