The Equations of Life

by Charles S. Cockell

  At the freezing temperatures found at the Earth’s polar regions, life’s membranes, made of lipids, freeze solid. Place a chunk of butter in a warm kitchen, and it is soft, easy to spread on your toast. After eating your breakfast and putting the butter in the fridge, you will notice that once the butter has sat there for an hour or so, it goes solid. Like the fatty acids in the butter, the fatty acids in a microbe’s lipid membranes, in the refrigerated temperatures of the Antarctic ice sheet, freeze solid. The lipids contain long chains that, when cooled, line up side by side, serried ranks of molecules that pack together tightly, providing little room for movement.

  There is a way to make them more mobile. The fatty acid chains comprise many carbon atoms attached to one another by single bonds, one carbon after another. Introduce a double bond into the chain, and you create a kink. The molecule, rather than being a long straight chain, now kicks out to one side. Lay these unsaturated fatty acids, as they are known, side by side, and they will not pack together. Because the rogue chains keep the fatty acids separated, they can now move around more freely and are less able to pack up tight in the freeze.

  Back in the kitchen, we can illustrate this idea with another common item. Safflower oil is full of unsaturated fatty acids. Put it on the kitchen table, and it is runny. Place it in the fridge, and it remains liquid. Unlike the fatty acids in butter, the oil’s kinked chains keep the molecules moving. In just the same way, microbes in the polar regions fill their membranes with unsaturated fatty acids. These branched molecules keep the membrane flexible and fluid. When exposed to subfreezing temperatures, the membranes, instead of freezing solid, now retain the malleability that allows food to move in and waste to get out of the cell.

  This example is one of stunning beauty. By the mere switch of a single bond to a double bond in a chain of carbon compounds, life can now colonize the frozen wastelands of Earth. Within this modification at the atomic level, we see how evolution can use simple tricks of chemistry and physics to master entirely new worlds.

  Yet magnificent though this simple invention may be, its accomplishment is to help push the lower limit of life down toward the hard barrier set by physics. At that lower perimeter, no change in membranes, no mere alteration in a few bonds, can escape the twilight zone, where even the most energy-rich environments are trapped in a chemical slow lane. In this zone, reactions are so tardy that no living entity can do chemistry fast enough to keep up with the inevitable assault of damage, molecular transformation, and disassembly.

  Thus, life is trapped between a hot and a cold place, its boundaries of operation constrained by simple principles. Invention and chance may well vary the exact limit at which life operates in any time or place in which it finds itself, but these limits are narrow. Even with wildly optimistic assertions about its temperature tolerance, the percentage of a planet that life can master is a tiny proportion of the total volume. Changing its capacities by hundreds of degrees merely alters the volume of a planet it may occupy by a few tenths of a percent. Life is hardy and persistent once it gets hold of a planet, but its dominion is small. Between absolute zero and the temperature of the interior of a star, say, the Sun, life occupies only 0.007 percent of this temperature range. This narrow vista is not a consequence of historical facts of evolution it might break free from if the evolutionary process were run again. It results from the laws of physics operating on a small but interesting branch of chemical compounds we call life.

  Across the universe, and even on a small rock like ours, life meets a bewildering variety of other extremes. Perhaps temperature is an anomalous example, an unusual set of physical boundaries. Do other extremes narrow the range of life?

  Travel to Guerrero Negro, in Baja California Sur, on the west coast of Mexico, and there you will find the biggest salt works in the world. This large, flat area near the coast contains thirty-three thousand hectares of salt fields, enormous expanses in which seawater is evaporated under the careful watch of its owners to make pancakes of white salt, glittering, dazzling in the sun. Each year, this industrial-scale operation makes nine million metric tons of salt.

  Like the salty ponds deep in Boulby mine, life here has a huge challenge. It must deal with the process of osmosis, which relentlessly causes the salty environment to suck water from the cells, transforming them into puny microscopic prunes. No random evolutionary adaptations can simply escape osmosis. It is an inescapable process, and wherever it exerts its effects, life must adapt or die.

  No one should be surprised to learn, then, that like thermophiles, these regions of the world contain life that has adapted to such an extent that it now needs high salt to survive. The halophiles, microbes that need between 15 and 37 percent salt to grow, inhabit Baja and Boulby and thrive in waters that would make most life shrivel up and collapse.

  Osmosis will not just go away. Its effects are uncompromising. It can be stopped only by exerting a pressure, the osmotic pressure (π), against it equal to:

  π = imRT

  where m is the number of moles of the substance per liter (the molarity), R is the universal gas constant, and T is the temperature. The value i is the curious van ’t Hoff factor, which has to be worked out using experiments. It is the degree of dissociation of the salt ions added to the water.

  The osmotic pressure that you must exert merely to stop pure water from being sucked up by salty seawater across a membrane nearly equals a crushing twenty-eight atmospheres of pressure.

  Faced with the trauma of having water extracted by force, life has evolved two effective ways to respond. You could just take up ions into the cell. The so-called salt-in microbes took the approach: if you can’t beat osmosis, join it. By allowing potassium ions to accumulate in the cell, the osmotic potential on the inside and outside of the cell is equalized and the cell can continue life as normal. The side effect is that the cell now has a high concentration of salty ions inside, and it must evolve its proteins to deal with this problem. Ions can disrupt bonds, interfere with the folding of proteins, and change the availability of water.

  Evolution’s response to this problem is ingenious. Many proteins have within them hydrophobic portions. As salts will displace water, these water-hating regions of proteins become even more attracted to each other as the water between them is pushed aside by the salts. This increased attraction is a problem because now these protein units may be too stuck together. By evolving proteins whose hydrophobic contact areas are reduced, the connection is made a little weaker, compensating for the salt that strengthens it. This subtle little trade-off brings the proteins back into normal operation. By changing charges and bonds in the key proteins of life, other adaptations add to the repertoire of modifications to live in salt.

  However, some life forms eschew salt completely. Instead, to maintain the osmotic balance, they produce compounds that act like salts but that are slightly kinder to the cell. Sugars such as trehalose and some amino acids can increase the concentration of compounds inside the cell so that the osmotic pressure equalizes with the outside without the damaging effect of salt ions. These “salt-out” microbes are very common and inhabit the salt crusts of Baja and the brine seeps of Boulby.

  Push the salt concentrations high enough, and eventually the problem is not so much all that intense osmotic pressure, but now just a general lack of water altogether. The cells become so depleted in water that this essential solvent for life, the liquid that makes the chemistry of life possible, is no longer in enough supply to maintain the machinery of life in a working state.

  This lower floor of water stress is set by the water activity (aw), a measure of the availability of water. In more exact terms, it is the ratio between the water vapor pressure above the salt and that of pure water. The smaller the water activity, the less water available. Pure water, distilled water, has a water activity of 1. A saturated salt solution, the sort of briny pond in Boulby, has a water activity of 0.75. Most microbes need a water activity of about 0.95 or higher; any lower than t
his, and osmotic stress sets in, and the molecules of life cease to function. However, the halophiles and many other microbes that can tolerate dry environments can push that limit much below 0.75. Some fungi can reach a water activity of just below 0.6.

  Like scientists enamored with finding the temperature extremes of life, there is a quest to find the water activity limit of living things, and no doubt with enough scurrying around the world and digging around in extreme environments, we’ll see that limit drop. But from the viewpoint of this book, the rat race of microbiologists interested in defining the new limit is less important than the more general point that water availability restricts life. Once the availability of that most fundamental of life’s requirements, liquid water, is pushed below a water activity of about 0.6, the diversity of living things that can persist in that realm is diminished. By a water activity of about 0.5, there are unlikely to be any active living things. There just is not enough water. A familiar culinary analogy is honey, whose water activity is usually below 0.6. That is why you can leave this sweetener on the kitchen table without its going moldy. Honey is a parched desert for a microbe.

  Water activity, and, indeed, honey, shows us that some places that contain liquid water are uninhabitable. We are all generally used to thinking of any watery environment as hospitable to life. You will often hear planetary scientists say that the search for life on other planets is about “following the water.” This is sometimes colloquially said as “where there’s water, there’s life.” The aphorism comes from our everyday observation of the essential role that water plays in living things, but you can see that this claim is not 100 percent accurate.

  Aside from honey, there are other watery solutions too extreme for life. A saturated solution of magnesium chloride at 25°C has a water activity of 0.328, well below the preferences of biology. These solutions can also cause disorder in biological molecules. Therefore, even on Earth, we can find places, such as deep brines in the Mediterranean, that contain biologically dangerous levels of magnesium chloride. When investigated by microbiologists, these brines are found to be at the limits of life.

  Deep in Boulby, channels of water cut hither and thither, dissolving their way through sodium chloride here, making their way through sulfate salts there. Throughout the mine, almost all the brines contain active life, halophiles making a living on their depauperate resources. Sometimes, these rivulets of salty water cut through a vein of magnesium chloride, and when they do that, the water activity plummets. Within these small trickles of water, there is no evidence of life. A small detour through a vast sequence of rocks, through hundreds of meters of salt that is extreme but benign to salt lovers, into one particular salt pushes life beyond its capacities.

  Similarly, Don Juan Pond, an undistinguished-looking water-filled hole in the McMurdo Dry Valleys of Antarctica, is thought to be devoid of active life. Since the 1970s, scientists have been intrigued by this rare water hole. Because it is filled with a brine of calcium chloride with a water activity of below 0.5, we would expect the hole to be lifeless. Indeed, microbiologists have had mixed results in growing things collected from Don Juan. The consensus is that the pond is empty of active life but that microbes washed in from the outside remain viable. When fished out and plated onto agar plates in more benign laboratory conditions, they will grow. It is quite possible that in the spring, when snow melts in Antarctica, the water flows into the pond, forming a lens of freshwater on its surface, where the water activity is above the threshold for life. Then, life may briefly take a respite and multiply before the pond is mixed and it returns to its uninhabitable state.

  These lifeless but watery habitats show us something profound and important. Water is a requirement for life. However, even on this planet, we have environments where there is plenty of liquid water but its availability to life is insufficient, not merely because the environment is dry, a solidified salt crust in the Baja Californian sun, but because even in a liquid state, some salt solutions fail to relinquish the water molecules needed for life. We need not visit alien worlds to find where life has reached its physical limits, where no amount of chance or evolution will push it beyond the barrier of salt. For over three and a half billion years, evolution has been experimenting with adaptations, yet when faced with a low water activity, evolution is impotent. You might fill a saturated magnesium chloride or calcium chloride brine with nutrients, organic material, and every conceivable energy source you can imagine, but still that watery environment will remain dead to multiplying cells.

  Startling though these restrictions are, let us continue to other extremes to see what else might stop life in its tracks. We need to explore some other extremes to construct a general picture of how physics might bound the biosphere. Travel to the south of Spain, near the ancient and architecturally stunning town of Seville, and you will come across the Rio Tinto, a bright orange and red river that cuts through the Iberian Peninsula. Flowing for more than a hundred kilometers, the river slices through a belt of sulfide rocks that oxidize to make sulfuric acid. The result is a highly acidic river, with an average pH of 2.3. Even this level of acidity is tame compared with Iron Mountain in California, where similarly acidic streams have a pH of 0 to 1, as low as battery acid. Considering such extreme chemistry, we might be forgiven for believing that conditions would again be too extreme for life.

  Yet we find life thriving in these locations. The pH of water is a measure of the concentration of protons. The more protons, the more acid the solution. Protons are not a bad thing for life. The flow of them through the machinery of the cell membrane is the basis of energy harvesting. However, if there are too many, the charge they build up will damage proteins and other crucial parts of the cell. The acid-loving microbes that live in the Rio Tinto and Iron Mountain, the acidophiles, have to work hard to keep those protons out, and they do this by pumping them from the cell to keep the interior of the cell at a near constant, almost neutral pH. To call them acidophiles is something of a misnomer, as the microorganisms have evolved to keep their cell interiors from becoming acidic, but despite their exhaustive efforts to keep protons out, they are adapted to these conditions. Place them in a less acidic environment, and many will die.

  At the other extreme are the alkaliphiles, microbes that can tolerate high-pH environments. A trip to Mono Lake just north of Death Valley, California, will give you a glimpse into the world of alkaline life. Here strange tubular carbonate mounds, called tufas, climb from the lake and the land around it in an eerie alien scene, chimneys that are testament to mineral precipitation in a lake with a pH of 10 and a saltiness three times that of the ocean. This high pH is no barrier to life. Not only do microbes grow in the lake’s waters, but alkali flies (Ephydra hians) run higgledy-piggledy along its shoreline, while brine shrimp (Artemia monica) flex and pulsate in its waters. Here, even animal life can thrive. The fly larvae, which begin their life in Mono Lake, have within them special organs that turn the alkaline waters into carbonate minerals. You can think of these biominerals sequestered in the larvae as a detoxification method, a clever way of removing the ions in the water and collecting them into minute grains and out of harm’s way.

  Mono Lake, although a source of fascination and a focus for many scientists, is not the most alkaline lake in the world. Other lakes around the world, such as Lake Magadi in the Rift Valley of Africa, with a pH over 11, also host ecosystems.

  So far, we know of no natural extreme pH environment that precludes life on Earth. Have we now found an extreme in the face of which life defies physics? Well, not really. We might appreciate this by thinking about the physical facts. High temperatures must eventually exclude life since in extremis, life is destroyed by the vast energies injected into the atomic bonds of its molecules. A fragile carbon-based life form just cannot hold those molecules together at 1,000°C, so we can easily and simplistically grasp the likelihood of an upper temperature extreme for life, although we can investigate where that limit might be and what molecular f
ailure ultimately defines it. Similarly with salt. In the simplest version of our understanding, the limit of salt or desiccation tolerance is set by the availability of water. Remove the water entirely or add salts so that the water molecules are all but unavailable, and life is denied the solvent it needs to operate. An edge of existence based on the availability of water is also easy to grapple with.

  When we turn to pH, there is nothing inherent that will shut down life. As long as cells have enough energy and good enough pumps to either remove protons from the cell or let them in, then the interior of the cell will remain near a neutral pH, undamaged by extremes of pH in the outside world. The ions themselves, provided they remain outside the cell, present no mortal threat. Perhaps not surprisingly, the different pH environments explored so far contain life.

  Not that pH is always kind to life. Add it to other extremes, such as high temperatures or salt stress, and the cell must now find enough energy to deal with multiple problems. In most of Earth’s environments, there is rarely just one extreme, although there are many environments where one will dominate. In the deep oceans, cold temperatures combine with saltiness. In volcanic pools, acidity often combines with high temperatures. Microbes have been found that can cope with salt, high pH, and high temperature. In any environment, an extreme might push life over the edge when the cell lacks sufficient energy to cope with the onslaught from a mishmash of them. Yet, on its own, pH does not seem to be a fundamental limit to life in the Earth’s natural environments.