A Process-based Approach to Evaluating the Role of Organic Ligands in Trace Metal Cycling in the Marine Environment

In addition to control by major nutrient elements (nitrogen, phosphorous, and silicon) growth and community composition of marine phytoplankton is also regulated by trace element nutrients (iron, copper, manganese, zinc, cobalt, nickel, and cadmium). Of these, iron is the most influential in the modern ocean, regulating phytoplankton growth and carbon export in high-nutrient low-chlorophyll regimes and exerting an important control on the marine nitrogen cycle through its role in di-nitrogen fixation. The distributions of these metals has the capacity to control primary production and phytoplankton community composition through differences in cellular quotas or metal sensitivities amongst species. The relationship between trace metal distributions and marine microorganisms is coupled; plankton and bacteria shape the distribution, speciation, and redox state of these metals through cellular uptake, recycling processes, and production of specific and non-specific metal chelators. The interplay between these chemical and biological processes has a profound influence on the modern ocean and global biogeochemical cycles. In this work the feedbacks between trace metal nutrients, natural metal-binding chelators, and marine microorganisms are examined in three distinct oceanic environments that encompass some of the major interfaces of trace metals to the ocean. In the upwelled waters of the California Current system field incubations examine the role of light on the uptake and speciation of metals; an important observation from this study was the increase in short-term Fe uptake rates for marine phytoplankton resulting from photochemical reactions of organically bound Fe, a process that may result in a competitive advantage for some phytoplankton species. In the surface waters of the West Florida Shelf a baseline study of the concentrations of bioactive trace metals and Fe-and-Cu binding organic ligands are reported between two seasons; the work identifies important regional and seasonal processes controlling the distributions of these metals and observed divergent mechanisms influencing the cycling of Fe-and-Cu-binding ligands that may act as a bottom-up control on phytoplankton communities in this region. In hydrothermal plumes along the Mid Atlantic Ridge field incubations examine the role of particles and Fe-binding ligands on the stabilization of dissolved Fe input from these vent systems; the experiments demonstrate the importance of colloids, exchange between particle phases, and Fe-binding ligand production in the stabilization of Fe far-field of these vent systems. The results of these studies present mechanistic frameworks to contextualize some of the basin-scale distributions of these metals generated from the GEOTRACES program. As the GEOTRACES program shifts progressively towards more process oriented studies this work may serve as a useful framework to test hypotheses and further characterize biogeochemical cycles of trace metals in these marine environments