Dissolved Organic Carbon Degradation Research Articles

Composition, Dynamic Changes, and Carbon Sequestration Effects of Organic Carbon in the Water of a Penaeus vannamei Culture Pond

To investigate the composition, dynamic changes, and carbon sequestration effects of organic carbon in the water of Penaeus vannamei aquaculture ponds, changes in organic carbon were assessed in the water of the P. vannamei pond monoculture and integrated P. vannamei–Mercenaria mercenaria pond aquaculture systems during an aquaculture period. RDOC (refractory dissolved organic carbon) was determined using the DOC (dissolved organic carbon) degradation method, and the organic carbon composition in the water and its relationship with environmental factors were analyzed. The results showed the following: (1) The aquaculture activities significantly increased the DOC, POC (particulate organic carbon), and RDOC contents in the water and decreased the proportions of RDOC in the water. The DOC, POC, RDOC contents, and RDOC proportions in the monoculture systems were higher than those of the integrated aquaculture systems. (2) DOC, POC, RDOC contents, and RDOC proportions in the water were significantly positively correlated with chlorophyll-a contents in both aquaculture systems and significantly negatively correlated with water temperature and salinity. (3) Based on the average content and proportion of RDOC in the water of the two aquaculture systems, it was calculated that approximately an average of 108.64 kg of RDOC was present per hectare of P. vannamei aquaculture water during the aquaculture period.

Long-term response of the microbial community to the degradation of DOC released from Undaria pinnatifida

With the aim to study the mechanism underlying the macroalgal carbon sequestration driven by microbes, we investigated the microbial community using metagenomics methods and its long-term degradation of dissolved organic carbon (DOC) derived from Undaria pinnatifida. It was observed that after removing U. pinnatifida, the concentration of the DOC decreased significantly (p < 0.05) within 4 days. Over a period of 120 days of degradation, the concentration of remaining DOC (26%) remained stable. The succession of microbial community corresponded to the three stages of DOC concentration variation. Moreover, the structure of microbes community and its metabolic function exhibited evident patterns of succession. The concentration of DOC was correlated negatively with the abundances of Planctomycetaceae (p < 0.01), and was correlated positively with the abundances of Roseobacteraceae and Rhodobacteraceae (p < 0.01). In addition, the metabolic pathways related to “Glycolysis/Gluconeogenesis”, “Alanine, aspartate, and glutamate metabolism”, “Citrate cycle (TCA cycle)” and “Tryptophan metabolism” was significantly correlated with the variations in DOC concentration (p < 0.05). These findings indicate that the variation in the DOC concentration was closely linked to the succession of Planctomycetaceae, Roseobacteraceae, Rhodobacteraceae, and the degradation of DOC derived from U. pinnatifida appeared to be influenced by metabolic functions.

A UVA light-emitting diode microreactor for the photocatalytic degradation of humic acids and the control of disinfection by-products formation potential

Chlorination is one of the most applied technologies for drinking water disinfection. However, the generation of disinfection by-products (DBPs) in the chlorination process is threatening drinking water safety, especially when the concentration of natural organic matter (NOM) is high in source water. Climate change increases the fluctuation of NOM in source water, which further challenges water safety in small and rural communities. In this study, environmental microfluidics was introduced to a UVA-LED photocatalytic oxidation system for a rapid oxidation of NOM. The results showed that HA adsorption and degradation, and DOC degradation were more favored in low pH. At pH 5, 56.9% of HA and 58.1% of dissolved organic carbon (DOC) were removed in 3.4 min by the microreactor. DBP formation potential was efficiently reduced, with the highest reduction of THM and HAA formation potentials by 76% and 70.7%, respectively. Compared with the bottle reactor, the degradation rate constants of HA and DOC by UVA-LED microreactor were increased by 2.99 – 15.8 times, and 6.8 – 170.2 times, respectively. Homogenous irradiation and rapid mass transfer in the microreactor accelerated the inhibited oxidation of HA in acidic and base solutions and increased the apparent quantum yields of photocatalysis. Further, the microfluidic photocatalytic oxidation was much more efficient in the reduction of DPBs formation potential in a source water sample. The process was rapid and had low chemical and energy consumption. Approaches on system scale-up, system simplification, and energy conservation would increase potential of microfluidic photocatalytic oxidation in water purification for households and small communities.

Dynamics of the Marine Dissolved Organic Carbon Reservoir in Glacial Climate Simulations: The Importance of Biological Production

AbstractThe marine dissolved organic carbon (DOC) reservoir rivals the atmospheric carbon inventory in size. Recent work has suggested that the size of the DOC reservoir may respond to variations in sea temperature and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in paleoclimate events including Cryogenian glaciation and Eocene hyperthermals. Despite these suggestions, the dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling has generally assumed this reservoir to be static. In this study, we utilize an Earth system model of intermediate complexity to assess the response of the marine DOC reservoir to various glacial boundary conditions. Our results indicate that the marine DOC reservoir is responsive to glacial perturbations and may shrink or expand on the order of 10–100 Pg C. In contrast to recent studies that emphasize the importance of DOC degradation in driving the mobility of DOC reservoir, our study indicates the importance of DOC production. In the experiment under full glacial boundary conditions, for example, a 19% drop in net primary production leads to an 81 Pg C reduction in the DOC pool, without which the atmospheric CO2 concentration would have been lower by approximately 38 ppm by dissolved inorganic carbon changes alone. Thus, DOC reservoir variability is necessary to fully account for the simulated changes in atmospheric CO2 concentration. Our findings based on glacial experiments are corroborated in a different set of simulations using freshwater flux to induce weakening of the Atlantic meridional overturning circulation.

Photo-produced aromatic compounds stimulate microbial degradation of dissolved organic carbon in thermokarst lakes

Photochemical and biological degradation of dissolved organic carbon (DOC) and their interactions jointly contribute to the carbon dioxide released from surface waters in permafrost regions. However, the mechanisms that govern the coupled photochemical and biological degradation of DOC are still poorly understood in thermokarst lakes. Here, by combining Fourier transform ion cyclotron resonance mass spectrometry and microbial high-throughput sequencing, we conducted a sunlight and microbial degradation experiment using water samples collected from 10 thermokarst lakes along a 1100-km permafrost transect. We demonstrate that the enhancement of sunlight on DOC biodegradation is not associated with the low molecular weight aliphatics produced by sunlight, but driven by the photo-produced aromatics. This aromatic compound-driven acceleration of biodegradation may be attributed to the potential high abilities of the microbes to decompose complex compounds in thermokarst lakes. These findings highlight the importance of aromatics in regulating the sunlight effects on DOC biodegradation in permafrost-affected lakes.

Unique biogeochemical characteristics in coastal ghost forests – The transition from freshwater forested wetland to salt marsh under the influences of sea level rise

Seawater intrusion by rising sea levels has created large areas of ghost forests along low-lying coastal wetlands in the southeastern USA, but more information is needed to better understand its soil biogeochemistry. Here, we characterized several soil and environmental parameters, including tree litterfall, surface and soil porewater quality, emissions of greenhouse gases, and microbial communities along a forest-to-marsh transect, including a freshwater forested wetland, a salt-impacted degraded ghost forest, and a salt marsh in Winyah Bay, SC, USA. General water quality parameters such as electrical conductivity, dissolved oxygen, temperature and aboveground productivity showed distinct trends along the freshwater forested wetland → degraded ghost forest → salt marsh transect, whereas there were no obvious trends in soil biogeochemical parameters. Concentrations of dissolved organic carbon (DOC) in the degraded ghost forest were generally similar to the freshwater forested wetland, but on average were higher than those in the salt marsh. More labile molecular features observed through Fourier transform ion cyclotron resonance mass spectrometry indicated an increase in the DOC biodegradability along the forest-to-marsh transect. Greater DOC biodegradability in the degraded ghost forest was observed and confirmed through its generation of the highest average electrical currents from sediment microbial fuel cells. The lowest CH4 and CO2 fluxes, but the highest degradable DOC, were observed in the degraded ghost forest, suggesting that lateral C export is important in this wetland. Moreover, the degraded ghost forest was dominated by a unique microbial community, including high abundance of Woesearchaeia, which enables carbon metabolism via symbiotic and/or fermentation-based lifestyles. Our study illustrates a ghost forest with very different characteristics compared to its parental freshwater forested wetland and its transitioned salt marsh. Data obtained from the two endmember ecosystems along the salinity gradient transect were not useful in predicting the unique biogeochemical processes in the degraded ghost forest.

The interaction between DOC released by cultured kelp (Saccharina japonica) and the bacterial community reveals the potential for increasing marine carbon sequestration by macroalgae culture

Macroalgae culture as a marine carbon sequestration strategy has been recently proposed. To understand the potential for carbon sequestration by macroalgae culture, we studied the interaction of dissolved organic carbon (DOC) released by Saccharina japonica, an important cultured kelp species in China, and the bacterial community. We found that S. japonica effectively released DOC containing both labile and refractory components, and the DOC released by S. japonica into the water decreased average molecular weight (Masswa) by 18 Da, meaning it was rich in low molecular weight matter. Degradation of labile DOC components occurred during the initial 2 months of the experiment. The Masswa and molecular diversity of the DOC was increased in turn by bacterial degradation, and the variation in the DOC composition resulted in the succession of the bacterial community. During labile DOC degradation, anaerobic Gammaproteobacteria, Bacteroidia and Campylobacteria were the dominant microbial classes in the water, and aerobic Iodidimonadaceae became the dominant family during the final stage of the experiment. Finally, 5% of the DOC was transformed into refractory DOC. Although the sinking biomass was not involved in the study, the produced refractory DOC through the interaction between releasing DOC and bacterial degradation indicates a potential of carbon sequestration by macroalgae culture.

Accumulation of recalcitrant dissolved organic matter in aerobic aquatic systems

AbstractAn oxygenated atmosphere changed life on Earth but it also provided a negative feedback to organic matter accumulation by increasing decomposition rates. Nonetheless, dissolved organic carbon (DOC) is a huge carbon pool (&gt; 750 Pg) and it can accumulate to high concentrations (20–100 mg C L−1) in some freshwater aquatic systems, yet it is not clear why. Here, we examine DOC in several Greenland lakes with varying DOC concentrations and identify processes that could alter its composition to make it increasingly recalcitrant. Δ14C aging of DOC corresponded with increased DOC concentrations, slower degradation rates, changes in isotope ratios and optical properties, all suggesting that photochemical and microbial degradation processes contributed to recalcitrance. Young DOC degradation was stimulated by phosphorus, but older DOC was not, suggesting an important role for nutrients early in degradation. Photochemical processing coupled with decreased habitat diversity in hydrologically isolated systems may enable recalcitrant DOC to accumulate with important implications for Earth's carbon and oxygen cycles.

Catchment Dissolved Organic Carbon Transport: A Modeling Approach Combining Water Travel Times and Reactivity Continuum

AbstractQuantifying the transfer of organic carbon from the terrestrial to the riverine ecosystems is of crucial importance to fully appreciate the carbon cycle at the catchment, regional and global scales. In this study, we propose a framework for modeling the flux of dissolved organic carbon (DOC) from hillslopes to stream and river networks which couples a transport model based on travel time distributions with the reactivity continuum (RC) approach to model DOC degradation. We test the model by applying it to the Plynlimon catchments (UK) exploiting both weekly and high‐frequency (7‐hr interval) time‐series. We use information about chloride to get an independent estimate of water travel times using the framework of StorAge Selection functions. Following the RC model, the composition and the degradation of DOC along the flowpaths, and its consequent concentration in the streamflow, is described assuming that DOC is composed by a mixture of compounds that follows a continuous spectrum of reactivity. For the high‐frequency data set, the model is able to reproduce DOC streamflow concentrations and to capture the complex hysteretic relation between DOC concentration and discharge. Weekly data are instead not frequent enough to properly describe DOC dynamics in this catchment. The distribution of the age of the water comprised in the streamflow proves thus a key variable to predict the quantity and the reactivity of the DOC exported from soils, and the effect of hydrologic variability on this process.

Carbon Sequestration in the Form of Recalcitrant Dissolved Organic Carbon in a Seaweed (Kelp) Farming Environment.

Under climate change scenarios, the contribution of macroalgae to carbon sequestration has attracted wide attention. As primary producers, macroalgae can release substantial amounts of dissolved organic carbon (DOC) in seawater. However, little is known about the molecular composition and chemical properties of DOC derived from macroalgae and which of them are recalcitrant DOC (RDOC) that can be sequestered for a long time in the ocean. In the most intensive seaweed (kelp) farming area (Sanggou Bay) in China, we found that kelp mariculture not only significantly increased DOC concentration, but also introduced a variety of new DOC molecular species, many of which were sulfur-containing molecules. A long-term DOC degradation experiment revealed that those DOC with strong resistance to microbial degradation, i.e., RDOC, account for approximately 58% of the DOC extracted from kelp mariculture area. About 85% (3587 out of 4224 with different chemical features) of the RDOC molecular species were steadily present throughout the long-term degradation process. 15% (637 out of 4224 with different chemical features) of the RDOC molecular species were likely newly generated by microorganisms after metabolizing macroalgae-derived labile DOC. All these stable RDOC should be included in the blue carbon budgets of seaweed.

Removal of chromophoric dissolved organic matter under combined photochemical and microbial degradation as a response to different irradiation intensities

Throughout the freshwater continuum, Dissolved Organic Carbon (DOC) and the colored fraction, Chromophoric Dissolved Organic Material (CDOM), are continuously being added, removed, and transformed, resulting in changes in the chromophoricity and lability of organic matter over time. We examined, experimentally, the effect of increasing irradiation-intensities on the combined photochemical and microbial degradation of CDOM and DOC. This was done by using a simulated mixed water column: aged water from a humic lake was exposed to four irradiation-intensities – representing winter, early and late spring, and summer conditions (0.10, 0.16, 0.36, and 0.58 W/m2) – and compared with dark controls over 37 days. We found a linear relationship between CDOM degradation and irradiation-intensities up to 0.36 W/m2; the degradation rate saturated at higher intensities, both at specific wavelengths and for broader intervals. After 37 days at high irradiation-intensity, CDOM absorption of irradiation at 340 nm had been reduced by 41%; 48% of DOC had been removed and DOC degradation continued to increase. Aromaticity (SUVA254) declined significantly over 37 days at the two lowest but not at the two highest UV- intensities; levels in unexposed control water remained constant. Direct observations of the humic lake showed that CDOM absorption of irradiation (340 nm) declined by 27% from winter to summer. A model based on hydrological CDOM input and CDOM degradation calculated from field measurements of UV-radiation and experimental CDOM degradation with UV-exposure from sunlight accurately predicted the annual course as observed in the lake. With no external CDOM input, 92% of the CDOM could be degraded in a year. The results support the notion that combined photochemical and microbial CDOM degradation can be remarkably higher in lakes than previously thought and that humic lakes retain their color due to light absorption by ongoing CDOM input.

Characterization of the vertical size distribution, composition and chemical properties of dissolved organic matter in the (ultra)oligotrophic Pacific Ocean through a multi-detection approach

This work presents a multi-analytical approach for the characterization of marine dissolved organic matter (DOM). The determination of marine dissolved organic carbon (DOC) was performed by size exclusion chromatography (SEC) and validated using a certified reference material (CRM) as well as through an intercomparison exercise. Multi-detection SEC, fluorescence and electrochemical methods were used in order to describe the size distribution spectra, the composition and chemical properties of marine DOM, in the (ultra)oligotrophic West Tropical South Pacific Ocean (WTSP). In this work, we defined the state of degradation of DOC in the different size fractions, operationally defined by SEC. We estimated that on average 62% of DOC was of humic nature (0.5–10 kDa), of which ~9% was able to complex trace elements, such as iron (Fe). Our results tend to support that non-refractory DOC is of high molecular weight (HMW), nitrogen (N)-rich, aliphatic, and has a weak fluorescence quantum yield and an enhanced binding capacity for Fe. The ageing of marine DOM occurring within mesopelagic waters is mainly driven by microbial respiration and alters these chemical properties. Although our results are in agreement with a paradigm describing oceanic DOM biogeochemistry known as the size-reactivity continuum, 3 μmolC L−1 of very HMW (> 10 kDa) were still observed in a water mass mainly composed of Pacific Deep Waters. This persistence could be explained by a significant content (5%) of aromatic carbon that may protect HMW DOM from long term biodegradation.

Characteristics of Dissolved Organic Carbon in Boreal Lakes: High Spatial and Inter‐Annual Variability Controlled by Landscape Attributes and Wet‐Dry Periods

AbstractConcentration and chemical composition of dissolved organic carbon (DOC) influence several lake functions; greenhouse gas exchange, nutrient cycling, food webs, and water treatability. To assess spatial and inter‐annual controls on DOC characteristics, 34 lakes were sampled annually for 8 years on the Boreal Plains, Western Canada—a region with heterogeneous surficial geology, and a sub‐humid climate with pronounced inter‐annual wet‐dry periods. Large spatial variability in long‐term average DOC concentration (10–49 mg C L−1) and aromaticity (SUVA254: 1.2–3.9 L mg−1 C m−1) among lakes was found. Higher DOC concentrations and aromaticity were associated with lakes in watersheds with fine‐textured surficial geology and with relatively large contributions through shallow, organic‐rich flow paths. Lake DOC aromaticity was also higher in lakes with lower evaporative enrichment, regardless of surficial geology, indicating shorter lake water residence times and less within‐lake degradation of allochthonous DOC. High inter‐annual variability for both DOC and aromaticity was observed, with coefficients of variation at 10.9 ± 4.6% and 11.1 ± 2.5% among lakes, respectively. Inter‐annual variability in DOC concentrations had low synchronicity among lakes, with patterns of variability linked to surficial geology and primarily responsive to short‐term cumulative precipitation. Conversely, inter‐annual variability in aromaticity had high synchronicity among lakes, driven by longer‐term cumulative precipitation and shifts in lake water residence times. Our study shows it is necessary to consider lake hydrogeomorphic setting and climate to understand spatial and inter‐annual variability in lake DOC characteristics and associated lake functions, and that Boreal Plains lakes have high climate sensitivity.

Enhanced Biodegradation of Dissolved Organic Carbon in the Western Boundary Kuroshio Current When Intruded to the Marginal South China Sea

AbstractThe advective supply of allochthonous dissolved organic carbon (DOC) from open ocean to marginal seas through western boundary current intrusion influences the regional carbon inventory and microbial activities. However, there is limited observation about this process and its biogeochemical impacts on marginal seas. In this study, we investigated the biodegradation of allochthonous DOC carried by the intrusion of the Kuroshio Current into South China Sea (SCS). Using an isopycnal mixing model, the exchange and biodegradation processes of Kuroshio‐intruded DOC were quantified. We estimated that approximately 10% of the surface DOC was remineralized due to the enhanced biodegradation in the SCS. This result was supported by the on‐deck bioassay experiments that were conducted under different environmental contexts. The results of modeling and on‐deck incubations indicate that DOC biodegradation was enhanced by the sharp gradient of environment factors, including nutrients supply, microbial species, and bio‐lability of DOC in the frontal zone during the surface water mass mixing. The amount of carbon released from the enhanced DOC degradation by Kuroshio intrusion was estimated to be approximately equal to 8.6 Tg C yr−1. Concomitantly, the amount of nitrogen released could contribute 0.19–0.70 mmol N m−2 d−1 to the surface of SCS which is comparable to the total supply from deeper water and nitrogen fixation in surface waters. This study suggests that the enhanced biodegradation of DOC during the western boundary currents intrusion could serve as an important sink of oceanic DOC, and thus provide an additional nutrient source to marginal seas.

High biodegradability of riverine dissolved organic carbon in late winter in Hudson Bay, Canada

Hudson Bay, at the southern margin of the Arctic Ocean, receives nearly one-third of Canada’s river discharge and approximately 5.5 Tg of riverine dissolved organic carbon (DOC) annually. Riverine DOC fluxes to Hudson Bay are expected to increase with climate change, but how this increase will influence the biogeochemistry of the coastal waters is largely unknown. In particular, the fate of riverine DOC that enters Hudson Bay during the dark, frozen winter period (roughly January to April) is poorly known despite high discharge from the large, regulated rivers of Hudson and James Bays at that time. Few studies have assessed the degradability of riverine DOC transported in winter anywhere across the Arctic, leaving unanswered questions regarding the impact of riverine DOC on the Arctic carbon budget, CO2 fluxes, and local food webs. Here, we assessed the biodegradability of DOC in riverine and coastal waters of southern Hudson Bay in late winter using 45-day incubation experiments. We found 24%–60% of the DOC in the rivers and on average 21% of the DOC in the immediate coastal waters to be biodegradable. Differences in biodegradability appeared to depend on properties of the rivers/watersheds and physical and biochemical processes in the aquatic environments. DOC biodegradability correlated strongly with DOC concentration, which was higher during winter than summer in all studied rivers and higher in the Nelson and Hayes Rivers, draining the Hudson Bay Lowlands than in most previously studied large rivers of the Arctic watershed. The Nelson River, regulated for hydropower production, had the highest winter DOC concentrations and most degradable DOC. The high biodegradability of Hudson Bay riverine DOC in late winter and high concentrations and fluxes of riverine DOC at that time imply strong leverage for future increases in DOC fluxes to impact the carbon cycle of these coastal waters.

A multi-component model for granular activated carbon filters combining biofilm and adsorption kinetics

Along with the rise of biological active granular activated carbon (bGAC) filtration as advanced treatment technology for wastewater treatment plant (WWTP) effluents, the mathematical representation of such systems is gaining increasing importance. This work introduces a model that describes the performance of bGAC-filters for Dissolved Organic Carbon (DOC) removal from a WWTP effluent. The DOC removal within bGAC-filters is accomplished by two mechanisms: adsorptive removal and biological transformation. An appropriate representation of the adsorptive removal requires the DOC to be divided into fictive fractions according to its adsorbability. Likewise, a further DOC classification according to its biodegradability is necessary. Modeling a bGAC-filter then becomes a multi-component adsorption problem, with the simultaneous occurrence of DOC degradation within a biofilm. For dealing with this modeling task, this work integrated the Ideal Adsorbed Solution (IAS) theory into a traditional biofilm model compatible with the Activated Sludge Model (ASM) Framework. For the description of the adsorption dynamics, a Freundlich isotherm for the equilibrium and a pseudo first order model for the kinetics were selected. The biofilm consisted of heterotrophic bacteria able to oxidize DOC using oxygen as electron acceptor. The correctness of the model was evaluated using experimental data from a pilot plant. The predicted DOC breakthrough curve satisfactorily fitted the experimental measurements for empty bed contact times (EBCT) of 6, 12, 24 and 33 min. Moreover, the model predicted the relationship between EBCT, DOC removal and bGAC-filter lifespan. The developed model is the first that combines multi-component adsorption and biofilm kinetics in a wastewater treatment context.

Microbial Population Dynamics in Model Sewage Treatment Plants and the Fate and Effect of Gold Nanoparticles.

Adequate functioning of a sewage treatment plant (STP) is essential to protect the downstream aquatic environment (ECHA 2017), and information on the degradability of chemicals and their toxicity to activated sludge microorganisms is required. An environmental realistic higher tier test is a STP simulation test as described in OECD 303A (2001) which for nanoparticles can also be used to study their sorption behavior to activated sludge. However, information is limited on the influence of synthetic sewage on the microbial community of the activated sludge. A modified community can result in modifications of the sludge floccules affecting the sorption behavior. The main objective of our study was to show whether a representative microbial diversity remains under standardized test conditions as described in OECD 303A (2001) using synthetic sewage as influent. Furthermore, we investigated whether just considering the functional properties of a STP (elimination of dissolved organic carbon; nitrification), is sufficient for an assessment of gold nanoparticles (AuNPs) or whether the influence on microbial diversity also needs to be considered. AuNPs were used as a case study due to their rising medical applications and therefore increasing probability to reach the sewer and STP. The results can provide significant input for the interpretation of results from the regulatory point of view. To deliver these objectives, the general changes of the microbial population in activated sludge and its influence on the degradation activity (dissolved organic carbon (DOC) and inorganic nitrogen) using freshly collected sludge from the municipal STP in an artificial test system as a model STP in accordance with OECD 303A (2001) were assessed. Additionally, we evaluated the potential impact of AuNPs and its dispersant on the microbial composition and the overall impact on the function of the STP in terms of DOC degradation and nitrogen removal to observe if an assessment based on functional properties is sufficient. The bacteria composition in our study, evaluated at a class level, revealed commonly described environmental bacteria. Proteobacteria (β, α, δ) accounted for more than 50% but also nitrifying bacteria as Nitrospira were present. Our results show that mainly within the first 7 days of an acclimatization phase by addition of synthetic sewage, the bacterial community changed. Even though AuNPs can have antibacterial properties, no adverse effects on the function and structure of the microorganisms in the STP could be detected at concentrations of increased modeled PEC values by a factor of about 10,000. Complementary to other metallic nanomaterials, gold nanomaterials also sorb to a large extent to the activated sludge. If activated sludge is used as fertilizer on agricultural land, gold nanoparticles can be introduced into soils. In this case, the effect on soil (micro)organisms must be investigated more closely, also taking into account the structural diversity.

Export of photolabile and photoprimable dissolved organic carbon from the Connecticut River

Dissolved organic carbon (DOC) impacts water quality, the carbon cycle, and the ecology of aquatic systems. Understanding what controls DOC is therefore critical for improving large-scale models and best management practices for aquatic ecosystems. The two main processes of DOC transformation and removal, photochemical and microbial DOC degradation, work in tandem to modify and remineralize DOC within natural waters. Here, we examined both the photo- and microbial remineralization of DOC (photolability and biolability), and the indirect phototransformation of DOC into biolabile DOC (photoprimed biolability) for samples that capture the spatiotemporal and hydrological gradients of the Connecticut River watershed. The majority of DOC exported from this temperate watershed was photolabile and the concentration of photolabile DOC correlated with UV absorbance at 254 nm (r2 = 0.86). Phototransformation of DOC also increased biolability, and the total photolabile DOC (sum of photolabile and photoprimed biolabile DOC) showed a stronger correlation with UV absorbance at 254 nm (r2 = 0.92). We estimate that as much as 49% (SD = 3.3%) and 10% (SD = 1.1%) of annual DOC export from the Connecticut River is directly photolabile and photoprimable, respectively. Thus, 2.82 Gg C year−1 (SD = 0.67 Gg C year−1) or 1.13 Mg C km−2 year−1 (SD = 0.27 km−2 year−1) of total photolabile DOC escapes photochemical degradation within the river network to be exported from the Connecticut River each year.

Photodegradation of autochthonous and allochthonous dissolved organic matter in a natural tropical lake

Dissolved organic carbon (DOC) is one of the most abundant fractions of organic matter in aquatic systems and plays an important role in the dynamics of aquatic environments, controlling both the penetration and the underwater light radiation climate. DOC can be photodegraded by light, thus facilitating biodegradation, especially in regions where the incidence of solar radiation is high, such as higher altitudes and lower latitudes. This study quantified the photodegradation of dissolved organic material in a natural tropical lake surrounded by native forests (Brazilian Atlantic Forest) through two experiments: i) the first experiment exposed concentrated autochthonous, allochthonous, and lake water to in situ solar radiation; ii) this experiment also exposed the same organic material to artificial UV radiation in an incubator under controlled conditions. The quality and quantity of dissolved organic carbon were measured using indices based on carbon absorbance and fluorescence spectrum. In the in situ experiment, it was observed that the DOC degradation profile of the concentrated allochthonous and autochthonous organic material were distinct from each other in the absorbance indices, and the lake water mostly resembled the latter one. On the other hand, we did not see evidence of any significant difference among treatments in the laboratory experiment. An increase in the SR index and a concomitant decrease in the fluorescence of humic compounds and SUVA254 over time were observed. In both experiments, the amount of degraded organic material over time was low and some possible explanations are discussed.

Assessing the priming effect of dissolved organic matter from typical sources using fluorescence EEMs-PARAFAC

Priming effect (PE) is increasingly recognized as an important mechanism in the microbial degradation of dissolved organic matter (DOM) from freshwater to the ocean. However, potential PE during the mixing of DOM from different sources and the effects on different DOM constituents are still largely unknown. This study examined the PE after adding DOM from typical natural and anthropogenic sources (rainwater, fresh plant, leaf litter, and wastewater) into pre-aged river DOM, using dissolved organic carbon (DOC) measurement, absorption spectroscopy, and fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). The plant-derived DOM had a low humic content and was dominated by benzoic acid-like and tyrosine-like fluorescent components (C4 and C5), which showed a high DOC bioavailability of 80%. DOC in rainwater and wastewater also had high bioavailabilities (45%–50%), while DOM in the leaf litter leachate showed high aromaticity, average molecular weight, and humic content but low DOC bioavailability (12%). There was generally limited PE (<5% of the initial values) on the degradation of DOC and chromophoric DOM (CDOM) for most samples. Two humic-like components (C2 and C3) showed little PE, while the humic-like C1 and C6, tyrosine-like C5, and tryptophan-like C7 showed variable PE after adding rainwater, wastewater, and plant leachate. Overall, the results revealed that the DOM from typical natural and anthropogenic sources had different composition and bioavailability, and their inputs to aquatic environments would result in variable PE on the bulk DOC and different DOM components.