Aerobic microbial Fe acquisition from ferrihydrite nanoparticles: effects of crystalline order, siderophores, and alginate.

This research compared the bioavailability of Fe associated with two forms of the hydrous Fe oxyhydroxide nanomineral ferrihydrite (Fh)--the smaller (1-3 nm), less ordered 2-line (2L) phase and the slightly larger, (2-6 nm) more ordered 6-line (6L) phase--to the common aerobic soil bacterium Pseudomonas mendocina ymp. P. mendocina can acquire Fe from minerals using high-affinity Fe(III) binding ligands known as siderophores and a cell-associated metalloreductase that requires direct cell-mineral contact. Wild-type (WT) P. mendocina and a siderophore(-) mutant were used to monitor siderophore -related and -independent Fe acquisition from 2L and 6L Fh. Both WT and mutant strains acquired Fe from Fh, although Fe acquisition and growth were substantially greater on the 2L phase than on the 6L phase. In the absence of bacteria, copious quantities of the biofilm exopolysaccharide alginate slightly promoted dissolution of 2L and 6L Fh. In biotic experiments, added alginate slightly enhanced growth and Fe acquisition from 6L Fh but not from 2L Fh. Recent research has led to an emerging understanding that Fe-oxide nanoparticle structure, stability, and reactivity are highly sensitive to size at the nanoscale; this research emphasizes how subtle differences in nanoparticle size-related properties can also affect bioavailability.