Photodegradation Of Dissolved Organic Matter Research Articles

Controlling factors of latitudinal distribution of dissolved organic matter in the upper layers of the Indian Ocean

We studied chromophoric (CDOM) and fluorescent (FDOM) dissolved organic matter (DOM) and dissolved organic carbon in surface waters to determine the factors controlling the geographical distribution of DOM along two meridional transects in the Indian Ocean. For CDOM, we calculated the absorption coefficients, spectral slope, and absorption coefficient ratio from the observed absorption spectra. For FDOM, we calculated the biological (BIX) and humification (HIX) indices from the excitation emission matrices (EEMs); parallel factor analysis of the EEMs identified three fluorescent components, i.e., two humic-like and one protein-like. Using these DOM parameters, a factor analysis extracted fewer latent variables than the observed variables to account for the geographical distributions. We obtained three factors (F1, F2, and F3), which explained ~ 84% of the variance in the observed data. From the factor loadings, F1, F2, and F3 were interpreted as the effects of net primary production-derived DOM and its horizontal transport, photodegradation, and vertical transport by physical processes. We characterized seven marine biogeochemical provinces by factor scores. F1 scores gradually decreased from the northernmost to the Antarctic province, with a small maximum around the subtropical front. F2 scores were highest in the subtropical province and decreased in both the northward and southward directions. F3 scores were high in the Antarctic and northernmost provinces, and lowest in the subtropical province. Only BIX was insufficiently explained by these factors. BIX was highest in the northern part of the subtropical province, where photodegradation of DOM was the most intense. This suggests that the possible interaction between photodegradation, autochthonous production, and reworking by heterotrophic bacteria of DOM occurs in the subtropical province.

Decreasing Photoreactivity and Concurrent Change in Dissolved Organic Matter Composition With Increasing Inland Water Residence Time

AbstractPhotochemical degradation of dissolved organic matter (DOM) has been the subject of numerous studies; however, its regulation along the inland water continuum is still unclear. We aimed to unravel the DOM photoreactivity and concurrent DOM compositional changes across 30 boreal aquatic ecosystems including peat waters, streams, rivers, and lakes distributed along a water residence time (WRT) gradient. Samples were subjected to a standardized exposure of simulated sunlight. We measured the apparent quantum yield (AQY), which corresponds to DOM photomineralization per photon absorbed, and the compositional change in DOM at bulk and individual compound levels in the original samples and after irradiation. AQY increased with the abundance of terrestrially derived DOM and decreased at higher WRT. Additionally, the photochemical changes in both DOM optical properties and molecular composition resembled changes along the natural boreal WRT gradient at low WRT (<3 years). Accordingly, mass spectrometry revealed that the abundance of photolabile and photoproduced molecules decreased with WRT along the boreal aquatic continuum. Our study highlights the tight link between DOM composition and DOM photodegradation. We suggest that photodegradation is an important driver of DOM composition change in waters with low WRT, where DOM is highly photoreactive.

Molecular insights into the impacts of acid mine drainage on dissolved organic matter dynamics in pit lakes

Pit lakes are artificial hydrological features created by mining operations that typically suffer from acid mine drainage (AMD), which not only endangers water quality but also exacerbates carbon loss. However, the impacts of AMD on the fate and role of dissolved organic matter (DOM) in pit lakes remain unclear. This study employed negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) combined with biogeochemical analysis to examine DOM molecular variations and environmental controls across the AMD-induced acidic and metalliferous gradients in five pit lakes. The results demonstrated distinct DOM pools in pit lakes characterized by the prevalence of smaller aliphatic compounds compared to other waterbodies. AMD-induced geochemical gradients promoted DOM heterogeneity among pit lakes, with acidic pit lakes containing more lipid-like compounds. Acidity and metals enhanced DOM photodegradation, reducing the content, chemo-diversity and aromaticity. Organic sulfur was detected in high abundance, potentially from sulfate photo-esterification and mineral flotation agent. Furthermore, microbial involvements in carbon cycling were revealed by DOM-microbe correlation network, but microbial contributions to the DOM pools decreased under acidic and metal stresses. These findings highlight abnormal carbon dynamics caused by AMD pollution and integrate DOM fate into pit lake biogeochemistry, thereby contributing to management and remediation.

Effects of Added Humic Substances and Nutrients on Photochemical Degradation of Dissolved Organic Matter in a Mesocosm Amendment Experiment in the Gulf of Finland, Baltic Sea.

Humic substances, a component of terrestrial dissolved organic matter (tDOM), contribute to dissolved organic matter (DOM) and chromophoric DOM (CDOM) in coastal waters, and have significant impacts on biogeochemistry. There are concerns in recent years over browning effects in surface waters due to increasing tDOM inputs, and their negative impacts on aquatic ecosystems, but relatively little work has been published on estuaries and coastal waters. Photodegradation could be a significant sink for tDOM in coastal environments, but the rates and efficiencies are poorly constrained. We conducted large-scale DOM photodegradation experiments in mesocosms amended with humic substances and nutrients in the Gulf of Finland to investigate the potential of photochemistry to remove added tDOM and the interactions of DOM photochemistry with eutrophication. The added tDOM was photodegraded rapidly, as CDOM absorption decreased and spectral slopes increased with increasing photons absorbed in laboratory experiments. The insitu DOM optical properties became similar among the control, humic- and humic+nutrients-amended mesocosm samples toward the end of the amendment experiment, indicating degradation of the excess CDOM/DOM through processes including photodegradation. Nutrient additions did not significantly influence the effects of added humic substances on CDOM optical property changes, but induced changes in DOM removal.

Photodegradation of humic acid from the surrounding soils of a sub-alpine Lake Tiancai

溶解性有机质（DOM）是湖泊生态系统的物质循环和能量流动的基础.典型亚高山湖泊DOM的主要来源为流域土壤腐殖酸（外源DOM），其具有较强的光吸收特性，光降解是其重要的转化过程，从而改变碳、氮、磷等物质的存在形式.在气候变暖的背景下，亚高山湖泊中DOM的浓度、pH、溶解氧及太阳辐射都可能发生变化；但这些环境变化对亚高山湖泊DOM光降解过程的影响尚不够清楚.由此本文研究了亚高山湖泊——天才湖（树线下200 m）周边土壤提取的腐殖酸（THA）的光降解过程：首先考察在光源为500 W汞灯、不通气以及pH=8.0条件下THA的降解过程中DOC浓度、有色溶解性有机质（CDOM，以a₂₈₀表征）和荧光溶解性有机质（FDOM）的变化，其后定性比较、定量分析不同光降解条件（pH、载气和光照波段）对THA的光降解过程的影响.结果表明，THA含有4种荧光组分，其中C1~C3均为类腐殖质，占总荧光强度83.64%，C4占比16.36%，为类蛋白质物质.在500 W汞灯的照射下，THA分子量逐渐降低，10 h后DOC浓度降低44.5%，a₂₈₀随光照时间的增加逐渐减少，CDOM含量较初始大幅度下降；而FDOM （以荧光组分强度表示）总体趋势表现为先增加后下降.相对于弱碱性的环境（pH=8.0），弱酸性环境（pH=6.0）更有利于THA的光化学过程.氧气的存在很大程度上促进了DOM光化学转化过程，通入空气的条件下a₂₈₀的降解速率是氮气条件下的3.25倍.在不同光波段的实验下表明，可见光和紫外光对THA降解的贡献差异较大，紫外光波段的表观光子降解率（η）约为可见光的5~11倍.结果表明在气候变化背景下，亚高山湖泊中DOM的光化学降解将可能加快，亚高山湖泊在碳循环中的作用可能增强.;Dissolved organic matter (DOM) in lakes is ubiquitous and plays an important role in the aqueous ecosystem. DOM in sub-alpine lakes is dominated by the terrestrially-derived humic acid in the soil. Photodegradation is an important transformation process of DOM because of its strong light absorption, changing the form of carbon, nitrogen, phosphorus. Under the scenario of climate warming, DOM concentration, pH, dissolved oxygen and solar radiation in subalpine lakes may change. However, the effects of these environmental changes on DOM photodegradation in sub-alpine lakes are still unclear. In this paper, the photodegradation of allochthonous DOM in a sub-alpine lake-Lake Tiancai (located 200 m below the tree line) were studied. Firstly, the humic acid (THA) was extracted from the soil around Lake Tiancai. Then the photodegradation of THA was investigated at different light bands (ultraviolet or visible light) and different pH conditions. The results showed that four components were identified by excitation-emission matrix fluorescence coupled with parallel factor analysis (EEM-PARAFAC), including C1-C3 were humic-like fluorescence, accounting for 83.64% of the total fluorescence intensity, and C4 was protein-like substance, accounting for 16.36%. Under the irradiation of 500 W mercury lamp, the molecular weight of THA decreased gradually and DOC concentration decreased by 44.5% after 10 h. The value of a₂₈₀ (a proxy for colored dissolved organic matter (CDOM)) decreased gradually with the irradiation time; while the fluorescent dissolved organic matter (FDOM) showed an overall trend of increasing at first and then decreasing. Compared with the weakly alkaline media (pH=8.0), the weakly acidic media (pH=6.0) facilitated for the photochemical process of THA and acidification may further promote DOM degradation. Oxygen played a crucial role in DOM photochemical reaction as the pseudo-first-order kinetic constant of a₂₈₀ in the presence of oxygen was 3.25 times that in the absence of oxygen. The ultraviolet light was much more efficient for THA degradation than visible light. The apparent quantum degradation ratios (η) of ultraviolet light were about 5-11 times that of visible light. This result of this study suggested that under the scenario of climate change, the photodegradation of DOM in sub-alpine lakes might accelerate, enhancing the role of sub-alpine lakes in the carbon cycle.

Bio- and photo-lability of dissolved organic matter in the Pearl River (Zhujiang) estuary

We investigated the bio- and photo-lability of dissolved organic matter (DOM) from the head, mixing zone, and mouth of the Pearl River estuary. At all three sites, bio- and photo-refractory dissolved organic carbon (DOC) and biorefractory chromophoric DOM (CDOM) dominated over the corresponding bio- and photo-labile constituents, while photolabile CDOM dominated over photo-refractory CDOM. Relative to the mixing-zone and mouth waters, the headwater was enriched with bio- and photo-labile DOC and photolabile CDOM and depleted with biolabile CDOM. Biolabile DOC was richer than photolabile DOC in the headwater, while photolabile CDOM was richer than biolabile CDOM at all three sites. Pre-biotransformation inhibited, stimulated, or had little impact on DOM photodegradation, depending on site. Ultra-violet absorption coefficients are indicators of bio- and photo-refractory DOC. The relative proportions of transparent and chromophoric DOM control the turnover of biolabile DOC and the effect of pre-biotransformation on DOM photodegradation.

Dissolved organic matter photodegradation in a water supply reservoir on temperate oceanic climate (Cfb): a case study of Passaúna reservoir, Brazil

Abstract Photodegradation is an important process for aquatic metabolism, related to the dissolved organic matter (DOM) transformation in water. For example, in a water supply reservoir, DOM is an important parameter since it can react and form disinfection by-products during water treatment. Thus, the understanding and determination of photodegradation rates are especially relevant for water resources management since they can influence DOM transformation in the water column. However, besides its relevance, few studies were conducted in the southern hemisphere for photodegradation rates determination. Therefore, we carried out field experiments to characterize and evaluate DOM photodegradation rate ‒ at different depths and in two solar irradiation periods ‒ through the application of absorption spectroscopy techniques in the UV-Vis region and fluorescence excitation-emission matrices (EEM) combined with the dissolved organic carbon (DOC) measurement. Low concentrations of natural DOM and nutrients in the aquatic environment were measured during the field monitoring. Our results indicate that photodegradation rates for a temperate oceanic climate reservoir were proportional to the availability of solar radiation, being more representative considering the time scale.

Distribution and Dynamics of Dissolved Organic Matter in the Changjiang Estuary and Adjacent Sea

AbstractLarge, river‐dominated estuaries provide important land‐ocean interfaces for biogenic element fluxes that impact global marine biogeochemistry. However, the reactivity and fate of dissolved organic matter (DOM) in this dynamic region remain not well understood. In this study, three cruises were conducted in February, May, and July 2017 in the Changjiang Estuary and adjacent sea (CEAS). Incubation experiments to determine photochemically and/or biologically mediated degradation of terrigenous and marine DOM were performed. Dissolved organic carbon (DOC), chromophoric DOM (CDOM) absorption, and total dissolved amino acids (TDAAs) were measured. DOM photodegradation and transformation were indicated by the CDOM spectral slope (S275−295) and diagenetic state by DOC‐normalized yields of TDAA [TDAA (%DOC)]. From the river to the open sea, DOC showed non‐conservative mixing and was rapidly removed in low salinity areas. Photodegradation, biodegradation, and photoenhanced biodegradation are important DOC removal mechanisms and exhibit specific responses depending on DOC source and composition. Terrigenous DOM is vulnerable to photodegradation, while marine DOM experiences rapid biodegradation. Contrary to DOC, TDAA (%DOC) increased from the river to the nearshore and then decreased toward the open sea, indicating the contribution of fresh marine plankton production. A box model showed that the annual DOC removal of CEAS was 2.6 ± 1.9 × 106 tons. Seaward increase of ammonium (NH4‐N) was observed in the CEAS; photoammonification, biomineralization of autochthonous and allochthonous DOM are potential NH4‐N regeneration mechanisms. A model was developed to calculate surface DOC concentrations based on CDOM absorption coefficients allowing high‐resolution monitoring of DOC in the CEAS.

Photodegradation of dissolved organic matter of chicken manure: Property changes and effects on Zn2+/Cu2+ binding property

Untreated livestock manure contains high concentrations of dissolved organic matter (DOM), which can enter the environment through leaching and eluviation, showing an important impact on the environment. In this research, fresh chicken manure from a large-scale chicken farm was collected as the source of DOM. The infrared spectrum of the original DOM was characterized. TOC analysis, UV spectrum and 3D fluorescence spectrum were used to measure the properties of DOM before and after photodegradation. Infrared spectroscopy results show that chicken manure DOM may contain aliphatic and aromatic compounds, alcohols, phenols, polysaccharides and some protein substances; In three systems, the order of TOC removal rates of DOM was water + UV system (85%) > > water + simulated sunlight system (7.2%) > ice + simulated sunlight system (4.5%); Changes of UV spectra, fluorescence spectra, molecular weight distribution and pH value show that, in three systems, as the illumination time increased, photodegradation reduced pH value of the systems, aromaticity and humus contents of DOM, while increased the proportion of medium and/or small molecular weight components of DOM. The amounts of all these changes were proportional to DOM photodegradation rates in the system. The binding ability of DOM with Cu2+ and Zn2+ in water solution decreased significantly after the photodegradation.

Contrasting responses of DOM leachates to photodegradation observed in plant species collected along an estuarine salinity gradient

A series of abiotic processes affected by salinity changes involve light-mediated reactions and different degradation pathways of organic compounds, including altered photo-oxidation, photo-degradation, and photolysis of organic matter (OM). Sunlight is known to degrade, oxidize, or mineralize dissolved organic matter (DOM) in waterways, creating large changes in compositional structure of DOM near the water surface and ultimately in the mixed layer. DOM derived from various vegetation types has differing levels of susceptibility to photolytic degradation depending on initial chemical composition and in what matrix degradation takes place (e.g., salinity). The effect of sunlight and salinity on degradation of leached DOM derived from three dominant vegetative species, Avicennia germinans, Juncus romerianus, and Taxodium distichum, along a riverine continuum in northeast Florida was determined. Leachates from these three sources in a deionized or seawater matrix were irradiated in a continuous flow-through photolytic system over the course of 20 h. Avicennia germinans and Juncus roemerinaus DOM readily degraded as indicated by decreases in absorbance across all wavelengths during the irradiation period, while Taxodium distichum DOM was found to increase in absorbance across all wavelengths in the freshwater matrix, but not in seawater. PARAFAC analysis indicated differences in photochemical components and % change of absorbance and fluorescence over time indicate the importance and variability of individual contributions to the DOM pool across an estuarine continuum. This work characterizes the photochemical properties of three individual DOM sources, exhibits the need for further research on this topic, and explores the salinity effect on photo-degradation of DOM from unique plant-derived DOM.

The photochemical release of dissolved organic matter from resuspended sediments: Insights from fluorescence spectroscopy

Sediments exposed to sunlight can serve as an important source of dissolved organic carbon (DOC) and nutrients to overlying waters. However, the photochemical release processes of dissolved organic matter (DOM) from resuspended sediments and the characteristics of photoreleased DOM are not fully understood. In this study, excitation-emission matrix fluorescence combined with parallel factor analysis (EEMs-PARAFAC) was used to study the photochemical release of DOM qualitatively and quantitatively. The EEMs-PARAFAC demonstrated that the photoreleased DOM is dominated by humic-like substances, and the photorelease process could be consist of the photoproduction and photodegradation of DOM. The concurrent photodegradation may result in the underestimation of photoreleased DOM. Moreover, the significant increases in DOC content and fluorescence intensity of humic-like components along with increasing nitrate and ferric ion indicated that nitrate and ferric ion could facilitate the photoproduction of DOM through the photochemical produced hydroxyl radical. However, the decreases in DOC and fluorescence intensity were also observed at high concentration of nitrate and ferric ion, owing to the photodegradation of DOM by redundant hydroxyl radical. All of these results suggest that EEMs-PARAFAC is an effective and sensitive analytical technique for evaluating DOM photoreleased from suspended lake sediments and previous studies may underestimate photochemical release of DOM from sediments due to the overlook of the subsequently photodegradation of these released DOM. Thus, the photochemical release of DOM and its associated pollutants from suspended particles in shallow and eutrophic lakes should be more significant then should be paid more attention.

Impact of UV radiation on DOM transformation on molecular level using FT-ICR-MS and PARAFAC.

Dissolved organic matter (DOM) is an omnipresent constituent of natural water bodies. Reuse and transformation of DOM compounds in the water column is driven by physicochemical and biological processes leading to the production of refractory DOM. Typically, breakdown of DOM chemical compounds into smaller or more condensed fragments is triggered by ultraviolet (UV) radiation. Here, we present a study on the photodegradation of DOM produced during an incubation experiment with a natural microbial community. At the end of the first incubation without UV irradiation, the samples from 3 mesocosms were filtered to remove microbes and particles and continuously exposed to UV radiation (280-365nm). We investigated DOM in depth via monitoring of dissolved organic carbon (DOC) concentrations, DOM molecular characterization by Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) and excitation emission matrix spectroscopy (EEMS). Analysis of variance indicated no significant differences in the DOC concentration between treatments. Main peaks in the fluorescent DOM (FDOM) were photo-bleached by UV radiation, and an increase in the fluorescent intensity of selected peaks was observed on irradiated samples toward the end of the experiment. Parallel factor analysis (PARAFAC) indicated the presence of three main components in all treatments: C1 (Marine humic M), C2 (Bacterial produced humic C), C3 (Tyrosine), and an additional component in the dark incubation of mesocosm 3, C4 (Tryptophan). Despite an intensive filtration protocol through 0.7, 0.2 and 0.1μm filters, low bacterial abundances were determined (<2.5×10-3 cells mL-1). We observed a direct correlation between structural indices and the intensity of PARAFAC components. Average double bond equivalent and aromaticity were strongly positively correlated with PARAFAC components C1 and C2 for one or more mesocosm. Moreover, FT-ICR-MS showed that under the tested conditions, the refractory character of the DOM assessed as the similarity to a deep ocean DOM reference did not increase on molecular level. Thus, mechanisms other than photochemical transformations of relatively recent DOM are likely necessary to facilitate long-term stability of DOM in the oceans.

Photodegradation disproportionately impacts biodegradation of semi‐labile DOM in streams

AbstractExposure of dissolved organic matter (DOM) to sunlight can increase or decrease the fraction that is biodegradable (BDOM), but conceptual models fail to explain this dichotomy. We investigated the effect of sunlight exposure on BDOM, addressing three knowledge gaps: (1) how fractions of DOM overlap in their susceptibility to degradation by sunlight and microbes, (2) how the net effect of sunlight on BDOM changes with photon dose, and (3) how rates of DOM photodegradation and biodegradation compare in a stream. Stream waters were exposed to sunlight, and then fed through bioreactors designed to separate labile and semi‐labile pools within BDOM. The net effects of photodegradation on DOM biodegradability, while generally positive, represented the balance between photochemical production and removal of BDOM that was mediated by photon dose. By using sunlight exposure times representative of sunlight exposures in a headwater stream and bioreactors colonized with natural communities and scaled to whole‐stream dynamics, we were able to relate our laboratory findings to the stream. The impact of sunlight exposure on rates of DOM biodegradation in streams was calculated using rates of light absorption by chromophoric DOM, apparent quantum yields for photomineralization and photochemical alteration of BDOM, and mass transfer coefficients for labile and semi‐labile DOM. Rates of photochemical alteration of labile DOM were an order of magnitude lower than rates of biodegradation of labile DOM, but for semi‐labile DOM, these rates were similar, suggesting that sunlight plays a substantial role in the fate of semi‐labile DOM in streams.

Photo-induced redox coupling of dissolved organic matter and iron in biochars and soil system: Enhanced mobility of arsenic

Dissolved organic matter (DOM) elucidated from biochars enhances the dissolution of iron oxides and reduction of iron. However, given that reduction mechanism of iron (Fe(III)) in the practical biochar applications for soil amendment and environmental remediation have not been fully elucidated, this study laid great emphasis on the photo-induced Fe(II) liberated from DOM-Fe(III) complexes. Thus, pyrolysis of biomass was carried out at 300 °C to maximize DOM release from biochars. Moreover, three different biomass samples (rice straw (R), granular sludge (G) from an anaerobic digester, and spent coffee grounds (C)) were chosen as carbon substrates for biochars preparation. To demonstrate the transformation of Fe(III), 1 and 5 wt% biochar was applied to the clean (S1) and arsenic-contaminated (S2) soil with/without the light. The results indicate that the light condition produces more Fe(II). The amount of Fe(II) accounts for 25.3, 28.6, and 30.7% of total iron under the light with 5 wt% GB, RB, and CB in S1, and 10.6, 13.1, and 13.8% in S2. This study demonstrates that Fe(II) is generated more under ultraviolet irradiation than visible light and dark condition. In addition, a control experiment without biochar showed that DOM plays an important role in the reduction of Fe(III). The mobility of arsenic increased under the light condition since the intermediates of DOM photo-degradation accelerates the dissolution of iron oxides and arsenic competes with DOM for the adsorption. Therefore, there was no significant correlation between the elution of arsenic and the formation of Fe(II) during the reductive dissolution of iron oxide under the light condition.

Assessment of variations in dissolved organic matter in contrasting streams in the Pampas and Patagonian regions (Argentina)

Dissolved organic matter (DOM) is the major source of carbon in aquatic environments and may be derived from allochthonous or autochthonous sources. This study presents the first DOM characterisation in streams from two contrasting Argentinean regions (the Pampas and Patagonia). We found that dissolved organic carbon (DOC) concentrations and absorption coefficients at 254 and 350nm (a254 and a350) were higher in Pampean than in Patagonian streams. DOM from streams in both regions contained compounds with high molecular weight and highly variable a350 per DOC; it has been suggested that this could be an indicator of lignin content. The characterisation of DOM reflects the agricultural signal and the effect of autochthonous sources in Pampean streams, whereas a more pristine condition with high input from terrestrial environments was recorded in Patagonian streams. No sign of DOM photodegradation was observed in either region. Fluorescence-based indices suggest a more recent input of DOM in Pampean streams, whereas a highly humified component was recorded in Patagonian streams. The combined use of absorbance and fluorescence-based techniques was useful for understanding the quantity and quality of DOM in these contrasting regions in Argentina.

Photodegraded dissolved organic matter from peak freshet river discharge as a substrate for bacterial production in a lake-rich great Arctic delta

Lake-rich Arctic river deltas are recharged with terrigenous dissolved organic matter (DOM) during the yearly peak water period corresponding with the solstice (24 h day−1 solar irradiance). Bacteria-free DOM collected during peak Mackenzie River discharge was exposed to sunlight for up to 14 days in June 2010. As solar exposure increased, carbon and lignin concentrations declined (10% and 42%, respectively, after 14 days), as did DOM absorptivity (62% after 14 days), aromaticity, and molecular weight. Photochemical changes were on par with those normally observed in Mackenzie Delta lakes over the entire open-water season. When irradiated freshet DOM was provided as a substrate, no significant differences were observed in community-level metabolism among five bacterial communities from representative delta habitats. However, bacterial abundance was significantly greater when nonirradiated (0 day) rather than irradiated DOM (7 or 14 days) was provided, while cell-specific metabolic measures revealed that per-cell bacterial production and growth efficiency were significantly greater when communities were provided irradiated substrate. This complex response to rapid DOM photodegradation may result from the production of inhibitory reactive oxygen species (ROS), along with shifts in bacterial community composition to species that are better able to tolerate ROS, or metabolize the labile photodegraded DOM.

Effects of precipitation on summer epilimnion thickness in tropical lakes

This study aimed to evaluate the importance of physical, chemical and biological factors (e.g., chlorophyll-a, and carbon) in the regulation of the summer epilimnion thickness (Zmix) of the tropical lakes focusing on trends of the current environmental variability. We sampled a set of tropical lakes from the middle Rio Doce lacustrine system (Minas Gerais, Brazil) for two consecutive summers. Besides Zmix, we analyzed the visible light attenuation (KdPAR), dissolved organic carbon (DOC), chlorophyll-a (Chl-a), total suspended solids (TSS), and fetch. We also analyze the quality of the dissolved organic matter (DOM) through colored dissolved organic matter (CDOM), spectral slope (S), DOC-specific absorbance (SUVA) and the ratio between the absorbance at 250 nm and 365 nm (E250:365). Our results showed quite different results during the two years studied. In the summer of 2012, when there was higher rainfall, variations in Zmix were correlated to the optical factors associated with DOM quality, while in the drier summer of 2013 Chl-a, TSS and fetch were the variables that more explained Zmix. It suggests that DOM regulates the Zmix in the rainy periods in the studied tropical lakes and that control is determined by the balance between the DOM input (runoff) and output (such as photodegradation) of the aquatic systems. In reduced rain conditions (summer 2013), the factors responsible by chemistry photodegradation of DOM were predominant, and the organic matter was no longer controlling Zmix, which was driven by internal mixing, productivity, and the wind (Chl-a, TSS and fetch). In this study we showed how changes in precipitation might cause shifts in the factors that regulate the epilimnion thickness in tropical lakes.

Photocatalytic degradation of DOM in urban stormwater runoff with TiO2 nanoparticles under UV light irradiation: EEM-PARAFAC analysis and influence of co-existing inorganic ions

In situ photocatalytic degradation of dissolved organic matter (DOM) of stormwater runoff can efficiently improve the aquatic environment quality and relieve the wastewater treatment pressure. In this work, photocatalytic degradation of DOM in TiO2 (AEROXIDE® P-25) photocatalyst under illumination of ultraviolet (UV) light was carried out, considering the influence of various factors like TiO2 dosage, solution pH along with the existence of co-existing ions (Cu2+ and H2PO4−). Generally, the variations of dissolved organic carbon (DOC), UV-based parameters and peak intensities of fluorescent constituents with UV exposure time fitted perfectly with the pseudo-first-order kinetics model. The total DOM removal efficiency was affected by diversiform factors like adsorption capacity of TiO2, UV light utilization efficiency, reactive free radicals produced and the influence of co-existing ions. The results of fluorescence excitation-emission matrix (EEM) coupled with parallel factor analysis (PARAFAC) modeling demonstrated that all the photodegradation rates for three identified fluorescent constituents (protein-like constituent 1 and 3, humic-like constituent 2) were faster than UV-absorbing chromophores, suggesting the DOM molecules in urban stormwater runoff contained much more π*-π transition structures. In addition, H2PO4− ions affected the photodegradation of DOM by capturing positive holes (h+) and hydroxyl radical (·OH), whereas Cu2+ ions were inclined to generate Cu-protein complexes that were more difficult to degrade than the other Cu-DOM complexes. This study supplied novel insights into the photocatalytic degradation mechanism of individual organic constituent in urban stormwater runoff and explored the influences of co-existing contaminants on their adsorption-photocatalysis processes.

Interactions between sunlight and microorganisms influence dissolved organic matter degradation along the aquatic continuum

AbstractCO2 emissions from inland surface waters to the atmosphere are almost as large as the net carbon transfer from the atmosphere to Earth's land surface. This large flux is supported by dissolved organic matter (DOM) from land and its complete oxidation to CO2 in freshwaters. A critical nexus in the global carbon cycle is the fate of DOM, either complete or partial oxidation. Interactions between sunlight and microbes control DOM degradation, but the relative importance of photodegradation vs. degradation by microbes is poorly known. The knowledge gaps required to advance understanding of key interactions between photochemistry and biology influencing DOM degradation include: (1) the efficiencies and products of DOM photodegradation, (2) how do photo‐products control microbial metabolism of photo‐altered DOM and on what time scales, and (3) how do water and DOM residence times and light exposure interact to determine the fate of DOM moving across the landscape to oceans?

Effect of silver sulfide nanoparticles on photochemical degradation of dissolved organic matter in surface water

Silver sulfide nanoparticles (Ag2SNPs) have shown photocatalytic activity, yet little is known about the effect of Ag2SNPs on the photochemical degradation of dissolved organic matter (DOM) in surface water, which seriously impairs understanding of Ag2SNPs' environmental risks. Herein, this study on the basis of electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) showed for the first time that photodegradation of natural organic matter (NOM, 2R101 N) could be accelerated by both bared and polyvinylpyrrolidone (PVP)-coated Ag2SNPs; the NOM with Ag2SNPs (e.g., 500 μg/L) exposed to light irradiation for 96 h showed molecular formulas with lower O/C ratios as compared to the NOM alone. Also, added number of points (ranging from 1 to 2 carboxyl groups) having the same Kendrick mass defect (KMD) (COO) values and higher intensity in smaller Kendrick mass (KM) (COO) values were observed in NOM with Ag2SNPs compared to NOM alone. However, negligible effects of Ag2SNPs on photodegradation of humic acid (HA, 2S101H) were observed, even when the concentration of Ag2SNPs was as high as 5 mg/L. Besides molecular characteristics, a great reduction in organic carbon content of NOM within 96 h was only observed in the presence of Ag2SNPs under light condition. More importantly, the enhanced photodegradation of DOM by Ag2SNPs even at a concentration of 100 μg/L was also validated in surface water. These findings suggest that Ag2SNPs have the potential to accelerate the photochemical degradation of DOM in surface water.