November 14, 2013 | 4
A few months ago, scientists revealed that some plucky mosses in Canada managed to do something long thought impossible: survive a 400-year close encounter with the business end of a glacier, and live to sprout another day. The conventional wisdom on glaciers was that they were Earth-scraping, life-razing forces of geology. Nothing could survive their passage, except perhaps bacteria and algae — and which didn’t really count, of course. When glaciers retreated, they left behind a “tabula rasa”: a clean slate. No more. The lowly moss, it turns out, can conquer a glacier.
Though scientists had observed mosses and other plants emerge in the soggy wake of retreating glaciers since the 1960s, all previous reports indicated that the plants were definitely dead. Mosses appearing in the shadow of Ellesmere Island’s melting Teardrop Glacier emerged intact, if a bit blackened, and exquisitely preserved right down to tiny spore capsules and delicate hairs, according to a team of three scientists from the University of Alberta who examined them and reported their findings in the Proceedings of the National Academy of Sciences in June. Radiocarbon dates ranged from 404.5 to 614.5 years old.
But that was not all. Some of the emergent mosses had developed green shoots or branches, suggesting that they were picking up right where they had left off when James I sat on England’s throne and the first English settlers were establishing a settlement in his name in Virginia.
The scientists who discovered them clipped samples of these plants and attempted to cultivate them in the lab. Amazingly, cultures from four different non-weedy moss species grew.
But how did these mosses manage to do it? In addition to the freezing temperatures and crushing weight of a glacier, there is the little matter of what 400 years without access to sunlight and exposure to whatever cosmic radiation manages to seep through the ice does to the life-support system of a delicate green plant with leaves in many cases only one cell thick.
The answer, it turns out, has something to do with the nature of Arctic glaciers. The mosses were not consumed by a grinding, scraping glacier, but by a “nonerosive, cold-based glacier margin”, the dominant type near the poles. In other words, the mosses were slowly buried by accumulating snow, a considerably gentler introduction to englaciation than you might expect from the movements of glaciers at lower latitudes.
But their feat also has a lot to do with the evolutionary path mosses followed. They have used their suite of ancient biological and anatomical traits to adapt to harsh environments — ones perhaps similar to the ancient environment in which their forms first evolved 470 million years ago — where unfortunate events like getting run over by a glacier are not remote possibilities, but facts of life.
For instance, if you grind up a petunia, you will have petunia puree. If you grind up a moss, and you can have thousands of little mosses, in Sorcerer’s Apprentice fashion. Each moss cell, be it in a leaf, root, or shoot, has the potential to de-specialize and grow an entire new organism. In animals, we would call these stem cells, but in humans only a select few cells maintain the ability to do this. In a moss, every cell can be a “stem cell.” Botanists call this ability “totipotency“, and while mosses and their friends have it, most “higher” plants do not. So it’s not necessary for moss spores to survive a trauma or even whole plants. Just a piece is all it takes to build a whole new moss from scratch.
To drive home the point that mosses are not “lesser” or “reduced” plants in spite of their “primitive” features — they’ve just deployed their talents in a different creative direction — La Farge, Williams and England pointed to a study of a flowering plant called Silene stenophylla. Colloquially called the narrow-leafed campion (and a member of the flowering Carnation Family), a specimen of this plant was recently resurrected after 31,800 years under 20-40 meters of Siberian permafrost. To get it to send up shoots, though, the scientists had to extract the seeds’ placental tissue, clone it, and then grow it on special food. The mosses exhumed from Teardrop Glacier just had to be ground up, spread on potting soil, and watered. Unlike the fancy-pants “higher” plants that evolved after them, cute, soft, fuzzy mosses are, at least in this case, the true rugged survivalists.
But mosses have another trick. Unlike most other plants, mosses cannot regulate water entering or leaving their bodies. Most “higher” plants you see have thick waxy coatings that only permit water to enter and leave by the roots or through holes in the leaves called stomata. In this way, they are able to remain alive and growing most of the time — even when it’s not damp or raining. But if they do run out of water — as during a drought — most wilt and die. They cannot tolerate being dried up.
Mosses, on the other hand, have a laissez-faire relationship with water. If it’s there, they instantly swell up and grow. If it’s not, they shrivel up, safely suspend their biology, and wait. They can wait a long time. This property is called “poikilohydry”, and together with totipotency, they enable mosses to grow entire new plants from bits of old, hard-knock, dried-up mosses.
Withstanding lengthy bouts of drying is an adaptation that has conferred extraordinary powers of resilience on other organisms too, including the tiny animals called rotifers and tardigrades or water bears. It’s long been suspected that water bears’ ability to withstand extreme radiation, temperature, and even the vacuum of space are inadvertant side effects of their systems for dealing with repairing the inevitable DNA damage caused by drying. Perhaps it’s no coincidence that these same organisms are often found living in tiny forests of moss (another name for water bears is the adorable “moss piglet”).
Now, scientists can add “extreme glacial encounter” to the impressive list of tolerances found in members of the moss ecosystem. And they also now know that retreating glaciers should not be considered biological blank slates. Thanks to their unique biology, mosses may lead the post-glacial Reconquista from positions already embedded within enemy lines.
La Farge C., Williams K.H. & England J.H. (2013). Regeneration of Little Ice Age bryophytes emerging from a polar glacier with implications of totipotency in extreme environments, Proceedings of the National Academy of Sciences, 110 (24) 9839-9844. DOI: 10.1073/pnas.1304199110