FIG. 2.

Fossilization of microorganisms by mineral replacement. (a) Schematic view of chemical bonding of minerals in solution to functional groups on the organic surface (microbial cell wall, EPS, etc.). (b) Scanning electron microscope and (c) transmission electron microscope view of an artificially fossilized, modern, chemolithotrophic microorganism, Pyrococcus abyssi, an analogue for early life on Earth and possible life on Mars (Orange et al., 2009). Arrow in (b) points to visible nanometer-sized deposits of silica on the cell wall. In (c), note the rugged surface and slightly irregular form of the microfossils (labeled “C”), as well as the fine-scale mineralization (Si, silica) of the cell wall (dark crust). (d, e) Fossilized (silicified), deflated (lysed) filament in the 3.45 billion-year-old Kitty's Gap sediments (see text in Section 3) in secondary electron view and EDX carbon mapping, respectively, showing the presence of carbon in the filament (Westall et al., 2006a). (f) Fossilized (silicified) coccoidal microfossils in the 3.45 billion-year-old Kitty's Gap sediments showing distinguishing features, such as cell division and lysis, as well as two sizes of cells (two species) (Westall et al., 2006a, 2011a). (g) For comparison with the fossilized coccoids in (f), abiotic silica spheres exhibit a similar morphology and, although spheres are juxtaposed, seemingly imitating cell division, they do not have many of the attributes of the biogenic coccoids (carbonaceous composition, fine-scale irregular surface, evidence of lysis, etc.; see text for discussion) (Westall et al., 2006a, 2011a). (Color graphics available at www.liebertonline.com/ast)