Organic dissolved copper speciation across the U.S. GEOTRACES equatorial Pacific zonal transect GP16
Introduction
Copper (Cu) is one of the essential trace metal micronutrients required for phytoplankton growth in the ocean, in addition to other bioactive trace metals such as iron (Fe), zinc (Zn) and cadmium (Cd). Copper is significant in that it facilitates various chemical and biological processes like Fe acquisition (Peers et al., 2005), photosynthesis (Peers and Price, 2006), ammonia oxidation (Walker et al., 2010), and nitrous oxide reduction (Stiefel, 2007). These identified roles establish Cu as a micronutrient whose abundance can be beneficial to plankton. Nevertheless, Cu can also have a negative effect on the growth of plankton if its free ion (Cu2+) concentration increases beyond certain thresholds. Cyanobacteria for example, are especially sensitive to Cu toxicity and can experience toxicity at Cu2+ levels as low as 10−11 M (Brand et al., 1986; Moffett et al., 1997). In natural oceanic waters, over 99% of the dissolved Cu is bound by organic ligands (L) which generally maintain Cu2+ in low concentrations (Coale and Bruland, 1988; Moffett and Dupont, 2007; van den Berg et al., 1987), thus buffering against Cu toxicity. Though the identity of these ligands is not entirely known, it has been shown that micro-organisms like coccolithophores produce thiols capable of binding Cu (Dupont et al., 2004). Cyanobacteria have been shown to produce strong Cu-binding ligands to alleviate Cu stress (Moffett et al., 1996). Other compounds like humic acids have been found to also bind Cu in both freshwater and seawater systems (Kogut and Voelker, 2001; Whitby and van den Berg, 2015). Coastal marine sediments have been suggested as a non-biological source of Cu-binding ligands (Skrabal et al., 2000), though the ligands detected were found to be “weak” ligands relative to those produced by cyanobacteria during Cu stress.The sources, sinks, and cycling of Cu ligands are not well defined, and until recently virtually no ocean basin scale Cu ligand datasets were readily available.
Dissolved Cu is affected by both biological and chemical process as Cu exhibits a “hybrid” type vertical profile with biological drawdown in surface waters and particle scavenging throughout the water column. Dissolved Cu concentrations increase gradually with depth, due to remineralization, but continue to undergo scavenging processes at intermediate depths (Bruland and Lohan, 2003). Currently there are a limited but growing number of studies with deep water Cu-binding ligand data to provide insight as to whether the observed increase in total dissolved Cu concentration with depth also leads to an increase in Cu2+ concentrations with depth. One early study showed that Cu2+ concentrations increase with depth as ligand concentrations decreased (Bruland and Franks, 1983), but more recent studies have shown that organic ligands throughout the water column are capable of buffering Cu2+ to low concentrations, approximately 10−14 M, below levels considered to be toxic (Buck et al., 2012; Moffett and Dupont, 2007), and in some cases limiting for growth for some organisms such as archaea (Amin et al., 2013). These studies highlight the importance of understanding the controls on Cu-binding ligand distributions in the water column and the changes ligands undergo, particularly between surface waters and deep waters (> 2000 m). To date, the primary tool employed for the study of Cu speciation has been electrochemical competitive ligand exchange analysis utilizing a known ligand to compete with the natural ligands (Buck and Bruland, 2005; Campos and van den Berg, 1994). The electrochemical technique enables characterization of the natural ligands with regard to both concentration and conditional binding constants for Cu, and with these two measurements (plus the total dissolved Cu concentration) the free Cu ion concentration can also be calculated.
This study, conducted on the U.S. GEOTRACES GP16 Eastern Pacific Zonal Transect (GP16) between Peru and Tahiti, provides insights into the cycling of Cu-binding ligands across a range of ocean conditions. Samples from full depth profiles were collected along the transect, generating speciation data for samples as deep as 5000 m while crossing an oxygen deficient zone (ODZ; stations 5-13), an oligotrophic subtropical gyre off the coast of Tahiti (stations 13-34), and a hydrothermal vent (stations 17-25) in the deep waters of the western section of the transect. Strong upwelling off the coast of Peru brings nutrients and trace metal-rich waters to the surface, fueling one of the most biologically productive coastal regions in the Pacific Ocean (Johnson et al., 2001; Pennington et al., 2006). The open ocean waters covering the western half of the transect were characterized by low macro-nutrient and low trace metal concentrations, which can influence the structure of microbial communities (Moore et al., 2013), while the offshore waters west of Peru include a region of high macro-nutrients and low chlorophyll (HNLC; stations 7-13) and are generally considered to be iron-limited for phytoplankton growth (Hutchins and Bruland, 1998).
In recent years, in part due to GEOTRACES activities, there has been a significant increase in the availability of Cu speciation data from the Atlantic (Heller and Croot, 2015; Jacquot and Moffett, 2015) and the Pacific (Jacquot et al., 2013; Thompson et al., 2014b). The work presented here contributes to the growing knowledge of Cu speciation in the oceans and gives insights into how unique ocean regimes can influence the speciation and distribution of Cu-binding ligands as well as elucidating some of the large-scale similarities and differences in organic Cu speciation across different ocean basins.
Section snippets
Sample collection
Dissolved Cu organic speciation samples were collected aboard the R/V Thompson during the U.S. GEOTRACES Eastern Pacific Zonal Transect (GP16) cruise at 22 stations from full depth profiles ranging from the surface to 5000 m (Fig. 1A). Samples were collected with the U.S. GEOTRACES carousel equipped with twenty-four 12 L Teflon-coated GO-Flo bottles (General Oceanics) and a sensor package for pressure, conductivity, temperature, and oxygen (Cutter and Bruland, 2012). Upon recovery of the
Dissolved copper
The dCu concentrations across the transect exhibited typical distribution patterns with depletion at the surface and near linear increases in concentration with depth (Fig. 1D). The dCu concentrations ranged from 0.22 nM in the offshore region of the transect to over 5 nM near the ocean bottom, with average concentrations of approximately 2 nM. Near surface dCu concentrations were elevated, approximately 1–2 nM, just off the coast of South America and within Equatorial Subsurface Waters (ESSW)
Continental margins as a source of strong Cu-binding ligands
Some of the strongest Cu-binding ligands detected on the GP16 transect were over the shelf, corresponding with a plume of 228Radium (228Ra) in intermediate waters (1000–2500 m) coming from the continental shelf off the coast of Peru (Sanial et al., 2018). This suggests that shelf sediments may be the ultimate source of the strong ligands (log KCuL, Cu2+cond > 14; Fig. 2B). High levels of 228Ra were also observed near the ocean bottom just off Peru, and this region also contained ligands of
Conclusions
This study provides the second large-scale Cu speciation data set for the U.S. GEOTRACES program, and highlights the effects of water mass age on Cu speciation and differences in Cu speciation between ocean basins. The GP16 and GA03 GEOTRACES transects share similar oceanographic features and in both ocean basins the hydrothermal vents that were sampled did not appear to be significant sources of dCu and were a modest source of Cu-binding ligands. Common features between the datasets are the
Acknowledgements
Funding for this project was provided through a National Science Foundation NSF OCE-1233733 to KAB and by a National Science Foundation Graduate Research Fellowship AR. We thank the captain and crew of the R/V Thompson as well as Cheryl Zurbrick for collecting the copper speciation samples during the two-month GEOTRACES cruise.
References (73)
- et al.
Competition between copper and iron for humic ligands in estuarine waters
Mar. Chem.
(2015) - et al.
Organic complexation and its control of the dissolved concentrations of copper and zinc in the Scheldt estuary
Estuar. Coast. Shelf Sci.
(1987) - et al.
Analysis of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in seawater using the Nobias-chelate PA1 resin and magnetic sector inductively coupled plasma mass spectrometry (ICP-MS)
Mar. Chem.
(2012) - et al.
Reduction of marine phytoplankton reproduction rates by copper and cadmium
J. Exp. Mar. Bio. Ecol.
(1986) Oceanographic distributions of cadmium, zinc, nickel, and copper in the North Pacific
Earth Planet
(1980)- et al.
Controls of trace metals in seawater
- et al.
Intercomparison of voltammetric techniques to determine the chemical speciation of dissolved copper in a coastal seawater sample
Anal. Chim. Acta
(2000) - et al.
Iron, macronutrients and diatom blooms in the Peru upwelling regime: brown and blue waters of Peru
Mar. Chem.
(2005) - et al.
Copper speciation in San Francisco Bay: a novel approach using multiple analytical windows
Mar. Chem.
(2005) - et al.
Organic complexation of iron in the eastern tropical South Pacific: Results from US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16)
Mar. Chem.
(2018)