Cu transport and complexation by the marine diatom Phaeodactylum tricornutum: Implications for trace metal complexation kinetics in the surface ocean.

Elucidating whether dissolved Cu uptake is kinetically or thermodynamically controlled, and the effects of speciation on Cu transport by phytoplankton will allow better modeling of the fate and impact of dissolved Cu in the ocean. To address these questions, we performed Cu physiological and physicochemical experiments using the model diatom, Phaeodactylum tricornutum, grown in natural North Atlantic seawater (0.44 nM Cu). Using competitive ligand equilibration-cathodic stripping voltammetry (CLE-CSV), we measured two organic ligand types released by P. tricornutum to bind Cu (L1 and L2) at concentrations of ~0.35 nM L1 and 1.3 nM L2. We also established the presence of two putative Cu-binding sites at the cell surface of P. tricornutum (S1 and S2) with log K differing by ~5 orders of magnitude (i.e., 12.9 vs. 8.1) and cell surface densities by 9-fold. Only the high-affinity binding sites, S1, exhibit reductase activity. Using voltammetric kinetic measurements and a theoretical kinetic model, we calculated the forward and dissociation rate constants of L1 and S1. Complementary 67Cu uptake experiments identified a high- and a low-affinity Cu uptake system in P. tricornutum, with half-saturation constant (Km) of 154 nM and 2.63 µM dissolved Cu, respectively. In the P. tricornutum genome, we identified a putative high-affinity Cu transporter (PtCTR49224) and a putative ZIP-like, low-affinity Cu transporter (PtZIP49400). PtCTR49224 has high homology to Homo sapiens hCTR1, which depending on the accessibility to extracellular reducing agents, the hCTR1 itself is involved in the reduction of Cu2+ to Cu+ before internalization. We combined these physiological and physicochemical data to calculate the rate constants for the internalization of Cu, and established that while the high-affinity Cu uptake system (S1) is borderline between a kinetically or thermodynamically controlled system, the low-affinity Cu transporters, S2, is thermodynamically-controlled. We revised the inverse relationship between the concentrations of inorganic complexes of essential metals (i.e., Ni, Fe, Co, Zn, Cd, Mn and Cu) in the mixed layer and the formation rate constant of metal transporters in phytoplankton, highlighting the link between the chemical properties of phytoplankton metal transporters and the availability and speciation of trace metals in the surface ocean.

Contrasted redox-dependent structural control on Fe isotope fractionation during its adsorption onto and assimilation by heterotrophic soil bacteria.

Despite the importance of structural control on metal stable isotope fractionation in inorganic and abiotic systems, the link between metal structural changes and related isotopic fractionation during reactions with organic surfaces and live cells remains poorly established. We conducted reversible adsorption of Fe(II) and Fe(III) on the surface of exopolysaccharide (EPS)-rich and EPS-poor Pseudomonas aureofaciens, and we allowed Fe intracellular uptake by growing cells. We analyzed the Fe isotopic composition of the remaining fluid and cell biomass, and compared the isotopic fractionation during adsorption and assimilation reaction with relative changes in Fe structural status between aqueous solution and bacterial cells, based on available and newly collected X-ray absorption spectroscopy (XAS) observations. Iron(III) adsorption onto P. aureofaciens at 2.8 ≤ pH ≤ 6.0 produced an enrichment of the cell surface in heavier isotopes with Δ57Fecell-solution ranging from +0.7 to +2.1‰, without a link to pH in EPS-rich cultures. In contrast, the magnitude of isotopic fractionation increased with pH in EPS-poor cultures. Iron(II) adsorption produced an even larger enrichment of the cell surface in heavier isotopes, by up to 3.2‰, tentatively linked to Fe(III) hydroxide precipitation. Intracellular assimilation of Fe(II) favored heavier isotopes and led to Δ57Fecell-solution of +0.8‰. In addition, Fe(III) cellular uptake produced an enrichment of the bacterial biomass in lighter isotopes with Δ57Fecell-solution of -1‰. The XAS analyses demonstrated the dominance of Fe(III)-phosphate complexes both at the cell surface and in the cell interior. We suggest that heavier isotope enrichment of the cell surface relative to the aqueous solution is due to strong Fe(III)-phosphoryl surface complexes and Fe complexation to ligands responsible for metal transfer from the surface to the inner cell. In case of Fe(II) adsorption or assimilation, its partial oxidation within the cell compartments may lead to cell enrichment in heavier isotopes. In contrast, loss of symmetry of assimilated Fe(III) relative to the aqueous Fe3+ ion and longer bonds of intracellular ions relative to aqueous Fe(III)-citrate or hydroxo-complexes could produce an enrichment of cells in lighter isotopes. The versatile nature of Fe(II) and Fe(III) fractionation without a distinct effect of pH and surface exopolysaccharide coverage suggests that, in natural soil and sedimentary environments, Fe isotope fractionation during interaction with heterotrophic bacteria will be primarily governed by Fe complexation with DOM and Fe redox status in the soil pore water.

Trace elements in the water column of high-altitude Pyrenean lakes: Impact of local weathering and long-range atmospheric input.

High altitude (alpine) lakes are efficient sentinels of environmental processes, including local pollution and long-range atmospheric transfer, because these lakes are highly vulnerable to ongoing climate changes and increasing anthropogenic pressure. Towards improving the knowledge of trace element geochemistry in the water column of alpine lakes, we assessed 64 physico-chemical parameters, including macro- and micronutrients, major and trace element concentrations in the water column of 18 lakes in the Pyrenees, located along the border between France and Spain. Lake depth, morphology, retention time and watershed rock lithology did not exhibit sizable impact on major and trace element concentrations in the water column. However, acidic (pH = 4.7 ± 0.2) lakes were distinctly different from circumneutral lakes (pH = 6.8 ± 0.5) as they exhibited >10 times higher concentrations of SO42- and trace metals (Fe, Mn, Zn, Cd, Pb, Co, Ni, Be, Al, Ga and REEs). While some of these elements clearly mark the presence of sulphide-rich minerals within the watershed (Fe, Zn, Cd and Pb), the increased mobility of lithogenic elements (Be, Al, Ga and REEs) in acidic lakes may reflect the leaching of these elements from silicate dust derived from atmospheric deposits or surrounding granites. At the same time, compared to circumneutral lakes, acidic lake water displayed lower concentrations of dissolved oxyanions (As, Mo, V, B and W) and elevated SO42- concentrations. The latter could lead to efficient Ba removal from the water column. The exploitation of metal ores within the watershed of three lakes clearly impacted high Zn and Cd concentrations observed in their water column, despite two of these lakes not being acidic. We conclude that local impacts have a greater effect on the water column than long-range atmospheric inputs and that dissolved trace element concentration measurements can be used for revealing sulphide-rich minerals or acid mine drainage within the lakes' watershed.

Influence of strong iron-binding ligands on cloud water oxidant capacity.

Iron (Fe) plays a dual role in atmospheric chemistry: it is involved in chemical and photochemical reactivity and serves as a micronutrient for microorganisms that have recently been shown to produce strong organic ligands. These ligands control the reactivity, mobility, solubility and speciation of Fe, which have a potential impact on Fe bioavailability and cloud water oxidant capacity. In this work, the concentrations of Fe-binding ligands and the conditional stability constants were experimentally measured for the first time by Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) technique in cloud water samples collected at puy de Dôme (France). The conditional stability constants, which indicate the strength of the Fe-ligand complexes, are higher than those considered until now in cloud chemistry (mainly Fe-oxalate). To understand the effect of Fe complexation on cloud water reactivity, we used the CLEPS cloud chemistry model. According to the model results, we found that Fe complexation impacts the hydroxyl radical formation rate: contrary to our expectations, Fe complexation by natural organic ligands led to an increase in hydroxyl radical production. These findings have important impacts on cloud chemistry and the global iron cycle.

Investigating the microbial ecology of coastal hotspots of marine nitrogen fixation in the western North Atlantic.

Variation in the microbial cycling of nutrients and carbon in the ocean is an emergent property of complex planktonic communities. While recent findings have considerably expanded our understanding of the diversity and distribution of nitrogen (N2) fixing marine diazotrophs, knowledge gaps remain regarding ecological interactions between diazotrophs and other community members. Using quantitative 16S and 18S V4 rDNA amplicon sequencing, we surveyed eukaryotic and prokaryotic microbial communities from samples collected in August 2016 and 2017 across the Western North Atlantic. Leveraging and significantly expanding an earlier published 2015 molecular dataset, we examined microbial community structure and ecological co-occurrence relationships associated with intense hotspots of N2 fixation previously reported at sites off the Southern New England Shelf and Mid-Atlantic Bight. Overall, we observed a negative relationship between eukaryotic diversity and both N2 fixation and net community production (NCP). Maximum N2 fixation rates occurred at sites with high abundances of mixotrophic stramenopiles, notably Chrysophyceae. Network analysis revealed such stramenopiles to be keystone taxa alongside the haptophyte diazotroph host Braarudosphaera bigelowii and chlorophytes. Our findings highlight an intriguing relationship between marine stramenopiles and high N2 fixation coastal sites.

Influence of secondary metabolites on surface chemistry and metal adsorption of a devitalized lichen biomonitor.

Despite the broad use of lichens as biomonitors of airborne trace elements, the surface chemistry and metal adsorption parameters of these organisms are still poorly known. The current investigation is aimed at (i) quantifying the acid-base surface properties and the first-order physical-chemical parameters of Cu2+ and Zn2+ adsorption of devitalized Pseudevernia furfuracea, a lichen commonly used in biomonitoring of airborne trace elements, and (ii) comparing the results with those available for moss biomonitors. Equilibrium constants and metal-binding site concentrations were calculated with a thermodynamic model by taking into account the presence/absence of ancillary extracellular cell wall compounds, namely melanin and acetone-soluble lichen substances. An acid-base titration experiment performed in the pH range of 3-10 showed that melanised and non-melanised P. furfuracea samples have lower pHPZC (3.53-3.99) and higher metal-binding site concentrations (0.96-1.20 mmol g-1) compared to that of the mosses investigated so far at the same experimental conditions. Melanin biosynthesis increased the content of carboxyl and phosphoryl groups and reduces that of amine/polyphenols. Cu2+ and Zn2+ adsorption was unaffected by the degree of melanisation while the removal of extracellular lichen substances slightly decreased Zn2+ adsorption. Although Cu2+ and Zn2+ adsorption parameters related to P. furfuracea surfaces were 3 times lower than in the mosses, lichen samples adsorbed the same amount of Cu2+ and 30% more Zn2+. The present study contributes in understanding the role of ancillary cell wall compounds in Cu2+ and Zn2+ adsorption in a model lichen. It also provides a first comparison between the surface physico-chemical characteristics of lichens and mosses frequently used as biomonitors of trace elements.

A call for refining the role of humic-like substances in the oceanic iron cycle.

Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.

Biosurface properties and lead adsorption in a clone of Sphagnum palustre (Mosses): Towards a unified protocol of biomonitoring of airborne heavy metal pollution.

Although mosses are widely used for active biomonitoring of air pollution, a unified protocol for their treatment before exposure in bags is still lacking. Here we used field- and laboratory-grown Sphagnum palustre L. moss, respectively, treated by EDTA and devitalized by oven drying at 100 °C, to elaborate a consistent procedure of metal and proton adsorption on moss surfaces. Acid-base titrations and Pb2+ adsorption experiments at different pH values and Pb2+ concentrations in solution were performed with both field-collected and laboratory cloned mosses. Devitalization and EDTA treatments did not produce any measurable difference in terms of H+ and Pb2+ adsorption capacities of moss surfaces. The stability constants for Pb2+ adsorption onto moss surfaces as a function of pH (pH-dependent adsorption edge) and at constant pH (5.5 and 6.5) as a function of Pb2+ concentration ("langmuirian" adsorption isotherm) were rather similar between different treatments. A Linear Program Modeling (LPM) of adsorption reactions revealed high similarity of adsorption constants regardless of treatments for both field-grown and cloned mosses. Therefore, in view of the use of S. palustre clone for biomonitoring lead in the environment, we recommend devitalization at 100 °C as unique treatment to perform with the aim to preserve the biomonitor before and after its exposure in bags.

Interactive Impacts of Silver and Phosphorus on Autotrophic Biofilm Elemental and Biochemical Quality for a Macroinvertebrate Consumer.

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. In particular, these biofilms represent a major resource for many invertebrate consumers and the first ecological barrier against toxic metals. To date, very few studies have investigated the indirect effects of stressors on upper trophic levels through alterations of the quality of biofilms for their consumers. In a laboratory study, we investigated the single and combined effects of phosphorus (P) availability and silver, a re-emerging contaminant, on the elemental [carbon (C):nitrogen (N):P ratios] and biochemical (fatty acid profiles) compositions of a diatom-dominated biofilm initially collected in a shallow lake. We hypothesized that (1) P and silver, through the replacement of diatoms by more tolerant primary producer species, reduce the biochemical quality of biofilms for their consumers while (2) P enhances biofilm elemental quality and (3) silver contamination of biofilm has negative effects on consumers life history traits. The quality of biofilms for consumers was assessed for a common crustacean species, Gammarus fossarum, by measuring organisms' survival and growth rates during a 42-days feeding experiment. Results mainly showed that species replacement induced by both stressors affected biofilm fatty acid compositions, and that P immobilization permitted to achieve low C:P biofilms, whatever the level of silver contamination. Gammarids growth and survival rates were not significantly impacted by the ingestion of silver-contaminated resource. On the contrary, we found a significant positive relationship between the biofilm P-content and gammarids growth. This study underlines the large indirect consequences stressors could play on the quality of microbial biomass for consumers, and, in turn, on the whole food web.

State of rare earth elements in the sediment and their bioaccumulation by mangroves: a case study in pristine islands of Indian Sundarban.

The mangrove ecosystems are known to efficiently sequester trace metals both in sediments and plant biomass. However, less is known about the chemistry of rare earth elements (REE) in the coastal environments, especially in the world's largest mangrove province, the Sundarban. Here, the concentration of REE in the sediment and plant organs of eight dominant mangrove species (mainly Avicennia sp.) in the Indian Sundarban was measured to assess REE sources, distribution, and bioaccumulation state. Results revealed that light REE (LREE) were more concentrated than the heavy REE (HREE) (128-144 mg kg-1 and 12-15 mg kg-1, respectively) in the mangrove sediments, with a relatively weak positive europium anomaly (Eu/Eu* = 1.03-1.14) with respect to North American shale composite. The primary source of REE was most likely linked to aluminosilicate weathering of crustal materials, and the resultant increase in LREE in the detritus. Vertical distribution of REE in one of the long cores from Lothian Island was altered by mangrove root activity and dependent on various physicochemical properties in the sediment (e.g., Eh, pH, organic carbon, and phosphate). REE uptake by plants was higher in the below-ground parts than in the above-ground plant tissues (root = 3.3 mg kg-1, leaf + wood = 1.7 mg kg-1); however, their total concentration was much lower than in the sediment (149.5 mg kg-1). Species-specific variability in bioaccumulation factor and translocation factor was observed indicating different REE partitioning and varying degree of mangrove uptake efficiency. Total REE stock in plant (above + live below ground) was estimated to be 168 g ha-1 with LREE contributing ~ 90% of the stock. This study highlighted the efficiency of using REE as a biological proxy in determining the degree of bioaccumulation within the mangrove environment.

Revisiting the distribution of oceanic N2 fixation and estimating diazotrophic contribution to marine production.

Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles.

Metal contaminations impact archaeal community composition, abundance and function in remote alpine lakes.

Using the 16S rRNA and mcrA genes, we investigated the composition, abundance and activity of sediment archaeal communities within 18 high-mountain lakes under contrasted metal levels from different origins (bedrock erosion, past-mining activities and atmospheric depositions). Bathyarchaeota, Euryarchaeota and Woesearchaeota were the major phyla found at the meta-community scale, representing 48%, 18.3% and 15.2% of the archaeal community respectively. Metals were equally important as physicochemical variables in explaining the assemblage of archaeal communities and their abundance. Methanogenesis appeared as a process of central importance in the carbon cycle within sediments of alpine lakes as indicated by the absolute abundance of methanogen 16S rRNA and mcrA gene transcripts (105 to 109 copies g-1 ). We showed that methanogen abundance and activity were significantly reduced with increasing concentrations of Pb and Cd, two indicators of airborne metal contaminations. Considering the ecological importance of methanogenesis in sediment habitats, these metal contaminations may have system wide implications even in remote area such as alpine lakes. Overall, this work was pioneer in integrating the effect of long-range atmospheric depositions on archaeal communities and indicated that metal contamination might significantly compromise the contribution of Archaea to the carbon cycling of the mountain lake sediments.

Chemical and structural characterization of copper adsorbed on mosses (Bryophyta).

The adsorption of copper on passive biomonitors (devitalized mosses Hypnum sp., Sphagnum denticulatum, Pseudoscleropodium purum and Brachythecium rutabulum) was studied under different experimental conditions such as a function of pH and Cu concentration in solution. Cu assimilation by living Physcomitrella patents was also investigated. Molecular structure of surface adsorbed and incorporated Cu was studied by X-ray Absorption Spectroscopy (XAS). Devitalized mosses exhibited the universal adsorption pattern of Cu as a function of pH, with a total binding sites number 0.05-0.06 mmolg(dry)(-1) and a maximal adsorption capacity of 0.93-1.25 mmolg(dry)(-1) for these devitalized species. The Extended X-ray Absorption Fine Structure (EXAFS) fit of the first neighbor demonstrated that for all studied mosses there are ∼4.5 O/N atoms around Cu at ∼1.95 Šlikely in a pseudo-square geometry. The X-ray Absorption Near Edge Structure (XANES) analysis demonstrated that Cu(II)-cellulose (representing carboxylate groups) and Cu(II)-phosphate are the main moss surface binding moieties, and the percentage of these sites varies as a function of solution pH. P. patens exposed during one month to Cu(2+) yielded ∼20% of Cu(I) in the form of Cu-S(CN) complexes, suggesting metabolically-controlled reduction of adsorbed and assimilated Cu(2+).

Metal and proton adsorption capacities of natural and cloned Sphagnum mosses.

Terrestrial mosses are commonly used as bioindicators of atmospheric pollution. However, there is a lack of standardization of the biomonitoring preparation technique and the efficiency of metal adsorption by various moss species is poorly known. This is especially true for in vitro-cultivated moss clones, which are promising candidates for a standardized moss-bag technique. We studied the adsorption of copper and zinc on naturally grown Sphagnum peat moss in comparison with in vitro-cultivated Sphagnum palustre samples in order to provide their physico-chemical characterization and to test the possibility of using cloned peat mosses as bioindicators within the protocol of moss-bag technique. We demonstrate that in vitro-grown clones of S. palustre exhibit acid-base properties similar to those of naturally grown Sphagnum samples, whereas the zinc adsorption capacity of the clones is approx. twice higher than that of the samples from the field. At the same time, the field samples adsorbed 30-50% higher amount of Cu(2+) compared to that of the clones. This contrast may be related to fine differences in the bulk chemical composition, specific surface area, morphological features, type and abundance of binding sites at the cell surfaces and in the aqueous solution of natural and cloned Sphagnum. The clones exhibited much lower concentration of most metal pollutants in their tissues relative to the natural samples thus making the former better indicators of low metal loading. Overall, in vitro-produced clones of S. palustre can be considered as an adequate, environmentally benign substitution for protected natural Sphagnum sp. samples to be used in moss-bags for atmospheric monitoring.

Phenolic profile of Dunaliella tertiolecta growing under high levels of copper and iron.

The present study investigates the phenolic profile of exudates and extracts of the green algae Dunaliella tertiolecta, harvested in natural seawater in the absence (control) and in the presence of Cu(II) (315 and 790 nmol L(-1)) and Fe(III) (900 nmol L(-1)) in order to identify and quantify the phenolic compounds produced under metallic stress conditions. The presence of metal ions modifies the growth of cells and changes cell metabolism by producing phenolic compounds adapted to the solution. The use of reversed-phase high-performance liquid chromatography (RP-HPLC) permitted the identification of 14 phenolic constituents. The concentration and type of polyphenols detected in cell extracts and in solution are directly related with the metal and its concentration during growth cultures, achieving 1.4 times higher levels of polyphenols under 790 nmol Cu(II) L(-1) with respect to the control experiments. Microalga excretes polyphenols to be adapted to the environmental conditions. Gentisic acid, (+) catechin and (-) epicatechin, the most prominent phenolic compounds detected in the algae extracts, showed high antioxidant activity in inhibiting 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals. This potent activity may be related to its presence in cells and exudates in high concentrations.

Silver nanoparticles impact phototrophic biofilm communities to a considerably higher degree than ionic silver.

Due to the significant increase in nanoparticle production and especially that of silver nanoparticles over the past decade, the toxicity of silver in both ionic (Ag(+)) and nanoparticulate (AgNPs) form must be studied in detail in order to understand their impact on natural ecosystems. A comparative study of the effect of AgNPs and ionic silver on two independent phototrophic biofilms was conducted in a rotating annular bioreactor (RAB) operating under constant conditions. The concentration of dissolved silver in the inlet solution was progressively increased every 4 days of exposure, from 0.1 to 100 µg L(-1). In the course of the 40-day experiment, biofilm samples were collected to determine the evolution of biomass, chlorophyll-a, as well as photosynthetic and heterotrophic enzymatic activities in response to silver addition. Analysis of both dissolved and particulate silver allowed quantification of the distribution coefficient and uptake rate constants. The presence of both AgNPs and Ag(+) produced significant changes in the biofilm structure, decreasing the relative percentage of Diatomophyceae and Cyanophyceae and increasing the relative percentage of Chlorophyceae. The accumulation capacity of the phototrophic biofilm with respect to ionic silver and the corresponding distribution coefficients were an order of magnitude higher than those of the phototrophic biofilm with respect to AgNPs. Higher levels of AgNPs decreased the biomass from 8.6 ± 0.2 mg cm(-2) for 0-10 µg L(-1) AgNPs to 6.0 ± 0.1 mg cm(-2) for 100 µg L(-1) added AgNPs, whereas ionic silver did not have any toxic effect on the biofilm growth up to 100 µg L(-1) of added Ag(+). At the same time, AgNPs did not significantly affect the photosynthetic activity of the biofilm surface communities compared to Ag(+). It can thus be hypothesized that negatively charged AgNPs may travel through the biofilm water channels, thereby affecting the whole biofilm structure. In contrast, positively charged Ag(+) is bound at the cell surfaces and EPS, thus blocking its further flux within the biofilm layers. On the whole, the phototrophic biofilm demonstrated significant capacities to accumulate silver within the surface layers. The main mechanism to avoid the toxic effects is metal complexation with exopolysaccharides and accumulation within cell walls, especially pronounced under Ag(+) stress. The significant AgNPs and Ag(+) uptake capacities of phototrophic biofilm make it a highly resistant ecosystem in silver-polluted river waters.

Oxidation of Cu(I) in seawater at low oxygen concentrations.

The oxidation of nanomolar copper(I) at low oxygen (6 µM) concentrations was studied as a function of pH (6.7-8.2), ionic strength (0.1-0.76 M), total inorganic carbon concentration (0.65-6.69 mM), and the added concentration of hydrogen peroxide, H(2)O(2) (100-500 nM) over the initial 150 nM H(2)O(2) concentration in the coastal seawater. The competitive effect between H(2)O(2) and O(2) at low O(2) concentrations has been described. Both the oxidation of Cu(I) by oxygen and by H(2)O(2) had a reaction order of one. The reduction of Cu(II) back to Cu(I) in the studied seawater by H(2)O(2) and other reactive oxygen intermediates took place at both high and low O(2) concentrations. The effect of the pH on oxidation was more important at low oxygen concentrations, where δlog k/δpH was 0.85, related to the presence of H(2)O(2) in the initial seawater and its role in the redox chemistry of Cu species, than at oxygen saturation, where the value was 0.6. A kinetic model that considered the Cu speciation, major ion interactions, and the rate constants for the oxidation and reduction of Cu(I) and Cu(II) species, respectively, was applied. When the oxygen concentration was lower than 22 µM and under the presence of 150 nM H(2)O(2), the model showed that the oxidation of Cu(I) was controlled by its reaction with H(2)O(2). The effect of the pH on the oxidation rate of Cu(I) was explained by its influence on the oxidation of Cu(I) with O(2) and H(2)O(2), making the model valid for any low oxygen environment.

Oxidation of Fe(II) in natural waters at high nutrient concentrations.

The Fe(II) oxidation kinetic was studied in seawater enriched with nutrients as a function of pH (7.2-8.2), temperature (5-35 °C), and salinity (10-36.72) and compared with the same parameters in seawater media. The effect of nitrate (0-1.77 × 10(-3) M), phosphate (0-5.80 × 10(-5) M) and silicate (0-2.84 × 10(-4) M) was studied at pH 8.0 and 25 °C. The experimental results demonstrated that Fe(II) oxidation was faster in high nutrient concentrations affecting the lifetime of Fe(II) in nutrient rich waters. Silicate displayed the most significant effects on the Fe(II) oxidation rate with values similar to those determined in seawater enriched with all the nutrients. A kinetic model was applied to the experimental results in order to account for changes in the speciation and to compute the fractional contribution of each Fe(II) species to the total rate constant as a function of pH. FeH(3)SiO(4)(+) played a key role in the Fe(II) speciation, dominating the process at pH over 8.4. At pH 8.0, FeH(3)SiO(4)(+) represented 18% of the total Fe(II) species. Model results show that when the concentration of silicate is 3 × 10(-5) M as in high nutrient low chlorophyll areas, FeH(3)SiO(4)(+) contributed at pH 8.0 by 4% increasing the rate to 11% at 1.4 × 10(-4) M. The effect of nutrients, especially silicate, must be considered in any further study in seawater media cultures and eutrophic oceanic areas.