A structural comparison of bacterial microfossils vs. `nanobacteriaʹ and nanofossils

The formation of bacterial microfossils results from the cell surface immobilization of soluble heavy metals (biomineralization) via passive ionic interactions or by the formation and release of chemical reactive metabolic by-products. These metal-encrusted cell surfaces are resistant to re-mobilization and are typically the only component of the cell that is preserved, for possibly as long as several billion years. The size and shape of microfossils are determined by bacterial morphology, which includes spherical, rod, filamentous, vibriod, helical and stalked structures. The examination of ultra-thin sections using transmission electron microscopy (TEM) reveals that mineralized bacterial cells have the basic shape of the original cell from which they formed and appear hollow. Even in rare cases when the cell envelope and the cytoplasm are mineralized, the cell envelope can be differentiated from the cytoplasm preserving the original cell morphology. Scanning electron microscopy (SEM) cannot differentiate between geochemical and geomicrobiological mineral precipitation. The term `nanobacteriaʹ has been used to describe spherical or rod-shaped minerals (tens of nanometers in diameter) observed using SEM. While these minerals may represent mineralized portions of bacteria, e.g., membrane vesicles, stalks or flagella, they are too small to be bacteria. Conversely, `nanobacteriaʹ may simply represent solid, inorganic precipitates.