Does a low-pH microenvironment around phototrophic FeII-oxidizing bacteria prevent cell encrustation by FeIII minerals?

Neutrophilic FeII-oxidizing bacteria precipitate positively charged FeIII minerals that are expected to sorb to the negatively charged cell surface, leading to encrustation and thus limiting the cells' accessibility to substrates and nutrients. However, electron-microscopy analysis of phototrophic iron-oxidizing Thiodictyon sp. strain F4 cells showed no encrustation, but mineral precipitation at a distance from the cell surface. In situ fluorescence microscopy analysis of F4 cells using a pH-sensitive fluorescent dye revealed a low cell surface pH (6.0 +/- 0.1) in contrast to the bulk pH (6.6 +/- 0.1). Biogeochemical modeling showed that the pH difference reduces FeIII sorption and FeIII precipitation rates at the cell surface, therefore directing mineral formation away from the cells. The results from this study therefore suggest that the establishment of a low cell surface pH could provide a mechanism for photoferrotrophs to successfully prevent FeIII mineral precipitation on the cell surface.