Title: Microbial Evolution and the Rise of Oxygen: the Roles of Contingency and Context in Shaping the Biosphere through Time

Speaker: Lewis Ward (Caltech, PhD Candidate)

Abstract:

The evolution of oxygenic photosynthesis, and the resulting oxygenation of the atmosphere during the Great Oxygenation Event ~2.3 billion years ago, was the single largest shift in the size and structure of the biosphere since the origin of life. Oxygen-fueled metabolisms are highly energetic, and drive most primary productivity, remineralization, and biogeochemical cycles on the surface of the Earth today. Before the evolution of oxygenic photosynthesis, however, the Earth must have functioned quite differently. The metabolisms available to the biosphere, the scale of primary productivity, and the structure of biogeochemical cycles were likely very different across the rise of oxygen as a result of both shifts in environmental context and evolutionary contingency in the form of accumulating evolutionary innovations, horizontal gene transfer of metabolic traits, and the radiation of new clades of organisms.
Here, I integrate multiple approaches, including geochemical modeling and comparative biology, to trace the evolution of microbial metabolisms and the productivity of the biosphere across the Great Oxygenation Event. In particular, I investigate the evolution of phototrophy in oxygenic Cyanobacteria and anoxygenic Chloroflexi, and demonstrate the importance of stem lineages in driving ancient productivity and the roles of horizontal gene transfer and relatively young adaptive radiations in shaping the diversity of extant phototrophs. Furthermore, I show that pre-GOE productivity was fundamentally constrained by geological supplies of electron donors to fuel metabolisms like methanogenesis and anoxygenic photosynthesis, and that the ability of oxygenic photosynthesis to use water as an essentially unlimited electron donor for carbon fixation was responsible for the largest and most rapid expansion of primary productivity in Earth history.