Award: OCE-1736280

Intellectual Merit Microbial cycling impacts the distribution and speciation of many trace elements and their isotopes (TEIs) in the ocean. While highly sensitive methods to make contaminant ?free total elemental concentrations have been in place for over a decade, we have not had the means to measure the molecular speciation of these TEIs. With support from this award, we completed our development of a very sensitive method to measure TEI molecular speciation using a combination of TEI extraction from seawater coupled to chromatographic analysis interfaced with inductively coupled and electrospray ionization mass spectrometry. We can now make quantitative molecular speciation measurements of pico-molar concentrations of TEI in as little as 1-2L of seawater. We used our method to survey the distribution of iron, copper, manganese, iodine, cobalt, nickel and zinc organic complexes in hundreds of samples across the US GEOTRACES Pacific Meridional Transect cruise at 152oW between Alaska and Tahiti. For iron, we found nearly all samples at depths < 1000 m contained siderophores, organic iron complexes used by microbes to acquire iron from the environment. We identified many of these siderophores as marinobactins, amphibactins, and ferrioxamines, as well as found new siderophores that had not been previously identified. Concentrations of siderophores were highest between 200-500 m through most of the section, with concentrations reaching values of 100 pM, indicating that microbes at these depths are iron deficient. We found copper ligands were also common across the section, with highest concentrations in regions characterized by high nitrite concentrations. We found a strong correlation between copper ligands and nitrite. We found high concentrations of distinct manganese ligands near the Alaskan continental margin, and high concentrations of organic iodine species in the upper 1000 m of the water column. Cobalt, nickel and zinc ligands were not correlated with any major water column feature, but were common in our samples. With support from the award we introduced a new method to measure thermodynamic stability constants of trace metal organic ligands using isotope exchange, and showed that different iron ligands in seawater have a range of stability constants, exchanging iron at different rates. We coded new mass filtering algorithms that allow us to assign masses to TEI-ligands, and described new iron and copper ligands in pure cultures of marine cyanobacteria and algae. These contributions should be of broad interest to environmental scientists, biogeochemists, and others requiring sensitive methods to monitor changes in the distribution and thermodynamic properties of organometallic and nonmetallic compounds. Broader Impacts The project made a number of scientific contributions that should be of broad interest to environmental chemists as outlined in the Intellectual Merit section of this report. The project supported the Ph.D. thesis research of three MIT/WHOI Joint Program students; Dr. Lydia Babcock-Adams, Mr. Jingxuan Li, and Ms. Iulia Madalina-Streanga. As part of this award, Dr. Babcock-Adams studies the molecular speciation and biogeochemical cycling of organic copper in the North Pacific Ocean. Dr. Babcock- Adams is currently a postdoctoral investigator at the National High Magnetic Field Laboratory at Florida State University. Mr. Li will complete his thesis research on the molecular speciation and cycling of iron in the North Pacific in late 2022. Ms. Madalina Streanga is analyzing our GEOTRACES data for organic iodine cycling, and is developing new methods to identify organic complexes incorporating monoisotopic elements (iodine, colbat, manganese, etc.). The project engaged a fourth graduate student, Dr. Daniel Muratore from Dr. Joshua Weitz?s laboratory at Georgia Tech who helped us code new algorithms to integrate datasets from inductively coupled and electrospray ionization mass spectrometers. Our ability to engage undergraduates in the project was severely impacted by COVID, which restricted our ability during the award to host and train visiting students in or laboratory. Nevertheless we were able to use the award to entrain two students into the project; Mr. Luis Valentin-Alvarado, from an under-represented group form the University of Puerto Rico, who worked in my laboratory in-person, and Ms. Sophie Zweifel from the University of Edinburgh who worked remotely. Mr. Alvarado is currently a Ph.D. student at UC Berkeley working on microbial biogeochemistry in the Banfield and Savage laboratories. Ms. Zweifel is currently a Ph.D. student at ETH in Zurich. Finally we performed a number of other outreach activities (instagram posts(@sea_the_chemistry), and other local and regional activities with K12 students. Last Modified: 08/03/2022 Submitted by: Daniel J Repeta