Modelling ROS formation in boreal lakes from interactions between dissolved organic matter and absorbed solar photon flux

Insight into the characterization of dissolved organic matter in shallow lakes with different trophic states and their net photo-generation capacity of reactive oxygen species

Reactive oxygen species (ROS) are ubiquitous in the aquatic environment, and they are closely related to several biogeochemical processes. Dissolved organic matter (DOM) is one of the main photosensitizers involved in the formation of ROS and it also serves as a sink for ROS by involving in scavenging, quenching, and antioxidant reactions. The net effect of these processes depends on the concentration, source, and composition of the DOM. Current studies have mainly focused on the steady-state concentration of reactive oxygen species ([ROS]ss) produced by the total DOM in lakes with different trophic states and ignored the net photo-generation capacity of ROS ([ROS]DOM, the net steady concentration of ROS generated per unit mass of DOM), leading to a vague understanding of the photochemical properties of DOM in aquatic systems, especially in shallow lakes with different trophic states. In this study, the optical composition of DOM was determined with optical characterization, such as specific UV-Vis and excitation-emission matrices with fluorescence regional integration (FRI-EEMs), and its molecular characteristics were analyzed by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results revealed that DOM in lakes with different trophic states had mixed endogenous and exogenous characteristics, accompanied by an increasing trend in endogenous characteristics with the increasing trophic state of lakes. Spectroscopic probes were used to detect the steady-state concentration of ROS and further calculate the [ROS]DOM, such as [3DOM*]DOM, [•OH]DOM, [1O2]DOM and [O2.-]DOM. The results indicated that the [ROS]DOM in lakes with light-eutrophic states was significantly higher than that in lakes with moderate-eutrophic and hyper-eutrophic states, which indicated that the DOM in lower trophic state lakes has a higher net photo-generation capacity of ROS. Pearson analysis results showed that [3DOM*]DOM, [•OH]DOM, [1O2]DOM and [O2.-]DOM had a significant positive correlation with lignin/CRAMs-like, aromatic, and tannin compounds, as well as the fluorescence components, fulvic- and humic-like substances and the UV-Vis indicator: SUVA254 revealed that DOM with higher humification and aromaticity had a higher net photo-generation capacity of ROS in different trophic state lakes. In addition, the molecular uniqueness of the DOM was dominated by lignin/CRAMs-like and aromatic compounds, which were positively correlated with [ROS]DOM, in the following order: [3DOM*]DOM > [•OH]DOM > [1O2]DOM > [O2.-]DOM. This study emphasizes the importance of focusing on the source, composition, and net photo-generation capacity of ROS by DOM, which would help evaluate the photochemical potential and other behaviors of DOM in lakes with different trophic states and provide guidance for the risk assessment of DOM input from different sources.

Phototransformation behavior of octachlorodibenzofuran (OCDF) on the surface of river suspended particulate matter: Kinetics and products formation mechanism

Octachlorodibenzofuran (OCDF) readily adheres to particulate matters in the aqueous environment owing to its strong hydrophobicity. Phototransformation is a key abiotic transformation process for organic pollutants. Here, we systematically investigated the phototransformation behavior of OCDF on the surface of suspended particulate matter (SPM) from the tributary of the Yangtze River. It was found that acidic conditions and the presence of NO3-, Ca2+, and Mg2+ had inhibitory effect on OCDF photodegradation. Through quenching experiments, we discovered that hydroxyl radical (HO•), superoxide anion (O2•-) and singlet oxygen (1O2) all contributed to the removal of OCDF, with HO• and O2•- being the main reactive oxygen species (ROS). A total of ten intermediates were identified using mass spectrometry, and three possible pathways were proposed. Theoretical calculations revealed that 1O2 dominated the dechlorination reaction, HO• was responsible for the generation of ring cleavage products, and both HO• and O2•- participated in the formation of hydroxylated products. Toxicity predictions by the T.E.S.T software showed that the toxicity and bioconcentration factors of intermediates in the photochemical process were reduced, thus decreasing the environmental risk. This research deepened the understanding of the phototransformation behavior of OCDF on the SPM surface, providing a scientific basis for assessing its environmental fate.

Monte Carlo Guidance for Better Imaging of Boreal Lakes in the Wavelength Region of 400-800 nm

Boreal lake depth, one of the most important parameters in numerical weather prediction and climate models through parametrization, helps in identifying notable environmental changes across the globe and in estimating its effect on the ecosystem in remote regions. However, there is no quantitative tool to effectively estimate lake depth from satellite images, leaving scientists to infer lake depth from extrapolation of statistics by relying on certain geological knowledge (such as those used in the Global Lake Database). The bottoms of boreal forest lakes are mainly composed of woody debris, and thus spectral imaging revealing contrast of woody debris can be used to estimate lake depth. Here, we use well-established Monte Carlo software to construct spectral images of boreal lakes that house woody debris, phytoplankton, and chlorophyll. This is accomplished by modeling the dynamic optical properties of selected boreal lakes and simulating the propagation of photons in the wavelength region of 400-800 nm. The results show that the spectral image contrast of boreal lakes is not only determined by the depth level and concentration level of phytoplankton and chlorophyll in water but is also affected by the spectral shape of background absorption, especially the contribution of pure water absorption in the total absorption of lake water.

The generation and transformation mechanisms of reactive oxygen species in the environment and their implications for pollution control processes: A review

Reactive oxygen species (ROS), substances with strong activity generated by oxygen during electron transfer, play a significant role in the decomposition of organic matter in various environmental settings, including soil, water and atmosphere. Although ROS has a short lifespan (ranging from a few nanoseconds to a few days), it continuously generated during the interaction between microorganisms and their environment, especially in environments characterized by strong ultraviolet radiation, fluctuating oxygen concentration or redox conditions, and the abundance of metal minerals. A comprehensive understanding of the fate of ROS in nature can provide new ideas for pollutant degradation and is of great significance for the development of green degradation technologies for organic pollutants. At present, the review of ROS generally revolves around various advanced oxidation processes, but lacks a description and summary of the fate of ROS in nature, this article starts with the definition of reactive oxidants species and reviews the production, migration, and transformation mechanisms of ROS in soil, water and atmospheric environments, focusing on recent developments. In addition, the stimulating effects of ROS on organisms were reviewed. Conclusively, the article summarizes the classic processes, possible improvements, and future directions for ROS-mediated degradation of pollutants. This review offers suggestions for future research directions in this field and provides the possible ROS technology application in pollutants treatment.

Effects of dissolved organic matter on the environmental behavior and toxicity of metal nanomaterials: A review

Metal nanomaterials (MNMs) have been released into the environment during their usage in various products, and their environmental behaviors directly impact their toxicity. Numerous environmental factors potentially affect the behaviors and toxicity of MNMs with dissolved organic matter (DOM) playing the most essential role. Abundant facts showing contradictory results about the effects of DOM on MNMs, herein the occurrence of DOM on the environmental process change of MNMs such as dissolution, dispersion, aggregation, and surface transformation were summarized. We also reviewed the effects of MNMs on organisms and their mechanisms in the environment such as acute toxicity, oxidative stress, oxidative damage, growth inhibition, photosynthesis, reproductive toxicity, and malformation. The presence of DOM had the potential to reduce or enhance the toxicity of MNMs by altering the reactive oxygen species (ROS) generation, dissolution, stability, and electrostatic repulsion of MNMs. Furthermore, we summarized the factors that affected different toxicity including specific organisms, DOM concentration, DOM types, light conditions, detection time, and production methods of MNMs. However, the more detailed mechanism of interaction between DOM and MNMs needs further investigation.

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

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

Acetylacetone effectively controlled the secondary metabolites of Microcystis aeruginosa under simulated sunlight irradiation

Inactivation of cyanobacterial cells and simultaneous control of secondary metabolites is of significant necessity for the treatment of cyanobacteria-laden water. Acetylacetone (AcAc) has been reported a specific algicide to inactivate Microcystis aeruginosa (M. aeruginosa) and an effective light activator to degrade pollutants. This study systematically investigated the photodegradation ability of AcAc under xenon (Xe) irradiation on the secondary metabolites of M. aeruginosa, mainly algal organic matter (AOM), especially toxic microcystin-LR (MC-LR). Results showed that AcAc outperformed H2O2 in destructing the protein-like substances, humic acid-like matters, aromatic proteins and fulvic-like substances of AOM. For MC-LR (250 µg/L), 0.05 mmol/L AcAc attained the same degradation efficiency (87.0%) as 0.1 mmol/L H2O2. The degradation mechanism of Xe/AcAc might involve photo-induced energy/electron transfer and formation of carbon center radicals. Alkaline conditions (pH > 9.0) were detrimental to the photoactivity of AcAc, corresponding to the observed degradation rate constant (k1 value) of MC-LR drastically decreasing to 0.0013 min-1 as solution pH exceeded 9.0. The PO43- and HCO3- ions had obvious inhibition effects, whereas NO3- slightly improved k1 value from 0.0277 min-1 to 0.0321 min-1. The presence of AOM did not significantly inhibit MC-LR degradation in Xe/AcAc system. In addition, the biological toxicity of MC-LR was greatly reduced after photoreaction. These results demonstrated that AcAc was an alternative algicidal agent to effectively inactivate algal cells and simultaneously control the secondary metabolites after cell lysis. Nevertheless, the concentration and irradiation conditions should be further optimized in practical application.

Evaluation of the fate of wastewater effluent organic matter in receiving water: Effect of sequential photochemical and biological processes

Effluent organic matter (EfOM) discharged from wastewater treatment plants (WWTPs) carry substantial risks to river ecosystems. The fate and role of EfOM in the receiving water is affected by its exposure to sunlight and microbial processes, but the extent of these processes remains unclear. In this study, three-phase sequence of irradiation and microbial incubation with EfOM were conducted to compare the behavior of EfOM with that of natural organic matter in receiving rivers (RNOM). The dissolved organic carbon (DOC) in EfOM was degraded by 23% after three sequential phases, while that in RNOM was degraded by 19%. In the first phase, the irradiation of EfOM stimulated microbial respiration and growth by producing easily metabolizable less aromatic lignin-type molecules, leading to a 21% increase in biodegradation. Conversely, the irradiation of RNOM removed biodegradable lignin-type molecules, causing a 50% decreased in biodegradation. The second and third irradiation phases of EfOM and RNOM produced biodegradable lignin-type molecules, making their molecular compositions increasingly similar. The acute toxicity of EfOM decreased by 55%, and differences in microbial species composition between EfOM and RNOM waters decreased by 82% after the three-phase sequence. These findings can improve understanding of the fate of EfOM discharged into receiving rivers.

Thermally enhanced biodegradation of benzo[a]pyrene and benzene co-contaminated soil: Bioavailability and generation of ROS

In this study, a set of comprehensive experiments were conducted to explore the effects of temperature on the biodegradation, bioavailability, and generation of reactive oxygen species (ROS) by thermally enhanced biodegradation (TEB) under benzene and BaP co-contaminated conditions. The biodegradation rates of benzene increased from 57.4% to 88.7% and 84.9%, and the biodegradation efficiency of BaP was enhanced from 15.8% to 34.6% and 28.6%, when the temperature was raised from the ambient temperature of 15 °C to 45 °C and 30 °C, respectively. In addition, the bioavailability analysis results demonstrated that the water- and butanol-extractable BaP increased with elevated temperatures. High enzymatic activities and PAH-RHD_α gene in gram-positive bacteria favored the long-term elevated temperatures (30 and 45 °C) compared to gram-negative bacteria. Moreover, ROS species (O₂^•- and •OH) generation was detected which were scavenged by the increased superoxide dismutase and catalase activities at elevated temperatures. Soil properties (pH, TOC, moisture, total iron, Fe³⁺, and Fe²⁺) were affected by the temperature treatments, revealing that metal-organic-associated reactions occurred during the TEB of benzene-BaP co-contamination. The results concluded that biodegradation of benzene-BaP co-contamination was greatly improved at 45 °C and that microbial activities enhanced the biodegradation under TEB via the increased bioavailability and generation and degradation of ROS.

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

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

Vertical characterisation of phylogenetic divergence of microbial community structures, interaction, and sustainability in estuary and marine ecosystems

The changes in the aquatic environmental conditions often influence the microbial community assemblages and genome repertoire. Studies investigating the aquatic diversity and ecosystem services were primarily conducted in horizontal environments while neglecting the microbial phylogenetic divergences, biotrophic interactions, and eco-sustainability at water vertical layers. We investigated the mechanisms of microbial transitions, and the ecological significance of water depth layers in the estuary and marine ecosystems. The results demonstrated that the salinity and turbidity increased with increasing water depth (0-50 m), while temperature and pH decreased significantly. The bacterial and eukaryotic diversity and composition significantly increased with an elevating water depth. Bacterial phyla such as Desulfobacterota, Acidobacteriota, Myxococcota, Gemmatimonadota, Campilobacterota, and Latescibacterota were increased significantly. However, niche preference occurred, and some microbes showed differential nestedness at water vertical layers. In the eukaryotic community, Eustigmatales group were the only clades predominantly phylogenetically nested at the surface water depth. c_Conoidasida, o_Gregarinasina, f_Eugregarinorida, and g_Lankesteria were the most predominant at the middle depth. While Mediophyceae clades, p_SAR, and the Animalia clades were the most predominant groups nested at the bottom depths. The microbial interaction, structure, and stability were increased with increasing depth. The vertical phylogenetic turnover of the microbial community was related to the feeding mechanisms. Phototrophic organisms were particularly adapted at the surface, and middle depth by parasitic and pathogenic organisms, while the bottom was inhabited by diatoms, decomposers, and detritus protists. This study demonstrated that the bottom depth was the most ecologically stable area with more profound ecosystem services.

Controls on the photochemical production of hydrogen peroxide in Lake Erie

In Lake Erie, toxin-forming harmful algal blooms (HABs) occur following high concentrations of hydrogen peroxide (H₂O₂). Correlation between H₂O₂ concentrations and HABs revealed knowledge gaps on the controls of H₂O₂ production in Lake Erie. One way H₂O₂ is produced is upon absorption of sunlight by the chromophoric fraction of dissolved organic matter (CDOM). Rates of this photochemical production of H₂O₂ may increase in proportion to the apparent quantum yield of H₂O₂ (Φ_H2O2,λ) from CDOM. However, the Φ_H2O2,λ for H₂O₂ production from CDOM remains too poorly constrained to predict the magnitude and range of photochemically produced H₂O₂, particularly in freshwaters like Lake Erie. To address this knowledge gap, the Φ_H2O2,λ was measured approximately biweekly from June-September 2019 in the western basin of Lake Erie along with supporting analyses (e.g., CDOM concentration and composition). The average Φ_H2O2,λ in Lake Erie was within previously reported ranges. However, the Φ_H2O2,λ varied 5-fold in space and time. The highest Φ_H2O2,λ was observed in the Maumee River, a tributary of Lake Erie. In nearshore waters of Lake Erie, the Φ_H2O2,λ decreased about five-fold from June through September. Integration of the controls of photochemical production of H₂O₂ in Lake Erie show that the variability in rates of photochemical H₂O₂ production was predominantly due to the Φ_H2O2,λ. In offshore waters, CDOM concentration also strongly influenced photochemical H₂O₂ production. Together, the results confirm prior work suggesting that photochemical production of H₂O₂ contributes but likely cannot account for all the H₂O₂ associated with HABs in Lake Erie.

Natural photoredox catalysts promote light-driven lytic polysaccharide monooxygenase reactions and enzymatic turnover of biomass

Lytic polysaccharide monooxygenases (LPMOs) catalyze oxidative cleavage of crystalline polysaccharides such as cellulose and chitin and are important for biomass conversion in the biosphere as well as in biorefineries. The target polysaccharides of LPMOs naturally occur in copolymeric structures such as plant cell walls and insect cuticles that are rich in phenolic compounds, which contribute rigidity and stiffness to these materials. Since these phenolics may be photoactive and since LPMO action depends on reducing equivalents, we hypothesized that LPMOs may enable light-driven biomass conversion. Here, we show that redox compounds naturally present in shed insect exoskeletons enable harvesting of light energy to drive LPMO reactions and thus biomass conversion. The primary underlying mechanism is that irradiation of exoskeletons with visible light leads to the generation of H₂O₂, which fuels LPMO peroxygenase reactions. Experiments with a cellulose model substrate show that the impact of light depends on both light and exoskeleton dosage and that light-driven LPMO activity is inhibited by a competing H₂O₂-consuming enzyme. Degradation experiments with the chitin-rich exoskeletons themselves show that solubilization of chitin by a chitin-active LPMO is promoted by light. The fact that LPMO reactions, and likely reactions catalyzed by other biomass-converting redox enzymes, are fueled by light-driven abiotic reactions in nature provides an enzyme-based explanation for the known impact of visible light on biomass conversion.

Characterization of dissolved organic matter processing between surface sediment porewater and overlying bottom water in the Yangtze River Estuary

Dissolved organic matter (DOM) exchange in the sediment-water interface of estuaries is essential for the global elemental cycle. To clarify the interface DOM processing, this study applies optical techniques and ultrahigh-resolution mass spectrometry to assess DOM composition of surface sediment porewater and bottom (overlying) water across the Yangtze River Estuary (YRE). Results suggested that DOM exchange in the sediment-water interface mainly followed from sediment porewater to bottom water driven by a significant dissolved organic carbon concentration gradient and hydrodynamic force. We also characterized two porewater DOM sources, including microbial production and byproducts of processed sediments. High microbial activities resulted in the enrichment of protein-like fluorescent components and N-bearing compounds in porewater, potentially decreasing the oxygen concentration of bottom water due to the high lability. And the deamination of N-bearing compounds in the sediment-water interface could likely serve as a N-bearing nutrient source to bottom water. Moreover, due to sediment-specific features in different areas driven by hydrologic sorting and local phytoplankton supply, porewater DOM of muddy areas accumulated more aromatic substances from the degradation of terrestrial organic matter. The release and oxic transformation of oxygen-deficient aromatic compounds could contribute to the refractory carbon pool of estuarine water (carboxyl-rich alicyclic molecules, CRAM), modulating the quality of organic carbon mobilized from the land to the coastal ocean. Considering strong hydrodynamic force in numerous estuaries worldwide, DOM exchange and processing at the sediment-water interface has a meaningful influence on the biogeochemistry of estuarine water columns, which warrants further studies.

The transformation of Benzophenone-3 in natural waters and AOPs: The roles of reactive oxygen species and potential environmental risks of products

Benzophenone-3 (BP-3) is a widespread emerging organic pollutant. However, little is known about the synergistic effect of various reactive oxygen species (ROS) in natural waters and wastewater treatment plants on its transformation. In this study, the indirect photochemical behavior of BP-3 in the natural aquatic environments and the degradation process in the AOPs system were investigated by theoretical chemistry calculations. Besides the potential eco-toxicity effects, health effects, and bioaccumulation of the transformation products were assessed by computational toxicology. Results of transformation mechanism and kinetics showed that OH· and ¹O₂ are the keys to the transformation of BP-3, whereas the role of HO₂· and O₃ can be ignored. AOPs based on OH· and ¹O₂ could lead to the rapid transformation of BP-3, while the transformation of BP-3 in natural waters is slow, and even environmental persistence can be observed. However, dissolved organic matter (DOM) promotes the indirect phototransformation of BP-3 in natural waters. A variety of transformation products are generated under the synergistic effects of ROS, H₂O, and ³O₂. Assessments of environmental risks indicated that the potential eco-toxicity and health effects of the main products are significantly lower than that of the parent BP-3. More importantly, low bioaccumulation of transformation products would not enlarge their eco-toxicity and health effects. This study not only gives valuable insights into the indirect phototransformation of BP-3 in natural waters but also provides theoretical support for the feasibility of BP-3 degradation in industrial wastewater by AOPs based on OH· and ¹O₂.

Nanocosm: a well plate photobioreactor for environmental and biotechnological studies

Phytoplankton are key primary producers at the bottom of the aquatic food chain. They are a highly diverse group of organisms essential for the functioning of our ecosystems and because of their characteristics, their biomass is considered for various commercial applications. A full appreciation of their abundance, diversity and potential is only feasible by using systems that enable simultaneous testing of strains and/or variables in a fast and easy way. A major bottleneck is the lack of a cost-effective method with the capacity for complex experimental set-ups that enable fast and reproducible screening and analysis. In this study, we present nanocosm, a versatile LED-based micro-scale photobioreactor (PBR) that allows simultaneous testing of multiple variables such as temperature and light within the same plate. Every well can be independently controlled for intensity, temporal variation and light type (RGB, white, UV). We show that our systems guarantee homogeneous conditions because of controlled temperature and evaporation and adjustments for light crosstalk. By ensuring controlled environmental conditions the nanocosm is suitable for running factorial experimental designs where each well can be used as an independent micro-PBR. To validate culture performances, we assess well-to-well reproducibility and our results show minimal well-to-well variability for all the conditions tested. Possible modes of operation and application are discussed together with future development of the system.

Biogenic Fenton process - A possible mechanism for the mineralization of organic carbon in fresh waters

To explore the mechanisms that mineralize poorly bioavailable natural organic carbon (OC), we measured the mineralization of OC in two lake waters over long-term experiments (up to 623 days) at different pH and iron (Fe) levels. Both the microbial and photochemical mineralization of OC was higher at pH acidified to 4 than at the ambient pH 5 or an elevated pH 6. During 244 days, microbes mineralized up to 60% of OC in the 10-µm filtrates of lake water and more than 27% in the 1-µm filtrates indicating that large-sized microbes/grazers enhance the mineralization of OC. A reactivity continuum model indicated that the acidification stimulated the microbial mineralization of OC especially in the later (>weeks) phases of experiment when the bioavailability of OC was poor. The reactive oxygen species produced by light or microbial metabolism could have contributed to the mineralization of poorly bioavailable OC through photochemical and biogenic Fenton processes catalyzed by the indigenous Fe in lake water. When Fe was introduced to artificial lake water to the concentration found in the study lakes, it increased the densities of bacteria growing on solid phase extracted dissolved organic matter and in a larger extent at low pH 4 than at pH 5. Our results suggest that in addition to the photochemical Fenton process (photo-Fenton), microbes can transfer poorly bioavailable OC into labile forms and CO₂ through extracellular Fe-catalyzed reactions (i.e., biogenic Fenton process).

Influence of hydro-morphologic variables of forested catchments on the increase in DOC concentration in 36 temperate lakes of eastern Canada

In the last decades, a worldwide increase in dissolved organic carbon (DOC) concentrations has been observed in temperate and boreal lakes. This phenomenon has several detrimental effects on the aquatic life and affect local C geochemical cycles. In this study, we measured DOC concentration in the water column of 36 lakes located in eastern Canada over a period of 35 years (1983-2017) and assessed the influence of climatic, hydrologic and morphometric variables on both DOC concentrations and on the rate of DOC changes (∆DOC). Our data show that morphometric and hydrologic variables have a stronger direct influence on lake water DOC concentrations than vegetation and climatic variables. DOC concentration strongly increased with the drainage ratio and the surface covered by organic deposits, which together explained 59% of the variance. As expected, we observed a significant increase in lake water DOC concentration in 72% of the surveyed lakes, which averaged 20% over the study period. Meanwhile, lake water SO₄^2- concentration decreased by 60%. ∆DOC was poorly influenced by the rate of changes in lake water SO₄^2- as well as by the rate of changes in mean annual air temperature and precipitation. ∆DOC was more related to the vegetation type and the morphometry of the catchment: a model including the percentage of conifers, terrestrial catchment area and ∆Cl yielded a variance explanation of 39%. This shows that the rate of increase was primarily driven by morphometric variables which did not change over the study period.

Photochemical Characterization of Surface Waters from Lakes in the Adirondack Region of New York

The Adirondack Mountain region of New York, a historical hotspot for atmospheric sulfur and nitrogen deposition, features abundant lakes that are experiencing browning associated with recovery from acidification. Yet, much remains unknown about the photoreactivity of Adirondack lake waters. We quantified the apparent quantum yields (Φ_app,RI) of photochemically produced reactive intermediates (RIs), such as excited triplet states of dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH), for surface waters collected from 16 representative Adirondack lakes. Φ_app,³DOM* and Φ_app,¹O2 for native Adirondack lake waters fell within ranges reported for whole waters and DOM isolates from various sources, while Φ_app,^•OH were substantially lower than those measured for other aquatic samples. Orthogonal partial least squares and multiple linear regression analyses identified the spectral slope coefficient from 290 to 400 nm (S_290-400) as the most effective predictor of Φ_app,RI among measured water chemistry parameters and bulk DOM properties. Φ_app,RI also exhibited divergent responses to controlled pH adjustment and aluminum or iron addition simulating hypothetical scenarios relevant to past and future water chemistry conditions of Adirondack lakes. This study highlights the need for continued research on changes in photoreactivity of acid-impacted aquatic ecosystems in response to browning and subsequent impacts on photochemical processes.

Dissolved organic carbon in eastern Canadian lakes: Novel patterns and relationships with regional and global factors

Long-term patterns in dissolved organic carbon (DOC) concentrations in 49 eastern Canadian lakes from four sites were re-examined with a ~ 35-year (~1980-2015) dataset. The study sites were Dorset (number of lakes, n = 8), Experimental Lakes Area (ELA, n = 4), Kejimkujik (n = 26) and Yarmouth (n = 11). Lake DOC patterns were synchronous within each site. However, comparisons of DOC patterns across sites showed that they were synchronous only between the Kejimkujik and Yarmouth locations. Hence, these two sites were pooled into a single Nova Scotia site (NS). Increases in DOC concentration were evident in Dorset, Ontario from 1988 (r² = 0.78, p < 0.001) and NS from 2000 (r² = 0.43, p = 0.006). DOC at the ELA in northwestern Ontario had a different pattern compared to the other sites, i.e., DOC had increased earlier (1983-2000), and then, unlike Dorset and NS, neither an increase nor decrease was detected between 2001 and 2015 (p = 0.78). Precipitation and sulfur deposition explained the greatest variance in DOC patterns at the Dorset and NS sites (i.e., precipitation: 21-49% and sulfur deposition: 24-54%). Precipitation was the most important driver of DOC at the ELA. Our results indicate that all the sites have gone through a process of increasing DOC, but at different times. The stabilizing pattern at the ELA since 2001 may suggest that DOC concentrations in ELA lakes have reached, or are approaching a new equilibrium, a phenomenon that was not observed at the other sites. Also, the increase in DOC was not always associated with declining sulfur deposition (e.g., ELA). Therefore, we conclude that there was considerable variation in DOC patterns across this large geographic region of Canada and potential drivers of these patterns were not consistent across these diverse sites.

Molecular differences between water column and sediment pore water SPE-DOM in ten Swedish boreal lakes

Boreal lakes are considered hot spots of dissolved organic matter (DOM) processing within the global carbon cycle. This study has used FT-ICR mass spectrometry and comprehensive data evaluation to assess the molecular differences of SPE-DOM between lake column water SPE-DOM and sedimentary pore water SPE-DOM in 10 Swedish boreal lakes of the Malingsbo area, which were selected for their large diversity of physicochemical and morphological characteristics. While lake column water is well mixed and fairly oxygenated, sedimentary pore water is subject to depletion of oxygen and to confinement of molecules. Robust trends were deduced from molecular compositions present in all compartments and in all 10 lakes ("common compositions") with recognition of relative abundance. Sedimentary pore water SPE-DOM featured higher proportions of heteroatoms N and S, higher average H/C ratios in presence of higher DBE/C ratios, and higher average oxygenation than lake column water SPE-DOM. These trends were observed in all lakes except Ljustjärn, which is a ground water fed kettle lake with an unique lake biogeochemistry. Analogous trends were also observed in case of single or a few lakes and operated also for compounds present solely in either lake column water or sedimentary pore water. Unique compounds detected in either compartments and/or in a few lakes showed higher molecular diversity than the "common compositions". Processing of DOM molecules in sediments included selective preservation for polyphenolic compounds and microbial resynthesis of selected molecules of considerable diversity.

Characterization of aquatic organic matter: Assessment, perspectives and research priorities

Organic matter (OM) refers to the largest reactive reservoir of carbon-based compounds on Earth. Aside of its role as a source of carbon, OM is also actively involved in a wide range of ecological functions. It also plays an important role in the solubility, toxicity, bioavailability, mobility and distribution of pollutants. Therefore, OM is a key component in the local and global carbon cycle. About 12,000 articles containing organic matter in the title were published during the past decade, with a continuous increasing number each year (ISI Web of Science). Although this topic was widely explored and its interest has significantly increased, some limitations remain. These limitations can be technical (e.g., pre-treatment processes, low-resolution instrument, data handling) and can be related to the current approach. In this review, we first present the current strategies and tools to characterize the organic matter in the aquatic environment, then we tackle several aspects of current characterization limitations. Finally, we suggest new perspectives and priorities of research to improve the current limitations. From our point of view, simultaneous studies of particulate and dissolved OM fractions should be prioritized and multi-disciplinary approach, creation of databases, controlled experiments and collaborative works should be the next targets for future OM research priorities.

Solar UV radiation in a changing world: roles of cryosphere-land-water-atmosphere interfaces in global biogeochemical cycles

Global change influences biogeochemical cycles within and between environmental compartments (i.e., the cryosphere, terrestrial and aquatic ecosystems, and the atmosphere). A major effect of global change on carbon cycling is altered exposure of natural organic matter (NOM) to solar radiation, particularly solar UV radiation. In terrestrial and aquatic ecosystems, NOM is degraded by UV and visible radiation, resulting in the emission of carbon dioxide (CO2) and carbon monoxide, as well as a range of products that can be more easily degraded by microbes (photofacilitation). On land, droughts and land-use change can reduce plant cover causing an increase in exposure of plant litter to solar radiation. The altered transport of soil organic matter from terrestrial to aquatic ecosystems also can enhance exposure of NOM to solar radiation. An increase in emission of CO2 from terrestrial and aquatic ecosystems due to the effects of global warming, such as droughts and thawing of permafrost soils, fuels a positive feedback on global warming. This is also the case for greenhouse gases other than CO2, including methane and nitrous oxide, that are emitted from terrestrial and aquatic ecosystems. These trace gases also have indirect or direct impacts on stratospheric ozone concentrations. The interactive effects of UV radiation and climate change greatly alter the fate of synthetic and biological contaminants. Contaminants are degraded or inactivated by direct and indirect photochemical reactions. The balance between direct and indirect photodegradation or photoinactivation of contaminants is likely to change with future changes in stratospheric ozone, and with changes in runoff of coloured dissolved organic matter due to climate and land-use changes.

The interactive effects of stratospheric ozone depletion, UV radiation, and climate change on aquatic ecosystems

Summary of current knowledge about effects of UV radiation in inland and oceanic waters related to stratospheric ozone depletion and climate change.

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.