Spherulite in the modern aragonitic travertine stromatolite has a calcite core with exopolymers

ABSTRACT Spherulites are spherical to ellipsoidal particles formed by various crystalline substances and can be abiotically synthesised in the laboratory. Among various spherulite types, spherulites composed of calcium carbonate are also found in various natural environments, but their abiotic or biotic origins are still controversial. This study investigated the origin of the spherulite found in travertine stromatolites from the Nagayu hot spring in Japan by observing the internal minerals and organic matter at the submicron scale. Powder X‐ray diffraction analysis shows that this stromatolite is exclusively aragonite in the bulk, as with other travertines in the study area, reflecting the high Mg 2+ and concentrations in the hot spring water. Scanning transmission X‐ray microscope (STXM) and transmission electron microscope (TEM) analyses of a focused ion beam (FIB)‐processed thin‐foil section reveal that the investigated spherulite contains organic matter with carboxy groups, amorphous calcium carbonate (ACC), and, unexpectedly, calcite in the core. These results suggest that microbially derived acidic extracellular polymeric substances (EPS) induce the assembly of ACC nanoparticles and local precipitation of calcite, and when EPS and ACC are embedded in minerals, the abiotic growth of radial fibrous aragonite dominates to form spherulites. Insights gained from this study are that microorganisms may play an important role in spherulite formation, even in the absence of obvious microbial signatures, such as mineral‐embedded cells, under conventional microscopy and that microbially derived acidic organic matter potentially affects polymorph selection in CaCO 3 .