For most of Earth's history, oxygen (O2) levels in the atmosphere and oceans were too low to support plant and animal life. Cyanobacteria are microorganisms that were responsible for oxygenating the atmosphere by producing O2 via photosynthesis, thus enabling life as it is exists today. However, the specific factors that drove the rise of oxygen in the atmosphere are unknown. In particular, little is known about the controls on cyanobacterial O2 production under the low-O2, sulfide-rich conditions that were widespread during Earth?s progressive oxygenation. This project will study the interplay between, light, hydrogen sulfide, O2 production, and microbiology in modern cyanobacterial mats that thrive under conditions that mimic those of the early Earth. The research and results will be integrated into efforts to recruit, support, and retain underrepresented students in the geosciences in an effort aimed at diversifying the workforce. In order to disseminate lessons learned, results of this outreach effort will be shared with the public through the visitor center at the Thunder Bay National Marine Sanctuary, presented at conferences and published in an education journal. Finally, this interdisciplinary project will establish a close international scientific collaboration between the U.S. and Germany.
This project will investigate geobiological controls on oxygen (O2) production by cyanobacterial mats under low-O2 and sulfidic conditions. Three central questions will be addressed to reveal the coupled microbial and geochemical processes. First, how do light and sulfide and their interactions control the balance of oxygenic and anoxygenic photosynthesis? Second, how are the observed shifts in these photosynthetic modes underpinned by metabolic pathways and activity of different cyanobacterial populations? Third, how do these photosynthetic modes affect the rate of sulfide production, which could represent a feedback on the balance of oxygenic and anoxygenic photosynthesis? The overall goal of the integrated approach behind addressing these questions is to reveal specific microbial populations, metabolic pathways, and geochemical processes that underpin mat biogeochemistry. Controlled experiments in mesocosms will be used to track rates of oxygenic and anoxygenic photosynthesis as a function of light, sulfide, and mat structure over a diel cycle. In parallel, state-of-the-art "omics" approaches will provide an unprecedented view of the dynamics of metabolic pathways in these microbial communities at the level of DNA, RNA, and protein. The same experimental framework will be used to measure the metabolic activity of sulfate reducing bacteria under oxygenic and anoxygenic photosynthesis across the diel cycle. These ex situ experiments will be rooted in reality via field investigations and direct measurements of mats in situ for parallel microprofiling of changes in geochemical parameters, assessment of metabolic processes, and proteomic analyses. More broadly, this project will advance the understanding of microbial geochemistry by forming an interdisciplinary team with diverse expertise to link geochemical processes to microbial populations and metabolic pathways with unprecedented resolution at the level of DNA, RNA, and protein.
