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.

Distribution of copper-binding ligands in Fram Strait and influences from the Greenland Shelf (GEOTRACES GN05)

The Fram Strait represents the major gateway of Arctic Ocean waters towards the Nordic Seas and North Atlantic Ocean and is a key region to study the impact of climate change on biogeochemical cycles. In the region, information about trace metal speciation, such as copper, is scarce. This manuscript presents the concentrations and conditional stability constants of copper-binding ligands (LCu and log KcondCu2+L) in the water column of Fram Strait and the Greenland shelf (GEOTRACES cruise GN05). Cu-binding ligands were analysed by Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) using salicylaldoxime (SA) as competitive ligand. Based on water masses and the hydrodynamic influences, three provinces were considered (coast, shelf, and Fram Strait) and differences were observed between regions and water masses. The strongest variability was observed in surface waters, with increasing LCu concentrations (mean values: Fram Strait = 2.6 ± 1.0 nM; shelf = 5.2 ± 1.3 nM; coast = 6.4 ± 0.8 nM) and decreasing log KcondCu2+L values (mean values: Fram Strait = 15.7 ± 0.3; shelf = 15.2 ± 0.3; coast = 14.8 ± 0.3) towards the west. The surface LCu concentrations obtained above the Greenland shelf indicate a supply from the coastal environment to the Polar Surface Water (PSW) which is an addition to the ligand exported from the central Arctic to Fram Strait. The significant differences (in terms of LCu and log KcondCu2+L) between shelf and coastal samples were explained considering the processes which modify ligand concentrations and binding strengths, such as biological activity in sea-ice, phytoplankton bloom in surface waters, bacterial degradation, and meltwater discharge from 79NG glacier terminus. Overall, the ligand concentration exceeded those of dissolved Cu (dCu) and kept the free copper (Cu2+) concentrations at femtomolar levels (0.13-21.13 fM). This indicates that Cu2+ toxicity limits were not reached and dCu levels were stabilized in surface waters by organic complexes, which favoured its transport to the Nordic Seas and North Atlantic Ocean and the development of microorganism.

Exploring the Effects of Organic Matter Characteristics on Fe(II) Oxidation Kinetics in Coastal Seawater

The iron(II) oxidation kinetic process was studied at 25 stations in coastal seawater of the Macaronesia region (9 around Cape Verde, 11 around the Canary Islands, and 5 around Madeira). In a physicochemical context, experiments were carried out to study the pseudo-first-order oxidation rate constant (k', min^-1) over a range of pH (7.8, 7.9, 8.0, and 8.1) and temperature (10, 15, 20, and 25 °C). Deviations from the calculated k_cal^' at the same T, pH, and S were observed for most of the stations. The measured t_1/2 (ln 2/k', min) values at the 25 stations ranged from 1.82 to 3.47 min (mean 1.93 ± 0.76 min) and for all but two stations were lower than the calculated t_1/2 of 3.21 ± 0.2 min. In a biogeochemical context, nutrients and variables associated with the organic matter spectral properties (CDOM and FDOM) were analyzed to explain the observed deviations. The application of a multilinear regression model indicated that k' can be described (R = 0.921 and SEE = 0.064 for pH = 8 and T = 25 °C) from a linear combination of three organic variables, k'^OM = k_cal^' -0.11* TDN + 29.9*b_DOM + 33.4*C1_humic, where TDN is the total dissolved nitrogen, b_DOM is the spectral peak obtained from colored dissolved organic matter (DOM) analysis when protein-like or tyrosine-like components are present, and C1_humic is the component associated with humic-like compounds obtained from the parallel factor analysis of the fluorescent DOM. Results show that compounds with N in their structures mainly explain the observed k' increase for most of the samples, although other components could also play a relevant role. Experimentally, k' provides the net result between the compounds that accelerate the process and those that slow it down.

CO2 fluxes in the Northeast Atlantic Ocean based on measurements from a surface ocean observation platform

The seasonal and spatial variability of the CO₂ system parameters and CO₂ air-sea exchange were studied in the Northeast Atlantic Ocean between the northwest African coastal upwelling and the oligotrophic open-ocean waters of the North Atlantic subtropical gyre. Data was collected aboard a volunteer observing ship from February 2019 to February 2020. The seasonal and spatial variability of CO₂ fugacity in seawater (fCO_2,sw) was strongly driven by the seasonal temperature variation, which increased with latitude and was lower throughout the year in coastal regions where the upwelling and offshore transport was more intense. The thermal to biological effect ratio (T/B) was approximately 2, with minimum values along the African coastline related to higher biological activity in the upwelled waters. The fCO_2,sw increased from winter to summer by 11.84 ± 0.28 μatm°C^-1 on the inter-island routes and by 11.71 ± 0.25 μatm°C^-1 along the northwest African continental shelf. The seasonality of total inorganic carbon normalized to constant salinity of 36.7 (NC_T) was studied throughout the region. The effect of biological processes and calcification/dissolution on NC_T between February and October represented >90% of the reduction of inorganic carbon while air-sea exchange described <6%. The seasonality of air-sea CO₂ exchange was controlled by temperature. The surface waters of the entire region acted as a CO₂ sink during the cold months and as a CO₂ source during the warm months. The Canary basin acted as a net sink of -0.26 ± 0.04 molC m^-2 yr^-1. The northwest African continental shelf behaved as a stronger sink at -0.48 ± 0.09 molC m^-2 yr^-1. The calculated average CO₂ flux for the entire area was -2.65 ± 0.44 TgCO₂ yr^-1 (-0.72 ± 0.12 TgC yr^-1).

Effect of Dunaliella tertiolecta organic exudates on the Fe(II) oxidation kinetics in seawater

The role played by the natural organic ligands excreted by the green algae Dunaliella tertiolecta on the Fe(II) oxidation rate constants was studied at different stages of growth. The concentration of dissolved organic carbon increased from 2.1 to 7.1 mg L(-1) over time of culture. The oxidation kinetics of Fe(II) was studied at nanomolar levels and under different physicochemical conditions of pH (7.2-8.2), temperature (5-35 °C), salinity (10-37), and dissolved organic carbon produced by cells (2.1-7.1 mg L(-1)). The experimental rate always decreased in the presence of organic exudates with respect to that in the control seawater. The Fe(II) oxidation rate constant was also studied in the context of Marcus theory, where ΔG° was 39.31-51.48 kJ mol(-1). A kinetic modeling approach was applied for computing the equilibrium and rate constants for Fe(II) and exudates present in solution, the Fe(II) speciation, and the contribution of each Fe(II) species to the overall oxidation rate constant. The best fit model took into account two acidity equilibrium constants for the Fe(II) complexing ligands with pKa,1=9.45 and pKa,2=4.9. The Fe(II) complexing constants were KFe(II)-LH=3×10(10) and KFe(II)-L=10(7), and the corresponding computed oxidation rates were 68±2 and 36±8 M(-1) min(-1), respectively.

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.

The submarine volcano eruption at the island of El Hierro: physical-chemical perturbation and biological response

On October 10 2011 an underwater eruption gave rise to a novel shallow submarine volcano south of the island of El Hierro, Canary Islands, Spain. During the eruption large quantities of mantle-derived gases, solutes and heat were released into the surrounding waters. In order to monitor the impact of the eruption on the marine ecosystem, periodic multidisciplinary cruises were carried out. Here, we present an initial report of the extreme physical-chemical perturbations caused by this event, comprising thermal changes, water acidification, deoxygenation and metal-enrichment, which resulted in significant alterations to the activity and composition of local plankton communities. Our findings highlight the potential role of this eruptive process as a natural ecosystem-scale experiment for the study of extreme effects of global change stressors on marine environments.

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.

Oxidation of nanomolar levels of Fe(II) with oxygen in natural waters

The oxidation of Fe(II) by molecular oxygen at nanomolar levels has been studied using a UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5-8.2), NaHCO3 (0.1-9 mM), temperature (3-35 degrees C), and salinity (0-36) on the oxidation of Fe(II) are presented. The first-order oxidation rates at nanomolar Fe(II) are higher than the values at micromolar levels at a pH below 7.5 and lower than the values at a higher pH. A kinetic model has been developed to consider the mechanism of the Fe(II) oxidation and the speciation of Fe(II) in seawater, the interactions between the major ions, and the oxidation rates of the different Fe(II) species. The concentration of Fe(II) is largely controlled by oxidation with O2 and O2.- but is also affected by hydrogen peroxide that may be both initially present and formed from the oxidation of Fe(II) by superoxide. The model has been applied to describe the effect of pH, concentration of NaHCO3, temperature, and salinity on the kinetics of Fe(II) oxidation. At a pH over 7.2, Fe(OH)2 is the most important contributing species to the apparent oxidation rate. At high levels of CO3(2-) and pH, the Fe(CO3)2(2-) species become important. At pH values below 7, the oxidation rate is controlled by Fe2+. Using the model, log k(i) values for the most kinetically active species (Fe2+, Fe(OH)+, Fe(OH)2, Fe(CO3), and Fe(CO3)2(2-)) are given that are valid over a wide range of temperature, salinity, and pH in natural waters. Model results showthatwhen H2O2 concentrations approach the Fe(II) concentrations used in this study, the oxidation of Fe(II) with H2O2 also needs to be considered.