Award: OCE-1736599

Metal are highly scarce in seawater, yet they are important for nutrition of marine ecosystems, and can influence marine carbon cycling. In this project we analyzed the distribution of dissolved cobalt and labile cobalt across the North and Central Pacific Ocean as part of the GEOTRACES program on the GP15 section. The meridonial section provides an exciting view of a number of processes that influence the distribution of cobalt including hydrothermal inputs, sedimentary and coastal inputs, scavenging in intermediate waters, and scarcity in surface waters. The Pacific transect suggests a substantial source of cobalt to the North Pacific basin along the Alaskan Shelf associated with a low salinity waters. Elevated concentrations of total and labile cobalt within oxygen minimum zones in the Equatorial North and South Pacific are likely due to the reduction of manganese oxide particles and release of incorporated cobalt. Additionally, increased cobalt concentrations above the Lō’ihi Seamount suggests a local hydrothermal source of dissolved cobalt. Low concentrations of cobalt are found in the surface ocean where cobalt is used as a nutrient for phytoplankton, particularly in the oligotrophic gyres. At depth, cobalt is depleted relative to phosphate throughout the transect, revealing a strong scavenging removal process in the deep Pacific. A number of parallel studies were conducted that contributed to our understanding of the biogeochemical interactions between metals and microbes in the Pacific Ocean and the Arctic Ocean. These included the discovery of abundant metal-containing enzymes that are involved in the transformation of nitrogen in the low oxygen waters of the oceans using new protein measurement techniques. A study of diatoms isolated in the North Pacific found they are particularly efficient at replacing cobalt for zinc, and this likely reflects the higher abundance of cobalt in the North Pacific environment. A study was conducted that compared the minimum requirement of cobalt within the abundant cyanobacterium Prochlorococcus at 60 atoms per cell, using both metal and protein analysis of Co requiring proteins. This last study compared these requirements to a GEOTRACES compliant cobalt section in the Central Pacific Ocean from the METZYME expedition and to a global Co biogeochemical model. Together this grant contributed to our understanding of the biogeochemical cycling of cobalt and other metals in the Pacific Ocean environment. Last Modified: 05/23/2020 Submitted by: Mak A Saito