Thomas Brock, a microbiologist, was driving west to a laboratory in Washington State in 1964 when he stopped off at Yellowstone National Park.
“I’d never seen Yellowstone before,” he said in an interview in 2017. “I came in the south entrance, got out of my car, and there were all these thermal areas spreading out from the hot springs into the lake. I was stunned by the microbes that were living in the hot springs, and nobody seemed to know anything about them.”
What fascinated him, on what would be the first of many trips to Yellowstone, were the blue-green algae living in a hot spring — proof that some life could tolerate temperatures above the boiling point of water.
It was the beginning of research that led to a revolutionary find in 1966: a species of bacteria that he called Thermus aquaticus, which thrived at 70 degrees Celsius (158 degrees Fahrenheit) or more.
The yellow bacteria — discovered by Dr. Brock and Hudson Freeze, his undergraduate assistant at Indiana University — survived because all their enzymes are stable at very high temperatures, including one, Taq polymerase, that replicates its own DNA and was essential to the invention of the process behind the gold standard in Covid-19 testing.
Dr. Brock died on April 4 at his home in Madison, Wis. He was 94.
His wife, Katherine (Middleton) Brock, known as Kathie, said the cause was complications of a fall.
“I remember running into Mullis at a meeting,” Dr. Freeze, now the director of the human genetics program at Sanford-Burnham Prebys Medical Discovery Institute in San Diego, said in an interview. “And I said, ‘I’m the guy who found Thermus aquaticus with Tom Brock,’ and he said that he used the very strain that we isolated in Yellowstone.” (Dr. Brock had deposited cultures at the American Type Culture Collection in Gaithersburg, Md.)
The PCR technology, which requires cycles of extreme heating and cooling, can multiply small segments of DNA millions or even billions of times in a short period. It has proved crucial in many ways, including the identification of DNA at a crime scene and, more recently, detecting whether someone has Covid-19.
“PCR is fundamental to everything we do in molecular biology today,” said Yuka Manabe, a professor of medicine in the division of infectious diseases at the Johns Hopkins University School of Medicine. “Mullis couldn’t have done PCR without a rock-stable enzyme.”
Thomas Dale Brock was born on Sept. 10, 1926, in Cleveland. His father, Thomas, an engineer who ran the boiler room at a hospital, died when Tom was 15, pushing him and his mother, Helen (Ringwald) Brock, a nurse, into poverty. Tom, an only child, took jobs in stores to help her.
When he was a teenager, his interest in chemistry led him to set up a small laboratory with a friend in the loft of a barn behind his house in Chillicothe, Ohio, where he and his mother lived after his father’s death. They experimented there with explosives and toxic gas.
After serving in the Navy’s electronics training program, Dr. Brock earned three degrees at Ohio State University: a bachelor’s in botany and a master’s and Ph.D. in mycology, the study of fungi.
With faculty jobs in short supply, Dr. Brock spent five years as a research microbiologist at the Upjohn Company before he was hired as an assistant professor of biology at Western Reserve University (now Case Western Reserve University) in Cleveland. After two years, he became a postdoctoral fellow in the university’s medical school. In 1960, he joined the department of bacteriology at Indiana University, Bloomington, where he taught medical microbiology.
When he arrived at Yellowstone, he did not have grandiose ambitions.
“I was just looking for a nice, simple ecosystem where I could study microbial ecology,” he said in an interview for the website of the University of Wisconsin, Madison, where he was a professor of natural sciences in the department of bacteriology from 1971 to 1990. “At higher temperatures, you don’t have the complications of having animals that eat all the microbes.”
Stephen Zinder worked with Dr. Brock as a student from 1974 to 1977, a period that included Dr. Brock’s last summer of work at Yellowstone and his research into the ecology of Wisconsin’s lakes, including Lake Mendota in Madison.
“He had an encyclopedic knowledge of microbiology and science in general,” said Dr. Zinder, now a professor of microbiology at Cornell University. “He was always learning and picking up new things.” He added, “I think his real ability was to see things simply and to figure out simple techniques to find out what the organisms were doing in their environment.”
Dr. Brock wrote or edited numerous books, including “Milestones in Microbiology” (1961); “Biology of Microorganisms” (1970), now in its 16th edition; and “A Eutrophic Lake: Lake Mendota, Wisconsin” (1985).
After retiring from the University of Wisconsin, Dr. Brock focused on ecological strategies to restore oak savanna, prairie and marshland on 140 acres that he and his wife had purchased in Black Earth, Wis., about 35 minutes from Madison.
The land, initially intended as a place for their two children to play, later became the Pleasant Valley Conservancy.
“It was less expensive than land nearer Madison, and it turned out to be more interesting,” said Mrs. Brock, who is also a microbiologist.
In addition to his wife, Dr. Brock is survived by their daughter, Emily Brock, and their son, Brian. His first marriage, to Mary Louise Louden, ended in divorce
To Dr. Brock, the discovery of Thermus aquaticus exemplified the benefits of being given the freedom to perform fundamental research without fixating on practical results.
“It’s kind of an interesting story,” he told Wyoming Public Radio in 2020, “how research that was being done for just basic research, trying to find out what kind of weird critters might be living in boiling water in Yellowstone,” would eventually lead to “extremely widespread practical applications.”