The evolution of oxygen tolerance

Photosynthesis is the source of all food and most energy resources on Earth. Purple phototrophic alphaproteobacteria perform anoxygenic photosynthesis in ecological niches that lack oxygen, and the process is driven by near-infrared light that has been filtered by the visible light-absorbing oxygenic phototrophs found higher in the water column/microbial mat.

Although they perform photosynthesis in the absence of oxygen, and its presence supresses pigment biosynthesis and photosystem assembly, many anoxygenic phototrophs display incredibly versatile metabolisms, some growing rapidly via aerobic respiration in the dark. How this tolerance to oxygen evolved as the atmospheric gas balance was drastically altered with the advent of oxygenic photosynthesis is of great interest, especially since the progenitor of the mitochondrion is believed to have been a phototrophic alphaproteobacterium able to perform both photosynthesis and aerobic respiration [1]; similarities still exist between extant purple bacteria and modern mitochondria, including the morphology of their energy transducing membranes and the respiratory enzymes they house.

Many purple bacteria are models for the study of aerobic respiration [2], and their ability to grow in the dark permits wholesale modification of genes involved in photosynthesis. However, despite having enzymes that should allow them to do so, others are either unable to grow in the presence of O2 or respire at rates that are impractical for study, even when O2 levels are reduced to a fraction of those in the atmosphere.

One such example is Blastochloris viridis, the lowest energy phototroph discovered to date, which is only able to slowly grow by aerobic respiration at 0.1% O2 [3]. The objectives of this synthetic biology project are:

- Increase expression of native Blc. viridis respiratory complexes and enzymes protecting against oxidative stress (e.g. cbb3 terminal oxidase, Fe-Mn superoxide dismutase), boosting O2 tolerance.

- Heterologously express recombinant terminal oxidases, with different affinities for O2, from aerobic purple bacteria in Blc. viridis (e.g. Rubrivivax gelatinosus aa3 oxidase).

- Perform long-term evolution experiments where Blc. viridis strains are exposed to increasing O2 in a bioreactor, with adaptation monitored by genome sequencing.

Engineering Blc. viridis to grow at higher O2 will permit the study, engineering, and exploitation of its unique long-wavelength photosynthesis. Knowledge derived from the project will be used for the future conversion of obligate anaerobes into organisms able to respire in the same way as complex life, elucidating the evolution of this fundamental ancient invention.