Microbial fossils preserved by early diagenetic chert provide a window into the Proterozoic biosphere, but seawater chemistry, microbial processes, and the interactions between microbes and the environment that contributed to this preservation are not well constrained. Here, we use fossilization experiments to explore the processes that preserve marine cyanobacterial biofilms by the precipitation of amorphous silica in a seawater medium that is analogous to Proterozoic seawater. These experiments demonstrate that the exceptional silicification of benthic marine cyanobacteria analogous to the oldest diagnostic cyanobacterial fossils requires interactions among extracellular polymeric substances (EPS), photosynthetically induced pH changes, magnesium cations (Mg2+), and >70 ppm silica.

Here we review the application of molecular biological approaches to mineral precipitation in modern marine microbialites. The review focuses on the nearly two decades of nucleotide sequencing studies of the microbialites of Shark Bay, Australia; and The Bahamas. Molecular methods have successfully characterized the overall community composition of mats, pinpointed microbes involved in key metabolisms, and revealed patterns in the distributions of microbial groups and functional genes. Molecular tools have become widely accessible, and we can now aim to establish firmer links between microbes and mineralization. Two promising future directions include “zooming in” to assess the roles of specific organisms, microbial groups, and surfaces in carbonate biomineralization and “zooming out” to consider broader spans of space and time. A middle ground between the two can include model systems that contain representatives of important microbial groups, processes, and metabolisms in mats and simplify hypothesis testing. These directions will benefit from expanding reference datasets of marine microbes and enzymes and enrichments of representative microbes from mats. Such applications of molecular tools should improve our ability to interpret ancient and modern microbialites and increase the utility of these rocks as long-term recorders of microbial processes and environmental chemistry.

Marine ooids have formed in microbially colonized environments for billions of years, but the microbial contributions to mineral formation in ooids continue to be debated. Here we provide evidence of these contributions in ooids from Carbla Beach, Shark Bay, Western Australia. Dark 100–240 μm diameter ooids from Carbla Beach contain two different carbonate minerals. These ooids have 50–100 μm‐diameter dark nuclei that contain aragonite, amorphous iron sulfide, detrital aluminosilicate grains and organic matter, and 10–20 μm‐thick layers of high‐Mg calcite that separate nuclei from aragonitic outer cortices. Raman spectroscopy indicates organic enrichments in the nuclei and high‐Mg calcite layers. Synchrotron‐based microfocused X‐ray fluorescence mapping reveals high‐Mg calcite layers and the presence of iron sulfides and detrital grains in the peloidal nuclei. Iron sulfide grains within the nuclei indicate past sulfate reduction in the presence of iron. The preservation of organic signals in and around high‐Mg calcite layers and the absence of iron sulfide suggest that organics stabilized high‐Mg calcite under less sulfidic conditions. Aragonitic cortices that surround the nuclei and Mg‐calcite layers do not preserve microporosity, iron sulfide minerals nor organic enrichments, indicating growth under more oxidizing conditions. These morphological, compositional, and mineralogical signals of microbial processes in dark ooids from Shark Bay, Western Australia, record the formation of ooid nuclei and the accretion of magnesium‐rich cortical layers in benthic, reducing, microbially colonized areas.

<p>The discovery of hydrated magnesium carbonates in a paleolake in Jezero Crater, Mars, has inspired the study of carbonates associated with lacustrine systems on Earth. Lake Salda, Turkey, contains hydromagnesite-dominated microbialites in a mafic-rock hosted terrain. This alkaline lacustrine system provides an excellent natural laboratory in which to characterize various depositional environments and biosignature preserving potential of microbialites and other magnesium carbonate deposits. Here we aim to describe the textures, mineral composition, and macroscopic morphology of hydromagnesite structures in Lake Salda. All microbialites are covered by a thick, sticky organic-rich layer dominated by diatoms and cyanobacteria. A close association of contemporary hydromagnesite precipitation with organic-rich layers in microbialites indicates biologically mediated precipitation. Microbialites around the lake exhibit a range of morphologies, including finger-like, mini columnar, domical, and structureless. The recently exposed microbialites commonly contain colloform and cerebroid structures with fine, crude lamination, vuggy and globular textures. Layered microscopic textures preserve evidence of partially mineralized filamentous bacteria that trapped detrital grains and also contain globules embedded in extracellular polymeric substances, and dense hydromagnesite. The differences in mineralogy, density, and organic preservation potential within these textures likely correspond to the depositional environment. For example, the globular textures are closely associated with the purple layers in microbialites, including the layers in the recently sampled microbialites from the deeper part of the lake (ca. 10 m). These layers may form in the presence of anoxygenic purple sulfur bacteria. Our preliminary results suggest that the textures and macroscopic structures of hydromagnesite microbialites may record and reflect relationships among carbonate minerals, environmental factors, and microbial community composition.</p>