Scientists find what makes some tap water smell like mold

Sometimes tap water smells like mold. Now scientists in Japan have found an enzyme^*1 that helps make the chemical^*2 behind this smell. Bacteria^*3 make the enzyme. The scientists are the first in the world to find it.

The team works at Tokyo University of Technology, a university in Japan. Its scientists include Taro Urase, Saki Goto and Masahiro Kimura. Their paper was published online on September 7, 2026, in the journal^*4 Water Research X.

The bad smell comes from chemicals called chloroanisoles^*5. They are sometimes found in tap water that has been cleaned with chlorine^*6. They are also found in some rivers. This happens when wastewater^*7 cleaned with chlorine flows into the river. One of these chemicals, 2,4,6-TCA^*8, is the best-known cause of the smell. People can smell it even when there is only a very small amount in the water.

Until now, scientists thought that fungi^*9 made most of the 2,4,6-TCA. But some scientists said that bacteria in water may also make it.

The team studied bacteria called Mycolicibacterium^*10. These bacteria make a lot of chloroanisoles. The team looked at all the genes^*11 of the bacteria. By studying the genes, they found an enzyme called O-methyltransferase^*12. This enzyme helps to make 2,4,6-TCA.

Then the team tested the enzyme. They put its gene into another kind of bacteria, E. coli^*13, and made it work there. The E. coli then changed a chemical called 2,4,6-TCP^*14 into 2,4,6-TCA. This showed that the enzyme makes 2,4,6-TCA.

The team now understands part of how bacteria make 2,4,6-TCA. They hope this will help people check the risk of bad smells in water.

1. *1 enzyme: a very small thing in a living body that helps change one chemical into another
2. *2 chemical: a kind of matter that things are made of, often made or studied by scientists
3. *3 Bacteria: very small living things that you can only see with a microscope
4. *4 journal: a magazine where scientists publish their work
5. *5 chloroanisoles: a group of chemicals that can make water smell like mold
6. *6 chlorine: a chemical put in water to kill germs and make it safe
7. *7 wastewater: used water from homes and factories
8. *8 2,4,6-TCA: one chemical in the chloroanisole group; it is the best-known cause of the moldy smell
9. *9 fungi: living things like mold and mushrooms
10. *10 Mycolicibacterium: a kind of bacteria that makes a lot of chloroanisoles
11. *11 genes: the parts inside a living thing that carry the instructions for how it works
12. *12 O-methyltransferase: the name of the enzyme that the team found
13. *13 E. coli: a common kind of bacteria that scientists often use in tests
14. *14 2,4,6-TCP: the chemical that the enzyme changes into 2,4,6-TCA