Stromatolites: Biogenicity, biosignatures, and bioconfusion

Stromatolites represent a multifarious system of nested, physically, chemically, and biologically influenced components that range in scale from microscopic to macroscopic. These components can include microorganisms, organic compounds of microorganisms, sediment grains, precipitated sediment, sedimentary textures (fabrics), microstructure, laminae, domes, columns, branched columns, and cones. Millimeter to meter scale edifices (stromatolites) are the result. Stromatolites once played a significant role in establishing life's presence on the early Earth, but now a shift away from reliance on stromatolites is occurring. There is a perception that Archean stromatolite-like structures have low reliability to signal life. This is likely due to (1) no unified theory on stromatolite morphogenesis, (2) no valid or appropriate modern analog to use in the interpretation of Archean stromatolites, and (3) disagreement on how to define the word stromatolite. No single feature or line of evidence has yet been found that can unequivocally indicate a biogenic nature for a stromatolite. However, a range of features and their combinations that are well documented for the vast majority of fossil stromatolites and are found in some living stromatolites, are difficult, if not impossible, to account for by inorganic processes. Morphology remains a valid criterion to indicate biogenicity.