I’m sure everyone else in BIO230 is as excited as I am over yesterday’s big press conference by NASA. I saw that today’s XKCD is certainly excited about it! In case you had your computer off yesterday, or didn’t watch any TV or listen to the radio, here is the gist of the press release. Researchers examining bacteria isolated from Mono Lake, California, found a species that appeared to tolerate tremendous levels of arsenic in their environment. Arsenic is poisonous to all living cells (well, after yesterday, almost all living cells,) and in fact was used successfully by Paul Ehrlich as a form of chemotherapy in the treatment of syphilis. This compound, known as Salvarsan, was the first so-called “magic bullet,” a compound designed to specifically target the pathogen with minimal damage to the host. So what is so exciting about the NASA-funded discovery which came out yesterday?
Arsenic is directly below phosphorus in the periodic table, which means that the chemistry that occurs with phosphorus can also occur with arsenic. However, arsenic is a larger element with an additional electron orbital, so the substitution of arsenic in a molecule will have profound effects on its function. Phosphorus is one of the “Big 6” elements found in all living cells, which also include carbon, hydrogen, oxygen, nitrogen, and sulfur. Of those 6 elements, the macromolecules of life are composed mainly of the first 4, sulfur is almost exclusively found only in proteins. Phosphorus, on the other hand is found mainly in phospholipids and nucleic acids, and in nucleic acids (DNA and RNA,) phosphorus forms the backbone of the nucleic acid molecule. Small molecules such as ATP also have phosphorus as an essential part of the molecule. So if a cell has arsenic present, arsenic can chemically substitute for the phosphorus, but the cell will end up having nucleic acids or lipids which are non-functional.
We’ve learned in this course about organisms that tolerate environments which we would perceive as being inhospitable to life. Thermophiles can grow in boiling water, halophiles can grow in extremely high levels of salt, and acidophiles can grow under pH conditions that would degrade human flesh. Thermophilic bacteria have developed enzymes that are stable at high temperatures, things are different in the case of halophiles and acidophiles. If one carefully examines the cytoplasm of these organisms where the enzymes and other cellular machinery is found, you will find that the cytoplasmic environment of those organisms is actually very close to that found in our cells, and that there enzymes have pH and salt optimal conditions very much like our enzymes. How do they do this? The cells use active transport to maintain a constant internal environment of low salt and neutral pH. This active transport can require a tremendous amount of energy (ATP) to maintain. So one might very logically assume that if an organism is arsenotolerant (can grow in high levels of arsenic, which is toxic!) that the mechanism for survival might be the same: the organism uses active transport to generate a cytoplasmic environment which is very low in arsenic. WRONG!
This is where the story gets really interesting. When researchers isolated this bacterium and grew it in the lab, they grew it in very high levels of arsenic, and found that it was very happy under those conditions. Furthermore, when they examined the DNA of these organism, they found that the DNA contained arsenic in the backbone of the molecule instead of phosphorus. This means that instead of actively trying to maintain a constant low physiological level of arsenic in the cell, these cells say “Bring it on!” and have developed a way to live with it. Although their biochemistry is only poorly understood as of now, in order to incorporate arsenic into DNA, these organisms must have enzymes that can bind to and catalyze reactions with precursors (ATP) that have arsenic in them instead of phosphorus. Also consider this: the structure of the double helix is essential for the transmission of heredity from generation to generation, and also as the template for the formation of messenger RNA in protein synthesis. Enzymes such as DNA polymerase and RNA polymerase must recognize the DNA double helix, bind to it, and catalyze an enzyme reaction. The distance between each base pair in this figure is 3.3 angstroms, and that distance is determined by the phosphodeoxyribose backbone. If we substitute arsenic for phosphorus, that distance will change, because arsenic is a bigger atom than phosphorus. It is incredible that this new molecule will even work in a living cell, yet is does so very successfully for this bacterium.
What does this mean in the big picture? Our thinking about the requirements for life is that all living cells have fundamental requirements. Prior to yesterday, we would not have considered that arsenic-based chemistry would be amenable to living systems. We now have an example of how living systems are flexible enough to adapt to an inhospitable ecological niche, take advantage of resources in that niche, and flourish. Our definition of habitable biospheres has now been expanded.