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Ultraviolet Radiation in Planetary Atmospheres

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15 Ultraviolet Radiation in Planetary Atmospheres and Biological Implications Petra Rettberg and Lynn J. Rothschild 15.1 Solar UV Radiation The extraterrestrial solar spectrum extends far into short wavelengths of UV-C (190280 nm) and vacuum UV (s atmosphere would probably be lethal to most living organisms without the shielding afforded by the atmosphere. Solar UV undergoes
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  15   Ultraviolet Radiation in Planetary Atmospheresand Biological Implications Petra Rettberg and Lynn J. Rothschild 15.1   Solar UV Radiation The extraterrestrial solar spectrum extends far into short wavelengths of UV-C (190-280 nm) and vacuum UV ( < 190 nm), wavelengths that no longer reach the surface of the Earth. The intensity of solar radiation reaching the Earth's atmosphere wouldprobably be lethal to most living organisms without the shielding afforded by the at-mosphere.Solar UV undergoes absorption and scattering as it passes through the Earth's at-mosphere with the absorption by carbon dioxide, molecular oxygen and ozone beingthe most important processes. Carbon dioxide has a peak absorbance at 190 nm, and soattenuates radiation below 200 nm. Ozone forms a layer in the stratosphere, thinnest inthe tropics (around the equator) and denser towards the poles. The amount of ozoneabove a point on the Earth's surface is measured in Dobson units (DU) - typically ~260DU near the tropics and higher elsewhere, though there are large seasonal fluctuations.It is created when ultraviolet radiation strikes the stratosphere, dissociating (or split-ting ) oxygen molecules (O 2 ) to atomic oxygen (O). The atomic oxygen quickly com-bines with further oxygen molecules to form ozone (O 3 ). Figure 15.1-A shows theabsorption cross section of ozone as a function of wavelength and Fig. 15.1-B a part of the extraterrestrial solar spectrum compared to terrestrial spectra calculated for differ-ent ozone concentrations. Increasing ozone concentrations result in lower irradiancesin the UV-B range of the spectrum. Surface UV-B radiation levels are highly variablebecause of sun angle, cloud cover, and also because of local effects including pollut-ants and surface reflections.Solar UV radiation affects life on Earth today, and probably even has had a strongerimpact on early evolution [1]. The composition of the Earth's atmosphere at that timediffered from that of today. Although its exact composition is not known, from modelcalculations it can be assumed that during the Archaean era, during which the diversifi-cation of early anaerobes took place and the first anaerobic photosynthetic bacteriaappeared (about 3.5 Ga ago), the amount of free oxygen in the atmosphere was signifi-cantly lower than today (see Chap. 14, Cockell)]. There was very little or no absorptionof solar UV radiation by ozone. The situation on the early Mars might have been com-parable (see Chap. 13, Lammer et al. and Chap. 14, Cockell). Taking  15 Ultraviolet Radiation in Planetary Atmospheres and Biological Implications233 AB Fig. 15.1 Absorption cross section of O 3 (A) and solar irradiance calculated for different O 3 concentrations (B), A = 440 DU, B = 400 DU, C = 360 DU, D = 310 DU, E = 258 DU, F = 185DU, G = 66 DU, H = extraterrestrial solar irradiance. the presumed composition of the early Martian atmosphere and the lower solar lumi-nosity into consideration radiative transfer calculations of the UV flux on the surfaceof Mars show a gradual increase of the irradiance including short-wavelength UV-Band UV-C over time until today. Thus, present-day solar UV irradiance on the surfaceof Mars may be similar to that on the surface of the Archaean Earth (see Chap. 14,Cockell ). 15.2   Biological Effects of Solar UV Radiation In biological systems, UV radiation causes photochemical reactions with differentbiological target molecules, the so-called chromophores. These interactions result intemporary or permanent alterations. The most important UV target in cells is the DNAbecause of its unique role as genetic material and its high UV sensitivity. The absorb-ing parts of DNA are the bases, the purine derivatives adenine and guanine, and thepyrimidine derivatives thymine and cytosine. Although the base composition of DNA isdifferent in different   genes and organisms, there are the common features of an ab-sorption maximum in the 260 nm region and a rapid decline toward longer wave-lengths (Fig. 15.2). Absorption of proteins between 240 and 300 nm is much lowerthan that of nucleic acids of equal concentration in weight per volume. Most proteinsare present in cells in higher numbers of identical copies. Therefore, photochemicalalterations in only a fraction of them do not disturb their biological function signifi-cantly. The same is true for molecules like unsaturated fatty acids, flavins, steroids,chinones, porphyrins, or carotenoids, which serve as components of the cell membrane,as coenzymes, hormones, or electron donor transport molecules.  234P. Rettberg and L. J. Rothschild Wavelength (nm)200220240260280300320    A   b  s  o  r   b  a  n  c  e 10 -3 10 -2 10 -1 10 0 DNAProtein Fig. 15.2 Absorption spectra of DNA (calf thymus DNA) and a protein (bovine serum albumin)at identical concentrations. The spectrum of UV radiation from the time that it first hits the surface of a bio-logical object changes while it passes the outer parts of the cell or tissue to reach thesensitive targets in the cells, the chromophores. Therefore the action spectrum de-scribing the wavelength dependence of a biological UV effect is often not identical tothe absorption spectrum of a chromophore. In Fig.15.3 examples for normalized bio-logical action spectra obtained with monochromatic radiation are given. These actionspectra show a remarkable similarity of the slopes of the curves in the UV-B range.However, the curves differ from each other significantly in the UV-A range. This iscaused by different photochemical reaction mechanisms. UV-B radiation is directlyabsorbed by the DNA molecules and causes photodamages. In contrast, UV-A radiationmainly excites so called photosensitizer molecules in the cell, which can either reactwith the DNA or with oxygen to give reactive oxygen species, which themselves cancause DNA damages.Due to the wavelength specificity of biological action spectra, especially in the UV-B range, and the highly wavelength-specific absorption characteristics of componentsof the atmosphere like ozone, the assessment of the influence of environmental (poly-chromatic) UV radiation on critical biological processes requires a biological weight-ing of the solar UV irradiance according to the biological responses under considera-tion. The biological effectiveness of solar UV radiation is determined by the shape of the action spectrum of the biological endpoint and the spectral irradiance [2, 3] usingequation 15.1  15 Ultraviolet Radiation in Planetary Atmospheres and Biological Implications235 λ  / nm250275300325350375400425   r  e   l  a   t   i  v  e  s  e  n  s   i   t   i  v   i   t  y 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 DNA damage erythema (CIE) erythema (laser)plantsmelanoma (fish)skin cancer(mouse) Fig. 15.3 Examples for different action spectra (from [2]). λλλ λλ d S E  E  eff  )()( ⋅= ∫  (15.1)with  E  λ ( λ ) = solar spectral irradiance (W/m² nm), S λ ( λ ) = action spectrum (relative units), and λ = wavelength (nm).The resulting biological effectiveness spectrum is shown exemplary for a terrestrialUV spectrum in Fig.15. 4. Integration of the biologically effective irradiance  E  eff  overtime (e.g., a full day, one year) gives the biologically effective dose  H  eff  (J/m²) eff  (e.g.,daily dose, annual dose).The significance of solar UV radiation as an environmental driving force for theearly evolution of life on Earth is reflected by the development of different protectionmechanisms against the deleterious biological effects of UV radiation   [1]. The mostimportant one is the development of several partly redundant enzymatic pathways forthe repair (see Chap. 17, Baumstark-Khan and Facius) of UV-induced DNA damagesvery early in evolution [4]. Examples are (i) the photoreactivation (PHR), that is theremoval of cyclobutane pyrimidine dimers and (6-4)pyrimidine-pyrimidone found inbacteria, Archaea and eukaryotes as a direct repair reaction in a single-step process, (ii)the nucleotide excision repair (NER) for the removal of bulky DNA lesions in bacteria,Archaea and eukaryotes, e.g., the UvrABCD pathway in  E. coli , (iii) the
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