Intellectual Merit: The current understanding of organic acid mediated phosphate dissolution is limited by the lack of a molecular scale characterization of its solid state speciation. Furthermore, phosphate dissolution has not been studied in context of its uptake without the complicating influence of metabolism as in microbial cells or plants. Our current knowledge can mainly be attributed to wet chemical studies in soils whose inherent complexity makes it difficult to characterize the effect of primary environmental variables (pH, concentration of phosphate, type of organic acid and its concentration). In addition, phosphate dissolution in binary and tertiary mixtures of iron, aluminum and Ca minerals (main sorbents of phosphate) that can serve as an effective analog for soils has not been investigated. Based on a molecular scale XANES based investigation of phosphate sorption in 1:1 (by mass) binary mixtures of Fe-oxide and Aloxide minerals or Ca containing minerals, we can now: 1) quantify the distribution of phosphate between individual mineral phases in binary and ternary mixtures (Khare et al., 2004; Beauchemin et al., 2003); 2) distinguish between adsorption and surface precipitation in single mineral and binary mixtures (Khare et al., 2005); and 3) determine phosphate bonding configuration and differentiate between surface complexes (Khare et al., 2007). Thus, we are now in a position to exploit these XANES based tools in uncovering molecular mechanisms of phosphate dissolution. This research will include a high affinity transporter for yeast cells reconstituted into proteoliposomes as a sink for dissolved phosphate to understand and realistically predict phosphate dissolution in natural systems.
The proposed research will take place over two years and will address two main hypotheses:
Hypothesis 1: Phosphate dissolution in single mineral, binary and tertiary mixtures is controlled by the solid state speciation (mode of phosphate bonding, adsorption vs. surface precipitation and the partitioning of phosphate in individual mineral phases), of phosphate in these minerals.
Hypothesis 2: Phosphate uptake will be adversely affected by Al3+ in binary mixtures of Fe and Al containing minerals however in ternary mineral systems the presence of Ca2+ will ameliorate Al toxicity.
Broader Impacts: Phosphorus is an essential plant macronutrient and also a potential water pollutant. Most terrestrial and marine ecosystems are P limited because phosphate minerals are sparingly soluble. Because organic acids citrate, malate released by plants roots or microbes are considered the main mode of P solubilisation in soils and other natural systems, this basic geochemical research is pertinent to improving soil fertility. This is particularly significant because food production needs to double in the next 20 years to sustain increasing world population. Characterizing organic acid mediated phosphate dissolution at the nano scale will also help with better predictions of phosphorus bioavailability and release. Recently phosphate release from heavily fertilized P enriched soils has degraded surface water quality, causing eutrophication. Furthermore, better estimates of phosphate bioavailability will help predict CO2 uptake rates critical to predicting global warming. This project will contribute to a graduate student's dissertation, to be supervised by the PI and co-PI jointly. An integrated wet chemical, spectroscopic (XANES), microscopic (TEM) and geochemical modeling approach will be used.
