Evolution of Cyanobacteria Through Geological Time

Abstract

Cyanobacteria have lived on Earth for longer than animals, plants, and fungi. In that time, they have oxygenated the atmosphere, rendering the planet habitable for complex life. Yet whilst the antiquity and diversity of Cyanobacteria is appreciated, the timing and evolution of genes guiding their interaction with the world are not well understood. In this thesis, sequencing technology, molecular clocks, and phylogenetic analyses are employed to search for evidence of ancient genes managing oxidative stress and their transition from unicellular to multicellular morphology. This is accomplished by interrogating hundreds of Cyanobacterial genomes for three antioxidant enzymes and eleven multicellularity genes. The evolutionary history of those genes is then reconstructed and mapped onto Bayesian molecular clocks calibrated with Precambrian microfossils, to predict when they first appeared in ancestral populations.

These analyses suggest that stem Cyanobacteria appeared approximately a billion years before the Great Oxidation Event and provide the first phylogenetic evidence that oxygenic phototrophs used CuZnSOD to manage oxidative stress before oxygen accumulated on a global scale. In this largely anoxic, microbial-dominated period of Earth’s history, genes underlying multicellular phenotypes emerged in stages. The first encoded a transcriptional regulator, permease, and components of septal junctions. Approximately 75 to 200 million years later, during the Great Oxidation Event, two further genes managing the frequency of differentiated and vegetative cells in filaments evolved. By sequencing five draft genomes, brackish Cyanobacteria from a coastal beach are also found genetically capable of synthesising glucosylglycerol to handle salinity in a similar manner to marine species. Altogether, this research expands a growing body of evidence describing how photosynthetic bacteria have adapted to their changing physical environments over the last 3.6 billion years. It also highlights the wealth of information in public data repositories, which can be analysed to better understand the co-evolution of life and Earth.

Date of Award	22 Mar 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Patricia Sanchez-Baracaldo (Supervisor) & Paul J Valdes (Supervisor)