Life and Light in the Deepest Depths
If asked to imagine a place on Earth untouched by humans, your first thoughts might be to mythical utopias such as Shangri-La and the Garden of Eden. But such a place does exist, almost as inaccessible and no less mysterious: The Deep Sea. How does life kilometers under the surface of the Earth adapt to cope with this harsh environment that never sees the light of day?
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Adapting to Life Down Under
It was first speculated that the deep ocean would contain little to no life at all, owing to the absence of light. When the first probes were sent to the bottom, however, scientists were shocked to discover thriving communities of living organisms. Light is associated with photosynthesis and life, therefore this discovery had big implications.
Implications like the possibility of life on planets and moons far from their sun, but will not be discussed here.
Dealing With Enormous Pressures
But exactly how deep is deep? The most popular destination for deep-sea research (to use popular very loosely here, as there have been more missions to the moon than to this part of Earth) is part of the Pacific Ocean called the Mariana Trench, measured to be about 11km in depth, deeper than Mount Everest is tall.
“‘Pssshh” you might say, “I can run that distance and be home in time for tea”. But remember that as you begin the descent, you will feel the cumulative force exerted by the water above. By the time you reach the bottom, the amount of pressure you would have to endure is equivalent to 1000 times that of standard atmospheric pressure. That’s the equivalent of having the weight of ten cows on your shoulders!
Remember when your high school science teacher said that a gas can be compressed but a liquid can’t? Well, you can throw that out the window. At that kind of pressure, even liquid water increases its density by about 5%. Living organisms present in these deep waters would have evolved special adaptations to cope with this intense pressure.
Firstly, they have to ensure that their bodies contain no pockets of air. This means no cavities, no lungs and no swim bladders found in conventional not-so-deep-sea fish. This is to ensure there is no pressure differential between their insides and their outsides, otherwise, their bodies would cave in upon themselves.
This also means that if a deep-sea fish filled with air rose to shallower depths, the inverse would happen. They would literally explode! But wait, remember that water would also compress and expand at such pressure differentials. To avoid this issue, these organisms are able to decrease their tissue density instead, with their bodies high in fat content and low in skeletal mass.
Another thing becomes an issue in the deepest depths. At a biochemical level, many metabolic processes that occur on the surface are either accelerated or retarded under these pressures. To cope with this, our deep-sea friends have put to use a whole lot of amazing chemistry, with one peculiar molecule particularly important.
The cells of deep-sea creatures contain a variety of molecules known as osmolytes. Osmolytes serve to counteract osmosis (water moving in or out), thereby maintaining cellular volume. As an added benefit, osmolytes should play key roles in redox processes and metabolism that keeps the creature alive.
The organic molecule trimethylamine N-oxide (TMAO) is one such osmolyte. What makes TMAO really amazing is it can directly counteract the destabilization of protein structure and binding caused by intense pressures. Protein activity is a key part of every biological process, with in vitro studies showing that TMAO is able to offset the effects of high pressure by preserving enzymatic activity1.
The deeper down you go, you’ll find that organisms contain higher and higher concentrations of TMAO in their cells. What a marvelous protein protecting, pressure offsetting molecule.
A lot of the chemistry and life processes that occur in deep-sea organisms are yet to be studied, due to the difficulty of obtaining live specimens. Their chemical makeup is so different from creatures that live nearer to the surface that they often do not survive for long when brought into a laboratory.
Generating Light in the Darkness
As mentioned, light from the sun does not penetrate past a few hundred meters into the ocean. At the deepest depths, it would be completely pitch black if not for the amazing ability of some creatures to generate light. And not just white light, but an exciting array of fluorescent hues!
Bioluminescence is the ability of a living organism to produce its own light. Amazing in itself, but when we realize that this light comes solely from chemical reactions, as opposed to electricity, this ability becomes simply breathtaking.
Well, it wouldn’t be completely fair to compare bioluminescence with light emitted from a bulb. That would be sort of like comparing a maglev to a steam engine.
In a light bulb, ~90% of energy is converted to heat, giving it a measly 10% efficiency for energy emitted as light. In bioluminescence, the efficiency of light production can be much higher, to the tune of up to 41%3. In a deep-sea organism, this means that a much smaller amount of energy is spent sustaining this ‘cold light’.
Luciferins in Bioluminescence
The family of molecules responsible for all bioluminescence is known as luciferins, which release light via a catalyzed oxidation/excitation followed by an electron transfer reaction as the molecule returns to the ground state. These luciferins—as well as their catalysts, luciferases—differ from species to species, and many have yet to be studied in detail.
Deep-sea creatures are known to possess the luciferin known as vargulin, but they catalyze their vargulin using a variety of separate luciferases. These differences in enzymes are therefore what lend deep-sea creatures their myriad of exciting colors.
What, then do they use all this light that they produce for? I mean, they don’t have good eyesight, to begin with. So why the fuss? Shown below is a list of what these creatures can do, given a little light.
- Yue, L., Lan, Z., & Liu, Y. J. (2015). The theoretical estimation of the bioluminescent efficiency of the firefly via a nonadiabatic molecular dynamics simulation. The journal of physical chemistry letters, 6(3), 540-548.
- Haddock, S. H., Moline, M. A., & Case, J. F. (2010). Bioluminescence in the sea. Annual Review of Marine Science, 2, 443-493.
- P. H. Yancey, Organic osmolytes as compatible, metabolic and counteracting cytoprotectants in high osmolarity and other stresses, Journal of Experimental Biology, 208, 2819-2830.