Contrasting effects of solar radiation and nitrates on the bioavailability of dissolved organic matter to marine bacteria

We evaluated the role of nitrate (NO₃^?) as a potential photosensitizer and the bacterial responses to dissolved organic matter (DOM) phototransformation from coastal waters in the northwestern Mediterranean Sea. In spring, without any addition of NO₃^?, the exposure of 0.2 μm filtered seawater (DOM-solution) to natural solar radiation (i.e. Full Sun [FS], including photosynthetically available [PAR: 400–700 nm], ultraviolet-A [UVAR: 315–400 nm] and ultraviolet-B [UVBR: 280–315 nm] radiations) stimulated bacterial production (BP) and abundance (BA) in natural assemblages (0.8 μm filtered seawater) by 80 and 20% as compared to unexposed (Dark) DOM-solutions, respectively. This stimulation resulted primarily from the exposure to PAR. When NO₃^? (30 μM) was added to DOM-solution before irradiation, BP and BA increased by 150 and 65% in FS compared to Dark, respectively, due to both PAR and UVBR. By contrast, in summer, the exposure of DOM-solution caused a decrease in BP by 30% but an increase in BA by 23% in FS compared to Dark, regardless of the NO₃^? addition before irradiation. The inhibition of BP resulted mainly from UVAR, whereas the stimulation of BA resulted from PAR. These results suggest contrasting effects along seasons of solar radiation and NO₃^? on DOM bioavailability, depending on its initial chemical composition.     

Electrochemical methodology to study labile trace metal/natural organic matter complexation at low concentration levels in natural waters

A new electrochemical methodology to study labile trace metal/natural organic matter complexation at low concentration levels in natural waters is presented. This methodology consists of three steps: (i) an estimation of the complex diffusion coefficient (D_ML), (ii) determination at low pH of the total metal concentration initially present in the sample, (iii) a metal titration at the desired pH. The free and bound metal concentrations are determined for each point of the titration and modeled with the non-ideal competitive adsorption (NICA-Donnan) model in order to obtain the binding parameters. In this methodology, it is recommended to determine the hydrodynamic transport parameter, α, for each set of hydrodynamic conditions used in the voltammetric measurements.The methodology was tested using two fractions of natural organic matter (NOM) isolated from the Loire river, namely the hydrophobic organic matter (HPO) and the transphilic organic matter (TPI), and a well characterized fulvic acid (Laurentian fulvic acid, LFA). The complex diffusion coefficients obtained at pH 5 were 0.4 ± 0.2 for Pb and Cu/HPO, 1.8 ± 0.2 for Pb/TPI and (0.612 ± 0.009) × 10⁻¹⁰ m² s⁻¹ for Pb/LFA. NICA-Donnan parameters for lead binding were obtained for the HPO and TPI fractions. The new lead/LFA results were successfully predicted using parameters derived in our previous work.     

Photodegradation of soil organic matter and its effect on gram-negative bacterial growth

To learn more about the role of the reactive oxygen species (ROS) in the production of bioavailable products of the dissolved organic matter, we investigate here the effect of the photolysis (lambda(exc) > 320 nm) of a soil extract (SE) on the growth of bacteria isolated from the same soil as used for obtaining the extract. Comparative experiments with Aldrich humic acid (AHA) as substrate were performed. The photodegradation of the SE was evaluated with different techniques-UV-visible absorption spectroscopy, fluorescence excitation emission matrices (EEM) and Fourier transform infrared spectroscopy (FTIR). Known ROS scavengers were employed to study the effect of photochemically produced ROS on the photodegradation of the substrates. To evaluate the effect of irradiation on the bioavailability of the SE and AHA, photolyzed and nonphotolyzed substrates were added to different culture media and the growth of Pseudomonas sp. isolated from the soil and a strain of Escherichia coli were studied. The different results obtained were assigned to the dissimilar metabolisms of both bacteria.     

Capillary zone electrophoresis of natural organic matter

This paper presents an in-depth look at the use of capillary electrophoretic (CE) techniques for the fingerprinting and characterization of humic substances and natural organic matter. These materials are highly heterogeneous in structure and show all characteristics of mixtures unliked in analytical chemistry. The electrophoretic approach, however, allows the determination of mobility distributions in different solution conditions, representative of the effective charge and size distribution status of the components present. A tabulated review covers over 50 references on the subject and highlights the possibilities and problems encountered in the analysis of such polydisperse materials with CE methods. In a second part of the article the consequences of experimental and buffer parameters on the behavior of humic materials in CE are presented.     

Membrane distillation of NaCl solution containing natural organic matter

The concentration of NaCl solution containing natural organic matter by membrane distillation (MD) has been performed. The salt solution produced during animal intestines processing was used as a feed. The presence of organic compounds in the feed caused the fouling of MD membranes. The experiments were performed with polypropylene capillary membranes. A rapid flux decline caused by the deposition of organic matter on the membrane surface has been observed. The morphology and composition of the fouling layer was studied using scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS) and Fourier transform infrared with diffuse reflectance spectroscopy (FTIR-DRS). Protein and sodium chloride constituted the major components of the gel layer. Rinsing of the MD module with a 2 wt.% citric acid solution removed a part of the fouling layer. Boiling of spent NaCl solution followed by filtration resulted in the separation of the organic matter in the form of a deposit. This enabled a significant reduction in the occurrence of fouling phenomenon.     

Photodegradation of polyglycidol in aqueous solutions exposed to UV irradiation

The photodegradation of polyglycidol in aqueous solution with UV wavelength of 254 nm was investigated. The experiments were carried out in air at a constant temperature and the photodegradation of polyglycidol (PGl) was compared to that of poly(ethylene oxide) (PEO), the most widely studied polyether. Size exclusion chromatography with multiangle light scattering detection (SEC-MALLS) was used to measure the changes in the molar masses and molar mass dispersities of polymers during degradation. The molar mass of PGl decreased dramatically during the first period of UV irradiation and then gradually approached a limiting value of 17,000 g/mol, regardless of the initial polymer concentration. PEO was less sensitive to UV irradiation than polyglycidol however, both polymers degrade mainly via chain scission. The degradation of PGl and PEO leads to acidification of their water solution. The photooxidation products were analyzed by FTIR and NMR spectroscopy and the spectrophotometric results revealed that the irradiation of the polymers led to the formation of carbonyl groups in the macromolecular chains. A mechanism accounting for the main routes of PGl photooxidation is proposed.     

Feed-water pretreatment: methods to reduce membrane fouling by natural organic matter

The prevention of fouling of polysulphone ultrafiltration membranes, used for the purification of natural brown water, was investigated by pretreating the feed-water prior to filtration. Natural brown water was pretreated by changing the pH of the feed solution and by coagulation with metal-ions prior to filtration. Specific characterisation techniques, developed by Maartens et al. (1998) [A. Maartens, P. Swart, E.P. Jacobs, Humic membrane foulants in natural brown water: characterization and removal, Desalination 115 (3) (1998) 215–227] and Jucker and Clarke (1994) [C. Jucker, M.M. Clark, Adsorption of aquatic humic substances on hydrophobic ultrafiltration membranes, J. Membrane Sci. 97 (1994) 37–52], were used to determine and compare the effects induced by the adsorption of natural organic matter on the permeability of capillary ultrafiltration membranes. The extent of foulant adsorption and the quality of the resultant permeate solutions were determined by UV–VIS-light spectroscopy. Results indicated that adsorption of natural organic matter can be minimised by adjusting the pH of the feed solution to 7. The findings of this investigation provides information of importance for the operation of future natural brown water ultrafiltration plants.     

Formulation selection of aliphatic aromatic biodegradable polyester film exposed to UV/solar radiation

Aliphatic aromatic copolyester films, poly(butylene adipate-co-terephthalate) or PBAT, are susceptible to photodegradation, leading to main chain scission and crosslinking. The presence of crosslinked structures not only decreased the mechanical properties of the film due to embrittlement, but also hindered the biodegradation process by limiting access of water and microorganisms to the polymer chains. This has limited the use of PBAT for outdoor applications, such as mulch films. In this study, response surface methodology (RSM) was used to determine the optimal concentrations of carbon black (CB) and the chain breaking antioxidant butylated hydroxytoluene (BHT) for the design of mulch films that can prevent the formation of crosslinked structures from recombination of free radicals. An overlaid contour plot of suitable concentrations of CB and BHT for the formulation of mulch film for crop production in Michigan or regions with similar solar radiation was established using selection criteria of light transmission of less than 20%, final tensile strength of at least 6.35 MPa, maximum gel fraction of 0.30, and positive number average molecular weight reduction sensitivity in the early stage of degradation.     

Vertical migration and motility responses in three marine phytoplankton species exposed to solar radiation

Diurnal vertical migration in the water column and the impact of solar radiation on motility were investigated in three marine phytoplankton species: Tetraselmis suecica, Dunaliella salina and Gymnodinium chlorophorum. Cells were exposed to solar radiation either in ultraviolet radiation (UVR, 280-400 nm) transparent Plexiglas tubes (45 cm length, 10 cm diameter) or in quartz tubes under three radiation treatments: PAB (280-700 nm), PA (320-700 nm) and P (400-700 nm). The three species displayed different behavior after exposure to solar radiation. Tetraselmis suecica was insensitive to UVR and under high solar radiation levels, cells accumulated preferentially near the surface. Exposure experiments did not indicate any significant changes in swimming speed nor in the percentage of motile cells after 5 h of exposure. On the other hand, D. salina was sensitive to UV-B displaying a significant decrease in swimming speed and percentage of motile cells after 2-3 h of exposure. Moreover, D. salina cells migrated deep in the water column when irradiance was high. The response of G. chlorophorum was in between that of the other two species tested, with a slight (but significant) decrease in swimming speed and percentage of motile cells in all radiation treatments after 5 h of exposure. While G. chlorophorum cells were more or less homogenously distributed in the water column, a slight (but significant) avoidance response to high radiation was observed at local noon, with cells migrating deep in the water column. Our data clearly indicate that these sub-lethal effects of solar radiation are species-specific and they might have important implications for the aquatic ecosystem.     

Effects of adsorbents on membrane fouling by natural organic matter

Fouling by natural organic matter (NOM) is a major impediment to cost-effective operation of membrane processes in water treatment. This research investigated the removal of NOM by three adsorbents: heated iron or aluminum oxide particles (HIOPs and HAOPs, respectively) and powdered activated carbon (PAC). Although PAC removed a larger fraction of the DOC than did either HAOPs or HIOPs, it adsorbed non-fouling molecules preferentially over foulants, whereas the opposite was true for the metal oxide particles. In addition, when the oxide adsorbents were pre-deposited on a microfiltration membrane, foulants that were not adsorbed in batch tests were effectively removed from the solution before it reached the membrane, leading to excellent performance with respect to both NOM removal and fouling. SEM images showed that membranes under a layer of HAOPs and HIOPs were virtually as clean as a pristine membrane.     

Transmission electron microscopy of the natural organic matter of surface waters

Some components of aquatic natural organic matter (NOM) can be analysed effectively by methods of particle analysis employing transmission electron microscopy in conjunction with multi-method analytical approaches in the field, minimum perturbation techniques for sample handling and technology transfer from the biomedical sciences. The NOM components, include fulvic acids, colloidal fibrils and organic polymers of MW > 30 000. The use of a water-compatible embedding resin permits shape and size analyses of colloidal NOM (1–1000 nm) in ultrathin sections which minimize the misleading dehydration artifacts of the past. Experimentally induced perturbations allow one to follow aggregation/coagulation events at 1 nm resolution, while permitting the analyst to relate some components of coagulum structure to chemical entities. This review presents the current status of attempts to optimize a combination of analytical chemistry and transmission electron microscopy for describing NOM and its behaviour in surface waters.     

Patterns of DNA damage and photoinhibition in temperate South-Atlantic picophytoplankton exposed to solar ultraviolet radiation.

Natural marine phytoplankton assemblages from Bahía Bustamante (Chubut, Argentina, 45 degrees S, 66.5 degrees W), mainly consisting of cells in the picoplankton size range (0.2-2 microm), were exposed to various UVBR (280-315 nm) and UVAR (315-400 nm) regimes in order to follow wavelength-dependent patterns of cyclobutane pyrimidine dimer (CPD) induction and repair. Simultaneously, UVR induced photosynthetic inhibition was studied in radiocarbon incorporation experiments. Biological weighting functions (BWFs) for photoinhibition and for CPD induction, the latter measured in bare calf thymus DNA, differed in the UVAR region: carbon incorporation was reduced markedly due to UVAR, whereas no measurable UVAR effect was found on CPD formation. In contrast, BWFs for inhibition of photosynthesis and CPD accumulation were fairly similar in the UVBR region, especially above 300 nm. Incubation of phytoplankton under full solar radiation caused rapid CPD accumulation over the day, giving maximum damage levels exceeding 500 CPD MB(-1) at the end of the afternoon. A clear daily pattern of CPD accumulation was found, in keeping with the DNA effective dose measured by a DNA dosimeter. In contrast, UVBR induced photosynthetic inhibition was not dose related and remained nearly constant during the day. Screening of UVBR or UVR did not cause significant CPD removal, indicating that photoreactivation either by PAR or UVAR was of minor importance in these organisms. High CPD levels were found in situ early in the morning, which remained unaffected notwithstanding treatments favoring photorepair. These results imply that a proportion of cells had been killed by UVBR exposure prior to the treatments. Our data suggest that the limited potential for photoreactivation in picophytoplankton assemblages from the southern Atlantic Ocean causes high CPD accumulation as a result of UVBR exposure.     

Kinetics of Cu(II) reduction by natural organic matter

The kinetics of Cu(II) reduction by Suwannee River fulvic acid (SRFA) at concentrations from 0.25 to 8 mg L(-1) have been investigated in 2 mM NaHCO(3) and 0.7 M NaCl at pH 8.0. In the absence of oxygen, SRFA reduced Cu(II) to Cu(I) in a biphasic manner, with initial rapid formation of Cu(I) followed by a much slower increase in Cu(I) concentration over time. When present, oxygen only had a noticeable effect on Cu(I) concentrations in the second phase of the reduction process and at high [SRFA]. In both the absence and presence of oxygen, the rate of Cu(I) generation increased with increasing [SRFA]. At 8 mg L(-1) [SRFA], nearly 75% of the 0.4 μM Cu(II) initially present was reduced to Cu(I) after 20 min, although the yield of Cu(I) relative to [SRFA] decreased at [SRFA] > 1 mg L(-1). Two plausible kinetic modeling approaches were found to satisfactorily describe the experimental data over a range of [SRFA]. Despite some uncertainty as to which approach is correct, common features of both approaches were complexation of Cu(II) by SRFA and reduction of Cu(II) by two different electron donor groups within SRFA: a relatively labile electron donor (with a concentration of 1.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) relatively rapidly and a less labile donor (with a concentration of 3.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) more slowly.     

Immobilization of fission iodine by reaction with insoluble natural organic matter

Iodine-129 is a fission product and highly mobile in the environment. Along with other stable isotopes of iodine, ¹²⁹I is released during reprocessing of nuclear fuel and must be trapped to prevent the release of radioactivity to the environment. Past studies have provided evidence that iodine can become associated with natural organic matter (NOM). This research explores the use of NOM (sphagnum peat and humic acid) to sequester iodine from the vapor and aqueous phases. NOM-associated iodine may be stable for geological storage. NOM-sequestered iodine can be recovered by pyrolysis to prepare target materials for transmutation. The nature of the NOM-iodine association has been explored.     

Measurement and computation of zinc binding to natural dissolved organic matter in European surface waters

The zinc binding characteristics of natural dissolved organic matter (DOM) from five representative European surface freshwater sources were studied by square wave anodic stripping voltammetry (SWASV) and model simulation. Water samples were titrated with zinc and free zinc ion activity {Zn²⁺}, was calculated from the measurement of labile zinc by SWASV and other system conditions. Measured values of {Zn²⁺}, which were in the range 10⁻⁷ to 10⁻⁵ M, were compared with those simulated using Humic Ion-Binding Models V and VI. It was assumed that zinc speciation was controlled by the organic matter, represented by fulvic acid (FA), together with inorganic solution complexation. The models were calibrated by adjusting the parameter DOMFA, the proportion of DOM considered to behave as FA. Two modeling scenarios were used to obtain DOMFA values, both considering and not considering the competitive effects of Al, Fe(II) and Fe(III). The default Zn-DOM binding strength in Model VI (log K_MA = 1.6) was not able to provide realistic values of DOMFA and a log K_MA of 1.8 was tentatively proposed as a more plausible value in these waters. Models V and VI gave very similar fits to the data after optimization of DOMFA, in contrast to recent findings for copper. This may be due to the fact that the additional strong binding sites provided by Model VI are not important in complexing Zn in the Zn concentration range investigated in this study. Computed free Zn activities from both modeling scenarios were very similar; however, the consideration of Al and Fe competition is more realistic for natural waters.     

Dynamics of potentially protective compounds in Rhodophyta species from Patagonia (Argentina) exposed to solar radiation

The impact of solar radiation upon potentially protective compounds (i.e., UV-absorbing compounds and carotenoids) was assessed in four Rhodophyte species from Patagonia (i.e., Ceramium sp. Lyngbye, Corallina officinalis Linnaeus, Callithamnion gaudichaudii Agardh and Porphyra columbina Montagne) during short-term (i.e., 46 h) experiments. Algae were exposed to solar radiation under two treatments (PAR only: 400-700 nm, and PAR+UVR: 280-700 nm) and sub-samples were taken every 3 h (or longer periods at night) to determine the spectral absorption characteristics and concentration of UV-absorbing compounds, carotenoids and photosynthetic pigments. Except for C. gaudichaudii which displayed a decrease in chl-a concentration throughout the experiment, photosynthetic pigments had small variations in all species. UV-absorbing compounds concentration had species-specific responses: Ceramium sp. was the only species in which UV-absorbing compounds concentration varied as a function of solar irradiance, with maximum values around local noon. In C. officinalis and P. columbina UV-absorbing compounds concentration increased as compared to that of chl-a; in Ceramium sp. and C. gaudichaudii, however, there was no relationship between UV-absorbing compounds content and chl-a concentration. Carotenoids, on the other hand, did co-vary with chl-a in all species. Our data suggest that, with the exception of C. gaudichaudii, the differential responses of UV-absorbing compounds concentrations are more associated to the previous light history of the algae (i.e., in turn due to their position in the intertidal zone) rather than to the radiation treatment imposed to the samples. Based on our results, the variable impact of solar radiation upon productivity (and eventually biodiversity) of macroalgae from Patagonia might consequently differentially affect higher trophic levels of the aquatic food web.     

Naturally occurring organic matter as a chemical trap to scan an ecosystem for natural products

A model is proposed that tests an ecosystem for natural products (NPs) using a nonpolar extract of naturally occurring organic matter (NOM), which we demonstrate to be an efficient chemical trap for relatively nonpolar organic molecules. To test the model we collected twenty-six samples of NOM from various locations on the Suwannee River, from its headwaters in the Okeefenokee Swamp to the Gulf of Mexico. We have tentatively identified stearic acid, DDT, chincodine, and a potential precursor to bryostatin. Our data provide evidence that NOM can trap, hold for several decades, concentrate, and transport NP in the environment.     

Adsorption of organic matter at mineral/water interfaces: 5. Effects of adsorbed natural organic matter analogues on mineral dissolution

The effects of the adsorption of pyromellitate, an analogue for natural organic matter, on the dissolution behavior of corundum (alpha-Al2O3) have been examined over a wide range of pyromellitate concentrations (0-2.5 mM) and pH conditions (2-10). The adsorption modes of pyromellitate on corundum have first been examined using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and are shown to be dominated by a fully deprotonated, outer-sphere pyromellitate species ([triple bond]AlOH2+. . .Pyr4-) at pH >/= 5.0. At lower pH conditions, however, an additional protonated outer-sphere species ([triple bond]AlOH2+. . .H2Pyr2-) and an inner-sphere species are also evident. In accordance with the ATR-FTIR findings, modeling of macroscopic pyromellitate adsorption data using an extended constant capacitance treatment was possible using two outer-sphere ([triple bond]AlOH2+. . .Pyr4- and [triple bond]AlOH2+. . .H2Pyr2-) and one inner-sphere ([triple bond]AlPyr3-) adsorbed pyromellitate species. The presence of adsorbed pyromellitate strongly inhibited the dissolution of corundum under acidic (pH < 5) conditions, consistent with a mechanism previously proposed by Johnson et al. whereby outer-spherically adsorbed Pyr4- species sterically protect dissolution-active surface sites from attack by dissolution-promoting species such as protons. A reduction in the protolytic dissolution rate of corundum results. A reference Suwannee River fulvic acid, which also adsorbs to aluminum (oxyhydr)oxide surfaces in a predominantly outer-sphere manner, was similarly shown to strongly inhibit the dissolution of corundum at pH = 3.