Chromophoric Dissolved Organic Matter Components Research Articles

Indirect photodegradation of zinc pyrithione: The effect of chromophoric dissolved organic matter components and seawater factors

Chromophoric dissolved organic matter (CDOM) is the main photosensitizer in seawater. CDOM can absorb solar radiation extensively and generate reactive intermediates (RIs), including singlet oxygen (1O2), hydroxyl radicals (HO·), and CDOM triplet states (3CDOM*), that are favorable for indirect photodegradation of organic pollutants. In the current study, the effect and mechanism of CDOM components on zinc pyrithione (ZnPT) indirect photodegradation were studied. Among the three CDOM isolates (humic acid (JKHA), Suwannee River humic acid (SRHA), and Suwannee River natural organic matter (SRNOM)), JKHA had the highest fluorescence intensity per gram of carbon and the fastest indirect photodegradation rate for ZnPT. Furthermore, the fluorescent components were divided into four terrestrial humic-like components: C1, C2, C3, and C4, by using excitation–emission matrix spectroscopy combined with parallel factor analysis. A notable relationship was found between the four fluorescent components of CDOM and ZnPT indirect photodegradation. The indirect photodegradation rate of ZnPT varied in different types of CDOM solutions, which may be attributed to the different compositions of CDOM. The relative contents of C1 and C4 were significantly higher in JKHA than in the other two CDOMs, and they were the major contributors to the production of 3CDOM* and 1O2, which promoted ZnPT indirect photodegradation. The impact of seawater factors on ZnPT indirect photodegradation was also investigated. The ZnPT indirect photodegradation rate initially decreased and then increased as the pH increased from 5.00 to 11.00. The lowest rate of indirect photodegradation of ZnPT was observed at pH = 9.00. As the salinity was <25‰, in salinity the ZnPT indirect photodegradation rate increased concomitantly with an increase, and pronounced inhibition of ZnPT indirect photodegradation was observed when the salinity was >25‰. Nitrate had a promoting influence on ZnPT indirect photodegradation, whereas HCO3−, halogen ions, and metal ions all inhibited it.

Spectral Characteristics and Source Analysis of Chromophoric Dissolved Organic Matter in Surface Water of Taihu Lake Before Cyanobacterial Blooming

Chromophoric dissolved organic matter (CDOM) is an important part of the nutrient biogeochemical cycle in aquatic ecosystems. To explore the characteristics and sources of CDOM components in the surface water of Taihu Lake, UV-visible spectroscopy and excitation emission matrix fluorescence spectroscopy-parallel factor analysis were used to analyze CDOM components in surface water. Combined with CDOM optical parameters (a355, SUVA254, a250/a365, FI, BIX, and HIX), the spatial differences and pollution sources were identified, and a preliminary comparison was made between this study and the historical data of CDOM components in Taihu Lake. According to the results, a355, SUVA254, and a250/a365 showed the characteristics of high concentration, high aromatic ability, and low relative molecular weight of CDOM in the surface water of the eastern part of Taihu Lake; however, the northern part showed the opposite characteristics. Four components were isolated from CDOM using parallel factor analysis:one tyrosine-like (C1), two types of tryptophan (C2 and C4), and one fulionic acid (C3). The main component C1 had a strong linear relationship with the C2 and C3 components, suggesting that different components originated from similar pollution sources. The fluorescence index showed that CDOM in different areas of Taihu Lake were differently affected by endogenous and terrestrial inputs; however, the overall humification degree was low. This indicated that the CDOM components in Taihu Lake were primarily protein-like (C1, C2, and C4) (>85%) and autogenous, with good biochemical availability.

Indirect photodegradation of sulfadimidine and sulfapyridine: Influence of CDOM components and main seawater factors

This study investigated the indirect photodegradation of sulfadimidine (SM2) and sulfapyridine (SP) in the presence of chromophoric dissolved organic matter (CDOM), and studied the influences of main marine factors (salinity, pH, NO3− and HCO3−). Reactive intermediate (RI) trapping experiments demonstrated that triplet CDOM (3CDOM*) played a major role in the photodegradation of SM2 with a 58% photolysis contribution, and the contributions to the photolysis of SP were 32%, 34% and 34% for 3CDOM*, hydroxyl radical (HO·) and singlet oxygen (1O2), respectively. Among the four CDOMs, JKHA, with the highest fluorescence efficiency, exhibited the fastest rate of SM2 and SP photolysis. The CDOMs were composed of one autochthonous humus (C1) and two allochthonous humus (C2 and C3). C3, with the strongest fluorescence intensity, had the strongest capacity to generate RIs and accounted for approximately 22%, 11%, 9% and 38% of the total fluorescence intensity of SRHA, SRFA, SRNOM and JKHA, respectively, indicating the predominance of CDOM fluorescent components in the indirect photodegradation of SM2 and SP. These results demonstrated the photolysis mechanism: The photosensitization of CDOM occurred after its fluorescence intensity decreased, and a large number of RIs (3CDOM*, HO· and 1O2, etc.) were generated by energy and electron transfer, then these RIs reacted with SM2 and SP to cause photolysis. The increase in salinity stimulated the photolysis of SM2 and SP consecutively. The photodegradation rate of SM2 first increased and then decreased with increasing pH, whereas the photolysis of SP was remarkably promoted by high pH but remained stable at low pH. NO3− and HCO3− had little effect on the indirect photodegradation of SM2 and SP. This research may contribute to a better understanding of the fate of SM2 and SP in the ocean and provide new insights into the transformation of other sulfonamides (SAs) in marine ecological environments.

Assessment of UV-VIS spectra analysis methods for quantifying the absorption properties of chromophoric dissolved organic matter (CDOM)

Several ultraviolet-visible (UV-VIS) spectral analysis methods have been used to quantify the absorption properties of chromophoric or colored dissolved organic matter (CDOM). Different spectroscopic parameters can be used as surrogates of optical properties; furthermore, advanced mathematical tools have also been applied to investigate the absorption spectrum. This study evaluated the most commonly used spectroscopic parameters in remote sensing research and advanced mathematical methods using absorption data on primary biomass constituents (BCs) in aqueous states. We found that, out of the eight spectrometric parameters, the spectral slope in the 275–295 nm range (S275–295) had the strongest correlation with the hydrogen to carbon ratio (H/C), and the spectral slope ratio (275–295 to 350–400 nm) SR and the absorbance ratio between 465 and 665 nm (E4/E6) had a strong correlation with the oxygen to carbon ratio (O/C). Additionally, the spectroscopic parameter values for the solutions of the BCs exhibited distinguishable differences. Gaussian fitting was suitable for single CDOM components but not for complex mixtures. Derivative analysis can be used for single-component discrimination with an extensive investigation of the absorption properties of this component. Additionally, we propose a possible bottom-up perspective to track the origins of CDOM through the absorption spectrum.

Spectroscopic properties and driving factors of dissolved organic matter in the Yellow River Delta

AbstractRiver deltas are hot spots of biogeochemical cycling. Understanding sources and driving factors of dissolved organic matter (DOM) in river deltas is important for evaluating the role of river deltas in regulating global carbon flux. In this study, spectroscopic properties of soil DOM were analyzed in both freshwater and tidal areas of the Yellow River Delta. Five fluorescent components of soil DOM (two humic-like DOM, two protein-like DOM and one possible contaminant) were identified by parallel factor analysis and further confirmed by comparison with an online database. Concentration, spectroscopic properties and sources of soil DOM and its components differed between freshwater and tidal areas. DOM concentration was much higher in freshwater areas than in tidal areas. In freshwater areas, soil DOM was mainly derived from phytoplankton and microorganisms, while it was mainly derived from microorganisms and human activities in tidal areas. These differences in DOM between both areas were strongly driven by environmental factors, especially soil carbon (C), nitrogen (N) and its stoichiometric ratio C/N. These explained 80.7% and 69.6% of variations in DOM and chromophoric DOM (CDOM), respectively. In addition, phytoplankton also contributed to soil DOM, CDOM and fluorescent components C1–C4 as identified by significant positive correlations between them. These results imply that both the concentration and composition of soil DOM are strongly driven by soil properties and phytoplankton density in the Yellow River Delta.

Influences of Hydrological Scenarios on the Bioavailability, Fate, and Balance of Chromophoric Dissolved Organic Matter in Lake Poyang

Lake Poyang has significant differences in hydrological characteristics between the flood and dry seasons. Unraveling the optical composition, bioavailability, fate, and balance of chromophoric dissolved organic matter (CDOM) and organic carbon fluxes in Lake Poyang under different hydrological conditions can help provide advanced schemes on carbon cycling, the transfer and transformation of organic matter, and water resource management of the lake. Three fluorescent components, including a humic-like (C1), a tryptophan-like (C2), and a tyrosine-like (C3) component, were obtained using three-dimensional fluorescence spectroscopy coupled with parallel factor analysis. Prior to and after 28 days of laboratory biodegradation, the means of a254 and the terrestrial humic-like (C1) component in the flood season were both significantly higher than that in the dry season (t-test, P<0.01), indicating that the terrestrial humic-like (C1) component contributed importantly to the CDOM pool. The contribution percentages of protein-like components in the dry season were 81.7% of the summed fluorescent components of CDOM, indicating that there might be discharge of domestic wastewater from areas surrounding the lake in the dry season. The bioavailabilities of the humic-like (C1) component and DOC were 14.0% and 43.2%, respectively, in the dry season. This can be explained by a declined-dilution effect in the lake during the dry rather than in the flood season. We observed no significant difference in the bioavailability of protein-like components under different hydrological conditions. The bioavailability of C1 (i.e.,%ΔC1) showed a decreasing trend from the southern inflowing river mouths to the downstream northern outlet at Hukou in both the flood and dry seasons, indicating that the bioavailability of the C1 decreased following the migration of CDOM in the lake. In the dry season and flood season, Lake Poyang was the source of DOC with fluxes of 14.0×103 t·mon-1 and 1.4×103 t·mon-1, respectively, whereas CDOM fluxes in corresponding periods were the source and weak sink with corresponding fluxes of 9.3×1010 m3·(m·mon)-1 and 1.1×1010 m3·(m·mon)-1, respectively. Therefore, the lake released substantial organic matter to the downstream receiving waters during the dry season, whereas in the flood season, the higher water level in the Yangtze River resulted in a prolonged water residence time of the lake, and a fraction of CDOM was bio-degraded into inorganic nutrients, favoring the metabolisms and the eutrophication process of the lake ecosystem.

The impacts of intensive scallop farming on dissolved organic matter in the coastal waters adjacent to the Yangma Island, North Yellow Sea

In-situ field investigations coupled with incubation experiments were conducted in the coastal waters adjacent to the Yangma Island to explore the impacts of intensive bay scallop farming on the quantity and composition of dissolved organic matter (DOM). During the scallop farming period, the values of dissolved organic carbon (DOC), chromophoric dissolved organic matter (CDOM) and fluorescence dissolved organic matter (FDOM) in the mariculture area (MA) were generally higher than those in the non-mariculture area (NMA). Bay scallops released a large amount of DOM with the characteristics of high molecular weight and low degree of humification into the water column through excretion, which altered the DOM biogeochemical cycle. The DOM excretion fluxes by scallop were calculated based on incubation experiments. The results showed that, without considering the DOM transformation in the water, the excretion process of bay scallops in a growth cycle can increase the concentration of DOC, CDOM and fluorescent components C1–C4 in the seawater in MA by 19.7 μmol l−1, 0.048 m−1, 0.065 QSU, 0.164 QSU, 0.017 QSU and 0.015 QSU, respectively. Assuming that the labile part of DOM excreted by scallops was completely aerobic decomposed, it could reduce DO and pH in the seawater by ~13.4 μmol l−1 and ~ 0.018 in MA. This study highlights the impact of human activities (scallop farming activities) on DOM cycle in coastal waters, which can help guide future policy formulating of mariculture and ecological protection.

Assessing the potential to use CDOM as an indicator of water quality for the sediment-laden Yellow river, China

Chromophoric dissolved organic matter (CDOM) in rivers is mainly affected by natural conditions and human activities and can reflect the watershed pollution status to a certain extent. The Yellow River is one of the largest contributors to the global riverine sediment flux from the land to ocean, and there is a paucity of information on how the optical properties of CDOM have the potential to serve as an indicator of water quality for the sediment-laden Yellow River. In this study, a three-dimensional fluorescence parallel factor (PARAFAC) analysis method was applied to investigate the seasonal and spatial variations in CDOM fluorescence components and spectral characteristics from the source region to the estuary in the mainstream of Yellow River. The relationships of CDOM with water quality indicators and trophic state were also analyzed. Six PARAFAC components (C1–C6) were identified and grouped into two categories: humic-like components (C1–C4), which accounted for 85.8 %, and protein-like components (C5 and C6), which accounted for only 14.2 %. The CDOM components, spectral parameters, and their clear correlations with the main ions (Na+ and Cl−) all indicated that the humic-like components may be primarily derived from nonpoint source erosion, and the protein-like components were mainly derived from point source discharges in the watershed. The combination of the CDOM absorption coefficient at 254 nm (a(254)), spectral slope ratio (SR), specific UV absorbance SUVA254, and fluorescence index (FI) had a good predictive ability for the key water quality indicators (total nitrogen (TN), dissolved total nitrogen (DTN), total phosphorus (TP), dissolved total phosphorus (DTP), and chlorophyll a (Chl a)) and trophic state index (TSI). Therefore, some fluorophores and UV spectral parameters of CDOM in the Yellow River can be used for rapid water quality monitoring and pollution source indication, especially pollutants related to nitrogen and phosphorus nutrients in the basin.

Chromophoric dissolved organic matter (CDOM) in a subtropical estuary (Galveston Bay, USA) and the impact of Hurricane Harvey.

The landfall of Hurricane Harvey in August 2017 provided the opportunity to study the impact of extreme freshwater discharge on chromophoric dissolved organic matter (CDOM) properties in a subtropical estuary (Galveston Bay, Texas). Both fluorescence spectroscopy (excitation-emission matrices) and a three-component parallel factor analysis (PARAFAC) model identified changes in CDOM properties. Comparing to Coble's peaks, component 1 was similar to peak C, component 2 to peak M, and component 3 to peak B. Results clearly show three periods with distinct CDOM properties: a dry season, a wet season, and Hurricane Harvey. The dry season was characterized by higher values of the spectral slope and fluorescence and biological indices. The wet season was characterized by high values of PARAFAC components 1 and 2 (humic-like) and the absorption coefficient at 350 nm. Some CDOM components were highly correlated with salinity, indicating conservative mixing. Component 3 (protein-like) had a low correlation to salinity, suggesting degradation or production processes in the bay. Silicates and NO3- + NO2- had negative relationships with salinity and a positive one with PARAFAC components 1 and 2. PARAFAC component 3 was correlated with dissolved oxygen and chlorophyll a, suggesting a relationship between CDOM fluorescent components and phytoplankton activity. High values of the humification index were observed immediately after Hurricane Harvey, indicating increased input of terrestrial organic matter into the bay. Hurricane Harvey increased CDOM levels and humification, and the variability and changes seem to be mostly due to freshwater discharge from the San Jacinto River and not the Trinity River. The influx of freshwater was sufficient to eliminate the salinity gradient in Galveston Bay and significantly change CDOM properties. Galveston Bay recovered quickly from the hurricane and associated flux of freshwater, returning to pre-hurricane CDOM characteristics in less than 2 months.

Excitation-emission matrix fluorescence spectra of chromophoric dissolved organic matter reflected the composition and origination of dissolved organic carbon in Lijiang River, Southwest China

Aquatic photosynthesis transforms inorganic carbon to organic carbon (OC), which contributes to autochthonous organic carbon (AOC) in sediment and particulate organic carbon in surface karst aquatic systems. Aquatic plant and microbes are participated in autochthonous dissolved organic carbon (ADOC) formation in surface karst aquatic systems, but the composition and formation of ADOC remains little known, which leaves problem on calculation of organic carbon flux. In this study, the Lijiang River was chosen as typical surface karst river to identify the DOC composition and its origin, and to explore the environmental influencing factors. Samples were collected seasonally from the upper to lower reaches of the river to analyze hydrochemical parameters and the excitation-emission matrix (EEM) spectrum of chromophoricdissolved organic matter (CDOM). Three CDOM components were calculated by parallel factor analysis (PARAFAC) from EEM spectra, which were allochthonous soil-sourced DOC (SDOC), autochthonous aquatic plant-sourced DOC (APDOC), and microbial-sourced DOC (MDOC). Based on the DOC component concentrations, SDOC is induced by large amounts of precipitation causing soil erosion in summer. APDOC formation is encouraged by moderate water temperatures in spring and fall restricted by high water turbidity in summer. The significant positive linear correlations between APDOC and dissolved inorganic carbon (DIC) and MDOC indicate DIC fertilization effect in karst aquatic systems directly promotes APDOC formation and indirectly promotes MDOC. Our study develops a relatively simple way to calculate composition of karst aquatic DOC, which demonstrates the participation of aquatic plants and microbes in APDOC production and reiterate that autochthonous DOC should be considered when calculating the carbon sink in surface karst aquatic systems.

Absorption and Three Dimensional Fluorescence Spectra of CDOM in the Water of the Taibaishan Nature Reserve

It is very important to understand water ecology and the carbon cycle process by studying the composition, source, spectral characteristics, and influence factors of chromophoric dissolved organic matter (CDOM). The optical characteristics, composition, and source of CDOM in 71 water samples were collected from the lakes and four rivers of the Taibaishan Nature Reserve in the summer of 2019. The rivers included the Bawang, Heihe, Shitou, and Xushui Rivers. They were analyzed by UV-Vis spectral and three-dimensional fluorescence spectroscopy, combined with a parallel factor analysis model and redundant analysis. The results showed that CDOM in the water of the Taibaishan Nature Reserve contained two types of four fluorescent components, in which the humus-like components C1 and C2 were the main components of CDOM. The relative contribution of C1 and C2 to the rivers was 82%-96%, which was significantly larger than the lakes. All fluorescence indexes (FI) were larger than 1.8, the index of recent autochthonous contribution (BIX), and the index of freshness (β:α) were all approximately 0.6. The humification indexes (HIX) of the rivers were significantly larger than those of the lakes (P<0.01). The DOM in the rivers was mainly from the forest soil of the Nature Reserve, and the water quality of the lakes was affected by tourists to some extent. The results of the redundant analysis show that the CDOM spectral characteristic parameters were significantly influenced by EC for the lakes (P<0.05) and by EC, DTN, and DOC for the rivers (P<0.01).

Characteristics of the chromophoric dissolved organic matter of urban black-odor rivers using fluorescence and UV-visible spectroscopy.

Urban black-odor water (BOW) is a typical phenomenon seen in the urban water environment; it is caused by excessive pollution by organic matter and other pollutants, such as nitrogen and phosphorous. Chromophoric dissolved organic matter (CDOM) is a major optical fraction of dissolved organic matter. In this study, optical properties and components of CDOM were obtained from 178 river samples collected from five cities in China, the sample were investigated using absorption and fluorescence spectroscopy. The collected included 89 ordinary water (OW) samples, 63 mild BOW (MBOW), and 26 heavy BOW (HBOW) samples. Significant differences were found in the absorption spectra of the HBOW, MBOW, and OW samples, particularly in their optical parameters (the slope of the spectrum (S275-295), and the ratio of two absorption coefficients of CDOM (E2:E3)). Additionally, the fluorescence intensity of the humic acid-like component (F5) and soluble microbial by product-like component (F4) obtained via the fluorescence regional integration (FRI) method were 3 and 4.2 times higher in HBOW than in OW, respectively; this could be used as an indicator to distinguish OW from BOW in urban rivers. The results obtained using the redundancy method and the strong negative correlation between F4 and dissolved oxygen (DO) (r=- 0.56) suggested that the composition of CDOM could change significantly under different urban water environments (p<0.01). Different correlations were also found between F5, and a355, E2:E3, S275-295 in different BOW levels, suggesting that the optical parameters of CDOM were mainly determined by the polluted organic matter originating from terrestrial sources with large molecular humic acid-like compounds; optical parameter a355 could distinguish BOW from OW. These findings are conducive in understanding the dynamics of organic matter pollution and to discover the composition and optical properties of the CDOM in urban BOW and OW, thereby providing an effective method for tracking the spatial characteristics of BOW in urban rivers using remote sensing technologies in areas with multiple sources of pollution.

Indirect photodegradation of sulfathiazole and sulfamerazine: Influence of the CDOM components and seawater factors (salinity, pH, nitrate and bicarbonate)

Sulfonamides (SAs) are ubiquitous antibiotics that are increasingly detected in the aquatic environment, and may cause potential harm to the environment and humans. Indirect photodegradation has been considered to be a promising natural degradation process for antibiotics in the environment. Chromophoric dissolved organic matter (CDOM) is an important participant in the indirect photodegradation of antibiotics. Indirect photodegradation of sulfathiazole (ST) and sulfamerazine (SM) were studied in the presence of CDOM and marine factors (salinity, pH, nitrate (NO3−) and bicarbonate (HCO3−)) to simulate photodegradation of these compounds in the coastal seawater environment. The main findings are as follows. First, the indirect photodegradation rates of ST and SM in the presence of CDOM were significantly increased and followed the pseudofirst order kinetics. Second, 1O2 played a critical role in the indirect photodegradation of ST and its contribution rate was 54.2%; 3CDOM⁎ performed similarly in the case of SM with a 58.0% contribution rate. Third, CDOM was divided into four fluorescent components by excitation-emission matrix spectroscopy and parallel factor analysis (EEMs-PARAFAC), including three exogenous components and an autochthonous component. The exogenous components with high molecular weight and higher number of aromatic groups played a decisive role in the indirect photodegradation of ST and SM due to their ability to generate higher levels of reactive intermediates (RIs). Finally, seawater factors (salinity, pH, NO3− and HCO3−) influenced the indirect photodegradation of ST and SM by influencing the steady-state concentrations of RIs. This report is the first study of indirect photodegradation of ST and SM from the perspective of the CDOM components and simulated coastal waters.

Absorption characteristics of CDOM in treated and non-treated urban lakes in Changchun, China

In urban settings, one may find (i) lakes that are non-treated (NT) and impacted by recurrent discharges of pollutants and nutrients, and (ii) lakes that, through restoration measures and active management, are treated (T) from external inputs. The optical properties of chromophoric dissolved organic matter (CDOM) have been used to assess the anthropogenic impact on lakes ecology, but their application in comparative assessments of urban lakes has not been attempted. For 2 years, we measured nutrients and CDOM properties in water samples collected from NT and T lakes in the city of Changchun, China. Significant differences in CDOM properties were found between the two types of lakes, and these results were supported by redundancy analysis. The NT lakes were eutrophic while the T lakes were mesotrophic, with mean trophic status index (TSI) of 74.2 and 50.3, respectively. The CDOM absorption coefficient at 350 nm, a(350), was 2-fold higher in NT than in T lakes (6.59 vs 3.21 m-1). In the NT lakes, CDOM components predominantly comprised large molecular weight (MW > 1000-Da) humus-like substances of allochthonous origin, whereas in the T lakes CDOM was dominated by low MW (<1000-Da) substances from autochthonous production. Seasonal fluctuation has a great influence on the CDOM concentration, but a little influence on its molecular composition. The CDOM concentration were higher in summer than in other seasons. Weather conditions (rainfall, temperature) and biophysical processes (biodegradation, photo-bleaching) likely contributed to these variations. We found the water quality of the treated lakes was getting better from 2016 to 2018. In summary, the study results, not only revealed seasonal effects, but most importantly documented the impact of human activities on the characteristics of CDOM in urban lakes. Most specifically, the sharp difference between the lakes in regard to a(350) (2-fold lower in T than in NT lakes) demonstrated the suitability CDOM absorption coefficient as an early indicator of the impact of treatment measures on the hydrochemistry of DOM in urban lakes.

Linking chromophoric organic matter transformation with biomarker indices in a marine phytoplankton growth and degradation experiment

The production and transformation of marine chromophoric dissolved organic matter (CDOM) provides a window into the marine biological pump as it is present at all depths and can be measured both in the field and via satellite. However, outside of lignin for terrestrial DOM, few studies have linked marine CDOM characteristics with biomarker indices. In this study, we quantified five fluorescent components of marine CDOM and base-extractable particulate organic matter (BEPOM) in a growth and degradation experiment using a natural plankton assemblage, and compared those results to bacterial abundances, hydrolytic enzyme activities, and amino acid concentrations and associated diagenetic indices. Rotating glass bottles containing plankton were sampled initially (day 0), during the mid-exponential (day 13) and stationary (day 20) growth phases, and again following a dark degradation period that lasted 42 days. Protein-like fluorescence (tryptophan-like and tyrosine-like) was correlated with the total amino acid concentrations for both the DOM and BEPOM through all phases of the incubation. However, tryptophan-like fluorescence showed a stronger correlation for aromatic amino acids. The concentration of particulate organic carbon changed significantly during each phase of the experiment and this substrate correlated with hydrolytic enzyme activities and bacterial abundance. This heterotrophy diagenetically altered the POM during the stationary phase and ultimately resulted in the increased production of more humic-like CDOM after degradation in the dark. Results from this study indicate that CDOM formation and cycling may play a prominent role in the ocean's nitrogen cycle.

Linking heterotrophic bacterioplankton community composition to the optical dynamics of dissolved organic matter in a large eutrophic Chinese lake

Elucidation of the linkages between the bacterial community composition and chromophoric dissolved organic matter (CDOM) in lake ecosystems is critical for the understanding of the inland water carbon cycling. Despite substantial research into the relationship between the bacteria community and the bulk DOM pool, knowledge of the specific relationship between the optical dynamics of DOM and the bacterioplankton community in lake ecosystems is still poor. We investigated the linkages between the optical dynamics of DOM and bacteria composition in shallow eutrophic Lake Taihu, China. Redundancy Analysis (RDA) indicated that besides water temperature and phytoplankton biomass, also CDOM was an important factor determining the composition of the bacterial community. Generalized Additive Models (GAM) showed that terrestrial humic-like C1 and tyrosine-like C4 were the key factors explaining the abundance of the main bacterial clades. C1 was closely correlated with Verrucomicrobia, Actinobacteria, Alphaproteobacteria, Betaproteobacteria and Planctomycetes, and C4 was closely related to the latter two and to Bacteroidetes. At family level, the dominant families – Pelagibacteraceae (Alphaproteobacteria) and Gemmataceae (Planctomycetes) – were related to both allochthonous and autochthonous CDOM fluorophores but responded differently to the various CDOM components. Tryptophan-like C2 was significantly and positively correlated with Gemmataceae and Ellin6075 (Acidobacteria). Additionally, we found that the biomasses of Cyanophyta, terrestrial humic-like C1, tryptophan-like C4 and C5 were significantly related to the richness of heterotrophic bacterioplankton. Our results provide new insight into the relationship between bacteria and DOM optical dynamics although the mechanisms leading to these relationships need further experimental investigations.

Response of Chromophoric Dissolved Organic Matter Composition to Different Hydrological Scenarios in Large Eutrophic Lake Taihu

Chromophoric dissolved organic matter (CDOM) is a fraction of dissolved organic matter that can strongly absorb light in the ultraviolet and blue regions and plays an important role in the biogeochemical cycling of carbon, nitrogen, and phosphorus. Unraveling the sources, optical composition, and corresponding spatial variabilities of CDOM can improve our understanding of carbon, nitrogen, and phosphorus cycling in lakes and lake water quality management. CDOM spectral absorption and fluorescent excitation-emission matrices were measured to investigate the compositional dynamics of CDOM under different hydrological scenarios. Our results showed that the mean value of dissolved organic carbon (DOC) concentration of (8.11±1.26) mg·L-1 in the rainy season was significantly higher than that in the dry season (3.53±1.19) mg·L-1 (t-test, P<0.01), whereas the mean spectral slope S275-295 of (20.89±1.90) μm-1 in the dry season was significantly greater than that in the rainy season (19.09±1.81) μm-1 (t-test, P<0.001). Three fluorescent components were identified using parallel factor analysis, and we further found that the dynamics of the three CDOM components were strongly influenced by hydrological conditions. Fluorescence intensity (Fmax) of the terrestrial humic-rich component C2 increased with increasing water levels and rainfall. Significant negative relationships were found between all three fluorescent components and dissolved oxygen (P<0.01), suggesting that all three components served as important substrates for microbial processing. Significant positive relationships were found between the terrestrial humic-rich C2 and tryptophan-like C1 and chlorophyll-a and chemical oxygen demand, indicating that anthropogenic inputs and algal degradation contributed significantly to the CDOM pool in Lake Taihu. We further found a significant positive relationship between DOC concentration and Fmax of terrestrial humic-rich C2 (r2=0.58, P<0.001), suggesting that DOC in Lake Taihu was primarily derived from allochthonous input.

Composition, Spatial Distribution Characteristics and Source Analysis of Chromophoric Dissolved Organic Matter in the Lanzhou Reach of the Yellow River

Chromophoric dissolved organic matter (CDOM) in riverine systems can be affected by environmental conditions and land-use, and can thus provide important information regarding anthropogenic activities in surrounding landscapes. It can modify the optical properties of waters and affect the balance and availability of dissolved nutrients and metals in water bodies. However, the characteristics of CDOM in the Lanzhou reach of the Yellow River have not yet been reported. In this study, the optical properties of water samples collected at 32 locations during April 2017 across the Lanzhou reach of the Yellow River were examined using UV-VIS and excitation-emission matrix fluorescence spectroscopy-parallel factor analysis (EEM-PARAFAC), to determine CDOM compositional changes, spatial distribution characteristics, and sources. Cluster analysis was used to categorize samples into groups of similar pollution levels within a study area. Results showed that CDOM was primarily comprised of low molecular weight organic substances with aromatic structure belonging to complex "protein-like-humic-like" substances, and dominated by protein-like substances (organism sources). Two humic-like components (C1, C4), one tryptophan-like component (C2), and one non-humic-like component (C3) were identified by PARAFAC. Tryptophan-like substances were predominant in the components of CDOM, accounting for 51.06% of average total fluorescence intensity. Humic-like materials and non-humic-like substances accounted for 36.74% and 12.20%, respectively. Weak correlations were observed between protein-like substances and humic-like substances, indicating different sources of these components. The distribution of total fluorescence intensity showed a distinct spatial pattern; trends in fluorescence intensity were weak-strong-weak along an upstream to downstream continuum, mainly affected by changes in the content of protein-like substances. The spatial variation of the CDOM in the Lanzhou reach of the Yellow River can therefore be assessed based on protein-like materials dynamics. Public spaces along rivers offer opportunities for community gatherings and recreational activities. However, high-intensity anthropogenic activities strongly influence CDOM concentration and composition in this area in different ways; sources include increased residential/commercial wastewater, catering, water recreation facilities pollution, shipping, and a small amount of industrial discharge. In addition, it was concluded that endogenetic pollution may become the main source of internal loading in the Lanzhou reach of the Yellow River, implying that stronger endogenetic pollution control is needed to alleviate CDOM pollution and improve water quality.

Evaluation of CDOM sources and their links with antibiotics in the rivers dividing China and North Korea using fluorescence spectroscopy.

Rivers act as carriers where active cycling of chromophoric dissolved organic matter (CDOM) affected by anthropogenic activities and land cover variation may occur. Little is known about the optical properties of CDOM in relation to antibiotics; these properties could provide valuable insights into CDOM transformation processes and biogeochemical reactivity. The spatial distribution of CDOM absorption, fluorescence intensities of CDOM components, and correlations between fluorescence indices (FI370, humification index (HIX)) and biological index (BIX)) with water quality and antibiotics were investigated in three rivers, namely, two rivers (Yalu River and Tumen River) dividing China and North Korea, and the Songhua River. Four humic-like components (C1 and C3-5), and one tryptophan-like component (C2), were identified via fluorescence excitation-emission matrices-parallel factor analysis (EEM-PARAFAC). The correlation between Fmax for five EEM-PARAFAC extracted CDOM components, C1-C5, FI370, HIX, BIX, and water quality parameters, and four antibiotics (i.e., Norfloxacin, Enrofloxacin, Sulfamethoxazole and Metronidazole), were determined through a redundancy analysis (RDA), with species-environment correlations of 0.887 and 0.833, respectively. The results showed that spatial variation in land cover, pollution sources, and terrestrial contribution in water quality affected Fmax for the fluorescent components C1-C5 and the fluorescence indices, indicating a high diverse chemical composition and transformation history. The Fmax for terrestrial humic-like components, C4 and C5, showed spatial variation depending on land cover and anthropogenic impacts. Further correlation and regression analyses indicated that CDOM soil fulvic-like component C5 correlated with Sulfamethoxazole and Metronidazole (t test, p < 0.01). Our results indicate that the spatial distributions of Fmax for CDOM fluorescent terrestrial components, evaluated by EEM-PARAFAC, have potential implications for the monitoring of Sulfamethoxazole and Metronidazole in surface waters. Further, these findings can be used to understand the biogeochemical cycling of CDOM and its effects on antibiotics pollution in the environment.

Dissolved Organic Matter (DOM) in the open Mediterranean Sea. I. Basin–wide distribution and drivers of chromophoric DOM

Chromophoric dissolved organic matter (CDOM) in the open Mediterranean Sea (MedSea) is barely documented, remaining the basin–wide patterns in intermediate and deep waters still enigmatic. Here, full–depth distributions of CDOM absorption coefficients and spectral slopes recorded during the HOTMIX 2014 cruise are presented and their respective environmental drivers resolved. General Additive Models (GAMs) in surface waters and Optimum MultiParameter (OMP) water mass analysis in deep waters were applied. In the surface, apparent oxygen utilisation (AOU), a proxy to cumulative net community respiration, explained most of the variability of dissolved organic carbon (DOC) and the absorption coefficient at 254 nm (a254), whereas the absorption coefficient at 325 nm (a325), and the spectral slopes were mostly explained by potential temperature, a proxy to stratification and solar radiation, indicating that both water column stability and photobleaching may drive the variability of the UV–A absorbing CDOM components. In deep waters, the effect of water mass mixing and basin–scale mineralization were discerned from local mineralization processes. Water mass mixing and basin–scale mineralization contributed more substantially to explain the variability of DOC, a254 and a325 (82–91%) than the variability of the spectral slopes (35–64%). Local mineralization processes indicate that DOC and CDOM play a more relevant role in the carbon cycle in the Eastern (EastMed) than in the Western (WestMed) Mediterranean: whereas DOC contributed to 66 ± 10% of the oxygen demand in the EastMed, it represented only 24 ± 4% in the WestMed. Independently of basins and layers, a254 revealed as an excellent proxy to the concentration of DOC in the MedSea. Also, the unexpected inverse relationship of a325 with AOU indicates that the consumption of the UV–A absorbing CDOM fraction prevails over their production.