Oxygen in photosymbiosis evolution

Oxygen (O₂) is now essential for the survival and growth of many organisms. Although the current atmosphere is rich in O₂ due to the action of photosynthetic organisms, this was not the case in the past, and even now, many existing ecosystems are oxygen-poor. Under these conditions, appropriating a continuous source of O₂ through photosymbiosis can represent an important selective advantage. In this review, we first present the advantages that hosts can gain from O₂ production by photosymbionts. However, O₂ production is also associated with oxidative stress, and we discuss how organisms hosting phototrophs have evolved a wide range of strategies to control these effects. Secondly, we examine how existing hypoxic areas are relevant ecosystems for observing the evolution of early and current photosymbioses, and how photosynthetic O₂ can drive their evolution.