THE ROLE OF EARLY LIFE IN THE PROTEROZOIC PHOSPHORUS CYCLE: EVIDENCE FROM THE BARAGA GROUP, MICHIGAN

Phosphorus is fundamental to life as a component of organic molecules such as ATP and DNA, but rarely mineralizes because biologic processes continuously recycle it. Much of what is known about microbial phosphorus biomineralization, however, comes primarily from bacteria in Phanerozoic marine environments. It is unclear how long the relationship between bacteria and phosphorus cycling has existed, but exquisitely preserved phosphatized bacteria from the 1.85 billion-year-old Baraga Group in the Upper Peninsula of Michigan provide new insight into the longevity and nature of this association. The Baraga Group accumulated during the apex of the Earth’s first phosphogenic episode following the initial oxygenation of the atmosphere-ocean system. Bacterial communities consist of rod, spherical, and filamentous forms and are the oldest reported fossil bacteria preserved in sedimentary phosphate. Phosphate concentrated in microenvironments led to passive mineralization of the bacteria and their associated extracellular polysaccharide substances. This authigenic mineralization suggests that microbes have played a fundamental role in fixing phosphate in marine sediments for nearly two billion years, highlighting the importance of this often-overlooked mode of microfossil preservation. Such biomineralization could only occur after the Great Oxidation event (ca. 2.4 Ga) when nearshore environments became sufficiently enriched in photosynthetic oxygen to facilitate Fe-redox pumping of pore water phosphate. The advent of coupled microbial and redox concentration of bioavailable phosphorus in coastal settings may have also been a factor in the evolution of eukaryotes, and would help explain their abundance in shallow-water sedimentary rocks. Before this relationship developed phosphorus may have remained bound to organic compounds and remained dissolved in seawater.