A structural comparison of bacterial microfossils vs. 'nanobacteria' and nanofossils

The formation of bacterial microfossils results from the cell surface immob ilization of soluble heavy metals (biomineralization) via passive ionic int eractions or by the formation and release of chemical reactive metabolic by -products. These metal-encrusted cell surfaces are resistant to re-mobiliza tion and are typically the only component of the cell that is preserved, fo r possibly as long as several billion years. The size and shape of microfos sils are determined by bacterial morphology, which includes spherical, rod, filamentous, vibriod, helical and stalked structures. The examination of u ltra-thin sections using transmission electron microscopy (TEM) reveals tha t mineralized bacterial cells have the basic shape of the original cell fro m which they formed and appear hollow. Even in rare cases when the cell env elope and the cytoplasm are mineralized, the cell envelope can be different iated from the cytoplasm preserving the original cell morphology. Scanning electron microscopy (SEM) cannot differentiate between geochemical and geom icrobiological mineral precipitation. The term 'nanobacteria' has been used to describe spherical or rod-shaped minerals (tens of nanometers in diamet er) observed using SEM. While these minerals may represent mineralized port ions of bacteria, e.g., membrane vesicles, stalks or flagella, they are too small to be bacteria. Conversely, 'nanobacteria' may simply represent soli d, inorganic precipitates. (C) 1999 Elsevier Science B.V. All rights reserv ed.