Posted by: Rob Viens | August 9, 2013

An Affirmation of Extreme Habitability!

Following up on Darwin’s visit to the Rio Negro salt flats yesterday, I want to touch on one last thing that he describes – life on the salt.  Although a minor note in his diary, I think this observation has some pretty profound effects on Darwin’s thinking, and leads to a fairly radical idea for the time (read on). Here is what Darwin has to say about life in the salt flats in Voyage of the Beagle:

“The border of the lake is formed of mud… The mud is black, and has a fetid odour. I could not at first imagine the cause of this, but I afterwards perceived that the froth which the wind drifted on shore was coloured green, as if by confervæ: I attempted to carry home some of this green matter, but from an accident failed. Parts of the lake seen from a short distance appeared of a reddish colour, and this perhaps was owing to some infusorial animalcula. The mud in many places was thrown up by numbers of some kind of worm, or annelidous animal. How surprising it is that any creatures should be able to exist in brine, and that they should be crawling among crystals of sulphate of soda and lime! And what becomes of these worms when, during the long summer, the surface is hardened into a solid layer of salt? Flamingoes in considerable numbers inhabit this lake, and breed here; throughout Patagonia, in Northern Chile, and at the Galapagos Islands, I met with these birds wherever there were lakes of brine. I saw them here wading about in search of food—probably for the worms which burrow in the mud; and these latter probably feed on infusoria or confervæ. Thus we have a little living world within itself, adapted to these inland lakes of brine. A minute crustaceous animal (Cancer salinus) is said to live in countless numbers in the brine-pans at Lymington; but only in those in which the fluid has attained, from evaporation, considerable strength—namely, about a quarter of a pound of salt to a pint of water.” (Voyage of the Beagle)

Flamingos in Patagonia (from Wikipedia Commons):

flamingos

What Darwin had discovered in his big salty “pond” are what we today call extremophiles – life forms that can live in extreme conditions. There are a lot of different types of extremophiles including thermophiles (orgamisms found in temperatures up to about 120°C), xerophiles (those that live in extremely dry conditions), and cryophiles (critters that can survive extended temperatures down to about -15°C) and acidophiles (yup – you guess it – acid lovers).  Organisms that live in brine and extremely salty conditions, such as the ones Darwin was describing, are called halophiles. A lot of these critters are bacteria or archaea, but there are some algae that fall in the realm of extremophiles.  This may be what Darwin was mistakenly identifying as “confervæ” – another type of green algae.

Dunalielia – a halophilic species of green algae (from the Microscope web site)

halophile

Extremely salty conditions presents a particular problem for living things. Living cells exist in balance with their environment, and the barrier (membrane) between living and non-living is semipermeable. (Semipermeable in this case means water can pass through but the dissolved salt ions cannot.) The process of osmosis, the movement of water through this membrane, is controlled in part by the saltiness of the water.  Water tends to move from low salt concentrations toward high concentrations (an attempt to balance things out, if you will). So if you drop a cell into water that is more salty than it is, it will effectively lose its water and “dry out”.  If you put it into water that is less salty, it will absorb water from the environment and explode. You might even remember doing this experiment in high school biology.

So to live in extremely salty conditions you need to have adaptations that help offset this problem. In a very generally sense, most halophiles tend to build up a large internal concentration of dissolved ions or organic compounds that behave like salts.  Since there is then a high concentration of dissolved material on both side of the membrane, everything is in equilibrium and the cell does not dry out.

For the last few decades, extremophiles have been studied in great detail (and there are new ones being discovered all the time).  One reason for this interest is simply because they are so unique, but there are also some more practical applications. One is an interest from the medical community – some of these extreme cells have “figured out” how to do some pretty useful things (such as carrying out certain chemical reactions under high temperatures). In addition, the study of extremophils forms the basis for the search for life on other planets and moons.  Because lets face it, you can’t get much more extreme than the cold, dry surface of Mars, the liquid methane lakes of Titan, or the subterranean oceans of Europa. If life is out there in our solar system, it is almost certainly an extremophile. (See a short 2-min video from SETI on halophiles in space.)

Methane lakes of titan (radar image in false color, from NASA) – can you imagine what Darwin would have thought of these!

lakes on Titan

Now all of these discoveries came long after Darwin’s trip to the great Salinas, but none-the-less, you can see that the fascination with extremophiles is already stewing around in his head back in the 1830s. I find the next statement (also from Voyage) to be quite powerful, and even highly emotional:

“Well may we affirm, that every part of the world is habitable! Whether lakes of brine, or those subterranean ones hidden beneath volcanic mountains—warm mineral springs—the wide expanse and depths of the ocean—the upper regions of the atmosphere, and even the surface of perpetual snow—all support organic beings.” (Voyage of the Beagle)

This is one of those passages in Darwin’s writings where you can see the twinkle in his eye, and get the sense that he really gets it.  I mean, he is really tapped into a deep understanding of life on Earth – even anticipating the things that would not be discovered for a long time to come. You also get the sense that this is a man who really loves being a scientist – this is the joy of science at its best.

I can almost see him stewing over the meaning of this statement – How is it possible that life as we know it could live in these extreme environments?  How could they possibly have adapted to these conditions? What wonders are out there yet to be discovered?

I wish we could show him all that we have learned in the last 100+ years since he died.  He would be amazed…(RJV)

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