PRESERVATION OF BIOORGANIC MOLECULES IN AMORPHOUS SILICA IN NATURE

Microbial and cellular components may become incorporated into amorphous silica either by complexation with metals or entombment. For instance, polysaccharides found in bacterial cell walls contain linear polymers of sugars that may be preserved even in arid conditions when complexed with Ca, Al, Si, Fe and Mn. Silicic acid can form a variety of complexes with ions and organic molecules, including mucopolysaccharides, glycoproteins that are enriched in hydroxyl amino acids (serine and threonine), glycine, aspartic and glutamic acids. These amino acids have been found in significant quantities in rock coatings from the Mojave and Sonoran deserts. In addition, silicic acid is expected to form organic silicate complexes (Si-O-C) with hydroxyl centers from cis-1,2-diols that are fixed at ca. 0.26nm. Candidates include some sugars, unsaturated polyhydroxy compounds, catechols (1,2-diphenols), and other compounds with rigid structures and a correct “bite” that matches the O-Si-O angle and thus make stable complexes. Flexible sugar-related substances, such as polyols and sugar acids, also make Si-O-C complexes with silicic acid if they possess at least four hydroxyl groups in a particular stereochemical arrangement. From these examples it is possible to see how various organic compounds, bacteria and fungi, or their remains, can make Si-O-C complexes with silicic acid and contribute to the crosslinking and hardening of the silicate polymers. Significantly, silicic acid also makes Si-O-metal complexes, such as the Si-O-Fe complex with ferrihydrate. This means that not only organic substances, but metals as well, can participate in polymerizing, crosslinking and hardening (by elimination of water) of silicic acid. The process of formation of desert varnish, rock coatings, and silica glazes may be silicification via dissolution of silicates. Small quantities of silicic (Si(OH)₄) or (di)silicic ((HO)₃Si-O-Si(OH)₃) acids form particles of condensed silica that fuse by gelling. Bacteria, fungi, and microcolonial fungi (arid extremophiles), might become silicified and incorporated into the coatings and glazes. The study of organic components in silica coatings may aid in understanding the process of formation of rock coatings, biomineralization, bacterial fossilization, and their past environments.