ORGANO-MINERAL STRUCTURES, BIOMINERALS, AND EXCEPTIONAL FOSSIL PRESERVATION—A MICROCOSM OF THE INFLUENCE OF IRON OXIDIZING MICROBES ON THE FOSSIL RECORD

The expanding role microbes play in the authigenesis and dissolution of minerals can in the right environmental circumstances be extensive and significant. This geomicrobial significance is on full display in recently discovered exceptionally preserved fossils from the Ediacaran-Cambrian Deep Spring Formation, Nevada. Consisting of tubicolous organisms, this fossil biota provides evidence for the influence of iron-oxidizing microbial communities in the synthesis of authigenic iron oxyhydroxide biominerals which provided a mineralogical outline of labile tissues. This mineralization likely occurred within the microaerophilic iron-oxidation zone of the sedimentary column during early stages of microbial degradation of organic matter. The authigenic precipitation of iron-rich clays also acts as the primary preservation in some specimens. An iron gradient within clays outlining tissues in these specimens provides direct evidence of the importance of the early adsorption of Fe(II) and Fe(III) ions onto structural biopolymers during soft-tissue preservation.

A single specimen of a filamentous organo-mineral structure was found within the mineral cavity of a three dimensionally preserved tubicolous fossil from the Deep Spring Formation. Similar to the minerals forming the cavity, abundant iron is found throughout the filament. The presence of iron without clear mineral growth suggests an intimate association between organic matter and mineral within the filament. However, crystal growth in both cubic and fibrous morphologies are also found growing within a substance reminiscent of Extracellular Polysaccharide. Modern iron-oxidizing bacteria are known to produce similar intricate organo-mineral structures in both marine and freshwater environments, however, like the preserving environment of the encasing fossil, this always occurs within microaerophilic conditions. The filament found within the Deep Spring tubicolous fossil superficially resembles such structures and likely contains organic matter. Determining the origin of this filament (whether recent or ancient) is paramount in understanding the long-term survival of such structures as well as assessing their usefulness as bioindicators in the search for life on Mars and the deep-time history of life on Earth.