Imagine a swimming pool full of battery acid. Would you want to dive in for a quick swim? What about just sticking your hand in? As humans, we have a tough time tolerating contact with environments of extreme pH, environments that are either very acidic or basic. We measure pH as a function of the concentration of hydrogen ions (H+) or protons in solution. Most life forms are unable to survive environments of either low or high pH. Take for example, proteins, a key building block of life. When placed in a solution of low pH, proteins can quickly become denatured. Despite the ability of pH to antagonize some of life's most fundamental molecules, scientists have identified life forms that not only survive in regions of low pH; they thrive there. These organisms, called acidophiles, are just another type of life form more broadly known as extremophiles.
The more we explore Earth, the more we are astounded by the diversity of environments and life we find. We have identified more than just the hot and the cold places, the wet and the dry ones. Now we have identified and are studying new extremes, like those of extreme pH. From the boiling sulfuric hot pots in Yellowstone National Park to puddles of acid mine drainage, life is proving to exist in increasingly acidic environments. In fact, life has been discovered at pH levels close to zero! How does life survive such conditions? And what do we stand to learn from studying acidophiles?
Acidophiles have adapted many unique characteristics to successfully combat their acidic surroundings. When acidophiles were first recovered, it was anticipated that their cytoplasm would differ significantly from life in more neutral conditions. However, the cytoplasm of some acidophiles was surprisingly found to be neutral like other cells. Incidentally, these acidophiles were also found to have more proton transporters in their cell membranes than other organisms. This way, the acidophile could literally pump protons out of the cell. Other acidophiles had cytoplasm that behaves as a much better buffer solution against large changes in pH than mesophilic cytoplasm. Yet other acidophiles exhibited cell membranes with a high concentration of positive charge. All of these adaptations allow these organisms to thrive in an environment that would be extremely inhospitable to "normal" life.
In our search for life beyond Earth, our studies of acidophiles may give us some clues as to the true limits for life as we know it. By better understanding how life has adapted to fill specific niches on Earth, we can extrapolate our knowledge to surmise about similar environments on other planets and moons. For instance, the conditions on Venus and the extreme volcanic activity on Io are two extraterrestrial environments in which we might expect to find areas of high acidity. Knowledge of acidophiles on Earth will assist our search for and possibly the identification of new life forms on other planets.