In a Place for the Dead, Studying a Seemingly Immortal Species
PETERSHAM, Mass. — 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 Dr. Pringle, a mycologist at Harvard, believes they may help answer one of science’s greatest questions: Is immortality biologically possible?
For eight years, Dr. Pringle, 42, has been returning to this cemetery each fall, to measure, sketch and scrutinize 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.
Once attached, they hardly lead a carefree life. They may face chemical warfare from neighboring lichens, as well as the menace of a hard rain.
While lichens are communities, Dr. Pringle is largely interested in the fungi. Mycologists, the scientists who study fungi — not the most glamorous corridor of biology — 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, Mich., holds an annual Humongous Fungus festival, complete with the world’s largest mushroom pizza, to celebrate a 38-acre, 1,000-ton 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. (“A bus can still run over them,” Dr. Pringle said.) But the concept has yet to catch on in the wider world of biology, dominated by scientists who study plants and animals.
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. Dr. Pringle says that way of looking at the world does not account for the realities of life as a fungus.
“What you know is based on the organisms you study,” she said. “What would you say about the evolution of senescence if instead of working with insects, you worked with modular organisms, which is what lichen are?”
Daniel Doak, a University of Colorado ecologist, agrees that the question is worth asking. Research like Dr. Pringle’s — along with other studies of species including the bristlecone pine tree and the wandering albatross, a bird, both of which may avoid senescence — suggests another possible path.
“It’s saying something fundamental,” Dr. Doak said, “that senescence is not an inevitable part of life. Which means there might be ways to prevent it.” That idea could eventually have implications for human medicine.
“There is variation in the natural world,” said Deborah Roach, a biologist at the University of Virginia who studies aging in plants. “One of the cool things to ask is: How do these other species escape this process? What rules that confine us to be an aging species don’t exist in other species?”
Before Dr. Pringle started the study, a colleague suggested she was wasting her time: to prove that something doesn’t age would take far longer than the duration of a research grant, perhaps longer than a researcher’s career or even life. But she stood her ground.
“I wanted to learn about fungi because those are the organisms that broke all the rules,” she said. “I wanted to know their natural history, their biology, so I could go back to fundamental principles of ecology and say, ‘Yeah, but in fungi it works like this.’ “
At the Frenches’ grave, Dr. 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 Dr. 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.”
Dr. 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.
After she finished monitoring the lichens on the Frenches’ grave marker, Dr. Pringle walked across a field, past her blue Prius with its “Mycologists Have More Fungi” bumper sticker. She followed a dirt path into the forest and stopped before a stone wall snaking through the woods.
“This is the largest individual I’ve seen, and it’s probably around 30 years old,” she said, pointing out a Xanthoparmelia roughly six inches in diameter. Nearby on the wall, she had taken samples of a half-dozen lichen the previous autumn, to see if they would grow back. A few showed no signs of regrowth, but in two of the bare patches, tiny dots of lichen were beginning to fill in.
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.