Trace Element Biogeochemistry
The solubility and bioavailability of dissolved micronutrient metals, in particular iron, in seawater is governed in large part by organic chelating agents (ligands) that are produced by microorganism. While these ligands are fundamental to our understanding of global metal cycling and biological productivity, their chemical composition, reactivity, and sources have remained a mystery. Much of our work focuses on the development of new analytical approaches for studying these molecules at the molecular level using chromatography and mass spectrometry. The fundamental mechanistic insights gained from these studies are needed to understand metal distributions in the environment today and predict how they will change and affect ocean ecosystems in the future.
How does iron from dust affect marine ecosystems?
Iron (Fe) is a crucial nutrient for microbial growth in the oceans, impacting the carbon cycle and the climate system, but Fe does not dissolve readily in seawater and so its availability limits phytoplankton growth over much of the surface oceans. One of the most significant ways by which Fe reaches the surface ocean is through deposition of wind-blown dust. However, for this Fe to be available for biological growth, it must dissolve and be kept in solution bound to organic molecules. Despite this known importance, the mechanisms of dust dissolution and the identity of the organic molecules solubilizing Fe remain poorly understood. One of our current studies focuses on using laboratory and field experiments to better understand how natural organic molecules present in seawater enhance the release of Fe from Saharan desert dust, what organic molecules are produced by microbes in response to dust, what is the isotopic fractionation of Fe associated with dust dissolution, and ultimately what is the role of ligand-mediation dissolution in determining the effect of dust on marine nutrient and carbon cycles.
Antarctic GEOTRACES: What are the sources of ligands (and the metals that they solubilize) to the chronically metal-starved Southern Ocean?
The Southern Ocean is one of the largest chronically iron limited biomes on the planet, and is currently undergoing unprecedented rapid changes as ice sheets melt due to warming temperatures. Assessing future changes to the Southern Ocean ecosystem relies on knowledge of the processes that supply iron to the surface waters along the Antarctic continent. We will participate in the Antarctic GP17-OCE and GP17-ANT cruises to determine what are the major sources and sinks of the organic ligands that solubilize and supply micronutrient Fe.
What biochemical processes solubilize trace metals across dynamic redox boundaries?
The fate and transport of metal contaminates in soils and sediments is controlled by a complex network of biogeochemical reactions coupled with hydrologic processes. Determining the controls over metal dispersal in dynamic watersheds is particularly important given their roles as conduits between surface and groundwater and vulnerability to perturbation. Dissolved organic matter (DOM) has a major but poorly understood control over the mobility of metals in these environments. Although redox conditions (aerobic vs. anaerobic vs. sulfidic) have a major effect on DOM composition, the impact on metal binding properties and associated migration remains unknown. We are currently developing new analytical methods for assessing the composition of metal-binding ligands within DOM and determine how they vary across different redox conditions, with the goal of developing models of metal speciation and migration that account for variations in DOM ligand chemistry.