Finding immortality in lichens

On a sparkling New England afternoon, as hawks coasted overhead and yellow leaves drifted to the ground, Anne Pringle stood before a large granite obelisk that marked the graves of a family called French.

In this bucolic cemetery, steps from the headquarters of Harvard’s research forest, she was pondering mortality. But she wasn’t thinking about the Frenches. She was thinking about lichens.

Pale green and vaguely ruffled, like calcified doilies, lichens grow all over the tombstones and the old stone walls that fringe properties in this part of the world. Most people barely notice them. But Pringle, a mycologist at Harvard, believes they may help answer one of science’s greatest questions: Is immortality biologically possible?

For eight years, Pringle, 42, has been returning to this cemetery each fall, to measure, sketch and scrutinise the lichens, which belong to the genus Xanthoparmelia. She wants to know whether they deteriorate with the passage of time, leaving them more susceptible to death.

Biologists call it senescence: the grim reality of decline with age. Are the lichens more likely to break apart as the years pass? Does their chemistry or bacterial composition change, leaving them more vulnerable to pathogens?

Lichens are not individuals but tiny ecosystems, composed of one main fungus, a group of algae and an assortment of smaller fungi and bacteria. To reproduce, they can either launch a single fungal spore that must then find new algae to join with, or they can send out fingerlike projections called isidia, which contain the whole lichen package and need only a nice rock to land on.

While lichens are communities, Pringle is largely interested in the fungi. Mycologists, the scientists who study fungi have long assumed that many of these organisms don’t age. The clear exception is yeast, a single-cell fungus that does senesce and that researchers use as a model to study aging. But most multicellular fungi, the assumption goes, don’t senesce.

No one has ever proved that, though, or even collected much data. The belief in fungi everlasting has been buoyed in part by Armillaria bulbosa, a species known as the “humongous fungus,” which grows to be as heavy as a blue whale and can take over acres of forest. (Crystal Falls, Michigan, holds an annual Humongous Fungus festival, complete with the world’s largest mushroom pizza, to celebrate a 38-acre, 1,000-tonne Armillaria that grows nearby and is thought to be 1,500 to 10,000 years old.)

Does that mean Armillaria and many of the world’s other fungi are not aging? Some experts believe it does. If true, such organisms would be the fungal equivalent of vampires, able to die only by external means. In the world beyond fungi, whether organisms can escape aging is a matter of scientific controversy. A longstanding explanation for aging pins the blame on built-up genetic mutations activated once fertility begins to taper off. But this theory doesn’t work for fungi, which reproduce more, not less, as they grow older.

According to a second theory, aging occurs because some traits that make us more reproductively successful may also set the stage for our demise. High testosterone levels, for instance, might help males make more babies—but also predispose them to prostate cancer.

Both theories explain aging as a biological imperative, a cellular commandment no life form can escape. Pringle says that way of looking at the world does not account for the realities of life as a fungus.

At the Frenches’ grave, Pringle held up a transparent sheet of plastic with the penciled outlines of about 60 lichens from a year earlier. She painstakingly located each on the tombstone, took notes on its appearance and traced it onto a new sheet. After so many years watching the lichens, she thinks of them “like old friends,” she said.

Most were alive and healthy—lacy green circles growing predictably year by year. Most, but not all. No. 94 was a tiny dot on last year’s tracing. Now it was covered over by another type of lichen, a different genus.

“That’s mortality,” said Pringle, “but not from senescence.” No. 59 was dead too; probably it was “swallowed by No. 8,” but it may have simply slid off the obelisk. “The hardest thing for a lichen here is hanging on,” she said. “It’s a vertical cliff face.”

Pringle’s preliminary results show that as a lichen grows older and larger, it is less likely to die. “If you made me answer the question now,” she said, “I’d say there can be senescence of parts of an individual. But I don’t think an individual ever senesces.” The definition of aging, then, may differ from organism to organism. Death as we know it rests on an animal-centered idea of individuality.

 “This is the largest individual I’ve seen, and it’s probably around 30 years old,” she said, pointing out a Xanthoparmelia roughly 6 inches in diameter. Until now, most of her work has involved such observations. But she plans to begin more direct experiments, like putting a tracer on part of a lichen to watch how it moves. After all, she said, “this is a slow system,” and while a lichen may live forever, a biologist will not.

Article orginally from the Indian Express