Tom Brock (September 10, 1926 − April 4, 2021) can readily be considered the father of extremophile microbiology. He pioneered the study of high temperature-loving microbes, by investigating, in a nearly frolicsome way, the hot springs of Yellowstone Park. He carried out this work pretty much alone, with both experimental ingenuity and conceptual gusto, laying down the basis for a new way to appreciate the microbial world. He also wrote a trailblazing textbook, Biology of Microorganisms (which other authors carried through 15 editions.)
His legacy certainly includes the insight, gained through painstaking experimental work, that microorganisms need to be studied in their natural environment in order to really get to know them. But rather than paraphrase, let's let him speak himself:
(excerpts from the chapter "Adaptation to the upper temperature limit by Synechococcus", p43 ff, from Tom Brock's book A Scientist in Yellowstone National Park. ©Thomas D. Brock 2017)
One of the striking things was the observation that near the upper temperature limit for phototrophic life, the microbial development was very meager. A few degrees below the upper temperature, defined mats started to form, but right at the upper limit there was just a thin film, barely visible. The cyanobacterium was microscopically similar throughout a wide temperature range from 70‑73 C (the upper temperature limit) to 50‑55 C, where the mats were the thickest.
One of the most interesting things I discovered was that the thin film at the upper limit was not struggling to survive, but was optimally adapted to that temperature. It functioned better here than at lower temperatures. I showed this by measuring photosynthesis with radioactive carbon dioxide, doing what I called "temperature transfer" experiments, as will be explained below.
The results of this work were published in a short paper in 1967. (Brock, Micro‑organisms adapted to high temperatures. Nature: 214: 882-885.) This paper has been widely cited, but the broad implications for understanding evolution have not been followed up.
And he summarized
"Of the micro-organisms which can grow at environmental extremes, one must distinguish between those which are optimally adapted to the extremes and those which grow better in less extreme conditions. This is especially important because an extreme environment is usually recognized as one in which growth of the organism is slow. An experimental approach to this must permit the direct measurement of the environmental variable in the habitat in which the organism is growing and in which it has therefore evolved. It is essential to examine the organism directly in its natural environment rather than in cultures isolated from such habitats, because it is not possible to be certain that the cultures are indeed representative of the natural material, and it is usually impossible to duplicate in the laboratory all aspects of the natural environment."