There are a lot of ways for the world to end. It can go up in a ball of flame, as happened to the dinosaurs some 65 million years ago. It can get buried under snow and ice in a great downward trend in global climate. It can get burned away by a gamma ray burst from a distant super-nova.
It may not have killed in quite the way you assume, however.Historically, volcanoes have been a major suspect in the mystery of the Permian-Triassic Extinction, which killed roughly 90% of the worlds biodiversity 252 million years ago.
Historically, the rapid increase in carbon levels which caused, among other things, irregularities in climate and ocean acidification has been attributed to volcanic eruptions. This disaster, also called the Great Dying, cannot be fully explained with just volcanoes, however; the upward trend in global carbon levels continues too sharply for too long. This study lays out evidence that the real culprit was a microorganism called Methanosarcina.
With the help of a key upswing in volcanic activity, it multiplied enormously in a very short period of time. Increases in methane levels would lead to ocean acidification, and evidence shows that species more vulnerable to changes in pH did indeed die off more than others.
Many models for the impact of modern ocean acidification are derived from ancient data like this. Its important to note that Methanosarcina isn"t technically a bacterium, but part of a very similar group called archaea. These microbes are the most ancient form of life still living, having separated from bacteria long before the emergence of plant, animal, or fungal cell types. These days they are mostly found in extreme or extremely ancient environments like caves, deep sea vents, and the mouths of volcanoes, but at a certain time in ancient history they were quite abundant.
The global atmosphere at the time was quite different than today"s, and microorganisms made a big contribution to making it this way. The eventual oxygenation of the atmosphere likely came about in large part due to early photosynthesizing cyanobacteria.
Some of the most lucrative niches were left totally unfilled in the millennia that followed the extinction, and they were filled by species that went on to become mammals, and even most species of dinosaur; the final extinction of terrible lizards wouldn"t come about for almost another 200 million years. There is a trend in global extinction events, however, in that they seem to happen fairly often. The impacts of major shifts in global climate, especially those tied to carbon emissions, are obviously of great interest to modern scientists. This data could inform our understanding of both where life has come from, and where it is about to go.
It may have been caused by a microbe.