Martha Gledhill

The organic complexation of iron in the Marine Environment: A review

Iron (Fe) is an essential micronutrient for marine organisms, and it is now well established that low Fe availability controls phytoplankton productivity, community structure, and ecosystem functioning in vast regions of the global ocean. The biogeochemical cycle of Fe involves complex interactions between lithogenic inputs (atmospheric, continental, or hydrothermal), dissolution, precipitation, scavenging, biological uptake, remineralization, and sedimentation processes. Each of these aspects of Fe biogeochemical cycling is likely influenced by organic Fe-binding ligands, which complex more than 99% of dissolved Fe. In this review we consider recent advances in our knowledge of Fe complexation in the marine environment and their implications for the biogeochemistry of Fe in the ocean. We also highlight the importance of constraining the dissolved Fe concentration value used in interpreting voltammetric titration data for the determination of Fe speciation. Within the published Fe speciation data, there appear to be important temporal and spatial variations in Fe-binding ligand concentrations and their conditional stability constants in the marine environment. Excess ligand concentrations, particularly in the truly soluble size fraction, seem to be consistently higher in the upper water column, and especially in Fe-limited, but productive, waters. Evidence is accumulating for an association of Fe with both small, well-defined ligands, such as siderophores, as well as with larger, macromolecular complexes like humic substances, exopolymeric substances, and transparent exopolymers. The diverse size spectrum and chemical nature of Fe ligand complexes corresponds to a change in kinetic inertness which will have a consequent impact on biological availability. However, much work is still to be done in coupling voltammetry, mass spectrometry techniques, and process studies to better characterize the nature and cycling of Fe-binding ligands in the marine environment.

The toxicity of copper(II) species to marine algae, with particular reference to macroalgae

Ambient concentrations of dissolved copper(II) in seawater are very low. However, levels can increase as a result of natural and anthropogenic sources. Such increase can have profound effects on organisms in the vicinity resulting in inhibition of growth, reduced fecundity, and even death. This paper highlights the importance of pecieation when considering the toxic effects of cooper, with particular reference to macroalgae in a marine environment, and to encourage more biologists to take account of this in their studies of metal pollution. 104 refs., 2 figs., 4 tabs.

Measurement of the redox speciation of iron in seawater by catalytic cathodic stripping voltammetry

Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to determine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is measured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (Fe-R) in the absence and presence of 2 mu M Dp, Fe-R being defined as that which was complexed by 20 mu M NN at pH 6.9 in the presence of 1.8 mM H2O2 and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for Fe-R. Detection limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for Fe-R, iron(III) and iron(II), respectively, using a 60 s deposition time. The method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of Fe-R ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be present at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column.

Variability in the speciation of iron in the northern North Sea

Variations in the speciation of iron in the northern North Sea were investigated in an area covering at least two different water masses and an algal bloom, using a combination of techniques. Catalytic cathodic stripping voltammetry was used to measure the concentrations of reactive iron (FeR) and total iron (FeT) in unfiltered samples, while dissolved iron (FeD) was measured by GFAAS after extraction of filtered sea water. FeR was defined by the amount of iron that complexed with 20 μM 1-nitroso-2-napthol (NN) at pH 6.9. FeT was determined after UV-digestion at pH 2.4. Concentrations of natural organic iron complexing ligands and values for conditional stability constants, were determined in unfiltered samples by titration. Mean concentrations of 1.3 nM for FeR, 10.0 nM for FeT and 1.7 nM for FeD were obtained for the area sampled. FeR concentrations increased towards the south of the area investigated, as a result of the increased influence of continental run off. FeR concentrations were found to be enhanced below the nutricline (below ∼40 m) as a result of the remineralisation of organic material. Enhanced levels of FeT were observed in some surface samples and in samples collected below 30 m at stations in the south of the area studied, thought to be a result of high concentrations of biogenic particulate material and the resuspended sediments respectively. FeD concentrations varied between values similar to those of FeT in samples from the north of the area to values similar to those of FeR in the south. The bloom was thought to have influenced the distribution of both FeR and FeT, but less evidence was observed for any influence on FeR and FeD. The concentration of organic complexing ligands, which could possibly include a contribution from adsorption sites on particulate material, increased slightly in the bloom area and in North Sea waters. Iron was found to be fully (99.9%) complexed by the organic complexing ligands at a pH of 6.9 and largely complexed (82–96%) at pH 8. The ligands were almost saturated with iron suggesting that the ligand concentration could limit the concentration of iron occurring as dissolved species.

Comparison of sample storage protocols for the determination of nutrients in natural waters

There have been several reports on storage protocols for the determination of nutrients in natural waters but each one has been limited to a particular sample matrix and they have reached different, matrix specific conclusions. The aim of this study was therefore to systematically apply the various recommended storage protocols to a range of natural water matrices. Samples from four contrasting sites in the UK, collected in late winter (February, 1999), were filtered and stored under different conditions (-80 degrees C, -20 degrees C, 4 degrees C and at 4 degrees C and -20 degrees C with 0.1% (v/v) chloroform) for up to 247 days prior to analysis. The sites were the River Frome in Dorset (a chalk stream catchment) and three sites from the Tamar Estuary (draining a non-chalk catchment) with salinities of 0.5 per thousand, 10 per thousand and 34 per thousand, Samples and controls were analysed for total oxidised nitrogen (TON) and filterable reactive phosphorus (FRP) using a segmented flow analyser with spectrophotometric detection. To investigate possible seasonal effects (particularly changes in biological and chemical matrix composition). a second sampling campaign was undertaken in early autumn (October, 1999). The results showed that the optimum storage conditions for the determination of TON and FRP were highly matrix dependent. with significant differences in FRP stability between the Frome and Tamar catchments (due to different calcium concentrations) and between samples of different salinities (due to different bacterial populations and/or dissolved organic matter). General guidelines for sample handling and storage are listed and matrix specific recommendations presented for samples rich in calcium and dissolved organic matter.

THE RELEASE OF COPPER-COMPLEXING LIGANDS BY THE BROWN ALGA FUCUS VESICULOSUS (PHAEOPHYCEAE) IN RESPONSE TO INCREASING TOTAL COPPER LEVELS

The growth of Fucus vesiculosus L. germlings in chemically defined culture media containing a range of Cu concentrations (20–1000 nM) was monitored simultaneously with measurement of the Cu speciation in the media by competitive equilibrium‐adsorptive cathodic stripping voltammetry. Fucus vesiculosus germlings were found to exude Cu‐complexing ligands with conditional stability constants of the order of 1.6 × 1011. Ligand concentrations increased with increasing total dissolved Cu concentrations (CuT) until a concentration of 500–800 neq Cu·L−1 was reached. Concentrations of the ligand exceeded CuT in treatments containing 20 and 100 nM Cu, were similar to CuT in the 500‐nM Cu treatment, but were less than CuT in the 1000‐nM treatment. Therefore, [Cu2+] were calculated to be at concentrations of 10−11− 10−10 M in the 20‐ and 100‐nM treatments, 10−9 M in the 500‐nM treatment, and 10−7 M in the 1000‐nM treatment. Growth rates were lowest at Cu2+ concentration > 10−9. These results are discussed within the context of the potential roles for exuded copper‐complexing ligands.

Production of phytochelatins and glutathione by marine phytoplankton in response to metal stress

Phytoplankton deal with metal toxicity using a variety of biochemical strategies. One of the strategies involves glutathione (GSH) and phytochelatins (PCs), which are metal‐binding thiol peptides produced by eukaryotes and these compounds have been related to several intracellular functions, including metal detoxification, homeostasis, metal resistance and protection against oxidative stress. This paper assesses our state of knowledge on the production of PCs and GSH by marine phytoplankton in laboratory and field conditions and the possible applications of PCs for environmental purposes. Good relationships have been observed between metal exposure and PC production in phytoplankton in the laboratory with Cd, Pb, and Zn showing the greatest efficacy, thereby indicating that PCs have a potential for application as a biomarker. Fewer studies on PC distributions in particulate material have been undertaken in the field. These studies show that free Cu has a strong relationship with the levels of PC in the particulate material. The reason for this could be because Cu is a common contaminant in coastal waters. However it could also be due to the lack of measurements of other metals and their speciation. GSH shows a more complex relationship to metal levels both in the laboratory and in the field. This is most likely due to its multifunctionality. However, there is evidence that phytoplankton act as an important source of dissolved GSH in marine waters, which may form part of the strong organic ligands that control metal speciation, and hence metal toxicity.

High temporal resolution field monitoring of phosphate in the River Frome using flow injection with diode array detection

The design and deployment of an in situ flow injection (FI) monitor for high temporal resolution monitoring of phosphate in the River Frome, Dorset, UK, is described. The monitor incorporates solenoid, self-priming micropumps for propulsion, solenoid-operated switching valves for controlling the fluidics and a miniature CCD spectrometer for full spectrum (200-1000 nm) acquisition and operates in a graphical programming environment. A tangential filtration unit is attached to the sample inlet line to remove suspended particulate matter and prevent blockage of the micropumps and valves. Detection (at 7 10 nm) is based on molybdenum blue chemistry with tin(II) chloride reduction. The detection limit is 0.67 muM PO4 and the linear range can be adjusted by using different wavelengths for detection. Pump noise is eliminated by subtraction of the signal at a non-absorbing wavelength (447 nm). Data from an intensive (sample every 30 min) field trial on the River Frome performed in October 2000 are presented, and the implications of the data for refining an export coefficient model for phosphorus from the catchment are discussed.

Responses of marine phytoplankton in iron enrichment experiments in the northern North Sea and northeast Atlantic Ocean

Short-term iron enrichment experiments were carried out with samples collected in areas with different phytoplankton activity in the northern North Sea and northeast Atlantic Ocean in the summer of 1993. The research area was dominated by high numbers of pico-phytoplankton, up to 70,000 ml−1. Maximum chlorophyll a concentrations varied from about 1.0 μg l−1 in a high-reflectance zone (caused by loose coccoliths, remnants from a bloom of Emiliania huxleyi) and about 3.5 μg l−1 in a zone in which the phytoplankton were growing, to about 0.5 μg l−1 in the northeast Atlantic Ocean. From the high-reflectance zone to the northeast Atlantic Ocean, nitrate concentrations increased from 0.5 μM to 6.0 μM. Concentrations of reactive iron in surface water showed an opposite trend and decreased from about 2.6 nM in the high-reflectance zone to <1.0 nM in the northeast Atlantic Ocean. In the research area, no signs of true iron deficiency were found, but iron enrichments in the high-reflectance zone, numerically dominated by Synechococcus sp., resulted in increased nitrate uptake. Ammonium uptake was hardly affected. Strong support for the effect of Fe on cell physiology is given by the increase in the f-ratio. Net growth rates of the phytoplankton (changes in cell numbers over 24 h) were almost unchanged. Phytoplankton collected from the northeast Atlantic Ocean, did not show changes in the nitrogen metabolism upon addition of iron. Net growth rates in these incubations were low or negative, with only slightly higher values with additional iron.

Evaluation of phosphorus concentrations in relation to annual and seasonal physico-chemical water quality parameters in a UK chalk stream

The aim of this paper was to examine historical physico-chemical water quality parameters (1990-1997) in the River Frome, East Stoke (NGR SY867868), in order to show both annual and seasonal (monthly) trends. EpCO2 (defined as the partial pressure of CO2 in natural water divided by the equilibrium partial pressure of CO2) levels ranged from mean values of 6.32+/-0.41 in spring/summer to 7.86+/-1.17 in autumn/winter. A decreasing trend in mean annual EpCO2 was also observed, with a high of 9.61 in 1990 and a low of 5.22 in 1996. The variations were attributed to changes in pH, which showed an inverse relationship with river discharge (r2=0.47). Both pH and EpCO2 levels were strongly linked to biological activity with increases caused by primary productivity. Filterable reactive phosphorus (FRP) and total phosphorus (TP) concentrations correlated with river discharge. The results showed that the majority of the phosphorus load was transported during storm events, which agrees with results from an export coefficient model predicting phosphorus loading in the Frome catchment. Recent River Frome monitoring campaigns using an in situ flow-injection (FI)-based monitor were in agreement with phosphorus concentration and related physico-chemical trends observed during historical sampling and laboratory analysis.