| Biogenic iron sulfides (FeSx) are most commonly formed by a passive nucleation on the cells envelopes (process known as induced type of biomineralization) and well documented for Bacillus subtilis, or possibly by an active mechanisms (controlled type of biomineralization) claimed to exist in some magnetotactic bacteria. The induced biomineralization does not result in any thermodynamic advantages for the cells. Still, the reaction sequence between iron and sulfides is exergonic, has a high reducing power and in certain cases a significant fractionation of sulfur during the formation of biogenic pyrite (FeS2) was observed. Therefore, there is ground to question if this type of reactions can also represent an energy resource for some microorganisms as well. Formation of FeSx minerals has been proposed previously as the first source of energy for early life forms and for some extant microorganisms but it was never demonstrated due to experimental difficulties. Using a sulfidic hydrothermal system from Romania and springs from the Yellowstone Park as sources I obtained liquid enrichments and successfully isolated bacterial strains that grow by using the reaction: Fe+2+HS -x=FeSx+ H2 as an energy resource. Iron monosulfides and pyrite are formed as metabolic byproducts and deposited in the periplasmic spaces of the cells. One of the strains identified as a new species of Proteobacteria, phylogenetically related with Thiobacillus thermosulfatus displays a controlled type of biomineralization of iron sulfides, that is: temperature sensitivity, dependence on the metabolic state and results in increase of the cellular energy (ATP) when ferrous iron and sulfides were experimentally provided.; This study improved the resolution of the Cr2+ reduction method used for the discrimination of iron sulfide precipitates from microbial cultures, identified subsurface habitats and fingerprints characteristic for dissimilatory precipitation of iron-sulfides, showed that the Fe2+ /HS- couple can be used by extant microorganisms as a source of energy, showed that living organisms can derive part of their energy from precipitation of minerals and provided support for the FeS bioenergetic model of early life. |
