The origin of oxygenic photosynthesis and the rise of oxygen on Earth.
Two and a half billion years ago, Earth's surface went through a dramatic change. The concentration of oxygen in the Earth's atmosphere rose dramatically, from nearly absent, to almost 10% of present day levels. This event, the Great Oxygenation Event (GOE), permanently changed the Earth's surface and the future of life on Earth.
The GOE resulted from the evolution of oxygen producing photosynthesis in the Cyanobacteria. The Cyanobacteria were not the first organisms to do photosynthesis, but they were the first to link together two photosystems and split water to produce oxygen gas. Despite how important this evolutionary innovation is, we do not know much about the evolution of oxygenic photosynthesis. |
Our best clues come from the genomes of the Melainabacteria, a recently-discovered, non-photosynthetic group of Cyanobacteria and the earliest branching photosynthetic Cyanobacteria, the Gloeobacter. However, the evolutionary distance between these groups is vast.
We found several basal Cyanobacterial taxa in microbial mats from an ice-covered lake in Antarctica (shown in the header image). Recently, we assembled a genome of a new, basally branching phototrophic Cyanobacteria which we named Aurora vandensis. In a recent we interrogated its genome to gain insight in to the evolution of oxygenic photosynthesis. |
We found that the A. vandensis genome contained key photosynthetic genes but was missing may extrinsic or stabilizing genes. Additionally, it was missing the genes necessary to produce phycoerythrin, an important photosynthetic pigment.
Based on the A. vandensis genome, we developed an evolutionary model in which oxygenic photosynthesis is made more efficient through evolutionary time via the evolution of extrinsic proteins to stabilize photosystem II and I reaction centers and improve light capture. This model suggests that the evolution of oxygenic photosynthesis may have significantly preceded oxidation of Earth’s atmosphere due to low net oxygen production by early Cyanobacteria. Additionally, we predict that ancestral lineages ay have needed to occupy low light habitats because too much UV radiation inhibits photosynthesis. As the photosystems stabilized, photon capture efficiency improved, and oxygenic phototrophs expanded to higher-light environments. Both would have resulted in significantly higher primary productivity and rates of oxygen production. |
Where to go from here? I am beginning to dig in to some additional Melainabacteria genomes we recovered from our Antarctic lake sample, so stay tuned!