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

The Costa Rica upwelling dome (CRD; similar to8.67degreesN and 90.6degreesW) was characterized chemically for cobalt and nickel abundances and speciation and biologically using cyanobacterial abundances and phylogeny. Total dissolved cobalt was 93 pmol L-1 at 90 m depth and decreased in surface waters to 45 pmol L-1 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 L-1 to 12 pmol L-1. Detection window studies showed that natural cobalt-ligand complexes have conditional stability constants greater than 10161 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 x 10(6) and 3.7 x 10(6) cells ml(-1). 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 L-1), whereas cobalt added to a 0.2-mum filtered control remained predominantly labile (387 pmol L-1), demonstrating that the Synechococcus-dominated community is a source of strong cobalt ligands.