Now thatwe have passed through summer, replete with warnings about the health hazardsof exposure to ultraviolet radiation, the subdued light of autumn provides theambiance in which to take a more balanced look at what UV radiation has meantto life on Earth. We need UV radiation to synthesize vitamin D, which iscritical for calcium absorption. UV radiation is used by some organisms as anenvironmental cue, and it aids in some repair mechanisms for DNA damage.Further, UV-catalyzed reactions in the atmosphere and on the early Earth werecritical 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 shouldthis be so, and how has life overcome this obstacle to thrive on Earth?
Theapparent diversity of organisms masks the fact that all life on Earth, andpossibly in the universe, is based primarily on a few types of organiccompounds. Principal among these are proteins and nucleic acids (RNA and DNA),respectively the primary structural and hereditary components of terrestrialbiology. Unfortunately, the maximum absorption of radiation for both compoundsis in the UV portion of the solar spectrum, 280 nm for proteins andapproximately 260 nm for nucleic acids, and such absorption could destroy thesemolecules. While solar radiation below about 290 nm does not reach the surfaceof the Earth today, it is still dangerously close to these peak absorptions.
Ifthis weren?t enough, UV radiation can catalyze the production of reactiveoxygen species, such as the hydroxyl radical, which themselves damage organiccompounds. And the situation was far worse on early Earth, prior to theformation of a protective ozone shield. Without the ozone shield (but with CO2in the atmosphere, which we have had from the earliest times), we would bebathed in UV radiation down to 200 nm — a horrifically dangerous situation forlife.
Withthis background, one might forgive an extraterrestrial biologist from assumingthat all life on Earth seeks refuge underground. But yet we know this not to beuniversally true. In fact, life underground is at a disadvantage as it cannotaccess other portions of the solar spectrum, specifically the longer wavelengthsthat bacteria, algae and plants exploit for photosynthesis and weanimals use for vision.
Acommon evolutionary solution to this problem is to produce biological"sunscreens" for protection from UV radiation while allowing accessto the longer wavelengths, and indeed many organisms from prokaryotes to humansuse this approach. But it is also possible to exploit minerals that aretransparent to longer wavelengths, but attenuate UV radiation. My lab has foundthat organisms that live under sand grains do just that, as do organisms thatlive in salt crusts such as the ones in San Francisco Bay?s Cargill SaltCompany. In collaboration with SETI Institute Principal Investigator JaniceBishop, and under the auspices of an NAI grant to the SETI Institute, weare exploring the possibility that iron-based compounds were particularlyimportant in protecting the earliest organisms on Earth.
Whyiron? Iron is one of the most abundant metals in the universe, found in starssuch as our sun, in planets, and as aprincipal constituent of certain types of meteorites and asteroids known asiron meteorites and M-type asteroids. On Earth it accounts for about 5.6% ofthe crust, and nearly the entire core. Iron is arguably the most important metal for life because ofits role in many metabolic processes, including being the critical component ofhemoglobin, the compound that transports oxygen in red blood cells.
Nearthe surface of the ocean, iron concentrations exist in the nanomolar topicomolar range. But the iron compounds that are there, for example nanophaseferric oxides/oxyhydroxides, are capable of absorbing UV radiation. Thus, wehave proposed that such compounds allowed early organisms to becomephotosynthetic — on the one hand, the iron compounds were available for use inmetabolism, while on the other they attenuated harmful UV radiation whiletransmitting the longer wavelengths needed for photosynthesis. Throughcombining our expertise in biology and geochemistry, and through lab and fieldwork, we plan to test this hypothesis in the coming years.
Rothschildis a Research Scientist at the NASA Ames Research Center, and a frequentcollaborator with SETI scientists.
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