The Dead Sea is not really a sea , but an endorheic lake, into which all the rivers of its basin flow and which has no contact with the sea. It is located at the bottom of a tectonic depression more than 400 meters below sea level, and is characteristic for its salinity with a concentration of dissolved salts of up to 28% , much higher than the average seawater, which is around 3.5%. This high salinity is what allows a person to float in its waters without difficulty . But in addition, its saline composition is different ; Unlike seawater, whose salts are made up mainly of chlorine, sodium, sulfate, and magnesium ions, the Dead Sea is made up mostly of calcium, magnesium, potassium, and bromine salts .
This composition of salts , not only different, but also extraordinarily concentrated , makes fish and amphibians unable to maintain osmotic balance , that is, the balance of salts between the interior of their body and the water that surrounds them. This total absence of fish and amphibians gives the lake its name; already in Hebrew it was known by two names; especially Yām ja-Melaj which means “sea of salt” and, in some writings, Yām ja-Māvet or “sea of death” . However, the name “of death” or “dead”, as we call it now, not only because it is not a sea, but also because it is not dead.
If the history of evolution has taught us anything, it is that life cannot be contained , life always makes its way. And that he did in an environment as inhospitable as the Dead Sea ; an environment where extremophilic organisms —which can live in extreme environments— have found a niche to inhabit and have formed a peculiar microbial ecosystem . Today we will talk about three living beings that have managed to inhabit and thrive in the Dead Sea.
Haloferax volcanii , the salt archaea
It is an archaea , a type of prokaryote — single-celled organisms without a nucleus — other than bacteria, which are actually closer to us than to them. It is a living being that lives without difficulty in the hypersaline waters of the Dead Sea , but it also needs heat , something easy to get in an endorheic lake in the desert. For this reason, during the summer these archaea reproduce much more easily.
This archaea, moreover, can be cultivated relatively well in the laboratory. Adding to this its condition of extremophile makes it a fantastic model organism for exobiology , that is, the study and understanding of what extraterrestrial life could be like in inhospitable environments .
However, its strength is also part of its weakness. Haloferax volcanii has a very restricted salinity range . Not only do you need a high salinity, but if it is too high, it harms you.
During the last decades, the extraction of water and mud from the Dead Sea is causing its level to drop and its waters to become increasingly salty , which puts the conservation of this extremophile at risk .
Dunaliella parva, the red green algae
We went from a prokaryote to a eukaryote — an organism with nucleated cells —. Dunaliella parva is a small unicellular algae, with a pair of cilia , adapted to saline environments . It lacks a rigid cell wall; instead it has a starch covering . This gives it some flexibility , allowing it to expand and contract , absorbing or expelling water, depending on the salinity of the environment. In addition, its cytoplasm contains high concentrations of glycerol , which allow it to maintain fluidity despite having little water.
Although it is part of the group of chlorophytes or green algae , its color is rather reddish , due to the high concentration of ß-carotene in its chloroplasts, the same pigments that give color to tomatoes or carrots. In fact, when high proliferations of this algae occur in the Dead Sea, its waters turn pink . These pigments protect the chlorophyll from extreme lighting. These algae are therefore extremophiles adapted to high salinity and strong sunlight .
They are also easy organisms to grow, although in that situation they do turn green when they are in less stressful situations. These algae are cultivated for the extraction of ß-carotenoids used as a food or cosmetic colorant , or as a nutritional supplement. But its ease of cultivation also makes Dunaliella parva a good model organism for exobiology .
The Elusive Protozoa of the Dead Sea
During sampling in the 1940s, two protozoa were isolated and cultured from samples from the Dead Sea. One of them was a type of amoeba found in the mud , more than 300 meters deep. We are talking about a unicellular organism of less than 80 microns in length. It was found to have the peculiarity that, under certain conditions, it was capable of producing flagella . On the other hand, they discovered a ciliated protozoan that was in close association with the amoeba above and with the algae of the genus Dunaliella . That single-celled organism was even smaller, no more than 24 microns, and had two hardened spines on the back .
However, neither culture has been preserved to this day . Later studies tried to relocate these curious protozoa, without success. So at the moment, we can’t be sure if these creatures actually exist in the Dead Sea or if it was some kind of contamination from the original samples from eight decades ago.
REFERENCES:
Oren, A. 1988. The Microbial Ecology of the Dead Sea. En K. C. Marshall (Ed.), Advances in Microbial Ecology (pp. 193-229). Springer US. DOI: 10.1007/978-1-4684-5409-3_6Oren, A., & Shilo, M. 1982. Population dynamics of Dunaliella parva in the Dead Sea1. Limnology and Oceanography, 27(2), 201-211. DOI: 10.4319/lo.1982.27.2.0201
Pohlschroder, M., & Schulze, S. 2019. Haloferax volcanii. Trends in Microbiology, 27(1), 86-87. DOI: 10.1016/j.tim.2018.10.004
