Research Scientist, NASA Ames Research Center
Now that we have passed through summer, replete with warnings about the health hazards of exposure to ultraviolet radiation, the subdued light of autumn provides the ambiance in which to take a more balanced look at what UV radiation has meant to life on Earth. We need UV radiation to synthesize vitamin D, which is critical for calcium absorption. UV radiation is used by some organisms as an environmental cue, and it aids in some repair mechanisms for DNA damage. Further, UV-catalyzed reactions in the atmosphere and on the early Earth were critical to providing the conditions for life to arise. But the fact remains: UV radiation itself is hazardous to carbon-based life such as ours. Why should this be so, and how has life overcome this obstacle to thrive on Earth?
The apparent diversity of organisms masks the fact that all life on Earth, and possibly in the universe, is based primarily on a few types of organic compounds. Principal among these are proteins and nucleic acids (RNA and DNA), respectively the primary structural and hereditary components of terrestrial biology. Unfortunately, the maximum absorption of radiation for both compounds is in the UV portion of the solar spectrum, 280 nm for proteins and approximately 260 nm for nucleic acids, and such absorption could destroy these molecules. While solar radiation below about 290 nm does not reach the surface of the Earth today, it is still dangerously close to these peak absorptions.
If this weren't enough, UV radiation can catalyze the production of reactive oxygen species, such as the hydroxyl radical, which themselves damage organic compounds. And the situation was far worse on early Earth, prior to the formation of a protective ozone shield. Without the ozone shield (but with CO2 in the atmosphere, which we have had from the earliest times), we would be bathed in UV radiation down to 200 nm — a horrifically dangerous situation for life.
With this background, one might forgive an extraterrestrial biologist from assuming that all life on Earth seeks refuge underground. But yet we know this not to be universally true. In fact, life underground is at a disadvantage as it cannot access other portions of the solar spectrum, specifically the longer wavelengths that bacteria, algae and plants exploit for photosynthesis and we animals use for vision.
A common evolutionary solution to this problem is to produce biological "sunscreens" for protection from UV radiation while allowing access to the longer wavelengths, and indeed many organisms from prokaryotes to humans use this approach. But it is also possible to exploit minerals that are transparent to longer wavelengths, but attenuate UV radiation. My lab has found that organisms that live under sand grains do just that, as do organisms that live in salt crusts such as the ones in San Francisco Bay's Cargill Salt Company. In collaboration with SETI Institute Principal Investigator Janice Bishop, and under the auspices of an NAI grant to the SETI Institute, we are exploring the possibility that iron-based compounds were particularly important in protecting the earliest organisms on Earth.
Why iron? Iron is one of the most abundant metals in the universe, found in stars such as our sun, in planets, and as a principal constituent of certain types of meteorites and asteroids known as iron meteorites and M-type asteroids. On Earth it accounts for about 5.6% of the crust, and nearly the entire core. Iron is arguably the most important metal for life because of its role in many metabolic processes, including being the critical component of hemoglobin, the compound that transports oxygen in red blood cells.
Near the surface of the ocean, iron concentrations exist in the nanomolar to picomolar range. But the iron compounds that are there, for example nanophase ferric oxides/oxyhydroxides, are capable of absorbing UV radiation. Thus, we have proposed that such compounds allowed early organisms to become photosynthetic — on the one hand, the iron compounds were available for use in metabolism, while on the other they attenuated harmful UV radiation while transmitting the longer wavelengths needed for photosynthesis. Through combining our expertise in biology and geochemistry, and through lab and field work, we plan to test this hypothesis in the coming years.