Zinc and cadmium speciation in subantarctic waters east of New Zealand
Introduction
For certain oceanic regions, there is strong evidence demonstrating that the bioavailability of certain trace elements, namely iron, limits primary production Martin et al., 1990, Martin et al., 1994, Coale et al., 1996, Boyd and Abraham, 2001. The bioavailability of trace metals for uptake and utilisation by marine phytoplankton is dependent on their chemical speciation Hunter et al., 1997, Sunda and Huntsman, 1998b, Hunter and Boyd, 1999. For example, the uptake of iron by phytoplankton is highly dependent on its chemical form. Recent studies have shown that porphyrin-complexed iron can be more efficiently assimilated by eukaryotic algae, whereas iron that is bound to siderophores is more available to prokaryotic algae, although there is some species variability Hutchins et al., 1999, Weaver et al., 2003. These results suggest that these two plankton groups have fundamentally different iron-uptake strategies. Apart from iron, other trace elements that have the potential to limit primary production in the ocean are zinc, manganese and cobalt Bruland, 1989, Martin et al., 1990, Coale, 1991, Morel et al., 1994, Fukuda et al., 2000, Ellwood and van den Berg, 2001, Saito and Moffett, 2001. To date, only a handful of studies have looked at the chemical speciation of these potentially limiting metals.
Ocean profiles for dissolved zinc a similar to those of major nutrients (N, P and Si) with low concentrations in surface waters that increase with depth Bruland et al., 1978, Bruland, 1980. In certain open-ocean regions, surface zinc concentrations can range between 10 and 100 pmol/kg, levels that can potentially limit phytoplankton growth Bruland, 1980, Brand et al., 1983, Martin et al., 1989. Laboratory culture experiments involving marine phytoplankton grown in zinc ion-buffered media indicate that phytoplankton growth is strongly dependent on the free zinc ion concentration in solution Anderson et al., 1978, Brand et al., 1983, Sunda and Huntsman, 1992. Below an inorganic zinc (Zn′) concentration of about 2 pmol/kg, some species of phytoplankton become growth limited Brand et al., 1983, Sunda and Huntsman, 1992, Sunda and Huntsman, 1995, Morel et al., 1994, Ellwood and Hunter, 2000. Studies of zinc complexation in surface water of the North Pacific and North Atlantic Oceans reveal that greater than 95% of zinc is complexed to organic ligands of a natural origin Bruland, 1989, Donat and Bruland, 1990, Ellwood and van den Berg, 2000. Such complexation lowers inorganic zinc concentrations into the low picomolar range. Whether such zinc levels leads to growth limitation of open-ocean phytoplankton species is debatable (Ellwood and van den Berg, 2000). To date, no work has looked at whether such natural organic zinc chelates can be assimilated by marine organisms, although some researchers have tried isolating these ligands Mackey, 1983, Mackey and Higgins, 1988.
Fundamental research into whether phytoplankton can assimilate such zinc chelates is required to determine whether the free ion model for zinc uptake, shown to be important in laboratory culture studies, applies in the ocean. The free ion model for metal-organism interaction has been established through culture experiments in which the free metal ion concentration in solution is controlled with synthetic chelators such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetate (NTA) Sunda and Guillard, 1976, Anderson et al., 1978, Sunda and Huntsman, 1992. More recently, several studies have shown that marine organisms can assimilate neutral lipophilic organic metal complexes, thereby increasing their internal metal concentration Phinney and Bruland, 1994, Phinney and Bruland, 1997, Croot et al., 1999.
Like zinc, the dissolved cadmium concentration profile is nutrient-like Boyle et al., 1976, Bruland, 1980, Frew and Hunter, 1992. The underlying processes leading to such a profile are not fully understood, but cadmium is known to be biologically useful, especially when cultured phytoplankton encounter low free zinc ion concentrations Price and Morel, 1990, Morel et al., 1994, Lee and Morel, 1995, Lee et al., 1995, Sunda and Huntsman, 1998a. Recent work by Cullen et al. (1999) and Lane and Morel (2000) has shown that when the marine diatom Thalassiosira weissflogii is grown under zinc-limiting conditions production of a cadmium centred carbonic anhydrase protein can be induced. The production of such a protein allows the diatom to grow at a near maximum growth rate (Lane and Morel, 2000).
The oceanic distribution and speciation of trace metals is controlled through biological uptake and regeneration processes. At present, only a few of studies have looked at zinc speciation for open-ocean waters and even less have investigated cadmium speciation Sakamoto-Arnold et al., 1987, Bruland, 1989, Donat and Bruland, 1990, Bruland, 1992, Ellwood and van den Berg, 2000. This study presents chemical speciation results for zinc and cadmium for a station situated south of the Chatham Rise, sampled on four separate occasions spanning the four seasons. In addition, results for shipboard incubation experiments carried out in austral summer 2003, to determine whether zinc is limiting phytoplankton productivity in these waters, are presented.
Subantarctic surface waters southeast of New Zealand are characterised by low silicate concentrations but relatively high nitrate and phosphate concentrations Vincent et al., 1991, Butler et al., 1992. Biological production in subantarctic waters is intimately related to the availability of dissolved iron Sedwick et al., 1997, Sedwick et al., 1999. Zinc has been proposed to limit primary production in subantarctic waters; however this has yet to be demonstrated Frew and Hunter, 1992, Frew and Hunter, 1995, Cullen et al., 1999.
Most of the results presented in this paper are for surface water samples (0–100 m) collected at or near NIWA's mooring site located at 46°40′S 178°30′E (Fig. 1). Seafloor depth at the site is ∼2800 m and the seasonal mixed layer varies from about 150–200 m in winter/spring months (June–September) to 30–60 m in summer months (January–March). For three cruises, this site was only occupied for 12 h, while a mooring was serviced and redeployed. On the fourth cruise (FeCycle), a site 30′ north of NIWA's mooring site was occupied for 14 days in late January 2003 with the aim of conducting experiments to better understand the biogeochemistry of iron in this region. At the beginning of the occupation, a site survey was carried out followed by release of a conservative tracer, sulphur hexafluoride (SF6), into ∼50-km2 patch of water (Law, pers. comm.). Over the 2-week occupation, a number of biological and chemical parameters were monitored within the SF6 patch. Reported in this paper are chemical speciation measurements made for zinc and cadmium and incubation experiments conducted to investigate zinc limitation in these waters.
The underlying theory for determining zinc and cadmium speciation in seawater using rotating carbon disc anodic stripping voltammetry (ASV) has been described in detail by Bruland (1989), Donat and Bruland (1990) and Bruland (1992). Briefly, the oxidation current (ip) for kinetically labile inorganic metal complexes (M′) is related to the concentration of metal deposited into the mercury film through the following equation:where S is the sensitivity (current/concentration)—calibrated after all the natural ligand(s) has been titrated—and αM is the inorganic side reaction coefficient for the complexation of [Mn+] by inorganic species present in seawater. Seawater αZn and αCd of 2.1 and 34.7 were used here Turner et al., 1981, Byrne et al., 1988.
At equilibrium, the concentration of M′, free ligand (L′) and metal ligand complex (ML) are related to one another by the mass action expression:where Kcond,M′′ is the condition stability constant, with respect to inorganic metal concentration, for the metal-organic ligand complex. Kcond,M′′ is measured under specific conditions, i.e. seawater pH, ionic strength and concentrations of potential competing cations. [ML] is the concentration of metal complexed to the natural organic ligand. [ML] is calculated as the difference between the total dissolved metal concentration of the sample and measured [M′]. [L′] is defined as the concentration of a natural organic ligand that is capable of strongly binding [Mn+]. The mass balance equation for [L′] is as follows:where CL is the total ligand concentration present.
Combining , and rearranging gives and equation similar to the Langmuir relationship Ruzic, 1982, van den Berg, 1982By plotting experimental data of [M′]/[ML] versus [M′], one can obtain values for CL (slope) and Kcond,M′′·CL(intercept).
The product of combining Eqs. (2) and (3) can also be expressed asThe mass balance equation for total dissolved metal ([MT]) complexed to organic and inorganic ligands can be expressed as followsSubstituting Eq. (5) into Eq. (6) allows inorganic zinc and cadmium concentrations to be calculated from the speciation data.
Section snippets
Instrumentation and reagents
The voltammetric system used consisted of Metrohm, 746 VA Trace Analyzer with a 747 VA voltammetric stand. The reference electrode was a double-junction, Ag/AgCl with a saturated AgCl internal solution and a salt bridge filled with 3 M KCl. The counter electrode was a glassy carbon rod. The working electrode was a rotating disk electrode with a glassy carbon tip (2.0 mm φ) onto which a mercury film was deposited.
Water used to make up reagents was purified using an ion-exchange water
Total dissolved metal concentrations
Total dissolved zinc, cadmium and cobalt concentrations are presented in Fig. 3, Fig. 4 along with relevant nutrient data. Zinc, cadmium and cobalt concentrations were consistently low in surface waters (0–20 m) with values ranging between 6–60, 4–50 and 2–15 pmol/kg for each metal, respectively (Fig. 3). Although dissolved zinc and cadmium concentrations were low, a seasonal trend did appear for the surface mixed layer, with lower zinc and cadmium concentrations in summer–autumn months
Discussion
To date, little work has focused on determining zinc and cadmium speciation in open-ocean waters and the only a few enrichment experiments have been conducted to investigated zinc limitation in open-ocean waters. Here, I have tried to link chemical speciation results to incubation experiments with the aim of determining whether zinc is important in controlling algal growth in subantarctic waters.
Conclusions
Chemical speciation of samples collected from subantarctic waters east of New Zealand reveal that significant portions of zinc and cadmium are complexed to natural organic ligands. Such complexation lowers inorganic zinc and cadmium concentrations to low picomolar levels in the mixed layer. Such low Zn′ concentrations are close to the levels at which certain laboratory cultured phytoplankton become zinc limited in their growth. Interesting, results from perturbation experiments that involved
Acknowledgements
Thanks to the following people that helped at sea: Phil Boyd, Richard Richardson, Keith Hartel, Marieke van Kooten, Stu Pickmere, Keith Hunter, Russell Frew, Peter Croot, Sylvia Sanders, Robert Strzepek, Julie Hall, Andrea Cumming and the officers and crew of Tangaroa. This research was supported with funds from the New Zealand Foundation for Research Science and Technology Postdoctoral Fellowship scheme (NIWX0102). Funding for NIWA's Ocean Ecosystems Programme is also supported by the New
References (74)
- et al.
Iron-mediated changes in phytoplankton photosynthetic competence during SOIREE
Deep-Sea Research Part II: Topical Studies in Oceanography
(2001) Oceanographic distributions of cadmium, zinc, nickel, and copper in the North Pacific
Earth and Planetary Science Letters
(1980)- et al.
Sampling and analytical methods for the determination of copper, cadmium, zinc and nickel at the nanogram per liter level in sea water
Analytica Chimica Acta
(1979) - et al.
The influence of temperature and pH on trace metal speciation in seawater
Marine Chemistry
(1988) - et al.
Phytoplankton growth and biological response to iron and zinc addition in the Ross Sea and Antarctic Circumpolar Current along 170°W
Deep Sea Research Part II—Topical Studies in Oceanography
(2003) - et al.
An improved metal extraction procedure for the determination of trace metals in sea water by atomic absorption spectrometry with electrothermal atomization
Analytica Chimica Acta
(1978) - et al.
A comparison of 2 voltammetric techniques for determining zinc speciation in Northeast Pacific Ocean waters
Marine Chemistry
(1990) - et al.
Zinc speciation in the Northeastern Atlantic Ocean
Marine Chemistry
(2000) - et al.
Determination of organic complexation of cobalt in seawater by cathodic stripping voltammetry
Marine Chemistry
(2001) - et al.
Cadmium–phosphorus cycling at the subtropical convergence south of New Zealand
Marine Chemistry
(1995)
Macronutrient and trace-metal geochemistry of an in situ iron-induced Southern Ocean bloom
Deep Sea Research Part II—Topical Studies in Oceanography
Metal-organic complexes in seawater—an investigation of naturally occurring complexes of Cu, Zn, Fe, Mg, Ni, Cr, Mn and Cd using high-performance liquid chromatography with atomic fluorescence detection
Marine Chemistry
Reversed-phase chromatographic separation and analysis of marine metal-organic complexes
Journal of Chromatography A
VERTEX: phytoplankton/iron studies in the Gulf of Alaska
Deep-Sea Research Part A—Oceanographic Research Papers
Iron, primary production and carbon nitrogen flux studies during the JGOFS North Atlantic bloom experiment
Deep-Sea Research Part II—Topical Studies in Oceanography
Theoretical aspects of the direct titration of natural waters and its information yield for trace metal speciation
Analytica Chimica Acta
Complexation of cobalt by natural organic ligands in the Sargasso Sea as determined by a new high-sensitivity electrochemical cobalt speciation method suitable for open ocean work
Marine Chemistry
Iron and manganese in surface waters of the Australian subantarctic region
Deep-Sea Research Part I—Oceanographic Research Papers
Processes regulating cellular metal accumulation and physiological effects: phytoplankton as model systems
Science of the Total Environment
The equilibrium speciation of dissolved components in fresh-water and seawater at 25 °C and 1 atm pressure
Geochimica et Cosmochimica Acta
Determination of copper complexation with natural organic-ligands in sea-water by equilibration with MnO2: 1. Theory
Marine Chemistry
Determination of copper in seawater by cathodic stripping voltammetry of complexes with catechol
Analytica Chimica Acta
The complexation of ‘dissolved’ Cu, Zn, Cd and Pb by soluble and colloidal organic matter in Narragansett Bay, RI
Marine Chemistry
Growth limitation of a coastal diatom by low zinc ion activity
Nature
Role of iron, light, and silicate in controlling algal biomass in subantarctic waters SE of New Zealand
Journal of Geophysics Research-Oceans
On the marine geochemistry of cadmium
Nature
Pelagic ecosystem structure and functioning in the Subtropical Front region east of New Zealand in austral winter and spring 1993
Journal of Plankton Research
Limitation of marine phytoplankton reproductive rates by zinc, manganese, and iron
Limnology and Oceanography
Complexation of zinc by natural organic ligands in the Central North Pacific
Limnology and Oceanography
Complexation of cadmium by natural organic ligands in the Central North Pacific
Limnology and Oceanography
Mn, Ni, Cu, Zn, and Cd in the Western North Atlantic
Zinc in north-east Pacific water
Nature
Oceanography of the Subtropical Convergence Zone around southern New Zealand
New Zealand Journal of Marine and Freshwater Research
Effects of iron, manganese, copper, and zinc enrichments on productivity and biomass in the sub-Arctic Pacific
Limnology and Oceanography
Copper complexation in the Northeast Pacific
Limnology and Oceanography
A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean
Nature
Influence of zinc and iron enrichments on phytoplankton growth in the northeastern subarctic Pacific
Limnology and Oceanography
Cited by (104)
The distribution and speciation of dissolved Cd and Pb in the Bohai Sea and Yellow Sea, China
2023, Marine Pollution BulletinVoltammetric methods for speciation analysis of trace metals in natural waters
2021, Trends in Environmental Analytical ChemistryCitation Excerpt :However, Cd is toxic to phytoplankton at high concentrations [146]. The speciation analysis of Cd in water samples can only be determined using the ASV method with similar parameters to Zn [145,147]. Hg film electrodes are usually suggested as the working electrode for Cd speciation analysis.