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.
Natural
sunscreens
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.
About
the author
Rothschild
is a Research Scientist at the NASA Ames Research Center, and a frequent
collaborator with SETI scientists.
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