Dissolved cobalt speciation and reactivity in the eastern tropical North Atlantic Oliver Baars a,b, ⁎, Peter L. Croot a,c a Marine Biogeochemistry, GEOMAR, Germany b Princeton University, USA c Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland, Galway, Ireland abstract article info Article history: Received 15 June 2014 Received in revised form 4 October 2014 Accepted 6 October 2014 Available online xxxx Keywords: Marine cobalt cycle Cobalt reducibility Seawater cobalt complexes Cobalt distribution Trace metal speciation Kinetics Thermodynamics Reactivity Marine cobalt redox chemistry Recent studies highlight the role of cobalt (Co) as an important micro-nutrient with a complex scavenged type oceanic distribution. To better understand the biogeochemical cycle of Co we investigate the distribution, speci- ation and reactivity of dissolved Co in the eastern tropical North Atlantic in the upper 800 m of the water column. For this purpose, we complement classical Co ligand titrations that require a thermodynamic equilibrium with evaluations of ligand-exchange kinetics and reducibility of potential Co(III) species. The experiments include ad- ditions of the artificial Co binding ligands dimethylglyoxime or Nioxime and detection by cathodic stripping volt- ammetry. We find two pools of Co compounds: a labile fraction that exchanges Co within minutes and a strong/ inert fraction that does not react within a 24-h period. No intermediate, slowly exchanging fraction is observed. Detection window experiments to determine complex stability constants show that the labile Co fraction is weak and likely consists of Co(II) complexes with no detectable free Co(II) ligands. The fraction of inert Co is always highest at the depth of the chlorophyll-a maximum. Addition of the reductant ascorbate increases the fraction of Co with rapid ligand-exchange kinetics and indicates the presence of dissolved reducible Co(III). The apparent Co(III) reducibility is highest at the chlorophyll-a maximum and decreases in deeper waters. Our results are in agreement with phytoplankton and associated bacteria being a source of Co(III) species, such as vitamin B 12 . The presented results have important implications for our understanding of the biological availability and the marine cycle of Co. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Cobalt is the essential metal center in vitamin B 12 (Guillard and Cassie, 1963) and plays a role in proteins where it can substitute Zn in a variety of phytoplankton species (e.g. Carbonic Anhydrases, Alkaline Phosphatases) (Lane and Morel, 2005; Saito and Goepfert, 2008). In the marine environment, dissolved Co concentrations are in the low pico-molar range and may reach levels that can limit certain phyto- plankton species and (co-)limit primary productivity (Bertrand et al., 2007; Panzeca et al., 2008; Saito and Moffett, 2002). Consistent with this role of Co, oceanic depth profiles are often nutrient type in the upper water column. However, the low solubility of inorganic Co(III) prevents accumulation in the deep oceans and contributes to the inter- mediate depth concentration maxima of dissolved Co that are observed in some ocean regions (Noble et al., 2008; Saito and Moffett, 2002). In particular, Co(II) may be co-oxidized by Mn oxidizing bacteria and sub- sequently precipitate with Mn oxides (Moffett and Ho, 1996). Reductive dissolution of Co(III) at the continental shelf and advection with oxygen minimum zone waters may additionally contribute to the formation of intermediate depth maxima of Co in the open ocean (Noble et al., 2012; Saito et al., 2010). Organic cobalt complexes in seawater can strongly affect abiotic and biotic reactions. Therefore, to understand the cycle of Co and its interac- tion with micro-organisms, it is important to unravel the speciation and reactivity of Co in the dissolved phase. Previous Co speciation studies have been performed using electrochemistry, whereby an artificial Co ligand is added that competes with natural Co ligands in the sample and its complex can be directly determined by adsorptive cathodic stripping voltammetry (AdCSV) (Ellwood and Van den Berg, 2001; Saito and Moffett, 2001). In that way, it has been shown that a signifi- cant fraction of Co may be bound to organic ligands that do not ex- change the metal after overnight equilibration with the competing ligand dimethylglyoxime (DMG) (Noble et al., 2012; Saito et al., 2010), whereby the cyanobacteria Prochlorococcus and Synechococcus are among the possible sources of ligands (Saito et al., 2002, 2005). Ad- ditions of Co to the sample can be done to titrate unbound natural li- gands and calculate conditional complex stability constants and ligand concentrations (Ellwood and Van den Berg, 2001; Saito and Moffett, 2001). Free Co ligands have been detected in only a subset of the exam- ined regions and water masses, that were mainly located in the Marine Chemistry xxx (2014) xxx–xxx ⁎ Corresponding author at: Princeton University, Department of Geosciences, Guyot Hall, Princeton, NJ 08544, USA. Tel.: +1 609 356 8195. E-mail address: obaars@princeton.edu (O. Baars). MARCHE-03162; No of Pages 10 http://dx.doi.org/10.1016/j.marchem.2014.10.006 0304-4203/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Marine Chemistry journal homepage: www.elsevier.com/locate/marchem Please cite this article as: Baars, O., Croot, P.L., Dissolved cobalt speciation and reactivity in the eastern tropical North Atlantic, Mar. Chem. (2014), http://dx.doi.org/10.1016/j.marchem.2014.10.006