The Great Oxygenation Event, also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, or Oxygen Revolution, was the biologically induced appearance of dioxygen, O2, in Earth's atmosphere. Oceanic cyanobacteria, which evolved into coordinated macroscopic forms more than 2.3 billion years ago are believed to have become the first microbes to produce oxygen by photosynthesis. Before the Great Oxygenation Event, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The Great Oxygenation Event was the point in time when these oxygen sinks became saturated, at which point oxygen, produced by the cyanobacteria, was free to escape into the atmosphere. The increased production of oxygen set Earth's original atmosphere off balance. Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have destroyed most such organisms at the time. Cyanobacteria were therefore responsible for one of the most significant mass extinctions in Earth's history. Eventually, the evolution of aerobic organisms that consumed oxygen established an equilibrium in its availability. Free oxygen has been an important constituent of the atmosphere ever since.