Laboratory models of microbial biosignatures in carbonate rocks

Enigmatically shaped laminated carbonate rocks called stromatolites dominated shallow marine environments for the first 80% of Earth's history, and are potentially the oldest macrofossils. While these ancient rocky cones and domes occasionally resemble some modern microbial structures, it is unclear whether their formation required biological processes or they could have been produced abiotically. To develop criteria for assessing the biogenicity of ancient stromatolites, we precipitated calcium carbonate in the laboratory in the presence and absence of modern microorganisms under chemical conditions relevant for the early Earth. Using this novel approach, we disproved the paradigm that microbial sulfate reduction, a metabolism important for the formation of modern stromatolites, was responsible for the precipitation of their ancient counterparts. We also produced the first laboratory evidence that sub-micron and micron-sized pores occurred in rapidly precipitating carbonate rocks only when microbes were present. Applying a set of experimentally established criteria to modern environmental samples and ancient stromatolites, we found similar biogenic microporosity in some modern fast-precipitating carbonates and in ancient stromatolites. In our abiotic laboratory precipitates, we observed calcite grains that resembled putatively biogenic features from the rock record called peloids. We explained their shape and growth pattern by purely inorganic parameters, underscoring the need for caution when interpreting seemingly biogenic fabrics in the rock record of Earth and other planets. Finally, we showed that active anoxygenic photosynthesis by Rhodopseudomonas palustris could stimulate the precipitation of calcite even in solutions that were well-buffered by a high concentration of dissolved inorganic carbon. Future studies of the relationship between photosynthetic biofilms, the environmental parameters such as light and currents, and the morphology of carbonate precipitates are key to recognizing potential biosignatures produced by similar organisms in the in situ precipitated stromatolites and other microbialites.