What type of organism initially added oxygen to earth's atmosphere?

Answer 1

Photosythetic, marine prokaryotes likes today's cyanobacteria (blue-green algae). Fossil stromatolites, like in Shark Bay, Australia, are colonial cyanobacteria. The Earth's atmosphere originally contained no free oxygen (minerals like pyrite that cannot form in the presence of oxygen are common in rock layers from this time), so the first life on Earth must have been anaerobic (oxygen is toxic to them) Bacteria or Archea. Some of these simple organisms were photosynthetic, which means they gave off oxygen as a waste product. As these organisms multiplied, the amount of free oxygen began to accumulate. First this free oxygen reacted with the iron and other metals in the oceans to make iron oxides (rust) which precipitated out of the oceans to form alternating layers of rust THOUSANDS of meters thick, called Banded Iron Formations. When the oxygen in the oceans had rusted out all of the metals, free oxygen began accumulating in the atmosphere. Free oxygen (O2) reacts with ultraviolet radiation from the sun to form ozone (O3), which shields the planet from harmful UV radiation. The abundant atmospheric oxygen is thought to have allowed for the evolution of the complex cells of Eukaryotes like us!

Answer 2

Cyanobacteria, also known as blue-green algae, were the first organisms to produce oxygen through photosynthesis, which eventually led to the accumulation of oxygen in Earth's atmosphere.