The answer is High altitude
The compound that was formed by the reaction of the first oxygen released by Cyanobacteria and iron are the metals of the earths crust. Cyanobacteria was the first organisms that used water instead of hydrogen sulfide or other compounds as a source of electrons and hydrogen for fixing carbon dioxide. Early Cyanobacteria inhabited marine sediments where Archean banded iron formations were deposited; consisting of reddish layers rich in iron oxide.
ANSWER
A standard light microscope is used to view living organisms with little contrast to distinguish them from the background, which would be harder to see with the electron microscope.
Electron microscopes can be used to examine not just whole cells, but also the subcellular structures and compartments within them.
