Awardee: Mark A. Torres (University of Southern California)
Current Placement: Assistant Professor of Earth, Environmental and Planetary Sciences, Rice University
Degree: Ph.D. Geochemistry, University of Southern California (2015)
Advisor: A. Joshua West (University of Southern California)
Amount: $64,000.00
Award Dates: August 16, 2013 — August 15, 2015


The project focused on the efficacy by which microorganisms can obtain nutrient Fe from silicate minerals. Silicate minerals are a particularly abundant mineral phase in the oceanic crust and thus the bio-availability of silicate-bound nutrients has important implications for microbial activity in the deep subseafloor (C-DEBI theme 1) and the limits to microbial life (C-DEBI theme 3). The specific goal of this project was to quantitatively determine how metal-binding organic compounds (siderophores) produced by microorganisms under Fe-limited conditions affect the rate of Fe-silicate mineral dissolution using laboratory experiments. The exact effect of microbial activity on Fe-silicate mineral dissolution has previously been hard to discern due to the complicating effects of feedbacks associated with microbial growth, siderophore production, and mineral dissolution rates. To limit the effects of these feedbacks, my experimental design used purified microbial siderophores and a silicate mineral (olivine) that dissolves at a rate that is relatively insensitive to the accumulation of its constituent ions in solution. My results showed that sub-millimolar siderophore concentrations lead to an order of magnitude increase in olivine dissolution rates. The accelerating effect of siderophores was linked to the removal of an inhibiting surface Fe-oxide coating that forms during the reaction of olivine at circum-neutral pH in the presence of O2. By combining the experimental results with a numerical model of the relevant biological feedbacks, this work further constrained the maximum extent to which microbial activity may affect silicate mineral dissolution rates under conditions of Fe-limitation. The results of this study are presently under review for publication in Geobiology.