CEE professor David Stahl stands in front of the tank where a water sample was taken in 2005 that led to the discovery of a marine microbe. A plaque was recently installed to the right of the tank.

All discoveries start somewhere. A significant discovery by Civil & Environmental Engineering professor David Stahl 13 years ago can be traced back to a water sample collected from a freshwater fish tank at the Seattle Aquarium, where a commemorative plaque now resides.

In recognition of the magnitude of his findings, Stahl was honored at the Seattle Aquarium on December 21, surrounded by colleagues and current and former researchers from his lab.

For more than a century, it was unknown which marine microbes were responsible for converting the nitrogen in ammonia to forms used by algae that form the base of the marine food web. In 2005, Stahl and his coworkers at UW were the first to isolate ammonia-oxidizing archaea (AOA) microorganisms from a water sample taken from a fish tank at the Seattle Aquarium.

“We now know from follow-up studies by many groups, including ours, that the AOA are the dominant nitrifiers in most natural systems, controlling the conversion of ammonia to nitrate, a major step in the global nitrogen cycle,” Stahl said.

The global nitrogen cycle is essential to all organisms on both land and sea. Converted into various chemical forms that move between land, atmosphere and living organisms, nitrogen enables all living things to live, grow and reproduce.

With the Latin name Nitrosopumilus maritimus, which translates to “the dwarf nitrifier of the sea,” AOA are extremely small— as many as four million fit on the head of a pin. Too small to be seen by the unaided eye, AOA are among the most abundant organisms in the ocean, accounting for as much as 20% of total marine microbes.

Leading up to the discovery, researchers in Stahl’s lab worked for more than 11 years to gain conclusive evidence that AOA are responsible for turning ammonia into nitrite. Although bacteria also convert ammonia to nitrite, Stahl suspected the primary oxifiers had not yet been discovered after he began noticing something unusual in several water samples— ammonia nitrogen was being oxidized even without the usual bacteria associated with nitrification.

To provide conclusive evidence, Stahl and researchers in his lab worked to isolate AOA. This is a difficult task as the microorganisms grow slowly and are difficult to separate from contaminating bacteria. By taking samples at an aquarium, researchers in Stahl’s lab were ensured of a higher ammonia content, which enabled them to isolate a pure AOA culture.

Holding a dual appointment with the Environmental Molecular Sciences Laboratory, PNNL, Stahl was elected to the National Academy of Engineering in 2012. He is an adjunct professor in the UW Department of Microbiology.

Assistant professor Mari Winkler shares highlights about professor David Stahl's discovery.