Cold as ice – organisms that embrace the frigid spring
When Ella Fitzgerald crooned “Spring can really hang you up the most”, her notes rang true to the human ear and Nature alike. In temperate climes, spring is a difficult time. As T.S. Eliot lamented in his epic poem, The Waste Land, “April is the cruellest month”. Following a winter that “kept us warm, covering Earth in forgetful snow”, spring wrests the world from its static state, and forces it onward into the season of growth ahead.
The challenge of the early days of spring is one that Henry David Thoreau noted from his cabin on the shores of his beloved Walden Pond in the 1840s. Thoreau documented the occurrence of all four seasons in a single day. Spring was not merely the great continuous thaw. Instead, early spring was a daily cycle of thawing every morning, great warmth midday, and re-freezing every evening and through the frigid night. At the cusp of winter and spring, temperature is a tremendous tease – offering comfort during the day, but turning to lethal iciness in the eve.
It is the daily re-freeze that poses one of the biggest risks to organisms living in temperate zones in early spring. The formation of ice-crystals within cells disrupts the function of cellular machinery – lancing membranes, and removing the liquid water that is necessary for biochemical reactions.
In animals, ice tears cells asunder. In the woody plant species that persist throughout the winter, ice doesn’t break cells as they are in animals, but it irreparably damages plants in another, vital way. Ice destroys their capacity to soak up water.
During the non-icy, growing season, temperate zone trees are dependent on the flow of water from the roots up to their leaves, where the water will be used in photosynthesis and other facets of metabolism. Water and solutes flow from the roots to the aerial tissues through the trees plumbing – tiny capillaries, known as xylem cells, that function as conduits for liquid transport.
Xylem cells are very thin, long cells, joined end-to-end to make continuous pipes for water flow. Together, these xylem cells make up most of the woody trunk of the trees. When you cut a tree on the horizontal, you will see the cross-section of these pipes –each tree ring is made of thousands of these pipes side by side.
The thin diameter of the xylem cells is what makes them effective water conduits. Water is drawn through these pipes by virtue of adherence of water molecules to each other, through what is known as cohesion. Long columns of water can be sustained within these thin capillaries on account of molecular cohesion.
The function of these capillaries will fail if an air bubble – an embolism – is introduced into the water column. When an embolism breaks the water column, cohesion can no longer be used to draw the water through the pipe.
Ice destroys xylem function by creating embolisms through a process known as cavitation. When ice forms within xylem cells it traps gas in tiny bubbles. When thawing occurs, these bubbles expand until they eventually break the continuous column of liquid. And with an audible popping noise, you can hear cavitation occur, and, with it, xylem function fail.
While some trees have evolved means by which to refill embolisms, these mechanisms are by no means fail safe. Cavitation could occur anywhere along the length of the tree trunk – which is sometimes significant.
As Byard and colleagues discovered in 2010, birch trees, which experience dramatic changes in temperature over a given year, have evolved two means by which to prevent xylem cavitation.
Some birch tree species avoid cavitation through a process known as deep supercooling. As its name implies, deep supercooling allows the tree to become very cold, and to do so without allowing ice crystals to form within the xylem. Deep supercooling birch trees manage to maintain water in its liquid state down to -40oC. They do this by making special proteins that prevent ice crystal formation, and by otherwise storing water in a relatively pure state.
Deep supercooling is an excellent strategy to contend with cold, unless the temperature drops below --40oC. Below that temperature, water will spontaneously, homogeneously undergo ice formation. That is, all of the water will convert to ice at once. Consequently, deep supercooling is cold-defence strategy that is limited to tree species that reside in locations that do not experience temperatures less than -40oC. The thing is, some birch trees live in regions where the temperatures fall well below -40oC. How do they contend with such low temperatures?
Birch tree species that live in locations that experience temperatures lower than -40oC evolved another means by which to contend with ice. They embrace the ice.
Instead of leaving water inside the xylem cells, birch trees that have evolved to contend with really low temperature expel the water outside of the cells. In such trees, the inside of the xylem cells dehydrate. They remain full of constituents, but sugars and proteins make up for the missing water. With more abundant solutes, like salty water, the freezing point of water inside of the cells is lowered.
Meanwhile, all of the ice crystals form on the outside of the cells, where they cannot create an embolism. Once the time of daily thawing and re-freezing has passed, the xylem cells refill with water from the roots, and the column of xylem sap is restored. By embracing ice formation on the outside of the cells, these birch trees protect the intended function inside the cells.
Not surprisingly, birch tree species that use the ice-embracing mechanism to contend with cold reside in more frigid regions than the deep-supercooling birch species. It may seem counter-intuitive, but embracing the ice turns out to be a better strategy to beat its crystal lethality.
Like many great innovations, evolution has rediscovered the ice-embracing process to protect other organisms from ice. Just as birch trees form ice on the outside of their cells, other, completely unrelated organisms do the same. As is fitting with its name, one of these birch-like mimics is the wood frog.
Many frogs burrow their way deep within the mud and silt of ponds, lakes, rivers and streams to escape the cold of winter. Not the wood frog.
If the wood frog burrows at all in the final days of autumn, it is just beneath the leaf little and soft mud near the soil’s surface. There, the wood frog is protected somewhat from the winter’s freeze by an insulating layer of snow. Nevertheless, the wood frog is largely exposed to large drops in temperature. These temperatures can fall well below zero within the regions of the world where the wood frog resides – from the Appalachians in the south to within the Arctic Circle in the north. In its northern-most abodes, temperatures may be as low as -18oC, with temperatures lower than -10oC being sustained for weeks at a time.
When the temperature drops to these low levels, the wood frog freezes. Literally. At such low temperatures, up to 70% of the water in the wood frog’s body is converted to ice.
And the wood frog survives.
Like the birch tree, the wood frog avoids the injurious consequences of ice, by enabling it to form on the outside of cells. Like the xylem cells in birch trees, the cells in the frog’s body become dehydrated. When the temperatures drop, water flows out of the cells through the cell membrane and forms ice crystals on the outside of the cells. In the meantime, the inside of cells, as well as blood plasma, become sugary. In the autumn, wood frogs build up stores of glycogen – a complex carbohydrate made up of sugar building blocks all linked together. Just like us, wood frogs build up glycogen in their livers.
Within hours of temperatures dropping below zero, wood frogs begin to break glycogen down into its component parts – the simple sugar, glucose. Glucose flows out of the liver and is taken up by other tissues throughout the frog’s body.
The build up of sugar within the frog’s cells functions like an antifreeze. It protects the interior of cells from forming ice crystals. Wood frogs make at least one other antifreeze as well – urea. Urea is a nitrogen-containing compound that, like glucose, prevents ice from forming within the cells. Working together, glucose and urea enable the cells within the frozen frog to assume a state of suspended animation.
Remarkably, frogs that are frozen solid can be thawed, and refrozen many times throughout the winter. Every time they are frozen, their cellular metabolism has to change. In the frozen state, the cells rely on anaerobic metabolism to survive. Just like a human runner who relies on anaerobic metabolism to keep moving, the tissues of frozen frogs accumulate lactic acid as they use anaerobic metabolism to keep them alive.
Come springtime, when temperatures rise enough so that the frogs are not refrozen, they completely thaw and become reanimated again. Lactic acid is cleared from their muscles, and glucose is converted back to glycogen. The frogs go along their merry way.
Remarkably, the birch trees and the wood frogs have happened on a very similar strategy to contend with the challenges of spring. In a marked case of convergent evolution, the trees and the frogs have been bestowed with a mechanism that enables them to undermine the lethal effects of ice by embracing ice. They provide a wonderful illustration of how evolution frequently finds a very similar elegant solution when confronted with the same challenge, by making use of equivalent raw materials of cellular and metabolic structure and function.
Like the wing of a bat and the wing of a bird, the ice-embracing mechanism for survival shared by the trees and the frogs shows how evolution can hone similarly crafted strategies for living life in incredibly disparate organisms. Rather than being foiled by the year’s cruellest month, evolution finds a way to embrace it, and carry the wonders of nature into another season.
Images: All photographs by Malcolm M. Campbell.
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