Adsorption of biologically critical trace elements to the marine cyanobacterium Synechococcus sp. PCC 7002: Implications for marine trace metal cycling

Abstract

Marine bacterial plankton play a key role in elemental cycling through their ability to bind, assimilate, metabolize, and modify the redox state of trace metals in seawater. Of those processes, arguably the least studied are the mechanisms underpinning trace metal adsorption to planktonic marine bacteria, despite a plethora of literature pertaining to terrestrial species. Recently, Liu et al. (2015) demonstrated that the marine cyanobacterium Synechococcus sp. PCC 7002 has the capacity to remove appreciable amounts of Cd2+, a proxy for other divalent cations, from seawater by adsorption. In this study, we build on that work and employ a surface complexation modelling (SCM) approach using titration and pH adsorption edge experiments to calculate the thermodynamic binding constants of four bioessential transition metals (Co, Ni, Cu, Zn) to Synechococcus in simulated seawater. Based on the titration results, the major functional groups involved in metal binding were carboxyl groups with a pKa of 5.59 and phosphoryl groups with a pKa of 7.61. Metal adsorption experiments indicate that Synechococcus can bind considerable concentrations of Zn, Cu, Ni, and Co at pH 8. When all four metals are simultaneously added to solution, the same adsorption pattern of Zn > Cu > Ni > Co is maintained, and accurately predicted by the SCM. Based on average marine cell densities and turnover rates of Synechococcus cells in the photic zone, we calculate that Synechococcus, in the absence of competing ligands such as dissolved organic matter (DOM), has the theoretical capacity to remove nearly all of the free metal cations from seawater. These observations highlight the surface reactivity of marine cyanobacteria as a potentially important vector for the transfer of dissolved metals from the photic zone to deeper waters or the seafloor in modernoceans, but they also have implications for the Precambrian oceans as sinking cyanobacteria could have acted as an exit channel for trace elements into ancient sediments including banded iron formations (BIF).