Abstract:
Processes 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.
