Stromatolites laminated, lithified structures typically built by microbial mats represent the most abundant record of life in the first 7/8THS of earth history (the archean and proterozoic eons). As…

The University of Southern California (USC) has been awarded a $1,983,901.56 research grant by the National Aeronautics and Space Administration (NASA) to investigate the role of iron redox cycling in the formation and decline of stromatolites, which represent some of the earliest macroscopic evidence of life on Earth. The grant, titled "STROMATOLITES LAMINATED, LITHIFIED STRUCTURES TYPICALLY BUILT BY MICROBIAL MATS REPRESENT THE MOST ABUNDANT RECORD OF LIFE IN THE FIRST 7/8THS OF EARTH HISTORY (THE ARCHEAN AND PROTEROZOIC EONS)," aims to elucidate the fundamental processes governing stromatolite formation and their distribution through geologic time. The research will utilize a combination of modern molecular biology, geochemical, and imaging techniques to study stromatolite development in a siliceous hot spring system in Yellowstone National Park, as well as analyze iron distribution patterns in ancient stromatolite samples. The findings from this work may contribute to a better understanding of the early evolution of life on Earth and improve the ability to detect potential biosignatures on other planets.

Stromatolites laminated, lithified structures typically built by microbial mats represent the most abundant record of life in the first 7/8THS of earth history (the archean and proterozoic eons). As macroscopic manifestations of microbial life, they represent a clear target for exobiologic investigation. Despite a long history of study, many strikingly fundamental questions remain with respect to stromatolite formation and their distribution throughout geologic time. Two pertinent questions include: 1) how do microbial mats lithify to become stromatolites (i.e., rocks)? IT is still not clear how soft, organic-rich microbial mats transform into hard, lithified stromatolites. 2) what controlled the distribution of stromatolites through time? Stromatolites reached a form diversity peak CA. 1.0 billion years ago, crashed, and became comparatively rare throughout the remaining 600 million years of earth history. The most common hypothesis the evolution of burrowing/grazing metazoans disrupting the microbial mats is not tenable, as the decline initiated well before the advent of animals. Here, we propose to investigate how the redox cycling of iron, not previously considered important for the formation of stromatolites, may lead to the lithification of soft microbial mats, and how the changes in the dissolved iron in archean, proterozoic, and phanerozoic oceans may link to the decline in stromatolite abundance and form diversity well before the evolution of animals. Iron, lithification, ancient ocean chemistry, and mats: before widespread marine oxygenation, the oxygen produced locally by cyanobacteria in a microbial mat could induce iron oxide precipitation in the mat during sunlight hours. At night, when the mat became anoxic, iron-reducing bacteria, could use the iron oxides to respire, releasing iron into pore waters. During microbial iron reduction, a boost of alkalinity is provided more than any other common metabolism shifting the local geochemical environment in favor of calcium carbonate precipitation in the mat. Testable consequences are evident, including a predicted cyclic iron enrichment within stromatolites formed by this process, and a link between stromatolite abundance/decline and marine iron concentrations through geologic time. In testing the role of iron redox cycling in the lithification of stromatolites, we may finally understand the decline of stromatolite form diversity/abundance that has remained unresolved since IT was recognized. The novel link between microbial iron reduction and stromatolites will be tested in two ways: 1. By examining iron reduction in modern stromatolites growing in obsidian pool prime (OPP), a siliceous hot spring in yellowstone national park. Here, a suite of modern molecular (SSU rrna, metagenomic/transcriptomic), geochemical, and imaging tools will be used to investigate the importance of iron reduction in a tractable system. Although OPP is a silica spring, the information gained is applicable to other systems, including carbonate systems. Since the growth rate of the OPP stromatolites is known, the aforementioned approaches will be captured, for the first time, during stromatolite lamination formation/growth. 2. By using micro X-RAY fluorescence techniques to investigate the distribution of iron within a suite of stromatolites through the rise and fall of stromatolite abundance in geologic time, testing the prediction of iron incorporation during lamination formation versus marine iron concentrations. Coincidentally, the decline of stromatolites neatly tracks the loss of the ferruginous oceans, as indicated by MO and CR proxies, lending credence to the hypothesis. Our study would be relevant to two main foci in the exobiology program 1) early evolution of life and the biosphere (stromatolites represent the earliest macroscopic evidence of life on earth) and 2) biosignature and life elsewhere (stromatolites could act as biosignatures for life on early earth/elsewhere).