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The Cobalt Cycle in the Tropical Pacific Ocean

Nicholas Hawco, Ph.D., 2017
Mak Saito, Advisor

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. Large cobalt plumes within anoxic waters are maintained by a cobalt supply from organic matter remineralization, an amplified sedimentary source from oxygen-depleted coastlines, and low-oxygen inhibition of cobalt scavenging. Rates of scavenging are calculated from a global synthesis of recent GEOTRACES data and agree with cobalt accumulation rates in pelagic sediments. It is possible that oceanic cobalt inventories are dynamic on the span of decades.

Minimum cobalt requirements of a Prochlorococcus strain isolated from the Equatorial Pacific are quantified and related to demand for the ribonucleotide reductase enzyme, which catalyzes a critical step in DNA biosynthesis. Competitive inhibition of cobalt uptake by low levels of zinc imply that wild Prochlorococcus are not far from a cobalt limitation threshold.