The cyanobacterium Prochlorococcus has divergent light-harvesting antennae and may have evolved in a low-oxygen ocean

Significance The marine unicellular cyanobacterium Prochlorococcus is the most abundant photosynthetic organism on Earth. Members of this genus are classically thought to be adapted to high-oxygen and nutrient-poor ocean conditions, with a principle divergence between high-light and low-light ecotypes. We show that the most basal Prochlorococcus lineages are adapted to the low-oxygen, low-light, and high-nutrient conditions found in the dimly illuminated waters of anoxic marine zones. The most basal lineages have retained phycobilisomes as light-harvesting antennae—a characteristic of most other cyanobacteria—whose loss was thought to define all Prochlorococcus . As oxygenic photosynthesis drove ocean oxidation in the ancient Earth, oxygen appears to have played as much a role as light and nutrients in driving Prochlorococcus evolution.