AbstractIn natural and laboratory-based environments experiencing sustained counter fluxes of sulfide and oxidants, elemental sulfur (S⁰)—a key intermediate in the sulfur cycle—can commonly accumulate. S⁰ is frequently invoked as a biomineralization product generated by enzymatic oxidation of hydrogen sulfide and polysulfides. Here we show the formation of S⁰ encapsulated in nanometre to micrometre-scale tubular and spherical organic structures that self-assemble in sulfide gradient environments in the absence of any direct biological activity. The morphology and composition of these carbon/sulfur microstructures so closely resemble microbial cellular and extracellular structures that new caution must be applied to the interpretation of putative microbial biosignatures in the fossil record. These reactions between sulfide and organic matter have important implications for our understanding of S⁰ mineralization processes and sulfur interactions with organic carbon in the environment. They furthermore provide a new pathway for the synthesis of carbon-sulfur nanocomposites for energy storage technologies.