In these last days of summer, I’ve been having fun catching (and releasing) the fireflies that appear every night in the woods near our house. It’s only recently that I learned that these flashing bugs were the inspiration behind ATP fluorescent cell assays – a popular laboratory tool that helps scientists detect living cells by making them glow. The fascinating story of a biochemist turned astrobiologist who invented this technology is even more illuminating, twisting and inspiring than the zig-zag path of a firefly and so I decided to share it with you today.
Emmett Chappelle was born on August 14th, 1924, in Phoenix, Arizona. At this time Phoenix was a farming community and Mr. Chappelle grew up on a cotton and cow farm. He graduated top of his 25 student high school class before joining the army to fight in World War II.
In the army he was initially assigned to a Specialized Training Program where he was able to use his strong math skills and take a few engineering classes. These two opportunities would have been unlikely to occur in Arizona where school segregation and intense racial inequality limited the prospects of even valedictorians. He was later reassigned to the all-Black 92nd Infantry Division and served in Italy for three years. Nevertheless, this brief time in the STP helped him attend the University of Phoenix after the war and earn an associate degree in electrical engineering.
Mr. Chappelle then went to the University of California Berkley where he earned a bachelor’s degree in biochemistry. Next, he taught for three years at the Meharry Medical College in Tennessee. His research work here led him to be recruited by the University of Washington where he earned his MS. He then began a Ph.D. at Stanford University but left in 1958 to work at The Research Institute for Advanced Studies in Maryland before completing this degree.
At the RIAS Mr. Chappelle studied photosynthesis in single-cell organisms and was part of a team tasked with finding oxygen sources for astronauts. His work showed that raising algae in space would create a viable, stable, and renewable oxygen supply as well as backup food. At this time finding ways for humans to survive in space was a major focus of astrobiology, another was discovering whether life existed on other planets. Mr. Chappelle became intensely interested in the latter. In 1963 he began researching biochemical tests for life in outer space at Hazleton Labs. Three years later he was recruited by NASA where he remained for the next 35 years until he retired in 2001.
At NASA Mr. Chappelle began investigating bioluminescence – the ability of living organisms to glow. This ability has evolved several times – there are bacteria, algae, jellyfish, sea stars, fish, sharks, mushrooms, and insects that perform chemical reactions that emit light. Mr. Chappelle focused on a well-known and readily available study organism: fireflies. This was a boon for his children and the neighborhood kids who he paid to catch fireflies every summer (1cent for each captured bug).
It turns out that fireflies have a pretty simple and efficient system to create their glow. This system consists of two chemicals – luciferin and luciferase. Light is emitted when luciferase acts on the appropriate luciferin substrate. However, for this reaction to take place there needs to be energy in the form of ATP. Because ATP is the “energy currency” of life and found in all living cells it’s an ideal proxy that can be used both to detect living organisms and to estimate the number of viable cells in a sample.
Chapelle’s test, which is sometimes called an ATP fluorescent cell assay, uses a modified version of luciferase and luciferin to detect and quantify cell activity in a sample. Two strengths of the test are its sensitivity (even a minute level of cell activity can be easily detected) and its simplicity (very few other chemicals need to be present for the reaction to successfully occur). The test has not yet detected life in outer space. However, it has been used extensively in food & water quality testing and in medicine to detect bacteria. Today, many hospitals use a luciferase-based test to quickly detect bacteria in the blood or urine of patients on antibiotics. They also use related ATP monitors to scan equipment and detect contaminated surfaces. Luciferase is also used as a reported gene and is a particularly powerful way to look at cells in live animals.
At NASA Mr. Chappelle also helped develop a test that measures photosynthesis using laser-induced fluorescent and low flying plains. Today this test is used by farmers to detect areas that are stressed, calculate growth rates, time harvests, and modify watering conditions. This method is also used to monitor more remote forest lands.
The diverse applications of Mr. Chappelle’s research are in large part a reflection of his ingenuity and inventiveness. Discovering and unraveling the biochemistry behind firefly bioluminescence was an impressive scientific accomplishment in and of itself. However, Mr. Chappelle was also able to envision and then implement multiple ways to harness and use this reaction!
Mr. Chappelle was honored with a NASA Exceptional Scientific Achievement Medal in 1994 and was inducted into the National Inventors Hall of Fame in 2007. He had 15 patents related to biochemistry, medicine, food science, and agriculture. Chappelle died on October 19, 2019. But is remembered by family, colleagues, and those he mentored. One student and friend was Rodney Washington who remembered Mr. Chappelle’s intelligence and curiosity but also his kindness and commitment to helping others. “He was serious about his work because as a scientist, he loved doing what he did. But he was a true mentor to me. I was young, 23 years old at the time, and needed a car after the engine blew in mine. He took me to Berman’s and bought me a used Toyota. I said I’d pay it off, but he let me come over to his house and work it off. I never paid a cent. He didn’t have a selfish bone in his body.”