Dear readers, the month of Christmas is at the door and I think you all deserve to slide in one of the coldest parts of the planet. With that said, get cozy and have a nice journey.
“Ice is the beginning of Antarctica and ice is its end. This is a world derived from a single substance – water, in a single crystalline state, the snow, transformed into a lithosphere composed of a single mineral – ice.” -Stephen J. Pyne
I think there aren’t better words for beginning this article.
The Antarctic tells a story of a true “transfiguration” of the earth into ice. This kingdom of ice on the planet was certainly formed during the distribution of the continents throughout the different geologic eras. The influx of warm water masses towards the Antarctic was lost when Australia and the Antarctic broke apart, followed by the formation of the circumpolar current. Variable factors such as the decrease of atmospheric CO2, had contributed to the formation of ice during the Eocene.
Initially, the temperature ranged between 4 and 5℃, and then started decreasing only in the last 4-5 million years ago. The cooling down of the entire Antarctic region has certainly decreased the biodiversity, especially of fish, gastropods and bivalves, decreasing the predator pressure on the invertebrates such as crinoids and ophiuroids, which now dominate these waters.
This dramatic decrease of temperatures, in nature, has generated a real derangement of the previously existing harmony, and has created a completely new one. The new state, however, required organisms to adapt. Adaptations, in general, cannot occur due to just one factor, but most of the time, they bring about a real “rearrangement” of the organisms.
Beginning with the lowest level and, at the same time, the most complex, nucleic acids (DNA and RNA) are affected by low temperatures that generate highly stable bonds. This stability, anyhow, becomes an obstacle for the DNA to be flexible and “opened” precisely when proteins must be transcripted and other signals must be sent from the DNA for the proper cell function and survival.
A protein called High Mobility Group B1 Protein (HMGB1) regulates the transcription by promoting the opening of the DNA helix. Different studies on euriterm organisms (those able to tolerate and survive in a wide range of temperatures without any damage) show how organisms acclimated to constant low temperatures are able to produce high concentrations of this protein compared to those adapted to higher environmental temperatures. The concentration of this protein, though, remains constant in organisms exposed to daily temperature variation, like warm during the day and cold during the night.
Most probably, the evolutionary adaptations of the Antarctic organisms had led to an elevated expression of the HMGB1 protein, and not to a true alteration of the intrinsic structure of the DNA.
If this is a typical implementation model of the functionality of an existing molecule, is emblematic the “invention” case of the macromolecular complexes which prevent freezing, or the loss of other molecular systems.
In the last case, a typical example is the absence of hemoglobin (Hb) in all of the 15 species of the notothenioid family Channichthyidae, commonly known as icefish. A single mutational event has brought to the total loss of the gene codifying for hemoglobin in these icefish. Although the hemoglobin is fundamental for the transportation of oxygen, its loss has been compensated with changes, devoted to restoring the capacity to transport oxygen. Among these changes, the most common are:
- bigger heart,
- high blood volume (from two to four times the blood volume of teleosts of similar dimensions that have hemoglobin)
- low systemic resistance of the circulatory system.
The loss of hemoglobin in icefish supports the total reduction of the circulatory costs, facilitating the diffusion and influx of oxygen. In addition, the absence of a ferric protein also decreases the formation of free radicals that can be extremely harmful.
These compromises have probably turned out crucial for the successful mutation, to such extent of making it a feature of this teleost family.
The characteristics acquired during the life history of these organisms are closely related to two factors: the low metabolic level at low temperatures and therefore also to the maximization of growth in the cold.
It is very likely that the acquired characteristics make these species slow in responding to climate change such as the ones we are experiencing. If the metabolic rate of an organism is very slow, this indicates that even in the cellular apparatus, the ability to adapt to sudden external changes is shown to be slowed down.
Another factor that makes us understand the “slowness” of these species is certainly also the age at which the first reproductive event occurs and the ability to generate offspring. The Antarctic brachiopod Liothyrella uva reaches sexual maturity at 20 years and will be able to give life from 5 to 10 fewer generations than the temperate brachiopods which instead mature within 2-4 years.
That is why, probably the changes that are taking place will have the greatest impact on species belonging to extreme habitats, as they have adopted changes that make them highly specialized.
Hans O. Portner, Lloyd Peck and George Somero, 2007. “Thermal limits and adaptation in marine Antarctic ectotherms: an integrative view”. Trans R Soc Lond B Biol Sci. 2007 Dec 29; 362(1488): 2233–2258.
“The Ice: a Journey to Antarctica”, 1986, Stephen J. Pyne.