Award: OCE-1737342

The oceans waters have the periodic table of elements in them that range from major ions like sodium and chloride (salt) that are in high concentrations (e.g., 3 tenths of a percent) to trace metals like iron or zinc that are in parts per trillion concentrations, but still act as micro nutrients and stimulate microscopic plants, the phytoplankton, growth. At the same time there are toxic trace elements like mercury or arsenic that are toxic to aquatic organisms like phytoplankton. Interestingly, it is not just the total concentration that makes them toxic but their chemical forms like having a positive or negative charge, or being surrounded by other inorganic ions (e.g., chloride, sulfate) or organic compounds (alcohols, proteins, etc.); these are called complexes. Some complexes are stable and decrease the bioavailability or transfer across cell membranes of certain trace metals, while others may increase the metals transfer into cells and make them more toxic or facilitate the beneficial properties. The reason why metals that are in low concentrations are of interest to ocean scientists is that like the higher concentration nutrients nitrogen and phosphorus, they affect plant growth through photosynthesis and in turn this controls the removal or inputs of the greenhouse gas carbon dioxide that affects the earths global climate. The problem is how do we study trace metals when we use metal ships and wire cables to sample the water? Specialized sampling equipment and procedures were developed to allow studies of trace metals in the worlds ocean. The mission of the international GEOTRACES program, which begun its ocean-wide sampling program in 2010, is to identify processes and quantify the rates of transfer that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions. Specifically, we aim to determine global ocean distributions of selected trace elements and isotopes including their concentrations, and chemical and physical forms and to evaluate the sources, sinks, and internal cycling of these elements to characterize what regulates their distributions. As part of the international planning, United States oceanographers aimed to contribute 5 major sections (top to bottom measurements at ~25 stations along a ~4000 mile long ships track) during the 2010-2020 decade. The fourth US section discussed here was in the Pacific Ocean from Alaska to Tahiti in 2018 (Figure 1). Our research on this transect examined the reactive gas hydrogen sulfide (rotten egg smell) whose ions react with metals like zinc, cadmium and mercury, thus affecting their reactivity and bioavailability. The graduate student working on the hydrogen sulfide research was Nicole Buckley. In the northern waters near Alaska the concentrations of hydrogen sulfide were highest and then decreased as we headed south; it was concentrated in the upper 300 m of the water column where it appeared to react with cadmium and zinc to form insoluble metal sulfides on particles that sank out of the water column. Thus, hydrogen sulfides removed these biologically essential metals. Nicole made calculations to see what metals were affected in the central North Pacific and the data show that sulfide is primarily reacting with cadmium, lead, and zinc (Figure 2). Moreover, the sulfide itself appeared to be produced by phytoplankton as observed in the horizontal surface water transect (Figure3), with total dissolved sulfide showing maxima at the same locations where two phytoplankton species, Synechococcus and Prochlorococcus are most abundant. While it has been hypothesized that phytoplankton do this to remove toxic metals, in this case at least cadmium and zinc are essential, while lead is potentially toxic. Thus, this conundrum remains a mystery. Overall, Nicole established that hydrogen sulfide is produced by phytoplankton and definitely complexes with a variety of trace metals such as lead, zinc, and zinc, but it also serves as a removal mechanism for these metals via the formation of insoluble metal sulfides. Finally, she successfully defended her Ph.D. and graduated in early May 2024 (Figure 4). Last Modified: 05/21/2024 Submitted by: GregoryACutter