The answer is Permian-Triassic extinction.
The Permian-Triassic extinction was 252 million years ago. It is known as The Great Dying because about 95% of all species died out due to environmental changes (volcanic eruptions, dry climate, melting of ice sheets, etc). This is also the only event in which many insect species wiped out.
They help convert nitrogen to nitrate which is usable by living beings. (Living beings can't use regular nitrogen)
<span>the part of science that deals with microorganisms</span>