In an opinion paper published in Trends in Microbiology researchers examine why fungal longevity remains so difficult to study. To investigate the long-overlooked biological mystery, the team recommends leveraging new technologies such as "fungi-on-a-chip" and conducting long-term lab experiments with genetic tracking.

"We don't really know if 10 years or 500 years is 'old' for a fungus or how much it differs between fungal species and lifestyles," says senior author Kristin Aleklett of Lund University, Sweden.

A mushroom that decorates the forest floor is just a small part of a fungus. Beneath the surface lies its main body—a sprawling network of threadlike filaments called mycelium that spreads through soil or wood and sometimes connects with plant or tree roots. Some fungal networks are thought to live for hundreds to thousands of years.

Where one fungus ends

Because the mycelium continually branches, grows, recycles old tissues and sometimes breaks off into new networks, "one of the biggest difficulties lies in being able to define where a fungal individual begins and ends," says Aleklett.

That raises a series of questions: What exactly should scientists measure? Does a fungus' age begin when its underground network first forms, even if much of it is later replaced? If the underground network breaks apart into separate pieces that still share the same DNA, are they still one individual?

Unlike animals or trees, scientists can't simply count birthdays or growth rings. Instead, they often rely on genetic tools to identify individual fungi and estimate their age based on how quickly the mycelium grows in the lab. But researchers cannot directly observe how the mycelium expands and dies back over seasons and years in the real world.