The role of horizontal gene transfer in photosynthesis, oxygen production, and oxygen tolerance.

One of the pivotal events during the early evolution of life was the advent of oxygenic photosynthesis, responsible for producing essentially all of the free oxygen in Earth's atmosphere. This molecular innovation required the development of two tandemly linked photosystems that generate a redox potential strong enough to oxidize water and then funnel those electrons ultimately to cellular processes like carbon and nitrogen fixation. The by-product of this reaction, molecular oxygen, spawned an entirely new realm of enzymatic reactions that served to mitigate its potential toxicity, as well as to take advantage of the free energy available from using [Formula: see text] as an electron acceptor. These ensuing events ultimately gave rise to aerobic, multicelled eukaryotes and new levels of biological complexity. Remarkably, instances of horizontal gene transfer have been identified at nearly every step in this transformation of the biosphere, from the evolution and radiation of photosynthesis to the development of biological pathways dependent on oxygen. This chapter discusses the evidence and examples of some of these occurrences that have been elucidated in recent years.