Thermodynamics of thermal radiation from stars photoautotrophs and biospheres

Photoautotrophs are almost the exclusive providers of chemical free energy to the Earth biosphere. Their importance in coadjuvating the evolutionary development of higher forms of life in other planets is briefly discussed from this point of view. A simple analysis based on the nonequilibrium thermodynamics of thermal radiation fields is performed. The analysis relates well known standard parameters of stars of the main sequence to the thermodynamic bounds on the free energy acquisition of planetary photosynthetic processes activated by the star radiation. Upper bounds to permissible wavelengths, active in photosynthesis easily follow. Simple inferences can then be made about the possible types of main sequence stars with planetary systems where exobiological photoautotrophs might have evolved. As red dwarves constitute both the great majority of companions to the Sun and the majority of main sequence stars in our Galaxy, emphasis is placed on discussing the case of planetary systems of stars with low photospheric temperatures. Bounds to the free energy intake per unit area by the biospheres of planets in planetary systems of late type main sequence stars are estimated and compared. Some simple conclusions are drawn.Special Symposium on Photochemistry and the Origins of Life, Sixth International Congress on Photobiology, Bochum, Germany.     

Leaf epidermal transmittance of ultraviolet radiation and its implications for plant sensitivity to ulraviolet-radiation induced injury

Summary Leaf epidermal transmittance of ultraviolet radiation (280–400 nm) was examined in several plant species to determine the capability of the epidermis to attenuate solar ultraviolet radiation. Epidermal samples were mechanically isolated and examined with a spectroradiometer/integrating sphere for transmittance. A survey of 25 species exposed to natural insolation was conducted. Although the species differed in life form, habitat type, and epidermal characteristics, epidermal transmittance was generally less than 10%. Ultraviolet radiation was attenuated 95 to 99% in more than half of the species. In 16 species, flavonoid and related pigments in the epidermis accounted for 20 to 57% of the attenuation. Several species exposed to supplemental ultraviolet irradiation (288–315 nm) in a greenhouse exhibited significant (P0.05) depressions in epidermal transmittance of 31 to 47%, apparently resulting from an increase in ultraviolet-absorbing pigments.     

Sensitivity of Laminariales zoospores from Helgoland (North Sea) to ultraviolet and photosynthetically active radiation: implications for depth distribution and seasonal reproduction

Depth distribution of kelp species in Helgoland (North Sea) is characterized by occurrence of Laminaria digitata in the upper sublittoral, whereas L. saccharina and L. hyperborea dominate the mid and lower sublittoral region. Laminaria digitata is fertile in summer whereas both other species are fertile in autumn/winter. To determine the light sensitivity of the propagules, zoospores of L. digitata, L. saccharina and L. hyperborea were exposed in the laboratory to different exposure times of photosynthetically active radiation (PAR; 400–700 nm), PAR + UVA radiation (UVAR; 320–400 nm) and PAR + UVAR + UVB radiation (UVBR; 280–320 nm). Optimum quantum yield of PSII and DNA damage were measured after exposure. Subsequently, recovery of photosynthetic efficiency and DNA damage repair, as well as germination rate were measured after 2 and 3 d cultivation in dim white light. Photosynthetic efficiency of all species was photoinhibited already at 20 µmol photons m⁻² s⁻¹ PAR, whereas UV radiation (UVR) had a significant additional effect on photoinhibition. Recovery of the PSII function was observed in all species but not in spores exposed to irradiation longer than 4 h of PAR + UVA + UVB and 8 h of PAR + UVA. The amount of UVB-induced DNA damage measured as cyclobutane–pyrimidine dimers (CPDs) increased with exposure time and highest damage was detected in the spores of lower subtidal L. hyperborea relative to the other two species. Significant removal of CPDs indicating repair of DNA damage was observed in all species after 2 d in low white light especially in the spores of upper subtidal L. digitata. Therefore, efficient DNA damage repair and recovery of PSII damage contributed to the germination success but not in spores exposed to 16 h of UVBR. UV absorption of zoospore suspension in L. digitata is based both on the absorption by the zoospores itself as well as by exudates in the medium. In contrast, the absorption of the zoospore suspension in L. saccharina and L. hyperborea is based predominantly on the absorption by the exudates in the medium. This study indicates that UVR sensitivity of zoospores is related to the seasonal zoospore production as well as the vertical distribution pattern of the large sporophytes.     

Host egg pigmentation protects developing parasitoids from ultraviolet radiation

Parasites rely on their hosts not only for nutrition and reproduction, but also for protection against natural enemies and adverse climatic conditions. In host‐parasite interactions, protective characteristics of both players are important to consider regarding damaging effects of environmental hazards. While ultraviolet radiation (UVR) is pervasive and harmful to organisms in general, its impact on parasite fitness remains understudied. Moreover, studies that do examine the effects of UV exposure on parasitic organisms tend to neglect host traits, which may vary inter‐ or intra‐specifically and thus confer different levels of environmental protection. We examined in the laboratory whether the UV‐protective value of host egg pigmentation could also benefit parasitoids, using the egg parasitoid Telenomus podisi and the predatory stink bug Podisus maculiventris. This host species lays eggs of variable pigmentation levels from light to dark grey, an adaptation protecting its own embryos from UVR. We showed that higher levels of host egg pigmentation protect parasitoids subjected to a developmental exposure to UVR, increasing emergence rates by up to 86% and reducing development time by up to 4%. This protective effect of host pigmentation was context‐dependent, being less pronounced at low UVR intensity and towards the end of parasitoid development. Parasitoids that emerged from darker‐coloured eggs exposed to UVR were of slightly larger size than those developing in light‐coloured eggs, but other fitness‐related traits (fecundity, longevity, sex ratio) were unaffected. This study provides the first experimental evidence that host pigmentation can increase host suitability for parasitic organisms, and emphasizes the importance of considering trait variation in interacting species when investigating the susceptibility of ecological communities to important abiotic environmental factors.     

Fitness cost from fluctuating ultraviolet radiation in Daphnia magna

Solar ultraviolet radiation (UVR) is an important environmental threat for organisms in aquatic systems, but its temporally variable nature makes the understanding of its effects ambiguous. The aim of our study was to assess potential fitness costs associated with fluctuating UVR in the aquatic zooplankter Daphnia magna. We investigated individual survival, reproduction and behaviour when exposed to different UVR treatments. Individuals exposed to fluctuating UVR, resembling natural variations in cloud cover, had the lowest fitness (measured as the number of offspring produced during their lifespan). By contrast, individuals exposed to the same, but constant UVR dose had similar fitness to control individuals (not exposed to UVR), but they showed a significant reduction in daily movement. The re-occurring threat response to the fluctuating UVR treatment thus had strong fitness costs for D. magna, and we found no evidence for plastic behavioural responses when continually being exposed to UVR, despite the regular, predictable exposure schedule. In a broader context, our results imply that depending on how variable a stressor is in nature, populations may respond with alternative strategies, a framework that could promote rapid population differentiation and local adaptation.     

Repair of ultraviolet radiation damage in xeroderma pigmentosum cells belonging to complementation group F

DNA-repair characteristics of xeroderma pigmentosum belonging to complementation group F were investigated. The cells exhibited an intermediate level of repair as measured in terms of (1) disappearance of T4 endonuclease-V-susceptible sites from DNA, (2) formation of ultraviolet-induced strand breaks in DNA, and (3) ultraviolet-induced unscheduled DNA synthesis during post-irradiation incubation. The impaired ability of XP3YO to perform unscheduled DNA synthesis was restored, to half the normal level, by the concomitant treatment with T4 endonuclease V and ultraviolet-inactivated Sendai virus. It is suggested that xeroderma pigmentosum cells of group F may be defective, at least in part, in the incision step of excision repair.     

Sources and measurement of ultraviolet radiation

Ultraviolet (UV) radiation is part of the electromagnetic spectrum. The biological effects of UV radiation vary enormously with wavelength and for this reason the UV spectrum is further subdivided into three regions: UVA, UVB, and UVC. Quantities of UV radiation are expressed using radiometric terminology. A particularly important term in clinical photobiology is the standard erythema dose (SED), which is a measure of the erythemal effectiveness of a UV exposure. UV radiation is produced either by heating a body to an incandescent temperature, as is the case with solar UV, or by passing an electric current through a gas, usually vaporized mercury. The latter process is the mechanism whereby UV radiation is produced artificially. Both the quality (spectrum) and quantity (intensity) of terrestrial UV radiation vary with factors including the elevation of the sun above the horizon and absorption and scattering by molecules in the atmosphere, notably ozone, and by clouds. For many experimental studies in photobiology it is simply not practicable to use natural sunlight and so artificial sources of UV radiation designed to simulate the UV component of sunlight are employed; these are based on either optically filtered xenon arc lamps or fluorescent lamps. The complete way to characterize an UV source is by spectroradiometry, although for most practical purposes a detector optically filtered to respond to a limited portion of the UV spectrum normally suffices.     

Repair of damage by ultraviolet radiation in xeroderma pigmentosum cell strains of complementation groups E and F

The xeroderma pigmentosum fibroblast strains XP2RO, complementation group E, and XP23OS, group F, were compared with normal human primary fibroblasts with regard to repair of damage induced by 254-nm UV. In XP2RO cells, repair DNA synthesis, measured by autoradiography (unscheduled DNA synthesis = UDS), was about 50% of the value found in normal human cells. In these cells also the removal of UV-induced sites recognized by a specific UV-endonuclease proceeds at a reduced rate. By having BUdR incorporated into the repaired regions, followed by the induction of breaks in these patches by 313-nm UV, it was shown that the reduced repair synthesis is not caused by a shorter length of the repair regions in XP2RO, but is solely due to a reduction in the number of sites removed by excision repair. In XP23OS a discrepancy was observed between the level of UDS, which was about 10% of the normal value, and other repair-dependent properties such as UV survival, host-cell reactivation and removal of UV-endonuclease-susceptible sites, which were less reduced than could be expected from the UDS level. However, when UDS was followed over a longer period than the 2 or 3 h normally used in UDS analysis, it appeared that in XP23OS cells, the rate of UDS remained constant whereas the rate decreased in normal control cells. Consequently, the residual level of UDS varies with the period over which it is studied.     

Methods for exposure of laboratory animals to ultraviolet radiation

Two different sources of ultraviolet B (UVB) radiation, an electronically controlled UVB exposure unit, containing FS40 tubes, and a hand-held Kromayer lamp, were evaluated for actual irradiance in W/m2 and spectra (physical dosimetry and biological dosimetry (skin effects in rodents)). The technical studies of the FS40 sources demonstrated that the flux intensity of the lamps could be changed electronically, without affecting the spectrum. Thus it was possible to standardize UVB exposure electronically. The biologically effective doses of these sources were analysed in RIV-Tox Wistar rats and BALB/c mice. After low doses of UVB radiation, histopathological changes such as acanthosis, hyperkeratosis and dermal inflammation were observed in the skin without the presence of major side effects such as erythema and oedema. After higher doses of UVB radiation erythema and oedema were clearly visible. Quantitative studies showed that the minimal erythema dose, as a biological parameter, correlated well to the emission in J/m2. In addition, biological parameters such as acanthosis and inflammation in the skin correlated well to the actual exposure in J/m2 and were sensitive biomarkers for UVB-induced skin toxicity. Thus, in addition to minimal erythemal doses, acanthosis and inflammation may also be applied as biologically relevant doses for studies of the biological effects of UVB radiation.     

An experimental basis for carcinogenic effects of ultraviolet radiation

Several hypotheses to explain ultraviolet carcinogenesis have been advanced. One such theory contended that cholesterol was directly involved in actinic carcinogenesis. Although the hypothesis, in its original form, was generally abandoned by the scientific community, it has been revived and modified from time to time as structures and functions of steroid hormones become more clearly understood. In essence it suggests that carcinogenic substances, structurally related to steroid hormones might result from photochemical conversion of cholesterol. Although some compounds with such properties have been isolated under controlled chemical conditions, until recently the failure to find such compounds in biological systems had cast serious doubt upon the validity of this hypothesis. It has now been demonstrated that cholesterol-derived oxidation products are formed in human skin upon exposure to ultraviolet light. One of the products formed is known to possess carcinogenic properties when administered to experimental animals. Furthermore, it has been reported by other investigators that the control mechanism for cholesterol synthesis is absent in liver tumors. Whether this biochemical lesion plays a causal role in the etiology of this disease is unknown but altered cholesterol metabolism has also been implicated in actinically induced skin cancer. It has been demonstrated in this laboratory that skin sterol synthesis is inhibited by light. The principal site of action of light on sterol synthesis appears to be prior to the formation of acetyl coenzyme A in the biosynthetic pathway. Sterol-derived photoproducts produce similar effects as light upon sterol synthesis. These observations suggest more than just a coincidental role of light upon sterols and sterol metabolism in the etiology of skin cancer. Lunar Science Institute Contribution.     

Cytological damage to the red alga Griffithsia pacifica from ultraviolet radiation

Continuous exposure for 7–10 days to 60% of ambient levels (sea level at mid-day in December) of UV-A and UV-B radiation caused cytological damage to regenerating fragments of Griffithsia pacifica under laboratory conditions. There was high mortality of individual cells and entire fragments in UV treated filaments. Rhizoid initiation was slower and rhizoids grew more slowly following UV treatment. After 7 days, UV radiated thalli showed chloroplast and nuclear degeneration. In addition, filaments tended to disarticulate so that single or groups of apparently healthy cells were common in the medium. These data suggest that the subtidal habitat of G. pacifica is based in part on lack of tolerance to UV radiation, and that UV protection mechanisms are not inducible or insufficient to prevent the accumulation of damage in this species.     

Measurements of microbial protection from ultraviolet radiation in polar terrestrial microhabitats

Biological dosimeters made from a monolayer of Bacillus subtilis spores were used to investigate the penetration of ultraviolet radiation into some widespread terrestrial microbial microhabitats at polar latitudes: at Mars Oasis (72°S) and Rothera Station (67°S) (UK) in the Antarctic (November 2000) and on Devon Island, Canadian High Arctic (75°N) (July 2000 and 2001). Layers of soil or dust of 𔘬 µm thickness, particularly in ice-free regions of the Arctic, could reduce UV exposure such that no inactivation of spores could be measured after 3 days. Control spores were killed in 24 h. Spores in artificial cryptoendolithic habitats with ~1 mm rock covering obtained a reduction of UV radiation-induced inactivation of at least 2 orders of magnitude. Hypolithic spores were protected against any inactivation for at least 4 days. Snow covers of between 5 and 15 cm thickness, depending on age and heterogeneity, attenuated UV radiation by an order of magnitude, although snow cover is seasonal and subject to climatic factors. These dosimetric data demonstrate that, except for microbes on the surface of soil grains, many terrestrial microbial communities are well protected from incident UV radiation by a variety of physical and biological coverings. This is in contrast to data reported for many polar aquatic microbial taxa, and might imply a greater robustness of terrestrial microbial communities against the effects of ozone depletion.     

Untargeted effects of ionizing radiation: implications for radiation pathology

The dogma that genetic alterations are restricted to directly irradiated cells has been challenged by observations in which effects of ionizing radiation, characteristically associated with the consequences of energy deposition in the cell nucleus, arise in non-irradiated cells. These, so called, untargeted effects are demonstrated in cells that have received damaging signals produced by irradiated cells (radiation-induced bystander effects) or that are the descendants of irradiated cells (radiation-induced genomic instability). Radiation-induced genomic instability is characterized by a number of delayed adverse responses including chromosomal abnormalities, gene mutations and cell death. Similar effects, as well as responses that may be regarded as protective, have been attributed to bystander mechanisms. Whilst the majority of studies to date have used in vitro systems, some adverse non-targeted effects have been demonstrated in vivo. However, at least for haemopoietic tissues, radiation-induced genomic instability in vivo may not necessarily be a reflection of genomically unstable cells. Rather the damage may reflect responses to ongoing production of damaging signals; i.e. bystander responses, but not in the sense used to describe the rapidly induced effects resulting from direct interaction of irradiated and non-irradiated cells. The findings are consistent with a delayed and long-lived tissue reaction to radiation injury characteristic of an inflammatory response with the potential for persisting bystander-mediated damage. An important implication of the findings is that contrary to conventional radiobiological dogma and interpretation of epidemiologically-based risk estimates, ionizing radiation may contribute to malignancy and particularly childhood leukaemia by promoting initiated cells rather than being the initiating agent. Untargeted mechanisms may also contribute to other pathological consequences.     

Surface solar ultraviolet radiation for paleoatmospheric levels of oxygen and ozone

Many investigators have concluded that the level of solar ultraviolet radiation (200–300 nm) reaching the surface was a key parameter in the origin and evolution of life on Earth. The level of solar ultraviolet radiation between 200 and 300 nm is controlled primarily by molecular absorption by ozone, whose presence is trongly coupled to the level of molecular oxygen. In this paper, we present a series of calculations of the solar ultraviolet radiation reaching the surface for oxygen levels ranging from 10^–4 present atmospheric level to the present level. The solar spectrum between 200 and 300 mn has been divided into 34 spectral intervals. For each spectral interval, we have calculated the solar ultraviolet radiation reaching the Earth's surface by considering the attenuation of the incoming beam due to ozone and oxygen absorption. A one-dimensional photochemical model of the atmosphere was used for these calculations.