When red blood cells degrade after their six-month lifespan, a bright orange pigment called bilirubin is produced as a byproduct. Bilirubin is typically secreted into the gut, where it is destined for excretion but can also be partially reabsorbed. Excess reabsorption can lead to a buildup of bilirubin in the blood and can cause jaundice—a condition that leads to the yellowing of the skin and eyes. Once in the gut, the resident flora can convert bilirubin into other molecules.

“Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colorless byproduct called urobilinogen,” explained Hall, who has a joint appointment in the University of Maryland Institute for Advanced Computer Studies. “Urobilinogen then spontaneously degrades into a molecule called urobilin, which is responsible for the yellow color we are all familiar with.”

Urobilin has long been linked to urine’s yellow hue, but the research team’s discovery of the enzyme responsible answers a question that has eluded scientists for over a century.