Membrane Vesicle Formation Removes Iron Sulfide Mineral Crusts From the Cell Surface of Growing Sulfate-Reducing Bacteria.

Sulfate-reducing bacteria (SRB) drive the process of sulfate reduction in low-temperature sedimentary environments. Through the production of sulfide, they promote the formation of iron-sulfide (Fe-S) minerals when Fe(II) is available. The negative charge of the cell surface of bacteria can promote the binding of Fe(II), leading to the precipitation of Fe-S minerals at the surface of SRB when sulfide is released from cells. We evaluated interactions between Fe-S minerals and the surface of SRB using transmission electron microscopy (TEM) in cultures of Maridesulfovibrio hydrothermalis AM13 grown with 4 mM of Fe(II) over 1 month of incubation. On average, 18% ± 10% of cells were encrusted in cultures collected during the exponential phase. Fe-S mineral deposition occurred at the surface of cells while cells were growing and producing sulfide in the presence of Fe(II), but mineral crusts were removed from most cells shortly after deposition. Cells removed crusts from their surface through the formation of membrane vesicles, which were apparently only produced during growth. Mineralized and non-mineralized membrane vesicles were preserved in mineral aggregates in stationary-phase cultures. On average, 17% ± 7% of cells were encrusted in cultures collected during the stationary phase, indicating that Fe-S minerals precipitated during the exponential phase and removed from the cell surface did not aggregate back onto cells. On the contrary, they formed large aggregates away from cells. When Fe-S mineral precipitation occurred in non-growing cell suspensions that were first exposed to Fe(II) then to sulfide, the proportion of encrusted cells increased to 95% ± 6%, indicating that resting or non-growing cells were not able to remove mineral crusts from their surface. The metabolic status of SRB therefore plays a role in their ability to escape Fe-S mineral entombment.