Evidence That Loss-of-Function Filaggrin Gene Mutations Evolved in Northern Europeans to Favor Intracutaneous Vitamin D3 Production

Skin pigmentation lightened progressively to a variable extent, as modern humans emigrated out of Africa, but extreme lightening occurred only in northern Europeans. Yet, loss of pigmentation alone cannot suffice to sustain cutaneous vitamin D3 (VD3) formation at the high latitudes of northern Europe. We hypothesized that loss-of-function mutations in the epidermal structural protein, filaggrin (FLG), could have evolved to sustain adequate VD3 status. Loss of FLG results in reduced generation of trans-urocanic acid, the principal endogenous ultraviolet-B (UV-B) filter in lightly-pigmented individuals. Accordingly, we identified a higher prevalence of FLG mutations in northern European populations when compared to more southern European, Asian and African populations that correlates significantly with differences in circulating 25-OH-VD3 levels in these same populations. By allowing additional UV-B penetration and intracutaneous VD3 formation, the latitude-dependent gradient in FLG mutations, likely together with other concurrent mutations in VD3 metabolic pathways, provide a non-pigment-based mechanism that sustains higher levels of circulating VD3 in northern Europeans. At the time that FLG mutations evolved, xerosis due to FLG deficiency was a lesser price to pay for enhanced VD3 production. Yet, the increase in FLG mutations has inadvertently contributed to an epidemic of atopic diseases that has emerged in recent decades.     

Non-invasive measurements of leaf epidermal transmittance of UV radiation using chlorophyll fluorescence: field and laboratory studies

Ratios of chlorophyll fluorescence induced by ultraviolet (UV) and bluegreen (BG) radiation [F(UV)/F(BG)] were determined with a Xe‐PAM fluorometer to test the utility of this technique as a means of non‐intrusively assessing changes in the pigmentation and optical properties of leaves exposed to varying UV exposures under laboratory and field conditions. For plants of Vicia faba and Brassica campestris, grown under controlled‐environmental conditions, F(UV‐B)/F(BG) was negatively correlated with whole‐leaf UV‐B‐absorbing pigment concentrations. Fluorescence ratios of V. faba were similar to, and positively correlated with (r²=0.77 [UV‐B]; 0.85 [UV‐A]), direct measurements of epidermal transmittance made with an integrating sphere. Leaves of 2 of 4 cultivars of field‐grown Glycine max exposed to near‐ambient solar UV‐B at a mid‐latitude site (Buenos Aires, Argentina, 34° S) showed significantly lower abaxial F(UV‐B)/F(BG) values (i.e., lower UV‐B epidermal transmittance) than those exposed to attenuated UV‐B, but solar UV‐B reduction had a minimal effect on F(UV‐B)/F(BG) in plants growing at a high‐latitude site (Tierra del Fuego, Argentina, 55° S). Similarly, the exotic Taraxacum officinale did not show significant changes in F(UV‐B)/F(BG) when exposed to very high supplemental UV‐B (biologically effective UV‐B=14–15 kJ m^?2 day^?1) in the field in Tierra del Fuego, whereas a native species, Gunnera magellanica, showed significant increases in F(UV‐B)/F(BG) relative to those receiving ambient UV‐B. These anomalous fluorescence changes were associated with increases in BG‐absorbing pigments (anthocyanins), but not UV‐B‐absorbing pigments. These results indicate that non‐invasive estimates of epidermal transmittance of UV radiation using chlorophyll fluorescence can detect changes in pigmentation and leaf optical properties induced by UV‐B radiation under both field and laboratory conditions. However, this technique may be of limited utility in cold environments where UV and low temperatures can stimulate the production of BG‐absorbing pigments that interfere with these indirect measurements of UV‐transmittance.     

Restoration of cutaneous pigmentation by transplantation to mice of isogeneic human melanocytes in dermal–epidermal engineered skin substitutes

Autologous engineered skin substitutes (ESS) containing melanocytes (hM) may restore pigmentation and photoprotection after grafting to full‐thickness skin wounds. In this study, normal hM were isolated from discard skin, propagated with or without tyrosinase inhibitors, cryopreserved, recovered into culture, and added to ESS (ESS‐P) before transplantation. ESS‐P were incubated in either UCMC160/161 or UCDM1 medium, scored for hM densities, and grafted to mice. The results showed that sufficient hM can be propagated to expand donor tissue by 100‐fold; incubation of hM in tyrosinase inhibitors reduced pigment levels but did not change hM recovery after cryopreservation; hM densities in ESS‐P were greater after incubation in UCDM1 than UCMC160 medium; hM were localized to the dermal–epidermal junction of ESS‐P; and UCDM1 medium promoted earlier pigment distribution and density. These results indicate that hM can be incorporated into ESS‐P efficiently to restore cutaneous pigmentation and UV photoprotection after full‐thickness skin loss conditions.     

The roles of the shikimate pathway genes,aroA and aroB,in virulence,growth and UV tolerance of Burkholderia glumae strain 411gr‐6

Burkholderia glumae is the major causal agent of bacterial panicle blight of rice, which is a growing disease problem for rice growers worldwide. In our previous study, some B. glumae strains showed pigmentation phenotypes producing at least two (yellow–green and purple) pigment compounds in casein–peptone–glucose agar medium. The B. glumae strains LSUPB114 and LSUPB116 are pigment‐deficient mutant derivatives of the virulent and pigment‐proficient strain 411gr‐6, having mini‐Tn5gus insertions in aroA encoding 3‐phosphoshikimate 1‐carboxyvinyltransferase and aroB encoding 3‐dehydroquinate synthase, respectively. Both enzymes are known to be involved in the shikimate pathway, which leads to the synthesis of aromatic amino acids. Here, we demonstrate that aroA and aroB are required for normal virulence in rice and onion, growth in M9 minimal medium and tolerance to UV light, but are dispensable for the production of the phytotoxin toxoflavin. These results suggest that the shikimate pathway is involved in bacterial pathogenesis by B. glumae without a significant role in the production of toxoflavin, a major virulence factor of this pathogen.     

The effects of UV-B radiation on epidermal anatomy in loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.)

The effects of enhanced UV‐B radiation on the needle anatomy of loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.) were studied in the field under supplemental UV‐B radiation supplied by a modulated irradiation system. The supplemental UV‐B levels were designed to simulate either a 16 or 25% loss of stratospheric ozone over College Park, Maryland. Enhanced UV‐B radiation caused different responses in these two species. The needles of loblolly pine had larger amounts of tannin in the lumen of epidermal cells and more wall‐bound phenolics in the outer epidermal walls of UV‐B‐treated needles, whereas the most pronounced effect on Scots pine needles was increased cutinization. In both species, the outer epidermal cell walls thickened and the needle cross‐sectional and mesophyll areas decreased (statistically significantly only in Scots pine). This suggests that more carbon may have been allocated to the protection mechanisms at the expense of photosynthetic area. The difference in response between these species suggests that the response to UV‐B radiation is not mediated by a single mechanism and that no generalization with regard to the effects of UV‐B on conifers can be made.     

Multivariate analysis of intraspecific responses to UV-B radiation in white clover (Trifolium repens L.)

White clover (Trifolium repens L.) is experiencing increased levels of ultraviolet‐B (UV‐B) radiation in temperate pastures due to the depletion of the stratospheric ozone layer. Based on 17 morphological, morphogenetic and physiological attributes, this study analysed the consequences of enhanced UV‐B on 26 white clover populations using principal components analysis (PCA). After 18 d of exposure to 13·3 kJ m ^{? 2} d ^{? 1} UV‐B in controlled environments, UV‐B significantly decreased above‐ground and below‐ground plant growth attributes, epidermal cell surface area and maximum quantum efficiency of photosystem II photochemistry (F_v/F_m). Aspects of cell division and cell expansion both were negatively affected by UV‐B. Stomatal density, specific leaf mass, root‐to‐shoot ratio and levels of UV‐B‐absorbing compounds increased in response to UV‐B. In the multivariate analysis, the main dimension of UV‐B sensitivity was characterized by changes in plant growth attributes. Alterations in partitioning within and between plant organs constituted a secondary tier of UV‐B responsiveness. Plant characteristics related to UV‐B tolerance included lower growth rate, smaller epidermal cell surface area and higher UV‐B‐induced levels of UV‐B‐absorbing compounds. The results suggest overall UV‐B tolerance for slower‐growing populations from less productive habitats with higher natural UV‐B irradiance.     

THALLUS DIFFERENTIATION OF PHOTOSYNTHESIS,GROWTH, REPRODUCTION,AND UV‐B SENSITIVITY IN THE GREEN ALGA ULVA PERTUSA (CHLOROPHYCEAE)1

The chlorophyte Ulva is perceived as a simple and uniform algal form, with little functional differentiation within a thallus. We compared morphology, pigmentation, photosynthesis, growth, reproduction, and UV‐B sensitivity between different thallus regions of Ulva pertusa Kjellman. Thallus thickness and cell size were significantly greater, whereas cell number was less in the basal region than in other regions. Photosynthetic pigment contents were lowest in the basal region and increased toward the marginal region. Photosynthetic capacity and photosynthetic efficiency normalized to fresh weight, area, volume, and cell number showed a progressive increase from the basal to marginal parts; however, on a chl basis those values were equal regardless of thallus part. Values of light saturation point were not statistically different between regions. Growth rates increased from marginal to basal and to middle parts of the thallus, whereas sporulation was highest in marginal (100%) followed by middle (30%) and basal parts (0%). Daily observation over 9 days showed that 56% of the basal cells divided once and did not produce spores, whereas every marginal cell went through its first division and 89% of the primary daughter cells also divided, resulting in 100% sporulation. A 7‐day treatment with PAR and PAR + UV‐A caused a significant decrease in the effective quantum yield of all thallus regions, followed by a recovery toward the initial values, whereas PAR + UV‐A + UV‐B irradiation led to greater photoinhibition and less recovery. Marked differences in the UV‐B sensitivity were observed, with marginal parts being more sensitive and basal parts most resistant.     

Stem cell factor rescues tyrosinase expression and pigmentation in discreet anatomic locations in albino mice

The K14‐SCF transgenic murine model of variant pigmentation is based on epidermal expression of stem cell factor (SCF) on the C57BL/6J background. In this system, constitutive expression of SCF by epidermal keratinocytes results in retention of melanocytes in the interfollicular basal layer and pigmentation of the epidermis itself. Here, we extend this animal model by developing a compound mutant transgenic amelanotic animal defective at both the melanocortin 1 receptor (Mc1r) and tyrosinase (Tyr) loci. In the presence of K14‐Scf, tyrosinase‐mutant animals (previously thought incapable of synthesizing melanin) exhibited progressive robust epidermal pigmentation with age in the ears and tails. Furthermore, K14‐SCF Tyr^c2j/c2j animals demonstrated tyrosinase expression and enzymatic activity, suggesting that the c2j Tyr defect can be rescued in part by SCF in the ears and tail. Lastly, UV sensitivity of K14‐Scf congenic animals depended mainly on the amount of eumelanin present in the skin. These findings suggest that c‐kit signaling can overcome the c2j Tyr mutation in the ears and tails of aging animals and that UV resistance depends on accumulation of epidermal eumelanin.     

Effects of ultraviolet-B radiation on leaf elongation, production and phenylpropanoid concentrations of Deschampsia antarctica and Colobanthus quitensis in Antarctica

Stratospheric ozone depletion by anthropogenic chlorofluorocarbons has lead to increases in ultraviolet‐B radiation (UV‐B; 280–320 nm) along the Antarctic Peninsula during the austral spring. We manipulated UV‐B levels around plants of Antarctic hair grass (Deschampsia antarctica; Poaceae) and Antarctic pearlwort (Colobanthus quitensis; Caryophyllaceae) for one field season near Palmer Station along the west coast of the Antarctic Peninsula. Treatments involved placing frames over naturally growing plants that either (1) held filters that absorbed most biologically effective radiation (UV‐B_BE; ‘reduced UV‐B’, 22% of ambient UV‐B_BE levels), (2) held filters that transmitted most UV‐B_BE (‘near‐ambient UV‐B’, 87% of ambient UV‐B_BE levels), or (3) lacked filters (‘ambient UV‐B’). Leaves on D. antarctica exposed to near‐ambient and ambient UV‐B were 16–17% shorter than those exposed to reduced UV‐B, and this was associated with shorter epidermal cells at the leaf base and tip. Leaves on C. quitensis exposed to near‐ambient and ambient UV‐B tended to be shorter (P=0.18) and epidermal cells at the leaf base tended to be smaller than those under reduced UV‐B (P<0.10). In order to further explain reductions in leaf length, we examined leaf concentrations of insoluble (cell‐wall bound) phenylpropanoids, since it has been proposed that wall‐bound phenylpropanoids such as ferulic acid may constrain cell expansion and leaf elongation. In both species, HPLC analysis revealed that ferulic and p‐coumaric acid were major components of both insoluble and soluble phenylpropanoids. Although there were no significant differences in concentrations between UV‐B treatments, concentrations of insoluble ferulic acid in D. antarctica tended to be higher under ambient and near‐ambient UV‐B than under reduced UV‐B (P=0.17). We also examined bulk‐leaf concentrations of soluble (methanol extractable) UV‐B‐absorbing compounds and found that concentrations were higher in plants exposed to near‐ambient and ambient UV‐B than in plants exposed to reduced UV‐B. We also assessed the UV‐B‐screening effectiveness of leaves that had developed on plants at the field site with a fiber‐optic microprobe. Leaf epidermal transmittance of 300‐nm UV‐B was 4.0 and 0.6% for D. antarctica and C. quitensis, respectively, which is low compared to grasses and herbaceous dicotyledonous plants found in more temperate climates. While the leaves of Antarctic vascular plants are relatively effective at screening UV‐B, levels of UV‐B in Antarctica are sufficient to reduce leaf epidermal cell size and leaf elongation in these species, although the mechanisms for these reductions remain unclear.     

The cell biology of human hair follicle pigmentation

Although we have made significant progress in understanding the regulation of the UVR‐exposed epidermal‐melanin unit, we know relatively little about how human hair follicle pigmentation is regulated. Progress has been hampered by gaps in our knowledge of the hair growth cycle’s controls, to which hair pigmentation appears tightly coupled. However, pigment cell researchers may have overly focused on the follicular melanocytes of the nocturnal and UVR‐shy mouse as a proxy for human epidermal melanocytes. Here, I emphasize the epidermis‐follicular melanocyte pluralism of human skin, as research models for vitiligo, alopecia areata and melanoma, personal care/cosmetics innovation. Further motivation could be in finding answers to why hair follicle and epidermal pigmentary units remain broadly distinct? Why melanomas tend to originate from epidermal rather than follicular melanocytes? Why multiple follicular melanocyte sub‐populations exist? Why follicular melanocytes are more sensitive to aging influences? In this perspective, I attempt to raise the status of the human hair follicle melanocyte and highlight some species‐specific issues involved which the general reader of the pigmentation literature (with its substantial mouse‐based data) may not fully appreciate.     

Distinct physiological and metabolic reprogramming by highbush blueberry (Vaccinium corymbosum) cultivars revealed during long‐term UV‐B radiation

Despite the Montreal protocol and the eventual recovery of the ozone layer over Antarctica, there are still concerns about increased levels of ultraviolet‐B (UV‐B) radiation in the Southern Hemisphere. UV‐B induces physiological, biochemical and morphological stress responses in plants, which are species‐specific and different even for closely related cultivars. In woody plant species, understanding of long‐term mechanisms to cope with UV‐B‐induced stress is limited. Therefore, a greenhouse UV‐B daily course simulation was performed for 21 days with two blueberry cultivars (Legacy and Bluegold) under UV‐B_BE irradiance doses of 0, 0.07 and 0.19 W m^?2. Morphological changes, photosynthetic performance, antioxidants, lipid peroxidation and metabolic features were evaluated. We found that both cultivars behaved differently under UV‐B exposure, with Legacy being a UV‐B‐resistant cultivar. Interestingly, Legacy used a combined strategy: initially, in the first week of exposure its photoprotective compounds increased, coping with the intake of UV‐B radiation (avoidance strategy), and then, increasing its antioxidant capacity. These strategies proved to be UV‐B radiation dose dependent. The avoidance strategy is triggered early under high UV‐B radiation in Legacy. Moreover, the rapid metabolic reprogramming capacity of this cultivar, in contrast to Bluegold, seems to be the most relevant contribution to its UV‐B stress‐coping strategy.     

UV‐B INDUCTION OF UV‐B PROTECTION IN ULVA PERTUSA (CHLOROPHYTA)1

The green macroalga Ulva pertusa Kjellman produced UV‐B absorbing compounds with a prominent absorption maximum at 294 nm in response only to UV‐B, and the amounts induced were proportional to the UV‐B doses. Under a 12:12‐h light:dark regime, the production of UV‐absorbing compounds occurred only during the exposure periods with little turnover in the dark. There was significant reduction in growth in parallel with the production of UV‐B absorbing compounds. The polychromatic action spectrum for the induction of UV‐B absorbing compounds in U. pertusa exhibits a major peak at 292 nm with a smaller peak at 311.5 nm. No significant induction was detected above 354.5 nm, and radiation below 285 nm caused significant reduction in the levels of UV‐B absorbing compounds. After UV‐B irradiation at 1.0 W·m^?2 for 9 h, the optimal photosynthetic quantum yield of the samples with UV‐B absorbing compounds slightly increased relative to the initial value, whereas that of thalli lacking the compounds declined to 30%–34% of the initial followed by subsequent recovery in dim light of up to 84%–85% of the initial value. There was a positive and significant relationship between the amount of UV‐B absorbing compounds with antioxidant activity as determined by the α,α‐diphenyl‐β‐picrylhydrazyl scavenging assay. In addition to mat‐forming characteristics and light‐driven photorepair, the existence and antioxidant capacity of UV‐B absorbing compounds may confer U. pertusa a greater selective advantage over other macroalgae, thereby enabling them to thrive in the presence of intense UV‐B radiation.     

The inhibitory effects of UV-B radiation (280–315 nm) on Gunnera magellanica growth correlate with increased DNA damage but not with oxidative damage to lipids

Damage to DNA and disruption of membrane integrity by lipid peroxidation processes are two of the proposed causes of UV‐B‐induced growth inhibition in plants. However, the relative significance of these different types of molecular damage has not been established in experiments carried out under realistic physiological conditions. Plants of Gunnera magellanica (a native herb from southern Patagonia) were exposed to a gradient of biologically effective UV‐B doses (from 0 to 6.5 kJ m^?2 d^?1 of UV‐B_be) in a greenhouse study. Leaf expansion was measured and sensitive techniques were used to detect damage to DNA (in the form of cyclobutane pyrimidine dimers; CPDs) and lipid peroxidation (via electronic‐paramagnetic resonance; EPR). Leaf expansion decreased and the CPD density increased with increasing UV‐B doses, but the degree of lipid peroxidation remained unaffected. The highest UV‐B dose induced a transient oxidative stress situation (as evaluated using the ratio of ascorbyl radical to ascorbate, A^·/AH^–), which was rapidly controlled by an increase in the ascorbate pool. The present results suggest that under a range of UV‐B_be doses that overlaps the range of doses that G. magellanica plants experience in their natural environment, growth inhibition is better explained by DNA damage than by increased lipid peroxidation.     

UV‐A/BLUE LIGHT–INDUCED REACTIVATION OF SPORE GERMINATION IN UV‐B IRRADIATED ULVA PERTUSA (CHLOROPHYTA)1

Recent reduction in the ozone shield due to manufactured chlorofluorocarbons raised considerable interest in the ecological and physiological consequences of UV‐B radiation (λ=280–315 nm) in macroalgae. However, early life stages of macroalgae have received little attention in regard to their UV‐B sensitivity and UV‐B defensive mechanisms. Germination of UV‐B irradiated spores of the intertidal green alga Ulva pertusa Kjellman was significantly lower than in unexposed controls, and the degree of reduction correlated with the UV doses. After exposure to moderate levels of UV‐B irradiation, subsequent exposure to visible light caused differential germination in an irradiance‐ and wavelength‐dependent manner. Significantly higher germination was found at higher photon irradiances and in blue light compared with white and red light. The action spectrum for photoreactivation of germination in UV‐B irradiated U. pertusa spores shows a major peak at 435 nm with a smaller but significant peak at 385 nm. When exposed to December sunlight, the germination percentage of U. pertusa spores exposed to 1 h of solar radiation reached 100% regardless of the irradiation treatment conditions. After a 2‐h exposure to sunlight, however, there was complete inhibition of germination in PAR+UV‐A+UV‐B in contrast to 100% germination in PAR or PAR+UV‐A. In addition to mat‐forming characteristics that would act as a selective UV‐B filter for settled spores under the parental canopy, light‐driven repair of germination after UV‐B exposure could explain successful continuation of U. pertusa spore germination in intertidal settings possibly affected by intense solar UV‐B radiation.     

Two light‐activated neuroendocrine circuits arising in the eye trigger physiological and morphological pigmentation

Two biological processes regulate light‐induced skin colour change. A fast ‘physiological pigmentation change’ (i.e. circadian variations or camouflage) involves alterations in the distribution of pigment containing granules in the cytoplasm of chromatophores, while a slower ‘morphological pigmentation change’ (i.e. seasonal variations) entails changes in the number of pigment cells or pigment type. Although linked processes, the neuroendocrine coordination triggering each response remains largely obscure. By evaluating both events in Xenopus laevis embryos, we show that morphological pigmentation initiates by inhibiting the activity of the classical retinal ganglion cells. Morphological pigmentation is always accompanied by physiological pigmentation, and a melatonin receptor antagonist prevents both responses. Physiological pigmentation also initiates in the eye, but with repression of melanopsin‐expressing retinal ganglion cell activity that leads to secretion of alpha‐melanocyte‐stimulating hormone (α‐MSH). Our findings suggest a model in which eye photoperception links physiological and morphological pigmentation by altering α‐MSH and melatonin production, respectively.     

Hairless pigmented guinea pigs: a new model for the study of mammalian pigmentation

A stock of hairless pigmented guinea pigs was developed to facilitate studies of mammalian pigmentation. This stock combines the convenience of a hairless animal with a pigmentary system that is similar to human skin. In both human and guinea pig skin, active melanocytes are located in the basal layer of the interfollicular epidermis. Hairless albino guinea pigs on an outbred Hartley background (CrI:IAF/HA(hr/hr)BR; designated hr/hr) were mated with red-haired guinea pigs (designated Hr/Hr). Red-haired heterozygotes from the F1 generation (Hr/hr) were then mated with each other or with hairless albino guinea pigs. The F2 generation included hairless pigmented guinea pigs that retained their interfollicular epidermal melanocytes and whose skin was red-brown in color. Following UV irradiation, there was an increase in cutaneous pigmentation as well as an increase in the number of active epidermal melanocytes. An additional strain of black hairless guinea pigs was developed using black Hr/Hr animals and a similar breeding scheme. These two strains should serve as useful models for studies of the mammalian pigment system.     

Contribution of hydroxycinnamates and flavonoids to epidermal shielding of UV‐A and UV‐B radiation in developing rye primary leaves as assessed by ultraviolet‐induced chlorophyll fluorescence measurements

Epidermally located ultraviolet (UV)‐absorbing phenolic compounds, flavonoids and hydroxycinnamic acid esters (HCAs), can shield the underlying tissues in plants against harmful UV‐radiation. The relative importance of the two different classes of phenolic compounds for UV‐screening was a matter of recent debate. Using a non‐invasive method based on chlorophyll fluorescence measurements to estimate epidermal UV transmittance, the relationship between epidermal UV shielding and the content of the two different groups of secondary phenolic compounds in the epidermal layers and the underlying photosynthetic mesophyll of developing rye primary leaves grown under supplementary UV‐B radiation was investigated. From the fourth to the tenth day after sowing, epidermally located flavonoids increased in an age‐ and irradiation‐dependent manner, whereas mesophyll flavonoids and epidermal HCAs, mainly ferulic acid and p‐coumaric acid esters, were constitutively present and did not vary in their contents over the observed time period. There was an excellent correlation between epidermal UV‐A and UV‐B absorbances as assessed by chlorophyll fluorescence measurements and contents of epidermal flavonoids. However, HCAs showed an additional contribution to UV‐B shielding. In contrast, mesophyll flavonoids did not seem to play a respective role. When absorbances of the abaxial and adaxial epidermal layers were compared, it became apparent that in fully expanded primary leaves epidermal tissues from both sides were equally effective in absorption of UV‐radiation. However, the earlier and more UV‐exposed abaxial epidermis of young unrolling leaves showed a significantly higher absorption. It is shown that in early stages of development the epidermal HCAs are the dominant UV‐B protective compounds of the primary leaf. This function is increasingly replaced by the epidermal flavonoids during leaf development and acclimation. The application of chlorophyll fluorescence measurements has been proven to be a useful tool for estimating relative contents of these compounds in epidermal tissue.     

Response of two Antarctic bryophytes to stratospheric ozone depletion

We report a study which measured changes to the radiative environment arising from stratospheric O₃ depletion at Rothera Point on the western Antarctic Peninsula (67°S, 68°W) and subsequent associations between these changes and the pigmentation and maximum quantum yield of photochemistry (F_v/F_m) of two Antarctic bryophytes, the liverwort Cephaloziella varians and the moss Sanionia uncinata. We found a strong relationship between O₃ column depth and the ratio of UV‐B to PAR irradiance (F_UV‐B/F_PAR) recorded at ground level. Weaker, but significant, associations were also found between O₃ column depth and noon irradiances and daily doses of unweighted and biologically effective UV‐B radiation received at ground level. Regression analyses indicated that F_UV‐B/F_PAR and daily dose of unweighted UV‐B were best predictors for concentrations of total carotenoids and UV‐B screening pigments extracted from bryophyte tissues. Concentrations of these pigments were loosely but significantly positively associated with O₃‐dependent irradiance parameters. HPLC analyses of carotenoids also suggested that both species increased the synthesis of neoxanthin during periods of O₃ depletion. Violaxanthin, lutein, zeaxanthin and b,bββ‐carotene concentrations were also apparently influenced by O₃ reduction, but not consistently across both bryophyte species. Concentrations of chlorophylls a and b were apparently unaffected by O₃ depletion. No direct associations between F_v/F_m and O₃‐dependent irradiance parameters were found. However stepwise multiple regression analyses suggested that the production of UV‐B screening pigments conferred protection from elevated F_UV‐B/F_PAR on F_v/F_m in both species and that carotenoids conferred protection on F_v/F_m in Sanionia. Our data suggest that changes to the radiative environment associated with stratospheric O₃ depletion influence the pigmentation of two Antarctic bryophytes, but that F_v/F_m is unaffected, at least in part because of rapid synthesis of protective pigments.     

Factors that influence the cutaneous synthesis and dietary sources of vitamin D

The major sources of vitamin D for most humans are casual exposure of the skin to solar ultraviolet B (UVB; 290-315 nm) radiation and from dietary intake. The cutaneous synthesis of vitamin D is a function of skin pigmentation and of the solar zenith angle which depends on latitude, season, and time of day. In order to mimic the natural environment of skin to sunlight exposure, we therefore measured serum 25-hydroxyvitamin D levels in volunteers with different skin types following repeated UV irradiation. Because melanin pigment in human skin competes for and absorbs the UVB photons responsible for the photolysis of 7-dehydrocholesterol to previtamin D3, we also studied the effect of skin pigmentation on previtamin D3 production in a human skin model by exposing type II and type V skin samples to noon sunlight in June when the solar zenith angle is most acute. Vitamin D is rare in food. Among the vitamin D-rich food, oily fish are considered to be one of the best sources. Therefore, we analyzed the vitamin D content in several commonly consumed oily and non-oily fish. The data showed that farmed salmon had a mean content of vitamin D that was approximately 25% of the mean content found in wild caught salmon from Alaska, and that vitamin D2 was found in farmed salmon, but not in wild caught salmon. The results provide useful global guidelines for obtaining sufficient vitamin D3 by cutaneous synthesis and from dietary intake to prevent vitamin D deficiency and its health consequences, ensuing illness, especially, bone fractures in the elderly.