Significance Anoxic marine zones (AMZs) create expansive habitats for microbes whose anaerobic metabolisms help drive global nutrient cycles, for example, by removing nitrogen from the oceans by producing N ₂ gas. AMZ cycles may also be shaped by oxygen intrusion from outside the AMZ, creating opportunities for aerobic microbial metabolisms. Here we show that aerobic processes in AMZs are linked to oxygen production within the anoxic zone. Oxygen is produced during daytime in a layer of photosynthetic cyanobacteria near the top of the AMZ and then rapidly consumed by aerobic processes without accumulating. Oxygen turnover and carbon fixation rates are comparable to those of microbial N ₂ production, suggesting an important role for internal oxygen cycling in AMZ transformations of matter and energy.