Final answer:
Microorganisms, including cyanobacteria and phytoplankton, alongside plants, are significant contributors to Earth's atmospheric oxygen through photosynthesis, with phytoplankton alone responsible for over 50%.
Step-by-step explanation:
Microorganisms, particularly photosynthetic microorganisms such as cyanobacteria, along with plants, are responsible for producing a significant portion of the Earth's oxygen. Through the process of photosynthesis, these organisms use sunlight to convert carbon dioxide and water into glucose and oxygen. This oxygen is then released into the atmosphere. It's well-established that phytoplankton, tiny photosynthetic organisms in the ocean, alone release over 50% of the Earth's oxygen. Including other microbes and land plants, this percentage increases even further. Geological evidence supports that the rise of such organisms has transformed the Earth's atmosphere from being originally anoxic to its current state, with about 20% of it being molecular oxygen (O2).