Melchor González-Dávila

The interannual variability of oceanic CO2 parameters in the northeast Atlantic subtropical gyre at the ESTOC site

[1] The long-term trends and average seasonal variability in the upper ocean inorganic carbon observations were investigated at the ESTOC Station (the European Time Series in the Canary Islands), on the basis of an existing 10-year series (1995-2004). Hydrographic temperature and salinity, together with the pH in total scale at 25°C (pH T ), total alkalinity (AT), partial pressure of CO 2 expressed as fugacity (∫CO 2 ), computed dissolved inorganic carbon (C T ) and fluxes of CO 2 (FCO 2 ) reveal substantial variability over the years. Seasonal detrended data of salinity-normalized C T (NC T ) and experimental ∫CO 2 show upward trends of 0.99 ± 0.20 μmol kg -1 yr -1 and 1.55 ± 0.43 μatm yr -1 , respectively, indicating direct control over the C T concentration due to increased atmospheric CO 2 concentration. Our series of experimental pH T data confirm the acidification of surface waters in the east Atlantic Ocean, with an interannual decrease of 0.0017 ± 0.0004 pH units yr -1 . Interannual trends were examined by determining the variance in biogeochemical anomalies over time. The resulting anomalies in temperature and salinity revealed two scenarios in the ESTOC site, where there are periods of cooler and fresher water than the mean, driven by variations in winter mixed-layer depths, and periods with inverse temperature and salinity anomaly relationships, related to seasonal changes in the position of the subtropical gyre. Hydrographic and biogeochemical anomalies at ESTOC were linked to large-scale climate variability indexes, such as the North Atlantic Oscillation (NAO) and the East Atlantic pattern (EA). A delay of around 3 years in the oceanic response to the NAO best correlates with the anomalies observed for temperature (0.83), salinity (0.56), alkalinity (0.49), C T (0.41), ∫O 2 (0.57) and the depth of the mixed layer (-0.64) with p < 0.05. The seasonal variability and its link-in to the large-scale climate variability of the North Atlantic subtropical gyre has been studied using the two long series, BATS and ESTOC.

Tracking the variable North Atlantic sink for atmospheric CO2

A Happy Marriage The fluxes of CO2 between the atmosphere and ocean are large and variable, and understanding why the concentration of atmospheric CO2 changes as it does, depends on accurately determining the details of those fluxes. One of the major obstacles in the way of quantifying this exchange is that there are too few measurements available, both temporally and geographically. Watson et al. (p. 1391) report results from a happy marriage of science and commerce—data collected by instruments fitted onto commercial ships plying the waters of the North Atlantic Ocean—that has generated the largest and most comprehensive set of measurements of ocean pCO2 ever collected. These data allow the oceanic CO2 sink to be monitored with unprecedented accuracy and will help researchers precisely map regional interannual air-sea fluxes. Data from instrumented commercial ships reveal substantial interannual variations of carbon dioxide flux between the ocean and the air. The oceans are a major sink for atmospheric carbon dioxide (CO2). Historically, observations have been too sparse to allow accurate tracking of changes in rates of CO2 uptake over ocean basins, so little is known about how these vary. Here, we show observations indicating substantial variability in the CO2 uptake by the North Atlantic on time scales of a few years. Further, we use measurements from a coordinated network of instrumented commercial ships to define the annual flux into the North Atlantic, for the year 2005, to a precision of about 10%. This approach offers the prospect of accurately monitoring the changing ocean CO2 sink for those ocean basins that are well covered by shipping routes.

The effect of organic compounds in the oxidation kinetics of Fe(II)

Abstract The oxidation of Fe(II) has been studied in the presence of the natural organic compounds alanine, glutamic acid, cysteine, and two synthetic aminocarboxilates [(ethylenedioxi)diethylenedinitrilo]tetra-acetic acid (EGTA) and (ethylenedinitro)tetra-acetic acid (EDTA), as a function of pH t (6–8), ionic strength (0.2–1 m) and temperature (5–35°C), in NaCl solutions, at different Fe(II)–organic compound ratios. Alanine and glutamic acid did not affect the oxidation kinetics of Fe(II). For these compounds, a second order pH dependence is obeyed over the pH range studied, where log k obs =−16.29(0.16)+2.09(0.02)pH and log k obs =−15.26(1.3)+1.94(0.18)pH, for the alanine and glutamic acid, respectively. EGTA formed a strong ferrous complex that inhibited the oxidation of Fe(II) and EDTA increased the oxidation of ferrous iron forming a Fe(III)–EDTA complex that showed photoreduction in the presence of light, regenerating Fe(II). In the pH range from 6 to 8.2, the process was not affected by pH. The dependence with ionic strength was described by the equation log k =15.351+0.565 I 1/2 −1.388 I . Cysteine modified this behavior as a function of the Fe(II)–cysteine ratios. A Fe(III)–cysteine complex is formed through a one-electron transfer process that involved the thiol group and resulted in the reduction of Fe(III) back to Fe(II), and the oxidation of cysteine to cystine. The Fe(OH)L complex formation and reduction was affected by pH and cysteine concentration. A kinetic model that describes the behavior observed has been developed.

Binding of Cu(II) to the surface and exudates of the alga Dunaliella tertiolecta in seawater

The adsorption process and the organic matter interaction between the marine phytoplankton specie Dunaliella tertiolecta and copper ions were investigated by differential pulse anodic stripping voltammetry. Suspensions of living algae in natural Gran Canaria (Islas Canarias) seawater were titrated with Cu(II) as a function of pH, temperature, and salinity. The acid-base properties of the surface of the alga in a 0.7 M NaCI solution were characterized and interpreted as if the surface contained carboxylic acid groups (pK a,1 = 4.92 ± 0.16) and amino groups (pK a,2 = 6.28 ± 0.09, pK a,3 = 10.06 ± 0.10). The binding constant for the weaker ligands of the Gran Canaria seawater increased from 8.60 ± 0.03 to 9.30 ± 0.12 when 2.2 x 10 7 cell L -1 was added to 0.45-μm filtered seawater. The rate of uptake was found to occur in two steps. The adsorption equilibrium data correlate well with a two-site model which considers the algal surface as one which possesses two major functional groups : high-affinity binding and low-affinity binding. The high-affinity constant was of the same order of magnitude as the complexing capacity of the exudates excreted by the algae, showing that extracellular ligands play an important role in decreasing the concentration of the free metal concentration. Changes in the temperature, salinity, and pH of the seawater solution modified both the adsorption of metal and the amount of complexed copper.

The role of phytoplankton cells on the control of heavy metal concentration in seawater

Abstract An overview is presented of the ability of phytoplankton to passively adsorb and actively assimilate heavy metals from their aqueous environment and to release into the environment organic ligands capable of complexing metals. The uptake of all necessary trace metals by phytoplankton occurs via binding to a surface ligand and subsequent transfer across the cell membrane. This sorptive process can be explained by using surface complex formation equilibria; due to the heterogeneity of the algal surface, multi-site binding models must be developed. The production of extracellular organic matter with metal complexing properties plays an important role in decreasing the concentration of free metal ions and thus mitigating the potential toxic effects on organisms. However, while much research has been carried out on the uptake of single species of metal ions, little attention seems to have been given to the study of multimetal ion systems. Synergistic and antagonistic interactions between multiple trace metals are expected and could be very important in the oceans. These types of behaviours are extremely complex, influencing uptake and release of natural ligands and limiting plankton production and plankton species composition in the oceans. Future investigations should be carried out in order to gain understanding how the combinations of metal ions affect the physiological, biochemical and ecological processes of phytoplankton in seawater.

Adsorption of copper on Pseudomonas aureofaciens: protective role of surface exopolysaccharides.

Adsorption of copper on exopolysaccharide (EPS)-rich and (EPS)-poor soil rhizospheric Pseudomonas aureofaciens cells was studied as a function of pH and copper concentration at different exposure time in order to assess the effect of cell exopolysaccharides on parameters of adsorption equilibria. The surface properties of bacteria were investigated as a function of pH and ionic strength using potentiometric acid-base titration and electrophoresis that permitted the assessment of the excess surface proton concentration and zeta-potential of the cells, respectively. For adsorption experiments, wide range of Cu concentration was investigated (0.1-375 microM) in order to probe both weak and strong binding sites at the surface. Experimental results were successively fitted using a Linear Programming Model approach. The groups with pK(a) of 4.2-4.8 and from 5.2 to 7.2, tentatively assigned as carboxylates and phosphoryl respectively, are the most abundant at the surface and thus essentially contribute to the metal binding. The presence of exopolysaccharides on the surface decreases the amount of copper adsorbed on the bacterial cell wall apparently via screening the underlining functional groups of the cell wall. At the same time, dissolved EPS substances do not contribute to Cu binding in aqueous solution. Results of this study allow quantification of the role played by the surface EPS matrix as a protective barrier for metal adsorption on bacterial cell walls.

The adsorption of Cd(II) and Pb(II) to chitin in seawater

The adsorption of Cd2+ and Pb2+ to the surface of chitin has been studied in seawater as a function of pH (2 to 8), temperature (5 to 60°C), salinity (4 to 36), and the presence of other transition metals. The rates of adsorption were quite rapid (t12 = 20 min) and were affected by the presence of other metals such as Cu2+. For Pb2+, a Langmuir-type adsorption equilibrium was used to represent the experimental data. The Langmuir stability constant, KHs, was 1.9 × 10−10 mg mol−1 and the complexing capacity was 4.4 × 10−8 mol g−1. For Cd2+, a single linear adsorption with a slope of 515.1 ± 8.7 ml g−1 was found. A decrease in the temperature resulted in greater adsorption for both Cd2+ and Pb2+. For Pb2+, an increase in the salinity resulted in a decrease of the adsorption. For Cd2+, the reverse behavior was observed as a consequence of a direct competition between the chitin surface groups and the Cl− ions in solution.

Copper adsorption in diatom cultures

Abstract The organic ligands naturally present in seawater, on the cell surface groups, and those released by the marine phytoplankton species, Thalassiosira weissflogii and Phaeodactylum tricornutum , and their physico-chemical interaction with copper ions were studied using differential pulse anodic stripping voltammetry (DPASV) as a function of pH, temperature, salinity and biomass. The acid–base properties were characterized from titration curves of diatom suspensions with proton. Three p K a values were determined for each diatom, all of which were similar. Titration curves with copper allowed us to determine the specific adsorption of copper in a heterogeneous adsorption model. An iterative method that combines both Scatchard and Van den Berg–Ruzic approaches was used in order to determine the complexing capacity and the binding constants. High-affinity surface groups of both algae have similar affinity for copper but the concentration of these groups is 45% per cell higher for P. tricornutum as compared to T. weissflogii . The low-affinity groups in T. weissflogii (9.37) have higher stability constants than those in the P. tricornutum (9.0). After 36 h equilibrium, a ligand concentration of 18.6×10 −16 M cell −1 P. tricornutum and 25.0×10 −16 M cell −1 T. weissflogii with a conditional stability constant of log K ′=9.8 and log K ′=10.0, respectively, was exuded into the original seawater. In the presence of lead, high specificity was observed for the lowest stability constant ligands for copper, while the highest stability constant ligands were affected more by the presence of lead in solution. The ligands of T. weissflogii were less affected.

Reduction of Fe(III) with sulfite in natural waters

The Fe(III) in marine aerosols and rainwaters can be reduced to Fe(II) by photochemical processes and by reactions with sulfite. In this paper, measurements of the rates of reduction of nanomolar levels of Fe(III) with sulfite (without O2) have been determined in NaCl and seawater solutions as a function of temperature (0° to 40°C), pH (2 to 6.8), ionic strength (I = 0.1 to 6 M), and composition (Na+, Mg2+, Ca2+, F−, Cl−, Br−, HCO3−, SO42−). The overall rate constant (k, M−1 min−1) for the reaction, Fe(III)+S(IV)→k products, is given by d[Fe(III)]/dt = −k[Fe(III)] [S(IV)]. The reaction was found to be first order with respect to Fe(III) and S(IV). The rate constants as a function of pH increased from a pH = 2 to 4 and decreased at higher pH. The effect of temperature and ionic strength on the rates could be represented by log k = log k0 + AI0.5/(1 + I0.5), where A = −1.1 in NaCl and −2.2 in seawater and log k0 = 25.39 − 6,323/T. The energy of activation was found by 121±6 kJ mol−1. The measured rates in seawater as a function of salinity were lower than the rates in NaCl at the same ionic strength. Measurements in NaCl solutions with added sea-salt ions (Mg2+, Ca2+, F−, Br−, and SO42−) at pH = 3.5 indicate that the formation of inert FeF2+ may be responsible for the lower rates. The effect of changes in the composition on the rates was interpreted by examining the speciation of Fe(III) and S(IV). This analysis indicates that the rate-determining steps from a pH of 2.5 to 4.0 are FeOH2+ + HSO3− ↔ HOFeSO3H + and HOFeSO3H+⟶k1FeOH++HSO3· and at pH of 4 to 6, the reactions Fe(OH)2+ + HSO3− ↔ (HO)2FeSO3H and (HO)2FeSO3H⟶k2Fe(OH)2+HSO3· become important. The changes in the concentration of FeOH2+ and HSO3− as a function of pH and composition can account for most of the changes in the rates. These kinetic studies indicate that the rates of reduction of Fe(III) with S(IV) in acidic water droplets at natural levels of S(IV) may be an important source of Fe(II).

The adsorption of copper to chitin in seawater

The interaction of metal ions with particulate matter is important in limiting their concentration in seawater. Chitin is one of the constituents of the natural particulate organic matter than can interact with metal ions and therefore may serve as a reasonable model for natural organic solids. The adsorption of Cu2+ on the chitin surface has been studied in seawater as a function of pH, temperature, and salinity. The amphoteric properties of the surface of chitin were characterized in 0.7 M NaCl in terms of a two-protic acid-base system (pHPZC = 5.4) with acidity constants p∗Ka1s = 4.4 ± 0.2 and p∗Ka2s = 6.4 ± 0.2. The maximum proton exchange capacity of chitin was found to be 2.3 ± 0.3 mol kg−1, broadly similar to other solids. The rates of the adsorption were quite rapid (t12 = 8 min) and not strongly affected by the presence of other metals such as Cd2+ and Pb2+. The adsorption equilibrium data have been found to correlate well with surface complex-formation equilibria or the mathematically equivalent Langmuir- type adsorption equilibria. The value for the stability constant of Cu2+ on chitin was found to be log∗KHs = 8.95 ± 0.01, and the complexing capacity of chitin was found to be 6.9 and 5.9 μmol g−1, respectively, in the absence and in the presence of Cd2+ and Pb2+. An increase in the salinity and a decrease in the temperature result in greater adsorption of Cu2+ to chitin