The Acceleration of Dissolved Cobalt’s Ecological Stoichiometry due to Biological Uptake, Remineralization, and Scavenging in the Atlantic Ocean
Authors: Saito Mak A. , Noble Abigail , Hawco Nicholas , Twining Benjamin S. , Ohnemus Daniel C. , John Seth G. , Lam Phoebe , Conway Tim M. , Johnson Rod , Moran Dawn , McIlvin Matthew 
Published: November 2016

Abstract. Cobalt has the smallest oceanic inventory of all known inorganic micronutrients, and hence is particularly vulnerable to influence by internal oceanic processes including euphotic zone uptake, remineralization, and scavenging. Due to its small oceanic inventory, cobalt provides a unique case study for considering the stoichiometric coupling between dissolved and particulate phases in the context of Redfield theory. The ecological stoichiometry of total dissolved cobalt (dCo) was examined using data from a U.S. North Atlantic GEOTRACES transect and from a zonal South Atlantic GEOTRACES-compliant transect (GA03/3_e and GAc01), by Redfieldian analysis of its statistical relationships with the macronutrient phosphate. Trends in the dissolved cobalt to phosphate (dCo:P) stoichiometric relationships were evident in the basin scale vertical structure of cobalt, with positive dCo:P slopes in the euphotic zone and negative slopes found in the ocean interior and in coastal environments. The euphotic positive slopes were often found to accelerate towards the surface and this was interpreted as due to the combined influence of depleted phosphate, phosphorus sparing mechanisms, increased alkaline phosphatase metalloenzyme production (a zinc or perhaps cobalt enzyme), and biochemical substitution of Co for depleted Zn. Consistent with this, dissolved Zn (dZn) was found to be drawn down to only twofold more than dCo, despite being more than 18-fold more abundant in the ocean interior. Particulate cobalt concentrations increased in abundance from the base of the euphotic zone to become ~ 10 % of the overall cobalt inventory in the upper euphotic zone with high stoichiometric values of ~ 400 μmol Co mol−1 P. Metaproteomic results from the Bermuda Atlantic Time-series Study (BATS) station found cyanobacterial isoforms of the alkaline phosphatase enzyme to be prevalent in the upper water column, as well as a sulfolipid biosynthesis protein indicative of P sparing. The negative dCo:P relationships in the ocean interior became increasingly vertical with depth, and were consistent with the sum of scavenging and remineralization processes (as shown by their dCo:P vector sums). Attenuation of the remineralization with depth resulted in the increasingly vertical dCo:P relationships. Analysis of particulate Co with particulate Mn and particulate phosphate also showed positive linear relationships below the euphotic zone, consistent with the presence and increased relative influence of Mn oxide particles involved in scavenging. Visualization of dCo:P slopes across an ocean section revealed hotspots of scavenging and remineralization, such as at the hydrothermal vents and below the OMZ region, respectively, while that of an estimate of Co* illustrated stoichiometrically depleted values in the mesopelagic and deep ocean due to scavenging. This study provides insights into the coupling between the dissolved and particulate phase that ultimately create Redfield stoichiometric ratios, demonstrating that the coupling is not an instantaneous process and is influenced by the element inventory and rate of exchange between phases. Cobalt’s small water column inventory and the influence of external factors on its biotic stoichiometry can erode its limited inertia and result in an acceleration of the dissolved stoichiometry towards that of the particulate phase in the upper euphotic zone. As human use of cobalt grows exponentially with widespread adoption of lithium ion batteries, there is a potential to alter this dynamic biogeochemical cycling and ecology of cobalt in the oceanic euphotic zone.