AEROBIC FE-OXIDIZING BACTERIA IN THE TONIAN PERIOD

Bacteria are among the earliest organisms to have appeared in Earth history. They played key roles in regulating the Archean and Proterozoic ecosystems. Although bacteria have a long evolutionary history, crown-group bacterial fossils are sparse and difficult to be identified due to their morphological simplicity and convergent evolution with archaeans and even eukaryotes, hampering our ability to evaluate the early evolutionary history of bacteria. However, several bacterial groups have unique morphologies facilitating their identification. For example, some modern Fe-oxidation bacteria are characterized by a helical stalk. Here we report abundant Fe-oxidation bacteria from the Tonian Liulaobei Formation in North China. Our analysis using high-resolution scanning electron microscopy reveals that these bacteria are characterized by twisted nanometer-scale filaments forming a helical stalk characteristic of extant aerobic Fe-oxidizing bacteria. The stalk occasionally branches. These Fe-oxidation bacterial fossils are preserved in association of microbial mats and on the surface of organic-walled microfossils. In contrast to previously reported Proterozoic and Phanerozoic Fe-oxidation bacteria, which are all mineralized, the Liulaobei Fe-oxidation bacteria are preserved as carbonaceous remains. Our study provides the first record of mat forming Fe-oxidizing bacteria with carbonaceous preservation in the Tonian Period. They have significant implications for the redox history of the Tonian oceans. In addition, our study opens a new taphonomic window for Fe-oxidation bacteria preservation and offers a new search image for early life on Earth.