In the beginning there was no oxygen in atmosphere. The activity of which organism is supposed to have made the earth's atmosphere aerobic or oxygen-rich?

Cyanobacteria (blue-green algae) were the first organisms to perform oxygenic photosynthesis, gradually converting Earth's reducing atmosphere to an oxygen-rich one over billions of years. Monocotyledons are plants that evolved much later, while filamentous algae and mycoplasma do not produce oxygen through photosynthesis.