Influence of ocean acidification on the complexation of iron and copper by organic ligands in estuarine waters

ArticleinMarine Chemistry 177(3) · March 2015with 280 Reads
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    We compiled a data set of ~2000 published metal speciation measurements made on samples of fresh waters, estuarine and coastal waters, and open ocean waters. For each sample, we applied the chemical speciation model WHAM7 to calculate the equilibrium free metal ion concentrations, [M] (mol L–1), amounts of metal bound by dissolved organic matter (DOM), ν (mol g –1), and their ratio ν/[M] (L g–1), which is a kind of ‘local’ partition coefficient. Comparison of the measured and predicted speciation variables for the whole data set showed that agreements are best for fresh waters, followed by estuarine and coastal waters, then open-ocean waters. Predicted values of ν/[M], averaged over all results for each metal, closely follow the trend in average measured values, confirming that metal reactivity, and consequent complexation by DOM, in natural waters accord with the expectations of the speciation model. Comparison of model predictions with measurements by different analytical techniques suggests that competitive ligand–stripping voltammetry methods overestimate metal complexation by DOM, and therefore underestimate [M]. When measurements by other methods are compared with predictions, for all metals, reasonable agreement with little bias is obtained at values of ν > 10–6 mol g–1 DOM, but at lower values of ν, the model predictions of [M] are mostly higher than the measured values, and the predictions of ν and ν/[M] are mostly lower. Research is needed to establish whether this reflects analytical error or the failure of the model to represent natural high-affinity ligands.
  • Article
    Factors that affect the respiration of organic carbon by marine bacteria can alter the extent to which the oceans act as a sink of atmospheric carbon dioxide. We designed seawater dilution experiments to assess the effect of pCO2 enrichment on heterotrophic bacterial community composition and metabolic potential in response to a pulse of phytoplankton‐derived organic carbon. Experiments included treatments of elevated (1000 ppm) and low (250 ppm) pCO2, amended with 10 μmol L‐1 dissolved organic carbon from Emiliana huxleyi lysates, and were conducted using surface‐seawater collected from the South Pacific Subtropical Gyre. To assess differences in community composition and metabolic potential, shotgun metagenomic libraries were sequenced from low and elevated pCO2 treatments collected at the start of the experiment and following exponential growth. Our results indicate bacterial communities changed markedly in response to the organic matter pulse over time and was significantly affected by pCO2 enrichment. Elevated pCO2 also had disproportionate effects on the abundance of sequences related to proton pumps, carbohydrate metabolism, modifications of the phospholipid bilayer, resistance to toxic compounds and conjugative transfer. These results contribute to a growing understanding of the effects of elevated pCO2 on bacteria‐mediated carbon cycling during phytoplankton bloom conditions in the marine environment. This article is protected by copyright. All rights reserved
  • Article
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    Global ocean acidification is a prominent, inexorable change associated with rising levels of atmospheric CO2. Here we present the first basin-wide direct observations of recently declining pH, along with estimates of anthropogenic and non-anthropogenic contributions to that signal. Along 152°W in the North Pacific Ocean (22-56°N), pH changes between 1991 and 2006 were essentially zero below about 800 m depth. However, in the upper 500 m, significant pH changes, as large as -0.06, were observed. Anthropogenic and non-anthropogenic contributions over the upper 800 m are estimated to be of similar magnitude. In the surface mixed layer (depths to ˜100 m), the extent of pH change is consistent with that expected under conditions of seawater/atmosphere equilibration, with an average rate of change of -0.0017/yr. Future mixed layer changes can be expected to closely mirror changes in atmospheric CO2, with surface seawater pH continuing to fall as atmospheric CO2 rises.
  • Chapter
    Although a classification of aquatic organic matter is difficult, an examination of recent literature reviews (Williams (1975), Reuter and Perdue (1977), Buffle (1984)) indicates that the major groups of organic ligands are polysaccharides, proteins and peptides, “pedogenic” (soil derived) refractory organic matter (PROM) and “aquogenic” (formed in situ in the water body) refractory organic matter (AROM). Altogether, the last two groups form the so-called water fulvic and humic acids which represent a large proportion (~ 70–80%) of the organic matter in natural waters. The characteristics of PROM resemble those of soil fulvic acids (SFA: extracted from soil; Schnitzer (1978)), but differ markedly from those of AROM and of humic and fulvic fractions of sediments (Buffle (1984)). The complexation data reported in the literature concern mostly SFA and the fulvic fractions of PROM and AROM.
  • Article
    A Donnan-type model for nonspecific binding of electrolyte ions has been combined with the non-ideal competitive adsorption (NICA) model for specific binding to produce a model for ion binding to humic substances. The model considers site heterogeneity, non-ideality, multicomponent competition, and electrostatic interactions. The NICA-Donnan model was fitted to data for H, Ca, Cd, Cu, and Pb binding by a purified peat humic acid. The model fits were good and covered a wide range of pH and free metal concentrations. The parameters from these single metal data sets were then used to predict the competitive effect of Ca on Cd and Cu binding at various pHs. These predictions agreed well with the experimental data although there were some small but systematic differences. The new NICA-Donnan model also predicted reasonably well the increase in Cd and Cu binding on changing from a 0.1 M KNO3 background electrolyte to 0.01 M KNO3. A shortcoming of the model is that in some cases it significantly underestimated the H+/M2+ exchange ratio, especially at high pH and for Cu binding.
  • Article
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    It has been proposed that increasing levels of pCO2 in the surface ocean will lead to more partitioning of the organic carbon fixed by marine primary production into the dissolved rather than the particulate fraction. This process may result in enhanced accumulation of dissolved organic carbon (DOC) in the surface ocean and/or concurrent accumulation of transparent exopolymer particles (TEPs), with important implications for the functioning of the marine carbon cycle. We investigated this in shipboard bioassay experiments that considered the effect of four different pCO2 scenarios (ambient, 550, 750 and 1000 μatm) on unamended natural phytoplankton communities from a range of locations in the northwest European shelf seas. The environmental settings, in terms of nutrient availability, phytoplankton community structure and growth conditions, varied considerably between locations. We did not observe any strong or consis- tent effect of pCO2 on DOC production. There was a significant but highly variable effect of pCO2 on the production of TEPs. In three of the five experiments, variation of TEP production between pCO2 treatments was caused by the effect of pCO2 on phytoplankton growth rather than a direct effect on TEP production. In one of the five experiments, there was evidence of enhanced TEP production at high pCO2 (twice as much production over the 96 h incubation period in the 750μatm treatment compared with the ambient treatment) independent of indirect effects, as hypothesised by previous studies. Our results suggest that the environmental setting of experiments (community structure, nutrient availability and occurrence of phytoplankton growth) is a key factor determining the TEP response to pCO2 perturbations.
  • Article
    Here we demonstrate the use of reverse titration - competitive ligand exchange-adsorptive cathodic stripping voltammetry (RT-CLE-ACSV) for the analysis of iron (Fe) binding ligands in seawater. In contrast to the forward titration, which examines excess ligands in solution, RT-CLE-ACSV examines the existing Fe-ligand complexes by increasing the concentration of added (electroactive) ligand (1-nitroso-2-naphthol) and analysis of the proportion of Fe bound to the added ligand. The data manipulation allows the accurate characterisation of ligands at equal or lower concentrations than Fe in seawater, and disregards electrochemically inert dissolved Fe such as some colloidal phases. The method is thus superior to the forward titration in environments with high Fe and low ligand concentrations or high concentrations of inert Fe. We validated the technique using the siderophore ligand ferrioxamine B, and observed a stability constant [Formula: see text] of 0.74-4.37×10(21)mol(-1), in agreement with previous results. We also successfully analysed samples from coastal waters and a deep ocean hydrothermal plume. Samples from these environments could not be analysed with confidence using the forward titration, highlighting the effectiveness of the RT-CLE-ACSV technique in waters with high concentrations of inert Fe.
  • Article
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    The sensitivity of marine crustaceans to ocean acidification is poorly understood, but can be assessed by combining data from physiological and ecological studies. The species most at risk are exclusively marine and have limited physiological capacities to adjust to environmental change. They are poor iono- and osmoregulators and have limited abilities to compensate for acid-base disturbances. The problems are compounded in slow-moving, relatively inactive species because they have low circulating protein levels and low buffering capacities. Species living in low-energy environments, such as deep-sea and polar habitats, are particularly vulnerable, because they are metabolically limited with respect to environmental change. Elevated pCO 2 levels in seawater, such as those predicted for the year 2300, are known to have diverse effects on calcification rate, little effect on egg production and a negative effect on growth rate and moulting frequency in marine crustacean species. At these levels, embryonic development is negatively impacted, but larval and juvenile stages do not appear to be affected, unless the changes in pCO 2 are accompanied by rising temperatures. Overall, marine crustaceans are broadly tolerant to the seawater pCO 2 levels expected by 2100 and 2300, but only in the medium-term (weeks) and only in the more adaptable species. The reductions in growth rate are of concern, as these changes could affect species survival, distribution and abundance. Studies are urgently needed to evaluate whether the patterns of vulnerability identified here in crustaceans will still be relevant after long-term (months) exposure to the relevant pCO 2 levels, in combination with changes in other environmental factors.
  • Article
    Humic Ion-Binding Model VII aims to predict the competitive reactions of protons and metals with natural organic matter in soils and waters, based on laboratory results with isolated humic and fulvic acids (HA and FA). Model VII is simpler in its postulated multidentate metal binding sites than the previous Model VI. Three model parameters were eliminated by using a formal relationship between monodentate binding to strong- and weak-acid oxygen-containing ligands, and removing factors that provide ranges of ligand binding strengths. Thus Model VII uses a single adjustable parameter, the equilibrium constant for monodentate binding to strong-acid (carboxylate) groups (K-MA), for each metallic cation. Proton-binding parameters, and mean values of log K-MA were derived by fitting 248 published datasets (28 for protons, 220 for cationic metals). Default values of log K-MA for FA were obtained by combining the fitted values for FA, results for HA, and the relationship for different metals between log K-MA and equilibrium constants for simple oxygen-containing ligands. The equivalent approach was used for HA. The parameterised model improves on Model VI by incorporating more metals (40), providing better descriptions of metal binding at higher pH, and through more internally consistent parameter values.
  • Article
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    Within the context of the UK Ocean Acidification project, Emiliania huxleyi (type A) coccolith morphology was examined from samples collected during cruise D366. In particular, a morphometric study of coccolith size and degree of calcification was made on scanning electron microscope images of samples from shipboard CO2 perturbation experiments and from a set of environmental samples with significant variation in calcite saturation state (Omega_calcite). One bioassay in particular (E4 from the southern North Sea) yielded unambiguous results – in this bioassay exponential growth from a low initial cell density occurred with no nutrient enrichment and coccosphere numbers increased tenfold during the experiment. The samples with elevated CO2 saw significantly reduced coccolithophore growth. However, coccolithophore morphology was not significantly affected by the changing CO2 conditions even under the highest levels of perturbation (1000 μatm CO2). Environmental samples similarly showed no correlation of coccolithophore morphology with calcite saturation state. Some variation in coccolith size and degree of calcification does occur but this seems to be predominantly due to genotypic differentiation between populations on the shelf and in the open ocean.
  • Article
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    The ongoing oceanic uptake of anthropogenic carbon dioxide (CO2) is significantly altering the carbonate chemistry of seawater, a phenomenon referred to as ocean acidification. Experimental manipulations have been increasingly used to gauge how continued ocean acidification will potentially impact marine ecosystems and their associated biogeochemical cycles in the future; however, results amongst studies, particularly when performed on natural communities, are highly variable, which may reflect community/environment-specific responses or inconsistencies in experimental approach. To investigate the potential for identification of more generic responses and greater experimentally reproducibility, we devised and implemented a series (n = 8) of short-term (2–4 days) multi-level (>4 conditions) carbonate chemistry/nutrient manipulation experiments on a range of natural microbial communities sampled in Northwest European shelf seas. Carbonate chemistry manipulations and resulting biological responses were found to be highly reproducible within individual experiments and to a lesser extent between geographically separated experiments. Statistically robust reproducible physiological responses of phytoplankton to increasing pCO2, characterised by a suppression of net growth for small-sized cells (<10 μm), were observed in the majority of the experiments, irrespective of natural or manipulated nutrient status. Remaining between-experiment variability was potentially linked to initial community structure and/or other site-specific environmental factors. Analysis of carbon cycling within the experiments revealed the expected increased sensitivity of carbonate chemistry to biological processes at higher pCO2 and hence lower buffer capacity. The results thus emphasise how biogeochemical feedbacks may be altered in the future ocean.
  • Article
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    Hemes are iron containing heterocyclic molecules important in many cellular processes. In the marine environment, hemes participate as enzymatic cofactors in biogeochemically significant processes like photosynthesis, respiration, and nitrate assimilation. Further, hemoproteins, hemes, and their analogs appear to be iron sources for some marine bacterioplankton under certain conditions. Current oceanographic analytical methodologies allow for the extraction and measurement of heme b from marine material, and a handful of studies have begun to examine the distribution of heme b in ocean basins. The study of heme in the marine environment is still in its infancy, but some trends can be gleaned from the work that has been published so far. In this review, we summarize what is known or might be inferred about the roles of heme in marine microbes as well as the few studies on heme in the marine environment that have been conducted to date. We conclude by presenting some future questions and challenges for the field.
  • Article
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    Message: The updated R script to calculate the ligand characteristics of 1 or 2 ligand classes from titration data is available for everybody and can be requested from micha.rijkenberg@nioz.nl Abstract: The determination of the thermodynamic characteristics of organic Fe binding ligands, total ligand concentration ([Lt]) and conditional binding constant (K'), by means of titration of natural ligands with Fe in the presence of an added known competing ligand, is an indirect method. The analysis of the titration data including the determination of the sensitivity (S) and underlying model of ligand exchange is discussed and subjected to a critical evaluation of its underlying assumptions. Large datasets collected during the international Polar Year, were used to quantify the error propagation along the determination procedure. A new and easy to handle non-linear model written in R to calculate the ligand characteristics is used. The quality of the results strongly depends on the amount of titration points or Fe additions in a titration. At least four titration points per distinguished ligand group, together with a minimum of four titration points where the ligands are saturated, are necessary to obtain statistically reliable estimates of S, K' and [Lt]. As a result estimating the individual concentration of two ligands, although perhaps present, might not always be justified.
  • Article
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    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 (pHT), total alkalinity (AT), partial pressure of CO2 expressed as fugacity (fCO2), computed dissolved inorganic carbon (CT) and fluxes of CO2 (FCO2) reveal substantial variability over the years. Seasonal detrended data of salinity-normalized CT (NCT) and experimental fCO2 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 CT concentration due to increased atmospheric CO2 concentration. Our series of experimental pHT 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), CT (0.41), fCO2 (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.
  • Article
    Iron supply has been suggested to influence phytoplankton biomass, growth rate and species composition, as well as primary productivity in both high and low NO3 (-) surface waters. Recent investigations in the equatorial Pacific suggest that no single factor regulates primary productivity. Rather, an interplay of bottom-up (i.e., ecophysiological) and top-down (i.e., ecological) factors appear to control species composition and growth rates. One goal of biological oceanography is to isolate the effects of single factors from this multiplicity of interactions, and to identify the factors with a disproportionate impact. Unfortunately, our tools, with several notable exceptions, have been largely inadequate to the task. In particular, the standard technique of nutrient addition bioassays cannot be undertaken without introducing artifacts. These so-called 'bottle effects' include reducing turbulence, isolating the enclosed sample from nutrient resupply and grazing, trapping the isolated sample at a fixed position within the water column and thus removing it from vertical movement through a light gradient, and exposing the sample to potentially stimulatory or inhibitory substances on the enclosure walls. The problem faced by all users of enrichment experiments is to separate the effects of controlled nutrient additions from uncontrolled changes in other environmental and ecological factors. To overcome these limitations, oceanographers have sought physiological or molecular indices to diagnose nutrient limitation in natural samples. These indices are often based on reductions in the abundance of photosynthetic and other catalysts, or on changes in the efficiency of these catalysts. Reductions in photosynthetic efficiency often accompany nutrient limitation either because of accumulation of damage, or impairment of the ability to synthesize fully functional macromolecular assemblages. Many catalysts involved in electron transfer and reductive biosyntheses contain iron, and the abundances of most of these catalysts decline under iron-limited conditions. Reductions of ferredoxin or cytochrome f content, nitrate assimilation rates, and dinitrogen fixation rates are amongst the diagnostics that have been used to infer iron limitation in some marine systems. An alternative approach to diagnosing iron-limitation uses molecules whose abundance increases in response to iron-limitation. These include cell surface iron-transport proteins, and the electron transfer protein flavodoxin which replaces the Fe-S protein ferredoxin in many Fe-deficient algae and cyanobacteria.
  • Article
    A high temperature catalytic oxidation (HTCO) technique was used to measure dissolved organic carbon (DOC) during seasonal surveys of the Tamar Estuary, U.K. At the time of the programme, the field of DOC analysis had been plagued by numerous analytical difficulties. However, using thorough calibration of the analytical systems and the systematic analysis of an internal reference material, a valuable estuarine DOC data set was produced. The range of DOC concentrations observed (478–110 μM C) is consistent with the published data for riverine and coastal sea waters respectively. The Tamar Estuary is a freshwater DOC-dominated system, with strong correlation between lateral DOC distribution and salinity. However, mixing behaviour was not strictly conservative. During tidal cycle studies at a fixed station, DOC concentrations appeared to be uncoupled from salinity, and were inversely related to turbidity. It is concluded that tidally-induced resuspension of bottom sediments provided the dominant control mechanism for DOC concentration. The Tamar Estuary shows contrasting behaviour to the larger, more heavily impacted, Severn Estuary. Hence it is likely that the behaviour of DOC in estuaries cannot be classified as typical per se, but is a function of the natural and anthropogenic characteristics of the catchment and hydrology.
  • Article
    The uptake of anthropogenic CO2 by the oceans since the onset of the industrial revolution is considered a serious challenge to marine ecosystems due to ensuing carbonate-chemistry changes (ocean acidification). Furthermore, the CO2 uptake is reducing the ocean’s capacity to absorb future CO2 emissions. In order to follow the changes in the ocean’s carbonate system, high-quality analytical measurements with good spatial and temporal resolution are necessary. High-precision and accurate pH measurements are now possible, and allow us to determine the progression of ocean acidification. The spectrophotometric pH technique is now widely used and capable of the required high-quality measurements. Spectrophotometric pH systems are deployed on ships and in situ on remote platforms. Smaller and more rugged instruments are nevertheless required for more widespread in situ application to allow routine high-resolution measurements, even in the most remote regions.We critically review oceanic pH measurements, and focus on state-of-the-art spectrophotometric pH measurement techniques and instrumentation. We present a simple microfluidic design integrated in a shipboard instrument featuring high accuracy and precision as a key step towards a targeted pH microsensor system.
  • Article
    The osmotic coefficients of FeCl3 at 25 °C from 0.15 to 1.7 m [Rumyantsev et al., Z. Phys. Chem., 218, 1089–1127, 2004] have been used to determine the Pitzer parameters (β(0), β(1) and Cϕ) for FeCl3. Since the differences in the Pitzer coefficients of rare earths in NaCl and NaClO4 are small, the values of Fe(ClO4)3 have been estimated using the differences between La(ClO4)3 and LaCl3. The Pitzer coefficients for FeCl3 combined with enthalpy and heat capacity data for the rare earths can be used to estimate the activity coefficients of Fe3+ in NaCl over a wide range of temperatures (0 to 50 °C) and ionic strength (0 to 6 m).The activity coefficients of Fe3+ in NaCl and NaClO4 solutions have been used to determine the activity coefficients of Fe(OH)2+ in these solutions from the measured first hydrolysis constants of Fe3+ [Byrne et al., Mar. Chem., 97, 34–48, 2005]. The activity coefficients of Fe(OH)2+, Fe(OH)3 and Fe(OH)4- from 0 to 50 °C have also been determined from the solubility measurements of Fe(III) in NaCl solutions [Liu and Millero, Geochim. Cosmochim Acta, 63, 3487–3497, 1999]. These activity coefficients have been fitted to the Pitzer equations. These results can be used to estimate the speciation of Fe(III) with OH− in natural waters with high concentrations of NaCl from 0 to 50 °C.
  • Article
    Centric diatoms isolated from open ocean environments require higher concentrations of Cu for growth than their coastal counterparts. In artificial seawater medium containing ,1 nmol L 21 Cu, three coastal species maintained near maximum rates of growth, but the oceanic clones were unable to survive. Copper limitation was more severe in the diatoms grown in low- than in high-Fe seawater, suggesting that Cu and Fe were interacting essential resources. The interactive effect was in part the result of a Cu requirement for Fe transport. Thalassiosira weissflogii and Thalassiosira oceanica had lower Fe quotas and slower rates of Fe uptake when (Cu) was reduced in the medium. Brief exposure of Cu-limited cells to 10 nmol L 21 Cu increased the instantaneous Fe uptake rate by 1.5 times in T. oceanica. Steady-state uptake rates of both species at high, growth-saturating concentrations of Fe were also Cu dependent. Oceanic species appeared to have an additional Cu requirement that was independent of Fe acquisition and likely responsible for their higher requirements compared to coastal species. Evidence for the importance of Cu in natural communities of phytoplankton was obtained from an incubation experiment performed in the Fe-limited basin of the Bering Sea. Addition of 2 nmol L 21 Cu doubled the phytoplankton net growth rate compared to the untreated controls and, in the presence of extra Fe, increased the growth rate compared to the samples amended with Fe alone. The results suggest that Cu may be an important micronutrient for phytoplankton growth in low-Fe regions of the sea because of its role in Fe acquisition. Paradoxically, oceanic diatoms may be more susceptible to the effects of low Cu concentrations than coastal species.
  • Article
    Full-text available
    Uptake of half of the fossil fuel CO2 into the ocean causes gradual seawater acidification. This has been shown to slow down calcification of major calcifying groups, such as corals, foraminifera, and coccolithophores. Here we show that two of the most productive marine calcifying species, the coccolithophores Coccolithus pelagicus and Calcidiscus leptoporus, do not follow the CO2-related calcification response previously found. In batch culture experiments, particulate inorganic carbon (PIC) of C. leptoporus changes with increasing CO2 concentration in a nonlinear relationship. A PIC optimum curve is obtained, with a maximum value at present-day surface ocean pCO2 levels (∼360 ppm CO2). With particulate organic carbon (POC) remaining constant over the range of CO2 concentrations, the PIC/POC ratio also shows an optimum curve. In the C. pelagicus cultures, neither PIC nor POC changes significantly over the CO2 range tested, yielding a stable PIC/POC ratio. Since growth rate in both species did not change with pCO2, POC and PIC production show the same pattern as POC and PIC. The two investigated species respond differently to changes in the seawater carbonate chemistry, highlighting the need to consider species-specific effects when evaluating whole ecosystem responses. Changes of calcification rate (PIC production) were highly correlated to changes in coccolith morphology. Since our experimental results suggest altered coccolith morphology (at least in the case of C. leptoporus) in the geological past, coccoliths originating from sedimentary records of periods with different CO2 levels were analyzed. Analysis of sediment samples was performed on six cores obtained from locations well above the lysocline and covering a range of latitudes throughout the Atlantic Ocean. Scanning electron micrograph analysis of coccolith morphologies did not reveal any evidence for significant numbers of incomplete or malformed coccoliths of C. pelagicus and C. leptoporus in last glacial maximum and Holocene sediments. The discrepancy between experimental and geological results might be explained by adaptation to changing carbonate chemistry.
  • Article
    The solubility of iron(III) hydroxide as a function of pH was investigated in NaCl solutions at different temperatures (5–50°C) and ionic strengths (0–5 M). Our results at 25°C and 0.7 M in the acidic range are similar to the solubility in seawater. The results between 7.5 to 9 are constant (close to 10−11 M) and are lower than those found in seawater (>10−10) in this pH range. The solubility subsequently increases as the pH increases from 9 to 12. The solubility between 6 and 7.5 has a change of slope that cannot be accounted for by changes in the speciation of Fe(III). This effect has been attributed to a solid-state transformation of Fe(OH)3 to FeOOH. The effect of ionic strength from 0.1 to 5 M at a pH near 8 was quite small. The solubility at 5°C is considerably higher than at 25°C at neutral pH range. The effects of temperature and ionic strength on the solubility at low and high pH have been attributed to the effects on the solubility product and the formation of FeOH2+ and Fe(OH)4−. The results have been used to determine the solubility products of Fe(OH)3, K∗Fe(OH)3 and hydrolysis constants, β∗1, β∗2, β∗3, and β∗4 as a function of temperature (T, K) and ionic strength (I):log K∗Fe(OH)3 = −13.486 − 0.1856 I0.5 + 0.3073 I + 5254/T (σ = 0.08)log β∗1 = 2.517 − 0.8885 I0.5 + 0.2139 I − 1320/T (σ = 0.03)log β∗2 = 0.4511 − 0.3305 I0.5 − 1996/T (σ = 0.1)log β∗3 = −0.2965 − 0.7881 I0.5 − 4086/T (σ = 0.6)log β∗4 = 4.4466 − 0.8505 I0.5 − 7980/T. (σ = 0.2)Both strong ethylenediaminetetraacetic acid and weak (HA) organic ligands greatly affect iron solubility. The additions of ethylenediaminetetraacetic acid and humic material were shown to increase the solubility near pH 8. The higher solubility of Fe(III) in seawater compared to 0.7 M NaCl may be caused by natural organic ligands.
  • Article
    Copper (Cu) complexation and distribution were characterized using competitive ligand exchange adsorptive cathodic stripping voltammetry and isotope dilution inductively coupled plasma mass spectrometry along two transects (20 degrees S and 10 degrees S) in the eastern tropical South Pacific. In the southern and westernmost stations, Cu showed upper water column depletion to values as low as similar to 0.26 nmol L-1, the lowest concentrations ever reported. However, Cu levels were much higher within the secondary nitrite maxima of the oxygen minimum zone (OMZ) in the northern (10 degrees S) transect. The enrichment of Cu in the reducing conditions of the OMZ has not been reported before and probably reflects remineralization and offshore transport from the shelf. Free [Cu2+] was typically low throughout the water column, ranging from about 3.15 x 10(-15) mol L-1 to 1.34 x 10(-13) mol L-1, and depth profiles exhibited similar features to those for dissolved Cu, though they showed more variability near the surface. Offshore and beyond the influence of the OMZ, the lowest dissolved and free [Cu2+] was within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. This finding is of interest because the two competing explanations for the PNM-iron limitation of diatoms and high rates of ammonia oxidation relative to nitrite oxidation-have high Cu requirements. The low concentrations of free Cu2+ measured here could impose significant constraints on the rates of these processes.
  • Article
    A highly sensitive voltammetric technique was developed to examine Fe speciation in seawater. The technique involves adding an Fe(III)-complexing ligand, salicylaldoxime, which competitively equilibrates with inorganic and organic Fe(III) species in ambient seawater. The Fe(III)-salicylaldoxime complex then is measured by adsorptive cathodic stripping voltammetry (ACSV). This new method revealed that 99.97% of the dissolved Fe(III) in central North Pacific surface waters is chelated by natural organic ligands. The total concentration of Fe-binding ligands is approximately 2 nM, a value greatly in excess of ambient dissolved iron concentrations. The titration data can be modeled as consisting of two classes of Fe-binding ligands, a strong ligand class (L1) with an average surface-water concentration equal to 0.44 nM with a conditional stability constant KL1Fe′cond = 1.2 × 1013 M−1, and a weaker ligand class (L2) with an average concentration equal to 1.5 nM with KL2Fe′cond = 3.0 × 1011 M−1. The low concentration of dissolved Fe present in surface waters (~ 0.2 nM), coupled with the excess of strong Fe-chelators, results in extremely low equilibrium concentrations of dissolved inorganic iron, [Fe′] ≈ 0.07 pM. In the deeper waters there is a 2 nM excess of Fe-binding ligands with a stability constant similar to that of the L2 class of ligands observed in surface waters, resulting in dissolved Fe(III) existing primarily in the chelated form in deep waters as well. The stability constants of the natural ligands are comparable to the model ligands desferal, a siderophore, and the prosthetic heme group, protoporphyrin-IX. The high degree of organic complexation of iron makes it critically important to reevaluate our perceptions of the marine biogeochemistry of iron and the mechanisms by which biota can access this chelated Fe.
  • Article
    The solubilities of iron(III) hydroxides in seawater were determined in Gulf Stream seawater as a function of pH (2 to 9), temperature (5 to 50 °C) and salinity (0 to 36). Our results at S=36 and 25 °C near a pH of 8 are in agreement with the measurements of Byrne and Kester [Mar. Chem. 4 (1976a) 255] and Kuma et al. [Limnol. Oceanogr. 41 (1996) 396] (0.2 to 0.3 nM). The solubilities at 5 °C are considerably higher than at 25 °C and decrease with a decrease in salinity. Near a pH of 8, the solubilities as a function of temperature (T/K) and ionic strength [I=19.922S/(1000−1.005S)] can be estimated from
  • Article
    Oceans are the largest reservoirs of reactive organic carbon on the Earth. Most (97%) of the organic carbon in seawater resides in the operationally defined dissolved phase. The separation of the particulate and dissolved phases is most commonly accomplished by the passage of water samples through filters with pore sizes in the range 0.2–1.0 μm. During the past decade, it has become increasingly common to avoid the filtration of open ocean water samples for the separation of the particulate and dissolved phases. Filtration is often omitted because particulate organic carbon (POC) is a small fraction of the total organic carbon and the extra handling and processing of water samples during filtration aboard ship can lead to contamination. Concentrations of organic carbon, nitrogen, and phosphorus in unfiltered water samples are referred to as total organic concentrations (TOC, TON, and TOP respectively). Relatively small volumes (milliliters) of seawater are analyzed for these measurements and larger particles are not represented in these samples. Thus, the term “total” is somewhat of a misnomer for these analyses. In this chapter the data gathered on the concentrations and chemical composition of marine DOM includes some samples that were not filtered prior to the analysis. These samples come from the open ocean and the contribution of particulate organic matter (POM) to the reported data is considered to be minimal. In some cases, data for both filtered and unfiltered samples were available and the differences among these samples were often within the error of the measurement.
  • Article
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    Full-text available
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    Over the next century, elevated quantities of atmospheric CO2 are expected to penetrate into the oceans, causing a reduction in pH (-0.3/-0.4 pH unit in the surface ocean) and in the concentration of carbonate ions (so-called ocean acidification). Of growing concern are the impacts that this will have on marine and estuarine organisms and ecosystems. Marine shelled molluscs, that colonized a large latitudinal gradient and can be found from intertidal to deep-sea habitats, are economically and ecologically important species providing essential ecosystem services including habitat structure for benthic organisms, water purification and a food source for other organisms. The effects of ocean acidification on the growth and shell production by juvenile and adult molluscs are variable among species and even within the same species, precluding the drawing of a general picture. This is, however, not the case for pteropods, with all species tested so far, being negatively impacted by ocean acidification. The blood of shelled mollusks may exhibit lower pH with consequences for several physiological processes (e.g. respiration, excretion...) and, in some cases, increased mortality in the long term. While fertilization may remain unaffected by elevated pCO2, embryonic and larval development will be highly sensitive with important reductions in size and decreased survival of larvae, increases in the number of abnormal larvae and an increase in the developmental time. There are big gaps in the current understanding of the biological consequences of an acidifying ocean on shelled molluscs. For instance, the natural variability of pH and the interactions of changes in the carbonate chemistry with changes in other environmental stressors such as increased temperature and changing salinity, the effects of species interactions, as well as the capacity of the organisms to acclimate and/or adapt to changing environmental conditions are poorly described.
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    The use of in-line UV-digestion for breakdown of dissolved organic matter (DOM), prior to voltammetric determination of trace metals in natural samples, is discussed in this paper. Destruction of DOM is necessary to free trace metals that are organically complexed and to remove interfering organic surfactants. Complete breakdown of DOM in natural water samples is achieved by in-line UV-digestion using a 100 W medium pressure mercury vapour lamp and a silica coil, at a sample digestion time of 4.5 min. The efficiency of the system is tested with destruction of humic acid, conversion of Cr(III) to Cr(VI), and the release of Ni and Cu from organic complexes in sea water. The in-line application of UV is illustrated by automated voltammetry of nanomolar levels of Cu and Ni in samples from oceanic origin
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    The metal complexing ability of surface water of the Irish Sea has been measured by the MnO2 adsorption method. In all samples strong copper-chelating compounds are present at concentrations of 60-150 nM, with conditional stability constants (log values) of 10.0-10.4. The concentrations of Cu, Pb and Cd in the samples are 16-39 nM, 1-7 nM and 0.1-2 nM, respectively; much less than the ligand concentrations. The organic compounds form complexes with 94-98% of dissolved copper, and therefore constitute the major form of copper in surface water of the Irish Sea. Recalculation of speciation of the inorganic fraction of copper in seawater reveals that the major complex ion is that of CuCO30 (60%), followed by CuOH+ (16%) and Cu(OH)20 (16%). Complexes with borate ions form a small and rather insignificant fraction of 1%.
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    The theory is discussed which describes the distribution of copper ions between a weak ion exchanger, as exemplified by MnO2, and natural organic complexing material in seawater. Application of this theory and experimental procedures are outlined in part II of this series. It is apparent from the theory that titration with Cu2+ of one or more organic complexing ligands can be graphically represented by straight lines; slope and y-axis intercept provide information on the conditional stability constants and the ligand concentrations. Model calculations show that measurement of metal complexation at ligand concentrations higher than normally present in seawater may produce erroneous results because of possible changes in the metal to ligand ratio in the complexes. It is therefore advisable to measure metal complexation in the original, unaltered, water sample.
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    A new method, based on the direct detection of iron-humic substance (HS) species by cathodic stripping voltammetry (CSV), is used to determine the iron binding capacity and complex stability of fulvic acid (FA), humic acid (HA), and the natural HS in the seawater. The FA binds 16.7 ± 2.0 nmol iron (Fe) (mg FA)-1, whereas the HA and the marine HS bind 32 ± 2.2 nmol Fe (mg FA)21. The complex stabilities are (log K'Fe'HS values) 10.6 for FA and 11.1 for HA and coastal HS. Measurements of coastal waters (Irish Sea) show that the HS occur in a widespread fashion, the HS concentration decreasing with increasing salinity and occurring at levels of 400 (at salinity 30) to 70 (at salinity 34) μg L-1. A sample from the deep Pacific was found to contain 36 μg HS L-1, amounting to 4% of the dissolved organic matter. Comparative measurement of the total iron complexing capacity by CSV with competitive ligand exchange showed that the natural HS can account for the entire ligand concentration in the shallow coastal and deep ocean waters tested. Measurements of the iron solubility showed that FA added to seawater and the HS in coastal waters maintain iron in solution at a level just below the iron binding capacity. The preliminary data for the open ocean indicate that the same may be true for HS in deep ocean waters. The data are consistent with a mechanism by which iron is transported from land to sea, associated with land-derived HS. © 2009, by the American Society of Limnology and Oceanography, Inc.
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    Total and labile Fe measurements, and Fe³⁺ titrations were carried out both at sea and in the laboratory with adsorptive cathodic stripping voltammetry (CSV) methods using 1-nitroso-2-naphthol (1N2N) as a complexing ligand to study Fe(III) speciation and the kinetic interaction of Fe³⁺ with naturally occurring organic ligands. On the continental slope and at the shelf/slope front of the Northwest Atlantic ocean, the total dissolved (< 0.4 μm) Fe was predominantly 1N2N nonlabile, with 60% nonlabile at the mouth of Delaware Bay. The exact chemical speciation of this nonlabile Fe is not known; although some of this Fe is likely in strong organic complexes with a KFeL1023.22 as determined by a competitive ligand equilibration/cathodic stripping voltammetry (CLE/CSV) method.