MOLECULAR CONTROLS ON STROMATOLITE AGGREGATION ARE NOT UNIQUE TO LIFE

Controls on the formation of stromatolites are underdetermined by the biological processes that may mediate their aggregation. Much recent attention has been focused on the significant role that microbial extracellular polymeric substances (EPS) play in trapping and binding sediments, contributing to the formation of stromatolitic structures. However interesting and complex these microbial exudates may be for microbial ecology, microbial evolution leading to its modern complex level of organization is not directly indicated by the most ancient preserved stromatolites. Adsorption properties that are reciprocal between inorganic (mineral) surfaces and organic molecule functional groups provide much of the molecular-scale aggregational potential of biomediated stromatolites. Carbonyl functional groups, for example, show particular affinity to unbonded OH groups on silicate crystal surfaces. Adsorption and subsequent condensation of organic diols and diacids through step polymerization is further potentiated by dehydration, such as may occur in paralic zones of fluctuating salinity or in tidal zones of frequent and repeated subaerial exposure. These are a purely nonvital process and are not directly linked to other biological properties, such as genetics and metabolism. Although mucopolysaccharides are a common molecular motif of modern EPS, not all microbial exopolymers have yet been characterized, and the pertinent binding and mineral nucleating properties they possess are not unique to mucopolysaccharides. Because all naturally occurring organic molecules presently on Earth have been actively cycled through biological (i.e. enzymatic) processes, it is challenge to envision a time when this may not have been the case. But by articulating the molecular properties which contribute to the meso-scale binding of particulate material and the nucleation of minerals, we may at first model, then experiment with, possible prebiologic molecular and physical conditions that governed Paleoarchean stromatolite formation. Only upon the consistent failure of these attempts may we confidently conclude that Earth’s oldest preserved stromatolites are indicative of microbial life.