The biogeochemistry of cobalt in the Sargasso Sea

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2001Processes that enable marine phytoplankton to acquire trace metals are fundamental to our understanding of primary productivity and global carbon cycling. This thesis explored the biogeochemistry of cobalt using analytical chemistry and physiological experiments with the dominant phytoplankton species, Prochlorococcus. A high sensitivity method for Co speciation was developed using hanging mercury drop cathodic stripping voltammetry. Dissolved Co at the Bermuda Atlantic Time Series station (BATS) in the Sargasso Sea was bound by strong organic complexes with a conditional stability constant of logK=16.3l0.9. A depth profile of Co at BATS revealed a nutrient-like profile. Biweekly time series measurements of total cobalt near Bermuda from the MITESS sampler were 0-47pM throughout 1999, and averaged 20±10pM in 1999. A transect of total cobalt from BATS to American coastal waters ranged from 19- 133pM and correlated negatively with salinity (r2=0.93), suggestive of coastal waters as an input source. Prochlorococcus strains MED4-Ax and SS120 showed an absolute requirement for Co, despite replete Zn. 57Co uptake rates and growth rates were enhanced by additions of filtered low Co cultures, suggesting that a ligand is present that facilitates Co uptake. Bottle incubations from a Synechococcus bloom in the Pacific showed production of 425pM strong cobalt ligand. These and other lines of evidence support the hypothesis that a cobalt ligand, or cobalophore, is involved in cobalt uptake. Co-limited Prochlorococcus cultures exhibited an increase in the fraction of cells in G2 relative to other cell cycle stages during exponential growth, and the durations of this stage increased with decreasing cobalt concentrations. This effect was not observed with Fe, N, or P-limited cultures, suggestive of a specific biochemical function of cobalt that would interfere with the late stages of the cell cycle. The ligand Teta was explored as a means to induce cobalt limitation. The CoTeta complex was not bioavailable to the Sargasso Sea microbial assemblage in short-term experiments. Bottle incubations with Teta did not induce cobalt limitation of Prochlorococcus. These results are consistent with the lower conditional stability constant for CoTeta (logK=11.2l0.1) relative to natural cobalt ligands in seawater, and with culture studies that suggest uptake of cobalt via strong organic ligands.The work in this thesis was supported by a grant from the National Science Foundation (#OCE-9618729) for cyanobacteria metal interactions in the Sargasso Sea. I have been funded through WHOI on an NSF coastal traineeship (#DGE-9454129) for my first year, followed by an EP A STAR Graduate Fellowship for the subsequent years. Additional funding was supplied by the WHOI Educational Endowment Funds and by the WHOI Ditty Bag fund for part of the DNA/cell cycle work