A tale of two gyres: Contrasting distributions of dissolved cobalt and iron in the Atlantic Ocean during an Atlantic Meridional Transect (AMT-19)
Introduction
Cobalt (Co), like iron (Fe), is essential for phytoplankton growth (e.g. Morel et al., 1994, Saito et al., 2002, Sunda and Huntsman, 1995a, Sunda and Huntsman, 1995b, Timmermans et al., 2001, Rodriguez and Ho, 2015). Cobalt is the metal centre in the vitamin B12.(cobalamin) complex which is essential for the synthesis of amino acids, deoxyriboses, and the reduction and transfer of single carbon fragments in many biochemical pathways. Cobalt is required for the de novo synthesis of vitamin B12 by marine prokaryotes (Bonnet et al., 2010). However, the majority of eukaryotic marine phytoplankton are B vitamin auxotrophs, acquiring their vitamin B12 requirements through a symbiotic relationship with bacteria (Croft et al., 2005, Cruz-López and Maske, 2016), although this pathway might not be a simple linear flux from producer to consumer (Helliwell et al., 2016). Cobalt is also the metal co-factor in the metalloenzyme, carbonic anhydrase (CA), which is required for inorganic carbon acquisition by Prochlorococcus, and Synechococcus (Sunda and Huntsman, 1995a, Saito et al., 2002). In addition, Trichodesmium require Co for nitrogen fixation (Rodriguez and Ho, 2015), and Co can substitute for zinc (Zn) as the metal co-factor of the protein PhoA in the enzyme alkaline phosphatase (AP) (Gong et al., 2005, Sunda and Huntsman, 1995a). The production of AP facilitates acquisition of phosphorus (P) from the organic-P pool by phytoplankton and bacteria (e.g. Mahaffey et al., 2014). In addition, the strong correlation between dissolved Co (dCo) and inorganic-P (phosphate,PO4) in the upper water column, across diverse oceanic regimes (Saito and Moffett, 2002, Noble et al., 2008, Noble et al., 2012, Bown et al., 2011, Dulaquais et al., 2014a, Baars and Croot, 2015), indicates the nutritive role of Co.
The role of iron (Fe) as an essential requirement for phytoplankton growth is well documented (e.g. Martin, 1990, Coale et al., 1996, Boyd et al., 2007). For example, photosystems I and II are Fe intensive, and Fe is required for enzymatic process at nearly all stages of the microbial nitrogen cycle, including nitrogen fixation (Morel and Price, 2003, Küpper et al., 2008, Richier et al., 2012). Despite Fe being the fourth most abundant element in the Earth’s crust, dissolved Fe (dFe) is often only present at trace concentrations (<0.5 nM) in oxygenated surface waters of the open ocean (Blain et al., 2007, Measures et al., 2008, Ussher et al., 2013). Consequently, primary production is limited by low Fe-availability in 30–40% of the world’s oceans (Moore et al., 2002, Boyd and Ellwood, 2010). In the Atlantic Ocean, a number of studies have demonstrated that primary production can be under Fe-stress or limitation, seasonally in association with the spring bloom. (Moore et al., 2006, Nielsdottir et al., 2009), as well as in regions where subsurface nutrient supply is enhanced (Moore et al., 2013, and references therein). The supply of Aeolian Fe is also a key control on the distribution of diazotrophs (Mills et al., 2004, Moore et al., 2009). In addition to Fe, light, macronutrients (N, P, Si), vitamins (e.g. B12) and micronutrients (e.g. Co, Zn) may also (co-)limit marine productivity (Bertrand et al., 2007, Saito et al., 2008, Moore et al., 2013, Browning et al., 2014).
A major vector of trace elements (TEs) to Atlantic surface waters is atmospheric deposition (Jickells et al., 2005, Baker et al., 2006, Baker et al., 2007, Sarthou et al., 2007, Buck et al., 2010, Evangelista et al., 2010, Ussher et al., 2013, Shelley et al., 2015), much of which originates from Northwest Africa (Prospero and Carlton, 1972). An estimated 240 ± 80 Tg of dust is transported westwards annually (Kaufman et al., 2005), primarily during the summer months. Approximately 40% of annual global dust deposition occurs in the North Atlantic Ocean (Jickells et al., 2005); the majority of this into waters beneath the Saharan dust plume (∼5–30°N) (Mahowald et al., 1999, Prospero et al., 2002, Kaufman et al., 2005). Hence, it is between these latitudes that surface Fe concentrations are highest (Measures et al., 2008, Fitzsimmons et al., 2013, Ussher et al., 2013). Wet deposition in the Intertropical Convergence Zone (ITCZ) scavenges aerosols from the atmosphere, effectively preventing the southwards transport of North African aerosols (Schlosser et al., 2013). Thus the seasonal migration of the ITCZ drives the latitudinal gradient in aerosol dust loading (Prospero and Carlson, 1972, Doherty et al., 2012, Doherty et al., 2014, Tsamalis et al., 2013), and hence surface water Fe concentrations and results in a concomitant shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic-P depletion (Schlosser et al., 2013). Despite Co being less abundant in crustal material than Fe (Fe 3.9%, Co 0.002%; Rudnick and Gao, 2003), atmospheric deposition is a source of Co to surface waters. (Shelley et al., 2012, Dulaquais et al., 2014a). Consequently, we anticipated that Co concentrations would also be highest under the Saharan plume due to the sheer volume of dust that is deposited.
Another important source of trace metals to remote Atlantic surface waters is through vertical mixing. This mechanism reportedly provides ∼5–35% of the dFe input flux to the Atlantic mixed layer (Ussher et al., 2013). Vertical mixing is particularly important in the tropics where elevated sub-surface dFe concentrations are associated with low oxygen, upwelled water (Bergquist and Boyle, 2006, Measures et al., 2008, Fitzsimmons et al., 2013, Ussher et al., 2013). On the other hand, lateral advection of Fe from shelf regions to the remote Atlantic Ocean is reported to range from minimal (Laes et al., 2007, Ussher et al., 2007, Noble et al., 2012, Fitzsimmons et al., 2013) to significant in the vicinity of 20°N (Rijkenberg et al. (2012). For Co, understanding the contribution of these sources is hindered by a relative paucity of data. However, lateral transport has recently been reported in both the eastern and western basins of the Atlantic (Noble et al., 2012, Dulaquais et al., 2014a, Dulaquais et al., 2014b).
Iron and Co distributions are also strongly influenced by both redox speciation and organic complexation. Although Fe2+ is the more bioavailable form of Fe (Shaked and Lis, 2012), the thermodynamically favoured species of Fe in oxic seawater (pH 8) is Fe3+. However, Fe3+ is relatively insoluble under these conditions, and is rapidly scavenged from the water column and forms insoluble Fe3+ oxyhydroxides (Liu and Millero, 2002). Chelation by organic ligands increases the solubility of Fe in seawater; both strong (e.g. siderophores) and weaker ligand classes (e.g., humics) have been shown to be play a role in maintaining Fe in solution (Mawji et al., 2008, Croot and Heller, 2012, Heller et al., 2013, Buck et al., 2015). Similarly, Co2+ in also thermodynamically favoured in oxic seawater, and Co forms strong organic complexes (Ellwood and van den Berg, 2001, Saito and Moffett, 2001, Baars and Croot, 2015).
The primary removal mechanism for Co and Fe from the euphotic zone is through biological uptake (Martin and Gordon, 1988, Moffett and Ho, 1996). In addition, adsorptive scavenging on to particles (Moffett and Ho, 1996, Johnson et al., 1997, Wu et al., 2001, Bruland and Lohan, 2003) and aggregation and sinking (Croot et al., 2004) are also important removal pathways for both Co and Fe.
The Atlantic Meridional Transect (AMT) programme provides an ideal platform to investigate Co and Fe cycling in the upper Atlantic Ocean and the role of these metals on climate-relevant biological processes. Here we report the geographical distribution and biogeochemistry of Co and Fe in the upper water column along a 12,000 km, gyre-centred transect of the Atlantic Ocean (AMT-19) between ∼50°N and 40°S. As our knowledge of Fe biogeochemistry is arguably more advanced than for Co, the following discussion aims to develop our understanding of Co biogeochemistry in the upper water column (⩽150 m) of the Atlantic Ocean between 50°N and 40°S by making comparisons with dissolved Fe distributions from this and earlier studies.
Section snippets
Sampling
Twenty-nine stations were sampled during cruise AMT-19 (13/10/09–28/11/09) from Falmouth, UK to Punta Arenas, Chile, on board the R.R.S. James Cook (Fig. 1). Stations were sampled from the six biogeographical provinces listed in Fig. 1, described by Longhurst (1998). In this study, the distribution of salinity, temperature, dCo, dFe and macronutrients (nitrate and phosphate) were used to identify the province boundaries (Table 1). The assigned province boundaries are subject to small-scale
Hydrographic setting and macronutrient distributions
The six biogeographical provinces used in this study are shown in Fig. 1. Note that the North Atlantic gyre is divided into two separate provinces; the North Atlantic subtropical gyre (NAST) and the North Atlantic tropical gyre (NATR). In these provinces, the thermohaline structure of the upper water column (Fig. 2) is primarily determined by the water masses that occupy each region and the relative evaporation and precipitation rates. In the North Atlantic, the lowest upper water column
Discussion
Given that there are a number of similarities in the redox and organic speciation of Co and Fe, the difference in the distributions of these two elements in the Atlantic Ocean is stark. In the northern gyre provinces (NATR and NAST), where deposition and dissolution of atmospheric aerosols is the dominant source of Fe (e.g. Duce and Tindale, 1991, Duce et al., 1991, Sarthou et al., 2003, Jickells et al., 2005, Baker et al., 2006, Buck et al., 2010, Evangelista et al., 2010, Ussher et al., 2013
Conclusions
Dissolved Co and Fe distributions showed strong, and often contrasting, regional differences during AMT-19. Extremely low concentrations of dCo (NATR/NAST; ∼20–30 pM) were observed in the northern gyre provinces where dFe was high, whereas the opposite trend was observed in the SATL. Both dCo and dFe distributions were generally nutrient-like; highlighting the nutritive role of these two bioactive elements. However, the extremely low dCo of the northern gyre provinces is somewhat of a paradox
Acknowledgements
With many thanks to the Captain and crew of RRS James Cook, and Carolyn Harris, Malcolm Woodward and Claire Widdicombe for kindly providing the nutrient, and the chl-a data, respectively. Thank you also to Mike Zubkov and Manuela Hartmann for discussion of bacterial abundance during AMT-19. We thank two anonymous reviewers for their valuable comments and suggestions. Funding for this work was provided through a Marine Institute (Plymouth University), UK Studentship to RUS, a Natural Environment
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Laboratoire des Sciences de l’Environnement Marin, UMR 6539 LEMAR (CNRS/UBO/IRD/IFREMER), Institut Universitaire Européen de la Mer, Technopôle Brest-Iroise, Plouzané, 29280, France.
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University of Southampton Waterfront Campus, National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK.