Publication Date:
2022-05-25
Description:
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 2017
Description:
Although over a dozen elements are needed to support phytoplankton growth, only a few are
considered to be growth-limiting. As the central atom in vitamin B12, cobalt is crucial for
metabolism, but its status as a limiting nutrient is uncertain. This thesis investigates the
geochemical controls on oceanic cobalt scarcity and their biological consequences.
Analysis of over 1000 samples collected in the Tropical Pacific Ocean reveals a dissolved cobalt
distribution that is strongly coupled to dissolved oxygen, with peak concentrations where oxygen
is lowest. Large cobalt plumes within anoxic waters are maintained by three processes: 1) a
cobalt supply from organic matter remineralization, 2) an amplified sedimentary source from
oxygen-depleted coastlines, and 3) low-oxygen inhibition of manganese oxidation, which
scavenges cobalt from the water column. Rates of scavenging are calculated from a global
synthesis of recent GEOTRACES data and agree with cobalt accumulation rates in pelagic
sediments. Because both sources and sinks are tied to the extent of oxygen minimum zones,
oceanic cobalt inventories are likely dynamic on the span of decades.
Despite extremely low cobalt in the South Pacific gyre, the cyanobacterium Prochlorococcus
thrives. Minimum cobalt and iron requirements of a Prochlorococcus strain isolated from the
Equatorial Pacific are quantified. Cobalt quotas are related to demand for ribonucleotide
reductase and methionine synthase enzymes, which catalyze critical steps in DNA and protein
biosynthesis, respectively. Compared to other cyanobacteria, a streamlined metal physiology
makes Prochlorococcus susceptible to competitive inhibition of cobalt uptake by low levels of
zinc. Although phytoplankton in the Equatorial Pacific are subject to chronic iron-limitation,
widespread cobalt scarcity and vulnerability to zinc inhibition observed in culture imply that
wild Prochlorococcus are not far from a cobalt-limitation threshold.
Description:
I am lucky to have benefitted from major financial support of the Saito Lab by the National
Science Foundation and the Gordon and Betty Moore Foundation. Specifically, National Science
Foundation grants for the Center for Microbial Oceanography Research and Education
(CMORE, DBI-0424599), GEOTRACES Pacific and Artic projects (OCE-1233261 and OCE-
1540254), and OCE-1220484 funded my thesis work. National Science Foundation grants OCE-
1031271 and OCE-1337780 and Gordon and Betty Moore Foundation grants 3782 and 3934 to
the Saito lab also provided instrumentation and funded field expeditions that enabled this work.
Keywords:
Cobalt
;
Zinc
;
Plankton
;
Kilo Moana (Ship) Cruise KM1128
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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