Mercury (II) Adsorption on Three Contrasting Chinese Soils Treated with Two Sources of Dissolved Organic Matter: II. Spectroscopic Characterization

To reveal the influencing mechanism of dissolved organic matter (DOM) on mercury (Hg II) adsorption by black, red, and fluvo-aquic soils in China, Fourier transform infrared (FTIR) spectroscopy, ¹³C nuclear magnetic resonance (NMR) spectroscopy, and three-dimensional excitation emission matrix (3DEEM) fluorescence spectroscopy were employed to characterize the DOM samples and DOM-Hg complexes. FTIR spectra showed that the complexation of Hg (II) mainly acted on the C=O, COO^?, and O-H groups of DOM from swine manure (DOMs) and wheat straw (DOMw). The NMR spectra indicated that the complex reaction of Hg (II) and DOM corresponded with the change in carboxyl C. The NMR results also showed that the dominant C components in DOM were aromatic C, O-alkyl C, alkyl C, and carboxyl C, and that DOMw imposed more influence on Hg (II) adsorption than DOMs, which was consistent with that of FTIR spectroscopy. The 3DEEM showed that DOM contained both aromatic protein-like and fulvic-like substances, and that the protein-like properties of DOMs and UV fulvic-like fluorescence substances of DOMw can better participate in the formation of Hg complexes. This result provides strong direct evidence to elucidate the DOM-Hg (II) binding mechanism, and further interprets the effect mechanism of exogenous DOM on Hg adsorption by soil.     

Three-dimensional fluorescence as a tool for investigating the dynamics of dissolved organic matter in the Lake Biwa watershed

Quantitative and qualitative characterizations of dissolved organic matter (DOM) were carried out at the watershed level in central Japan by measuring dissolved organic carbon (DOC) concentration and the three-dimensional excitation–emission matrix (3-D EEM). DOC concentration was low (mean 37 ± 19 µM C) in the upstream waters, whereas, in general, it increased toward the downstream areas (mean 92 ± 47 µM C). Significant variations in DOC concentration were detected among rivers and channels. DOC concentration in the epilimnion of Lake Biwa increased during the summer period and decreased during the winter period. The lake hypolimnion has lower DOC concentration (mean 87 ± 7 µM C) compared with the epilimnion (107 ± 15 µM C). Fulvic acid (FA)-like substances in the DOM were directly characterized by 3-D EEM. The fluorescence peak for upstream DOM was found in regions with longer wavelengths (excitation/emission 386 ± 6/476 ± 5 nm) compared with downstream and lake DOM (351 ± 12/446 ± 15 nm and 341 ± 6/434 ± 6 nm, respectively). The DOC concentration is correlated with fluorescence peak intensity of FA-like substances in DOM in river waters. Such a relationship was not found in lake DOM. A blueshift of the fluorescence peak from upstream to lake DOM was observed. A decrease in fluorescence intensities was also detected during the summer period. These results may suggest that the degradation of FA-like substances in DOM occurs from natural solar irradiation. Protein-like fluorescence was significantly detected in the lake epilimnion during the summer period. A linear relationship between DOC concentration and protein-like fluorescence indicated that an autochthonous input of DOM gave rise to the increase in DOC concentration in the lake epilimnion during the summer. These results may suggest that the 3-D EEM can be used as a tool for the investigation of DOM dynamics at the watershed level with concurrent measurement of DOC concentration and the fluorescence properties of fulvic acid-like and protein-like substances.     

Polarized site-selective fluorescence spectroscopy of the long-wavelength emitting chlorophylls in isolated Photosystem I particles of Synechococcus elongatus

Isolated trimeric Photosystem I complexes of the cyanobacterium Synechococcus elongatus have been studied with absorption spectroscopy and site-selective polarized fluorescence spectroscopy at cryogenic temperatures. The 4 K absorption spectrum exhibits a clear and distinct peak at 710 nm and shoulders near 720, 698 and 692 nm apart from the strong absorption profile located at 680 nm. Deconvoluting the 4 K absorption spectrum with Gaussian components revealed that Synechococcus elongatus contains two types of long-wavelength pigments peaking at 708 nm and 719 nm, which we denoted C-708 and C-719, respectively. An estimate of the oscillator strengths revealed that Synechococcus elongatus contains about 4–5 C-708 pigments and 5–6 C-719 pigments. At 4 K and for excitation wavelengths shorter than 712 nm, the emission maximum appeared at 731 nm. For excitation wavelengths longer than 712 nm, the emission maximum shifted to the red, and for excitation in the far red edge of the absorption spectrum the emission maximum was observed 10–11 nm to the red with respect to the excitation wavelength, which indicates that the Stokes shift of C-719 is 10–11 nm. The fluorescence anisotropy, as calculated in the emission maximum, reached a maximal anisotropy of r=0.35 for excitation in the far red edge of the absorption spectrum (at and above 730 nm), and showed a complicated behavior for excitation at shorter wavelengths. The results suggest efficient energy transfer routes between C-708 and C-719 pigments and also among the C-719 pigments.Abbreviations Chl chlorophyll - FWHM full width at half maximum - PS I Photosystem I     

Assignment of the low-temperature fluorescence in oxygen-evolving Photosystem II

Low-temperature absorption and fluorescence spectra of fully active cores and membrane-bound PS II preparations are compared. Detailed temperature dependence of fluorescence spectra between 5 and 70 K are presented as well as 1.7-K fluorescence line-narrowed (FLN) spectra of cores, confirming that PS II emission is composite. Spectra are compared to those reported for LHCII, CP43, CP47 and D1/D2/cytit b₅₅₉ subunits of PS II. A combination of subunit spectra cannot account for emission of active PS II. The complex temperature dependence of PS II fluorescence is interpretable by noting that excitation transfer from CP43 and CP47 to the reaction centre is slow, and strongly dependent on the precise energy at which a ‘slow-transfer’ pigment in CP43 or CP47 is located within its inhomogeneous distribution. PS II fluorescence arises from CP43 and CP47 ‘slow-transfer’ states, convolved by this dependence. At higher temperatures, thermally activated excitation transfer to the PS II charge-separating system bypasses such bottlenecks. As the charge-separating state of active PS II absorbs at >700 nm, PS II emission in the 680–700 nm region is unlikely to arise from reaction centre pigments. PS II emission at physiological temperatures is discussed in terms of these results.     

Seasonal changes in the chemical quality and biodegradability of dissolved organic matter exported from soils to streams in coastal temperate rainforest watersheds

The composition and biodegradability of streamwater dissolved organic matter (DOM) varies with source material and degree of transformation. We combined PARAFAC modeling of fluorescence excitation–emission spectroscopy and biodegradable dissolved organic carbon (BDOC) incubations to investigate seasonal changes in the lability of DOM along a soil-stream continuum in three soil types: bog, forested wetland and upland forest. The percent BDOC ranged from 7 to 38% across all sites, and was significantly greater in soil compared to streamwater in the bog and forested wetland, but not in the upland forest. The percent BDOC also varied significantly over the entire sampling period in soil and streamwater for the bog and forested wetland, as BDOC peaked during the spring runoff and was lowest during the summer months. Moreover, the chemical quality of DOM in wetland soil and streamwater was similar during the spring runoff and fall wet season, as demonstrated by the similar contribution of protein-like fluorescence (sum of tyrosine and tryptophan fluorescence) in soil water and in streams. These findings suggest that the tight coupling between terrestrial and aquatic ecosystems is responsible for the delivery of labile DOM from wetland soils to streams. The contribution of protein-like fluorescence was significantly correlated with BDOC (p < 0.001) over the entire sampling period indicating DOM is an important source of C and N for heterotrophic microbes. Taken together, our findings suggest that the production of protein-rich, labile DOM and subsequent loss in stream runoff might be an important loss of labile C and N from coastal temperate watersheds.     

Use of ESI-FTICR-MS to Characterize Dissolved Organic Matter in Headwater Streams Draining Forest-Dominated and Pasture-Dominated Watersheds

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) has proven to be a powerful technique revealing complexity and diversity of natural DOM molecules, but its application to DOM analysis in grazing-impacted agricultural systems remains scarce. In the present study, we presented a case study of using ESI-FTICR-MS in analyzing DOM from four headwater streams draining forest- or pasture-dominated watersheds in Virginia, USA. In all samples, most formulas were CHO compounds (71.8–87.9%), with other molecular series (CHOS, CHON, CHONS, and CHOP (N, S)) accounting for only minor fractions. All samples were dominated by molecules falling in the lignin-like region (H/C = 0.7–1.5, O/C = 0.1–0.67), suggesting the predominance of allochthonous, terrestrial plant-derived DOM. Relative to the two pasture streams, DOM formulas in the two forest streams were more similar, based on Jaccard similarity coefficients and nonmetric multidimensional scaling calculated from Bray-Curtis distance. Formulas from the pasture streams were characterized by lower proportions of aromatic formulas and lower unsaturation, suggesting that the allochthonous versus autochthonous contributions of organic matter to streams were modified by pasture land use. The number of condensed aromatic structures (CAS) was higher for the forest streams, which is possibly due to the controlled burning in the forest-dominated watersheds and suggests that black carbon was mobilized from soils to streams. During 15-day biodegradation experiments, DOM from the two pasture streams was altered to a greater extent than DOM from the forest streams, with formulas with H/C and O/C ranges similar to protein (H/C = 1.5–2.2, O/C = 0.3–0.67), lipid (H/C = 1.5–2.0, O/C = 0–0.3), and unsaturated hydrocarbon (H/C = 0.7–1.5, O/C = 0–0.1) being the most bioreactive groups. Aromatic compound formulas including CAS were preferentially removed during combined light+bacterial incubations, supporting the contention that black carbon is labile to light alterations. Collectively, our data demonstrate that headwater DOM composition contains integrative information on watershed sources and processes, and the application of ESI-FTICR-MS technique offers additional insights into compound composition and reactivity unrevealed by fluorescence and stable carbon isotopic measurements.     

Distribution of mercury, methyl mercury and organic sulphur species in soil, soil solution and stream of a boreal forest catchment

Concentrations of methyl mercury, CH₃Hg (II), total mercury, Hg_tot = CH₃Hg (II) + Hg (II), and organic sulphur species were determined in soils, soil solutions and streams of a small (50 ha) boreal forest catchment in northern Sweden. The CH₃Hg (II)/Hg_tot ratio decreased from 1.2–17.2% in the peaty stream bank soils to 0.4–0.8% in mineral and peat soils 20 m away from the streams, indicating that conditions for net methylation of Hg (II) are most favourable in the riparian zone close to streams. Concentrations of CH₃Hg (II) bound in soil and in soil solution were significantly, positively correlated to the concentration of Hg_tot in soil solution. This, and the fact that the CH₃Hg (II)/Hg_tot ratio was higher in soil solution than in soil may indicate that Hg (II) in soil solution is more available for methylation processes than soil bound Hg (II). Reduced organic S functional groups (Org-S_RED) in soil, soil extract and in samples of organic substances from streams were quantified using S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Org-S_RED, likely representing RSH, RSSH, RSR and RSSR functionalities, made up 50 to 78% of total S in all samples examined. Inorganic sulphide [e.g. FeS₂ (s)] was only detected in one soil sample out of 10, and in none of the stream samples. Model calculations showed that under oxic conditions nearly 100% of Hg (II) and CH₃Hg (II) were complexed by thiol groups (RSH) in the soil, soil solution and in the stream water. Concentrations of free CH₃Hg⁺ and Hg²⁺ ions in soil solution and stream were on the order of 10^–18 and 10^–32M, respectively, at pH 5. For CH₃Hg (II), inorganic bi-sulphide complexes may contribute to an overall solubility at concentrations of inorganic sulphides higher than 10^–9M, whereas considerably higher concentrations of inorganic sulphides (lower redox-potential) are required to increase the solubility of Hg (II).     

Release and aggregation of the light-harvesting complex in intact leaves subjected to strong CO2 deficit

Tobacco plants were subjected to long-term CO₂ deficit. The stress caused photoinhibition of Photosystem (PS) II photochemistry and the aggregation of the light-harvesting complex of PS II (LHC II). The aggregation was shown by the appearance of the characteristic band at 698–700 nm (F₆₉₉) in 77 K fluorescence emission spectra. LHC II aggregates are considered to quench fluorescence and, therefore, the fluorescence yield was determined to verify their quenching capability. PS II photochemistry, measured as F_V/F_M, was largely depressed during first 4 days of the stress. Unexpectedly, the total fluorescence yield increased in this period. Fitting of emission spectra by Gaussian components approximating emission bands of LHC II, PS II core, PS I and F₆₉₉ revealed that mainly the bands at 680 and 699 nm, representing emission of LHC II aggregates, were responsible for the increase of the fluorescence yield. This shows an interruption of the excitation energy transfer between LHC II and both photosystems and, thus, a physical disconnection of LHC II from photosystems. PS II and PS I emissions were not quenched in this period. Therefore, it was concluded that these LHC II aggregates were accumulated out of PS II antenna, and, thus they cannot be involved in dumping of excess excitation. The total fluorescence yield turned to decrease only after the large depression of PS II photochemistry, when LHC II aggregation was considerably speeded up and the fluorescence yields of PS I and II turned to decline.     

Temperature dependence and polarization of fluorescence from Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803

To determine the fluorescence properties of cyanobacterial Photosystem I (PS I) in relatively intact systems, fluorescence emission from 20 to 295 K and polarization at 77 K have been measured from phycobilisomes-less thylakoids of Synechocystis sp. PCC 6803 and a mutant strain lacking Photosystem II (PS II). At 295 K, the fluorescence maxima are 686 nm in the wild type from PS I and PS II and at 688 nm from PS I in the mutant. This emission is characteristic of bulk antenna chlorophylls (Chls). The 690-nm fluorescence component of PS I is temperature independent. For wild-type and mutant, 725-nm fluorescence increases by a factor of at least 40 from 295 to 20 K. We model this temperature dependence assuming a small number of Chls within PS I, emitting at 725 nm, with an energy level below that of the reaction center, P700. Their excitation transfer rate to P700 decreases with decreasing temperature increasing the yield of 725-nm fluorescence.Fluorescence excitation spectra of polarized emission from low-energy Chls were measured at 77 and 295 K on the mutant lacking PS II. At excitation wavelengths longer than 715 nm, 760-nm emission is highly polarized indicating either direct excitation of the emitting Chls with no participation in excitation transfer or total alignment of the chromophores. Fluorescence at 760 nm is unpolarized for excitation wavelengths shorter than 690 nm, inferring excitation transfer between Chls before 760-nm fluorescence occurs.Our measurements illustrate that: 1) a single group of low-energy Chls (F725) of the core-like PS I complex in cyanobacteria shows a strongly temperature-dependent fluorescence and, when directly excited, nearly complete fluorescence polarization, 2) these properties are not the result of detergent-induced artifacts as we are examining intact PS I within the thylakoid membrane of S. 6803, and 3) the activation energy for excitation transfer from F725 Chls to P700 is less than that of F735 Chls in green plants; F725 Chls may act as a sink to locate excitations near P700 in PS I.Abbreviations Chl chlorophyll - BChl bacteriochlorophyll - PS Photosystem - S. 6803 Synechocystis sp. PCC 6803 - PGP potassium glycerol phosphate     

High-performance liquid chromatographic assay for amiloride in plasma and urine

A sensitive and simplified high-performance liquid chromatographic procedure has been developed for quantification of amiloride in rabbit plasma, as well as human plasma and urine. Following protein precipitation with perchloric acid, the supernatant was directly injected into a C₁₈ Nucleosil column. The mobile phase consisted of methanol—water (45:55) containing 0.1 M perchloric acid, and the compound was quantitated using a fluorescence detector at excitation and emission wavelengths of 286 and 418 nm, respectively. The average recovery was 97.6%. The calibration curve was linear over the range 2.0–20.0 ng/ml. The limit of detection was 0.5 ng/ml.     

Degradation of surface-water dissolved organic matter: influences of DOM chemical characteristics and microbial populations

The degree to which biodegradation of dissolved organic matter (DOM) depends on microbial community structure and source remains unknown. In this study, we concentrated the microbial biomass from two streams in northern Michigan and a dystrophic bog lake in northern Wisconsin with varying initial DOM concentration (6.7–78.8 mg C l^–1) and DOM chemical characteristics (e.g. DOM average molecular weights from 808–1887 Da). Each of the three microbial inocula was added to each of the three DOM sources at in situ population levels for a total of nine treatments. Changes in DOM concentration and bacterial productivity, along with chemical characteristics, were examined over 308 h. The [³H]-leucine incorporation method was used to measure microbial production. In two of three sampling sites, bacterial communities were most productive when metabolizing DOM in their native waters. A variable peak in productivity was seen between 16–48 h after inoculation, followed by a drop in productivity in most treatments, with periods of DOM production most likely due to microbial turnover. These data suggest that microbial communities are better able to degrade the DOM of their native habitats, suggesting that biodegradation of DOM is influenced by source-specific microbial species and DOM chemical characteristics.     

Low-Intensity picosecond fluorescence kinetics and excitation dynamics in barley chloroplasts

The fluorescence decays of barley chloroplasts have been measured by single-photon counting with tunable picosecond dye laser excitation. The fluorescence decays of dark-adapted chloroplasts are best fitted to a sum of three exponential lifetime components with lifetimes of 112, 380 and 2214 ps. The relative magnitude of each component is shown to be dependent on the excitation wavelength and collected emission wavelength. The excitation wavelength dependence is correlated with the Photosystem (PS) I and PS II action study of Ried [36] and with the measured pigment distributions in the photosynthetic unit [37,41]. Experiments varying the single excitation pulse intensity from 10⁸ to 10¹² photons/cm² pulse show that our results are not distorted by singlet-singlet annihilation. Unflowed samples where the cloroplasts are under constant illumination show 2-fold increases in quantum yield of fluorescence primarily in the two longer lifetime components. Theoretical calculations of Shipman [31] on an isolated reaction center with a homogeneous antenna are discussed and the principles extended to discussion of the measured barley chloroplast fluorescence decay components in terms of photosynthetic unit light-harvesting array models and earlier experimental work. Our data support a photosynthetic unit model in which 70–90% of the photons absorbed are quenched by either PS I or efficiently quenching PS II in a process where the fluorescence lifetime is 100 ps. The origin of the intermediate 380 ps. component is probably due to excitation transfer to a PS II reaction center in a redox state which quenches less efficiently.     

High-performance liquid chromatography of methamphetamine and its related compounds in human urine following derivatization with fluorescein isothiocyanate

A high-performance liquid chromatographic method with fluorescence detection for the determination of methamphetamine and its related compounds is reported. Methamphetamine, amphetamine, norephedrine, p-hydroxymethamphetamine and 1-phenylethylamine as an internal standard were extracted from human urine, derivatized with fluorescein-4-isothiocyanate, and then separated on a reversed-phase column within 36 min. The fluorescence intensity of the effluent was monitored at excitation and emission wavelengths of 496 and 518 nm, respectively. Calibration curves were confirmed to be linear up to at least 100 pmol on the column with a correlation coefficient (r) of 0.994–0.999 for the target compounds. The detection limits (S/N=3) were 55–105 fmol per 20-μl injection. The method was successfully applied to urine samples taken from methamphetamine addicts.     

The far red induced slow component of delayed light from chloroplasts is emitted from Photosystem II

In spinach chloroplasts illuminated with far red light, the relative intensity maximum during the decay of delayed light is emitted at 680–690 nm. This finding supports previous models predicting emission from Photosystem II, and contradicts earlier attributions to Photosystem I.Due to self absorption, the emission spectrum of the relative maximum is shifted to longer wavelengths and displays apparent Photosystem I characteristics in chloroplast samples of higher concentration or in leaves. This may have caused earlier investigators to ascribe the emission to Photosystem I.A differences between the spectral width of the emission spectra of delayed fluorescence and the relative maximum indicates that these two phenomena represent emission from different sub-populations of Photosystem II centers.Abbreviations PS I Photosystem I - PS II Photosystem II - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea     

Fluorescence spectroscopic characterization of dissolved organic matter in the waters of Lake Fuxian and adjacent rivers in Yunnan,China

The fluorescence properties of dissolved organic matter (DOM) in the water of Lake Fuxian and its adjacent rivers on the Yunnan Plateau, southwestern China, were studied to specify the characterization of DOM in the lake and river waters. The fluorescence properties with the excitation–emission matrix in the water of Lake Fuxian are different from those in the river water. The differences in these properties between the lake and river water could arise not only from their sources but also from the reactivity of the photobleaching of DOM. In the lake, the supplying of allochthonous fluorescent materials from inflowing rivers to the fluorescent DOM is less significant than the photobleaching of fluorescent substances.     

Confocal imaging of transmembrane voltage by SEER of di-8-ANEPPS

Imaging, optical mapping, and optical multisite recording of transmembrane potential (V_m) are essential for studying excitable cells and systems. The naphthylstyryl voltage-sensitive dyes, including di-8-ANEPPS, shift both their fluorescence excitation and emission spectra upon changes in V_m. Accordingly, they have been used for monitoring V_m in nonratioing and both emission and excitation ratioing modes. Their changes in fluorescence are usually much less than 10% per 100 mV. Conventional ratioing increases sensitivity to between 3 and 15% per 100 mV. Low sensitivity limits the value of these dyes, especially when imaged with low light systems like confocal scanners. Here we demonstrate the improvement afforded by shifted excitation and emission ratioing (SEER) as applied to imaging membrane potential in flexor digitorum brevis muscle fibers of adult mice. SEER—the ratioing of two images of fluorescence, obtained with different excitation wavelengths in different emission bands—was implemented in two commercial confocal systems. A conventional pinhole scanner, affording optimal setting of emission bands but less than ideal excitation wavelengths, achieved a sensitivity of up to 27% per 100 mV, nearly doubling the value found by conventional ratioing of the same data. A better pair of excitation lights should increase the sensitivity further, to 35% per 100 mV. The maximum acquisition rate with this system was 1 kHz. A fast “slit scanner” increased the effective rate to 8 kHz, but sensitivity was lower. In its high-sensitivity implementation, the technique demonstrated progressive deterioration of action potentials upon fatiguing tetani induced by stimulation patterns at >40 Hz, thereby identifying action potential decay as a contributor to fatigue onset. Using the fast implementation, we could image for the first time an action potential simultaneously at multiple locations along the t-tubule system. These images resolved the radially varying lag associated with propagation at a finite velocity.     

光系统Ⅱ核心复合物的稳态荧光光谱

在83K和160K两个温度下,通过激发波长对荧光发射谱的影响研究了光系统Ⅱ中核心复合物的荧光光谱特性。用不同波长的光激发,核心复合物的发射谱的最大发射峰值不变,用480、489、495和507nm的光分别激发核心复合物,其光谱最大峰值处的荧光强度随不同激发波长下β-胡萝卜素分子的吸收强度的增大而降低,在长波长区域光谱的变化依赖于首先被激发的色素分子。所以,激发波长的不同影响着核心复合物中能量传递的途径。通过高斯解析,分析出核心复合物中至少存在有7组叶绿素a组分,它们是Ch1 a660,Ch1 a670,Ch1 a680,Ch1 a682,Ch1 a684,Ch1 a687和Ch1 a690。     

Effects of desiccation on the excitation energy distribution from phycoerythrin to the two photosystems in the red alga Porphyra perforata

Low temperature (77°K) fluorescence emission and excitation spectra were recorded for wet and desiccated thalli of Porphyra perforata . The photosystem I (F730) and photosystem II (F695) fluorescence emission kinetics during photosystem II trap closure were also recorded at 77°K. Desiccation induced a lowering of the fluorescence yield over the whole emission spectrum but the decrease was most pronounced for the photosystem II fluorescence bands, F688 and F695. It was shown that the desiccation-induced changes of the phycoerythrin sensitized emission spectrum were due to 1) a decrease in the fluorescence yield of the photosystem I antenna, 2) an even stronger decrease in the fluorescence of photosystem II, which was mediated by an increased spillover (k_T(II→I)) of excitation to photosystem I and an increase in the absorption cross section, α, for photosystem I. We hypothesize that the increase of both k_T(II→I) and α are part of a mechanism by which the desiccation-tolerant, high light exposed, Porphyra can avoid photodynamic damage to photosystem II, when photosynthesis becomes inhibited as a result of desiccation during periods of low tide.     

Photooxidation of wetland and riverine dissolved organic matter: altered copper complexation and organic composition

In natural waters, the uptake of transition metals such as copper (Cu) by aquatic biota depends on the activity of the free cupric ion ({Cu²⁺}) rather than on total Cu concentration. Thus, an important ecological function of dissolved organic matter (DOM) in aquatic ecosystems is Cu–DOM complexation, which greatly decreases the {Cu²⁺}. However, Cu bioavailability is greatly modified by source and environmental history of DOM because DOM affinity for Cu varies by orders of magnitude among DOM sources; moreover, DOM is photochemically unstable. During 72-h irradiation experiments at intensities approximating sunlight with DOM from a palustrine wetland and a third-order river, we investigated photooxidative effects on DOM complexation of Cu as well as spectral and chemical changes in DOM that might explain altered Cu complexation. Irradiation decreased Cu complexation by riverine DOM, but unexpectedly increased Cu complexation by wetland DOM, resulting in 150% greater {Cu²⁺} in riverine DOM at the same dissolved organic carbon concentrations. The specific ultraviolet absorption (SUVa) and humic substances tracked photochemical changes in the conditional stability constants of Cu–DOM complexes, suggesting that the aromaticity of DOM influences its affinity for Cu. Carbonyl concentration in ¹³C nuclear magnetic resonance spectra (¹³C-NMR) covaried directly with Cu binding-site densities in DOM. However, no aspect of Cu–DOM complexation consistently covaried with fluorophores (i.e., the fluorescence index) or low molecular weight organic acids. Our results suggest that global increases in UV radiation will affect Cu–DOM complexation and subsequent Cu toxicity depending on light regime as well as DOM source. Handling editor: K. Martens