Group of Dr. Michelle Gehringer
My group investigates the most significant atmospheric change in the Earth’s history – the oxygenation of the Early Earth’s atmosphere ~2 400 Mya, during the Great Oxygenation Event (GOE). It is thought that ancient cyanobacteria, the only modern day prokaryotes capable of conducting oxygenic photosynthesis, released sufficient oxygen to alter the redox balance of the planet. Phylogenetic studies as well as geologic investigations suggest that cyanobacteria, and oxygenic photosynthesis, evolved before the GOE, raising questions as to what prevented the earlier oxygenation of the Earth’s atmosphere. Our research tries to replicate some of the conditions thought to have prevailed during the late Archean in both fresh and marine environments. By studying the growth responses of modern day descendants of the oldest Cyanobacteria under simulated Archean conditions, we obtain insights into the potential nutrient deficiencies they may have faced, as well as their photosynthetic responses, including oxygen release rates, salt tolerance and the ability to overcome iron toxicity. Studying the early Archean ecosystem is essential for finding out more about the onset of photosynthesis, the process primarily responsible for oxygenating the Earth‘s atmosphere. This research complements the interdisciplinary DFG funded Priority Programme SPP1833: Habitable Earth whereby researchers from a diverse range of fields are investigating the conditions that lead to the GOE and the establishment of life as we know it today.
Specific projects currently being undertaken:
Iron toxicity during the Archean: The earliest Cyanobacteria are thought to have evolved in a freshwater environment, acquiring the ability to live in a marine environment at a later stage. We have demonstrated that early single cellular strains of cyanobacteria would have survived a wash-out event into brackish and in some cases marine waters. We are now focusing our research on the ability of these cyanobacterial strains to grow in Archean ocean analogous media, rich in iron (II), under anoxic conditions. By utilizing microsensors, we record oxygen production and pH changes in our cultures. Traditional biochemical assays are employed to monitor changes in the medium, such as nitrate and phosphate levels while glycogen and protein levels of the cultures are recorded. Additionally, we utilize fluorescent microscopy to visualise the location of respiration and polyphosphate body formation within the cells, as well as iron deposition on the outer surfaces. These approaches will provide greater insight into the contribution of cyanobacteria in the period leading up to the great Oxygenation Event.
Cyanobacterial iron uptake during the Archean: Modern day Cyanobacteria are generally grown under iron limiting conditions, with many strains synthesizing siderophores to capture iron from the environment. Additionally, the iron that is present is oxidized due to the high oxygen content of the atmosphere, meaning that the cyanobacteria have to reduce Fe(III) prior to incorporation into biological molecules. This project focuses on the iron uptake mechanisms in an ancient marine cyanobacterium, Pseudanabaena PCC7367 whereby quantitative PCR methods are employed to determine the changes in expression of targeted genes involved in iron uptake and regulation in under both anoxic and oxic conditions. Combining this data with bioinformatic analysis of iron uptake genes within all Cyanobacterial genomes sequenced to date will provide greater insight and understanding into the evolution of iron uptake mechanisms in ancient cyanobacterial species.
Cyanobacteria are essential primary producers, both in marine and terrestrial systems by virtue of fixing CO2 during oxygenic photosynthesis. In addition, many strains are also able to fix N2 gas, providing ammonia or amino acids for further biosynthetic processes. Our current research focuses on Climate Change and Bioweathering processes. Specifically we investigate the effects of elevated CO2 levels, as predicted by current climate change models, on primary productivity, growth and secondary metabolite production in both terrestrial and marine cyanobacterial species. We work in close collaboration with geobiochemists to obtain and interpret carbon and nitrogen fractionation profiles under these different culture conditions and try and relate these back to the cyanobacterial fossil record. Additionally we investigate how terrestrial species are able to utilize mineral substrates to obtain essential nutrients via bio - weathering processes under modern-day climate change conditions and under the anoxic atmosphere of the Archean.