The Elements of Evolution (43-3) (Not) Freezing

(Not) Freezing

Physiological processes require liquid water. Freezing the water inside cells spells certain death unless you happen to be a well-fed Panagrolaimus davidi worm, in which case you just feel a bit stiff.

Despite the peril of being on the icy edge of death, many organisms live at temperatures below the equilibrium freezing point of their body fluids. To do so necessitates specific adaptations which vary among species, though many antifreeze solutions involve protein agents: those wily macromolecules who know how to keep the plumbing working.

The precise physiological challenge of not freezing differs between marine and terrestrial environments. Life in the sea is more thermally stable than on land, owing to the much greater specific heat capacity of water compared to air. If it’s cold in the water, it’s likely to stay that way for some time. Thermal shocks are a greater hazard on land.

Life evolved in the sea, so most organisms have a body fluid similar to seawater in osmotic strength. Hence, for most marine organisms, freezing is a relative problem: if the sea remains fluid, so do they.

Teleosts arose during the Triassic. These ray-finned fish make up 96% of all living fish, with abundant diversity: 26,840 extant species. Teleost’s great advantage is their jaws, which may protrude from their mouths, enabling them to grab prey and draw it in.

Teleosts are unusual in having thin blood: roughly half the osmotic strength of seawater. This dilute blood likely reflects an early evolutionary phase in fresh water, when thinner blood would have reduced osmoregulatory costs. It means that teleosts in polar waters are living with blood that would easily freeze without some serious compensatory devices.

Polar teleosts avoid freezing via a suite of anatomical, physiological, and chemical adaptations. One of them is an antifreeze protein (AFP). P. davidi also avoid catching a lethal cold via an AFP.

AFPs inhibit ice crystal formation and are effective in minute concentrations. Antifreeze proteins bind to specific faces of growing ice nuclei, preventing them from reaching sufficient size to achieve thermodynamic stability, and thereby inoculate bulk freezing. The proteins thermally assess cold spots and so can efficiently prevent freezing.

Icefish (notothenioids) live off the coast of Antarctica. They evolved AFPs once, when first adapting to the freezing waters, with a genetic recipe that is uniquely efficient in manufacturing antifreeze proteins. These fish also have aglomerular kidneys which prevent losing AFPs in their urine.

Representing a major evolutionary radiation, icefish are the dominant teleost on the continental shelf of Antarctica. Later-evolved notothenioids which live in warmer waters to the north of Antarctica have the AFP-generation gene, but do not express it, as the protection is superfluous.

Antarctic fish are ever in frigid waters, and therefore need antifreeze throughout their lives. In contrast, many fish on the fringes of the Artic basin only face freezing in winter. Many of these fish synthesize their AFPs seasonally.

Arctic fish employ a different AFP than Antarctic ones, with exception. Codfish created on their own AFP, selfsame as icefish, illustrating convergent evolution for not freezing to death in frigid waters.

The thermal environment on land is more volatile than in the sea, and the evolutionary responses have been correspondingly complex. Some land animals employ antifreeze proteins. Certain insects, frogs, turtles, and at least 1 snake can tolerate extracellular water freezing. Water in the cells remains fluid, and metabolism continues, albeit at a low level. A minority of these creatures actually induce extracellular freezing via ice-nucleating proteins as a form of virtual hibernation. Meantime, AFPs are employed within cells to prevent recrystallization, averting tissue damage from ice crystals growing while the animal is frozen.

Some insect protein antifreezes have been found to be many times more effective than fish AFPs. Such efficacy is needed to survive the wider range and rapidity of frigidity to which these insects are subjected.

Many arthropods exposed to extreme cold also produce various cryoprotectants sufficient to significantly drop the freezing point of their cells. Some species employ just 1 compound, while others use a complex chemical suite which both protects and minimizes cellular injury. Especially prominent in damage control is the use of trehalose, a double-glucose sugar which helps maintain membrane integrity during desiccation (anhydrobiosis).

Cellular dehydration is a significant problem for organisms whose extracellular fluids freeze, as the increased osmolarity of the residual unfrozen water is high, pulling water from the cell. Many unicellular beings residing in super-salty homesteads or able to hold up under drought are often incidentally able to withstand freezing. Maintaining cell membrane integrity plays a resounding role in such resistance. There appear to be significant physiological parallels and evolutionary convergences between drought and freezing tolerances, involving stress (chaperone) proteins and similar osmolytes (compounds affecting osmosis).

Whereas land animals are motile and may migrate to locales that lessen environmental stresses, plants are sessile, and must withstand whatever challenges the weather delivers. Using a variety of ice-nucleating agents, plants typically initiate freezing in xylem and extracellular fluids as a way to raise the odds of cell survival. Besides the potential insulating effect of an ice shell, extracellular freezing withdraws water from cells. So, fighting freezing to death involves similar responses to dealing with drought.

Besides their extensive precocious knowledge, plants learn the best ways to survive water shortages. For many plants, aridity is a common problem, whereas life-threatening cold is less frequent. Being able to apply honed skills to an infrequent hazard improves the probability that a plant can manage to survive.