pierre laszlo

Extreme halophiles

Our present knowledge divides living beings into three categories, the prokaryotes (bacteria whose cells have no nuclei), the eukaryotes (bacteria and all higher organisms whose cellĀ  have nuclei), and archaebacteria (primitive bacterial organisms). The latter are distinguished by their mitochondrial RNA sequences, by the chemical composition of the cell walls and membrane--the membrane does not consist of a lipid bilayer, but of glycerol bound by ether (and not ester) bonds to long chains of 20 or 40 carbons--and by the presence of RNA-polymerases whose degree of complexity is in-between those of the procaryotes and eukaryotes. Archaebacteria are not rare organisms: 30% of the oceans' pelagic organisms fall into this category.

Among these micro-organisms, there are bacteria that live in extreme environments such as the hot springs on midoceanic mountain ridges. Another class of archaebacteria, the extreme halophiles, thrives in areas with high salinity, which one might have a priori believed to rule out any life form: lakes or seas with salinity as high as 300 grams of salt per liter. For example, the African lakes, Nakuru and Simbi, are so full of sodium that their pH reaches 10! Lake Nakuru contains 8% sodium. Millions of flamingos live on its shores and feed on bacteria such as Spirulina.

More generally, one finds extreme halophiles in salt marshes, evaporation pools rich in salt, hypersalty seas like the Dead Sea or the Aral Sea, subterranean salt deposits, salt domes, brines and salt-curings, dried meats or fish. Bacteria such as Dunaniella salina live in aqueous solutions that can reach sodium chloride saturation, that is, the water is so laden with salt that it can no longer dissolve all any more.

One can only admire the adaptive mechanisms that allow these bacteria to thrive in areas a priori unsuited to life, with salt concentrations of between two and five moles per liter. The two chief problems to be solved by extreme halophiles (such is their name) are the considerable osmotic pressure on the outside of the cell, and the precipitation of intracellular proteins through the process of "salting out." One response to the first constraint is that of Dunaniella salina, which produces elevated intracellular concentrations of glycerol (glycerine) in order to help balance out the huge outer osmotic pressure. Another counter-move is to expel sodium and let potassium enter as an intracellular cation. As a response to the second constraint, bacterial proteins are modified and made more hydrophilic by the introduction of acid residues--in particular, glutamic and aspartic acids--to their surface; some of these acid residues form salt bridges to the base residues lysine and arginine, providing greater rigidity and structural stability to these protein molecules.

That life can adapt itself to such harsh and seemingly inhospitable environments prompts us to greater intellectual openness and tolerance: we will certainly discover other forms of life in other environments that seem a priori out of bounds (why not under very high pressures, for instance?). Genetic engineering technologies will modify existing organisms so as to make them more able to survive in inhospitable places. Will humankind one day be capable of constructing artificial life forms that thrive in surroundings unimaginable today?

I deem it important to think about what is excessive and beyond-the-norm: scientific thinking has such a playful and provocative turn of mind. Its role includes the imagining of monsters; in this way, the science community resembles other social groups whose solidarity is also founded on the occasional orgiastic or carnivalesque excess: "making merry" and thinking out the limits of one's own knowledge stem from one and the same dionysiac impulse.