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Extreme Halophiles

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 cells 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.