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.