Research in the Barbeau Lab

quick links

trace metal speciation  ●  trace metal limitation  ●  trace metal acquisition

Chemistry of trace metal speciation in seawater

     Dissolved iron(III) speciation in seawater is dominated  by complexation with strong organic ligands which mediate the biological availability and geochemical cycling of iron.  Some of these ligands are thought to be derived from intracellular material released via grazing or cell lysis. Tetrapyrroles, for example, derived from chlorophyll pigments and hemes, have been hypothesized to contribute significantly to iron(III) complexation in seawater, and to participate in iron cycling.  We are attempting to test this hypothesis by studying the structural chemistry of interaction between iron and model synthetic and naturally-occuring tetrapyrroles under seawater conditions, using spectroscopic methods.  We are also studying the potential for biological uptake of tetrapyrrole-bound iron, using structurally well-characterized iron-tetrapyrrole complexes. These investigations may also be relevant to the cycling of other biologically active trace metals, such as copper.  This work is funded by the American Chemical Society, Petroleum Research Fund (link to proposal abstract).

Examples of intracellular tetrapyrroles

Synthesis of an Fe-tetrapyrrole complex

     Another current area of our research which involves the study of iron speciation in seawater is a collaborative project with Dr. Lihini Aluwihare, an organic geochemist at SIO.  We are collaborating with the Aluwihare lab to study how metal complexation may influence the longevity of colored dissolved organic matter (CDOM) in marine systems.  This work involves isolation and characterization of natural dissolved organic matter, and photochemical experiments on different fractions with and without the presence of complexed iron to determine how Fe complexation may influence photochemical reactivity.  This project is funded by the Office of Naval Research (link to proposal abstract).

     The redox state of iron is a critical parameter in iron speciation and cycling. As a component of our field work in the Southern Ocean (see project description below) we are using flow injection analysis and chemiluminescence to characterise the distribution of reduced Fe (Fe(II)) in seawater.  As a labile intermediate in the redox cycling of iron in surface waters, the presence of Fe(II) could have important implications for the biological availability of iron.  We will be using the FeLume system (Waterville Analytical) to study the distribution, sources and sinks of Fe(II) in waters of the southern Drake Passage.  This work is funded by the Department of Energy (link to proposal abstract).

Role of trace metals in structuring diverse planktonic ecosystems - Southern California Bight, Eastern Tropical North Pacific, and Southern Ocean

     Iron availability is now widely recognized as an important factor potentially controlling phytoplankton productivity and community composition in the ocean.  Field studies have been instrumental in shaping our current understanding of the chemistry of iron in the ocean, and our concept of the role of iron as a limiting nutrient for phytoplankton.  We are actively involved in field research programs in several areas of the world ocean.  In conjunction with the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program, one of the longest and most comprehensive time series of marine observations in the world, we are studying the potential role of iron as a limiting micronutrient in the Southern California Bight. We hypothesize that iron supply is a significant, interannually variable, and previously uncharacterized factor in phytoplankton growth in some areas of the CalCOFI sampling region, particularly coastal areas at times when the nitricline shoals significantly. To test this hypothesis we are measuring iron concentration and speciation and conducting iron-addition incubation experiments on a series of CalCOFI cruises covering several annual cycles in productivity.  This work is funded by NASA, through the New Investigator Program (link to proposal abstract). 

     We are also studying the biogeochemical cycling of iron in the Eastern Tropical North Pacific off Mexico, where a typical oligotrophic near-surface oceanic ecosystem overlies a suboxic zone. The euphotic zone in this location has an unusual layered structure, extending from an oligotrophic surface mixed layer down through a steep nutricline to a low-light suboxic zone dominated by an atypical population of Prochlorococcus sp.  We are participating in two cruises to this area with Dr. Ralf Goericke, to study the speciation and cycling of iron in this unique regime.  This work is funded by the National Science Foundation, Chemical Oceanography program (link to proposal abstract).

      In collaboration with an interdisciplinary group of colleagues from SIO, U. Hawaii and Boston College, led by SIO biological oceanographer Greg Mitchell, we are also taking part in a field study of phytoplankton communities in the Southern Ocean.  Our study area in the southern Drake Passage is the site of a reproducible transition from low to high phytoplankton biomass.  We are attempting to determine whether a gradient in iron supply is responsible for this phenomenon.  This study is finded by the NSF Office of Polar Programs (link to proposal abstract).

Some pictures from our recent cruises... 

top left: Brian and Andrew on the fall 2002 CalCOFI cruise, R/V New Horizon fantail; top right: Andrew with Dave Wolgast (CalCOFI) recovering a GO-flo bottle on the summer 2003 CalCOFI cruise, R/V New Horizon fantail; bottom left: Kathy and Brian deploying GO-flo bottles on November 2003 Mexico cruise, R/V New Horizon starboard 1/4 deck; bottom right: Fe-lume chemiluminescence instrument in clean hood, November 2003 Mexico cruise.

Acquisition of trace metals by marine microorganisms

    We are studying the utilization of iron species traditionally thought of as non-bioavailable - ie. strongly chelated Fe(III), and iron in the form of oxyhydroxide solids - by several ecologically-significant groups of bacteria and cyanobacteria, including the colonial nitrogen-fixing cyanobacterium Trichodesmium.  In collaboration with Margo Haygood's group at SIO and John Waterbury at the Woods Hole Oceanographic Institution, we have demonstrated that Trichodesmium colonies collected in the field harbor a diverse assemblage of heterotrophic bacteria, many of which respond to Fe-limiting growth conditions by producing siderophores (strong Fe(III)-binding ligands). Moreover, uptake experiments performed on collected colonies indicate that under low-Fe conditions (as experienced episodically in the oligotrophic northwestern Atlantic) Trichodesmium colonies are able to readily take up Fe complexed to the siderophore desferrioxamine B. These findings have led to the hypothesis that Trichodesmium is able to utilize exogenous Fe(III) complexes derived from the siderophores produced by its associated microbial community.  We are conduct ing further laboratory and field work with Trichodesmium to document the role of exogenous microbial siderophores in Trichodesmium iron acquisition.  The tools that we are developing for this research, such as ⁵¹Cr(III) complexes with marine siderophores, are also shedding light on how bacteria interact with and utilize siderophores in the marine environment.  This work is funded through the NSF Biological Oceanography program (link to proposal abstract).

Trichodesmium utilization of exogenous Fe(III)-siderophore complexes

CAS indicator plate inoculated with rinsed Trichodesmium colonies show siderophore production associated with bacteria