Tough Neighborhood
Now that we’ve introduced a few of the residents of volcanic hot springs, we are forced to ask the question “How do they survive in such conditions?” The most basic problem of high temperature and low pH living is molecular stability. Under both regimes, the bonds that hold together vital organic substances, such as proteins, membrane lipids and DNA, face the danger of being torn apart, leaving useless fragments and a disabled cell.
Let’s examine the effects of hot springs conditions on a human cell with a hypothetical experiment. Our laboratory must contain a water bath; don’t think candles and bubbles, however, for we must bathe our cell in water that could boil potatoes. If that sounds inhospitable, we have yet to add the piéce de rèsistance, sulfuric acid at a concentration equal to that of battery acid. Moments after we placed our unfortunate cell into this nightmarishbath, the precisely organized structure that allows a cell to function would begin to fall apart.
First to go would be the cell membrane. This all-important structure regulates passage of substances to and from the cell, acts in cell-signaling and functions during respiration to transform cellular energy into useful ATP, the universal energy currency. Under these conditions, however, the phospholipid bilayer that makes up the membrane will peel apart, leaving the interior of the cell exposed. Embedded within the cytoplasm are a multitude of proteins, the heavyequipment of the cell. These macromolecules, built from a string of amino acids that are folded into complex 3-dimension forms, facilitate nearly all reactions within the cell, from DNA replication to metabolic activity. Further, proteins form the cell’s cytoskeleton, a structural lattice-work within the cytoplasm. Unfortunately for our doomed cell, both high temperatures and acidity attack the bonds that keep proteins folded into the proper conformation. With their precise 3-dimensional structure denatured, proteins lose all catalytic and structural capacity. Though the loss of its lipid membrane and all its proteins would be spell the end for any cell, one final insult remains. The DNA molecule that carries our cell’s genetic information will also fall before the combined onslaught of temperature and pH.
Heat Adaptations
Faced with these prospects, how can a thermoacidophile cope with hot springs conditions and avoid becoming a lifeless mass of organic molecules? The answers lie in a series of molecular adaptations shaped by evolution at the extremes. As noted above, the cell membrane is composed of phospholipids. These macromolecules are built of fatty-acid chains connected by a glycerol head, which usually form a double-layered envelope that surrounds the cell. In many thermophilic organisms, these fatty-acids show a high degree of saturation. Saturated fats in our foods have a higher melting temperature and exist as a solid at room temperature whereas unsaturated vegetable fats melt at a lower temperature and exist as oils. Similarly, highly saturated fatty acids in the cell membrane of thermophiles maintain their integrity even at high temperatures. For the hyperthermophilic Archaea that brave the highest livable temperatures recorded, even this adaptation is not enough. Organisms of these environments employ a unique lipid monolayer with covalent bonds preventing the separation of the membrane. A monolayer avoids the problem of delamination and shows a high degree of thermostability.
Thermophiles must also prevent their proteins from denaturing at high temperatures. Their amino acids building-blocks are no different from those of other proteins, and in fact, even the sequences of amino-acids in heat-stabile proteins are much the same as their temperature-intolerant cousins. The difference lies in the precise folding of the protein; its 3-dimensional conformation can increase its heat tolerance many degrees. Patterns of folding that increase the number of strong ionic bonds between different parts of the protein help to maintain its shape even under heat stress. Certain parts of a protein are hydrophobic, or water-avoiding. With these regions concentrated near its center, a protein can strongly resist the tendency to unfold, for that would bring the hydrophobic interior into contact with the aqueous environment of the cell. Even when high temperatures do partially denature cellular proteins, they may be refolded into the proper shape by a special heat-shock protein, called a chaperonin. Utilizing these adaptations, Sulfolobus gives us an example of a thermostable protein in NADH oxidase. This enzyme catalyzes an important metabolic reaction in all cells, but thanks to a specialized folding pattern, those found in Sulfolobus able to maintain function up to 90° C.
With proteins and lipids safe from the heat, hot springs microorganisms must still protect the DNA molecule from heat damage. One way to stabilize a long nucleic acid strand is to wind it into tight coils. In fact, the enzyme reverse DNA gyrase, isolated from hyperthermophilic Bacteria and Archaea, performs this very action. In addition to supercoiling, hyperthermophiles may use kinks to increase thermostability of DNA. Sulfolobus carries another protein in its toolkit, called Sac7d, that induces these sharp turns in its DNA, thereby increasing its melting temperature by 40°C.
Adaptations to low pH
Hot, sulphuric springs also punish inhabitants with extremely high acidity. Less is currently known about the physiological adaptations of low pH living than about thermophily but biologists do know how acidity affects cells. First, high acidity limits the amount of CO2 that can dissolve in water. This prevents photosynthesizing organisms from colonizing environments of extreme acidity. The remaining residents still face an attack on their cell membrane by hydrogen ions. To combat this, acidophiles employ longer fatty acid chains that inhibit the acid breakdown of membrane phospholipids.
To prevent protein and DNA breakdown, cells must maintain an internal environment much closer to neutral than their surroundings. While most membranes are somewhat permeable to small, inorganic ions, acidophiles tightly regulate their passage. By controlling ion transport they can maintain a neutral interior (pH 7) even in surroundings of pH 2. This means that they maintain an ion gradient of five orders of magnitude across their membrane. In other words, these acidophiles withstand the chemical pressure of hydrogen ion concentrations 100,000 times greater outside their cells than within!
It’s not all bad news for thermoacidiphiles, however. Higher temperatures actually increase the rate of chemical reactions and improve the efficiency of enzymes up to the denaturation point. Temperature is really just a measure of the kinetic energy of a molecule; increased temperatures mean that the molecules move around more. This increased movement means more collisions between molecules, a necessary first step in any reaction. So, if we see increased reaction rates at high temperatures, we might expect higher growth rates in thermophiles compared to other microorganisms. Sadly, this advantage is more than counteracted by the time spent repairing and replacing damaged cellular components and the growth rates in hot spring environs are generally more sluggish than those of more benign environments.
Despite the travails of life in boiling acid, thermoacidophiles not only survive, they build impressive ecosystems that reach into nearly every corner of their hot spring habitat. While examination of individual extremophiles yields information about their adaptations and cellular functions, true understanding of any living creature only comes when we place them in the context of their ecological relationships. It is to these all-important relationships that we now turn.
Our tour of life in Lassen Volcanic National Park continues with a look at: