Environmental Limits Of Life

Monarch caterpillar and Monarch butterfly. (Courtesy: Marlin E. Rice)

To be alive is to change. It is true that there exist resting stages of living things that do nothing, as far as we can tell, for thousands of years and are still viable after the experience. Such resting stages include bacteria deep within the Earth, spores of dinoflagellates and diatoms, plant seeds, dried out tardigrades (equidistant cousins of earth worms and centipeds). However, we do not count these organisms among the living because they rest, but because they come alive and change when the time is ripe, that is, when the environment is favorable.

A favorable environment invariably includes the availability of water. Why should this be so? Well, first of all, change (growth, maturation, reproduction) implies chemical reactions, that is, break-up and re-assembly of molecules. Such reactions take place within liquids. They cannot proceed when an organism is desiccated or frozen. Reaction in the gas phase is excluded because temperatures would be too high for the delicate molecules of life to survive. While the surfaces of solids are a good place to rearrange molecules, there must be a way to bring the molecules to the surfaces in the first place, to sustain the reaction. (A football stadium is a good place to arrange the seating of fan clubs, but offsite transportation is necessary for a stadium to fill and to empty.) In a cell, the necessary mobility is delivered by liquid water. The fact that many more fundamentally different types of organisms live within water than live outside of water suggests that life originated in a watery environment and only later invaded dry land, inventing mechanisms for coping with the limited supply of water.

The individual water molecule (on the left) is composed of two hydrogen atoms and one oxygen atom. The oxygen end of the molecule is negatively charged.The two hydrogen atoms bear a slight positive charge. This separation of charge within the molecule is called "polarity", and the molecule is said to be "polar". Because water is a polar molecule, it "sticks" to itself and has special properties. The positive end of one molecule is electrically attracted to the negative end of another water molecule. (Courtesy: Michael J. Pidwirny)

Granted that life-sustaining reactions have to take place within a liquid, why should that liquid required to be water, rather than liquid ammonia or liquid carbon dioxide or some liquid organic compound (as seen in petroleum, for example)? Why water? In fact, we cannot exclude the possibility that other liquids besides water can play a role in sustaining life processes. Carbon dioxide is out, because its stability range for being liquid is too narrow (it tends to be either gaseous or solid). But water is special. It has a number of properties that make it an ideal solvent, capable of holding and transporting molecules. Perhaps the most important is the electric polarity of single water molecules, with the two hydrogen atoms to one side of the oxygen atom. This makes for the presence of weak binding forces which keep the water liquid and provide for ways to envelope the molecules to be transported. Besides, water is very abundant. Hydrogen is the single most abundant element in the universe. The nucleus of the oxygen atom is made of precisely four helium nuclei, a very stable arrangement. Thus, oxygen also is very abundant. It seems unlikely, therefore, that water would not be present if other liquids are present on a planetary body at a temperature that is acceptable for the existence of reactive organic molecules (made of carbon, hydrogen and oxygen). One interesting liquid to study for its potential as a host for life is an intimate mixture of water and ammonia. It does not freeze until the temperature drops below –100°C.

The fact that we need a liquid environment for life's chemical reactions greatly limits the acceptable temperature range. At standard atmospheric pressure, water is liquid from 0°C to 100°C. (This is how the centigrade scale is defined: 100 degrees between freezing and boiling.) With increasing pressure, the boiling point moves to higher temperatures. As long as the water stays liquid, certain life forms can deal with high temperature. Extremophile bacteria and archea are abundant at deep-sea vents at water temperatures up to 300°C. They make a living as chemo-autotrophs, using chemical disequilibria between seawater and basalt in the vent environment as an energy source. They also provide food for the vent communities. Similar microbes are found in hot springs on land, albeit at lower temperatures (the pressures are not great enough to keep the water from boiling at 100°C).

Water stays liquid at high pressures; this fact allows living organisms to thrive right at the bottom of the deepest trenches, at pressures a thousand times greater than at sea level. (That there are no more creatures down there has more to do with the lack of food than with the high pressure.) At very low pressures and low temperatures water either exists as ice or as vapor, but not in liquid form. This is why we should not expect to find active life on Mars today - the pressure is too low for liquid water. Of course, this does not exclude finding fossil leftovers from a distant warmer period, when water may have been flowing on Mars. Apparently, resting stages (typically de-watered) can survive the zero pressure conditions in space: bacteria exposed on Apollo spacecraft and subsequently recovered were found viable. However, they can do nothing during their vacuum exposure other than wait for better circumstances, that is, conditions providing for liquid water.

Do you like salt herring? Salting the herring was invented in the Middle Ages as a solution to the problem of how to get herring to shore and to customers before it disintegrates from bacterial attack. Salt, on the whole, is inimical to bacterial proliferation. We have some trouble with salt ourselves: we cannot use seawater as drinking water. Drinking too much of it makes us very sick. Since our fish-like ancestors left the sea some time ago (400 million years or so) we have lost the ability to deal with so much unwanted salt - our kidneys cannot handle it. Marine organisms, of course, do just fine. But what about salt brine? Is it entirely inaccessible to living things? Well, no. There are brine shrimp and there are brine beetles populating the brine pools in the deserts of California. They do not seem to mind. But is there life in really concentrated brines? What about the Dead Sea? Yes, even so and despite its name. There is a class of microbes called "halophiles" (salt lovers) that can easily tolerate high salt density. In fact, because every one else is kept out of the habitat, they have the place to themselves and thrive.

Liquid water has some other ingredients, some of them potentially a lot more nasty than salt. Even so, some living organisms make a specialty out of coping with such (for us) unpleasant environments. Take sulfurous brines, or highly acidic or alkaline environments. Almost invariably, given an energy source, there is some archean life form growing in this stuff. One organism has been found thriving in the tanks of nuclear reactors! Thermococcus radiodurans can take a dose of radiation 10,000 times greater than that which is lethal to other organisms. How does it manage? How did it ever evolve the ability to do so when that level of radiation does not exist in nature? Perhaps it has learned, earlier, how to cope with some other disruptive process and has transferred these skills to coping with the effects from radiation.

The lesson from this survey of life's environment is simple. If there is a place not taken, where a living can be made, Life will find a way to grasp the opportunity. However, it is doubtful that Life could originate under the conditions which it can tolerate or conquer once it exists. Presumably, conditions for the origin of life are much more restrictive than the conditions acceptable for the expansion of life.