Production of cobalt binding ligands in a Synechococcus feature at the Costa Rica upwelling dome

The Costa Rica upwelling dome (CRD; ;8.678N and 90.68W) was characterized chemically for cobalt and nickel abundances and speciation and biologically using cyanobacterial abundances and phylogeny. Total dissolved cobalt was 93 pmol L21 at 90 m depth and decreased in surface waters to 45 pmol L21 at 10 m. Cobalt was 40% labile at 90 m but was completely complexed by strong ligands at 10 m. A surface transect out of the dome showed decreasing total dissolved cobalt from 57 pmol L21 to 12 pmol L21. Detection window studies showed that natural cobalt-ligand complexes have conditional stability constants greater than 1016.8 and that competition with nickel did not release cobalt bound to organic complexes, consistent with natural cobalt ligands being Co(III) complexes. Synechococcus cell densities at the CRD are among the highest reported in nature, varying between 1.2 3 106 and 3.7 3 106 cells ml21. Phylogenetic analysis using the 16S–23S rDNA internally transcribed spacer showed the majority of clones were related to Synechococcus strain MIT S9220, while the remaining subset form a novel group within the marine Synechococcus lineage. In a bottle incubation experiment chlorophyll increased with cobalt and iron additions relative to each element alone and the unamended control treatment. Cobalt speciation analysis of incubation experiments revealed large quantities of strong cobalt-ligand complexes in the cobalt addition treatments (401 pmol L21), whereas cobalt added to a 0.2-mm filtered control remained predominantly labile (387 pmol L21), demonstrating that the Synechococcus-dominated community is a source of strong cobalt ligands. Processes that influence the production of metal binding ligands in the surface ocean are not well understood. Many of the important biologically used metals (Fe, Cu, Zn, Co, and Cd) have a chemical speciation that is dominated by their complexation with strong organic ligands (e.g., Rue and Bruland 1995; Saito and Moffett 2001; Morel et al. and references therein; Morel et al. 2003). While the chemical structures of these metal-complexing ligands in the natural environment are currently unknown, their destruction by ultraviolet-irradiation provides evidence for their organic composition. Furthermore, conditional stability constants indicate these ligands are very strong, much stronger than most inorganic complexes (i.e., chloro or carbonate complexes). 1 Corresponding author ([email protected]).