Abstract:
Oxygenic photosynthesis is one of the most important innovations in the history of life on Earth, responsible for most primary productivity as well as providing molecular oxygen for aerobic respiration. It is widely accepted that Cyanobacteria invented oxygenic photosynthesis, and that this was a necessary precondition for the rise of oxygen by ~2.3 Gya, but it is debated whether this occurred immediately following the evolution of Cyanobacteria or whether they originated far earlier in Archean time. To address this question, we have utilized a combination of disparate approaches including geochemical modeling of the Archean Earth and phylogenetic and molecular clock analyses of extant Cyanobacteria and their nonphototrophic relatives. Taken together, results from these investigations support a consistent scenario in which Cyanobacteria evolved oxygenic photosynthesis shortly before 2.3 Gya, triggering the relatively rapid oxygenation of the atmosphere. Oxygenic photosynthesis in Cyanobacteria is derived from anoxygenic phototrophy, a much more ancient metabolism found in several groups of bacteria today. In order to understand the history of anoxygenic phototrophy, we are investigating the diversity and evolutionary history of extant anoxygenic groups as well as their ability to form stromatolites, sedimentary features with a record dating back 3.5 Ga.