EXAMINING A NOVEL ROLE FOR IRON REDOX CYCLING AND THE LITHIFICATION OF MICROBIAL MATS IN THE ROCK RECORD

Stromatolites, macroscopic laminated lithified structures commonly attributed to the activity of microbial mats, constitute the most abundant record of life from early Earth and can preserve information of their pre-lithified environment and biology, making them ideal for the study of early Earth and possibly other planets. However, the processes that drive lithification in microbial mats remain difficult to discern in the rock record. Carbonate mineral stromatolites are the most abundant type of stromatolite throughout geological time, with photosynthesis, sulfate reduction, or degradation of extracellular polymeric substances cited as the most common lithification drivers. However, if photosynthesis alone induced lithification, stromatolites would be ubiquitous throughout the ancient and modern eras, yet they are not. The most abundant records of stromatolites are from the Archean and Proterozoic, a time when sulfate concentrations were likely low, and lastly, not all biomats possess significant amounts of EPS nor are all EPS-rich biomats lithified. Thus, a different process may have occurred in ancient systems that fostered rapid lithification of stromatolitic mats. Here, we investigate the hypothesis that photosynthesis coupled to iron cycling in microbial mats may lead to lithification, an avenue not previously explored.

To test this hypothesis, we have constructed geochemical models examining the alkalinity boost provided by the proposed conceptual model and used a modern growing stromatolite to determine the genetic potential of stromatolite building microorganisms and geochemical changes under light and dark conditions. Geochemical modeling indicates a much larger alkalinity boost as a result of dissimilatory iron reduction compared to sulfate reduction. Furthermore, geochemical sampling performed at Obsidian Pool Prime Hot Spring (Yellowstone National Park) revealed that under dark conditions the stromatolites mobilized more iron in the form of Fe(II) compared to light conditions. Lastly, 16S rRNA gene and metagenome sequencing revealed the abundance of microorganisms possessing the required genes to perform iron reduction within the stromatolites. These results support the hypothesis that iron reduction may act as a potential mechanism for stromatolite lithification.