UVB-induced DNA and photosystem II damage in two intertidal green macroalgae: distinct survival strategies in UV-screening and non-screening Chlorophyta

Acclimation of intertidal macroalgae Ulva prolifera to UVB radiation: the important role of alternative oxidase

BACKGROUND

Solar radiation is primarily composed of ultraviolet radiation (UVR, 200 - 400 nm) and photosynthetically active radiation (PAR, 400 - 700 nm). Ultraviolet-B (UVB) radiation accounts for only a small proportion of sunlight, and it is the primary cause of plant photodamage. The use of chlorofluorocarbons (CFCs) as refrigerants caused serious ozone depletion in the 1980s, and this had led to an increase in UVB. Although CFC emissions have significantly decreased in recent years, UVB radiation still remains at a high intensity. UVB radiation increase is an important factor that influences plant physiological processes. Ulva prolifera, a type of macroalga found in the intertidal zone, is intermittently exposed to UVB. Alternative oxidase (AOX) plays an important role in plants under stresses. This research examines the changes in AOX activity and the relationships among AOX, photosynthesis, and reactive oxygen species (ROS) homeostasis in U. prolifera under changes in UVB and photosynthetically active radiation (PAR).

RESULTS

UVB was the main component of solar radiation impacting the typical intertidal green macroalgae U. prolifera. AOX was found to be important during the process of photosynthesis optimization of U. prolifera due to a synergistic effect with non-photochemical quenching (NPQ) under UVB radiation. AOX and glycolate oxidase (GO) worked together to achieve NADPH homeostasis to achieve photosynthesis optimization under changes in PAR + UVB. The synergism of AOX with superoxide dismutase (SOD) and catalase (CAT) was important during the process of ROS homeostasis under PAR + UVB.

CONCLUSIONS

AOX plays an important role in the process of photosynthesis optimization and ROS homeostasis in U. prolifera under UVB radiation. This study provides further insights into the response of intertidal macroalgae to solar light changes.

WHIRLY1-deficient chloroplasts display enhanced formation of cyclobutane pyrimidine dimers during exposure to UV-B radiation

The single-stranded DNA/RNA binding protein WHIRLY1 is a major chloroplast nucleoid-associated protein required for the compactness of nucleoids. Most nucleoids in chloroplasts of WHIRLY1-knockdown barley plants are less compact compared to nucleoids in wild-type plants. The reduced compaction leads to an enhanced optical cross-section, which may cause the plastid DNA to be a better target for damaging UV-B radiation. To investigate this hypothesis, primary foliage leaves, chloroplasts, and nuclei from wild-type and WHIRLY1-knockdown plants were exposed to experimental UV-B radiation. Thereafter, total, genomic and plastid DNA were isolated, respectively, and analyzed for the occurrence of cyclobutane pyrimidine dimers (CPDs), which is a parameter for genome stability. The results of this study revealed that WHIRLY1-deficient chloroplasts had strongly enhanced DNA damages, whereas isolated nuclei from the same plant line were not more sensitive than nuclei from the wild-type, indicating that WHIRLY1 has different functions in chloroplasts and nucleus. This supports the hypothesis that the compaction of nucleoids may provide protection against UV-B radiation.

Effects of UV-B radiation on epiphytic bacterial communities on male and female Sargassum thunbergii

The effects of increased UV-B radiation on macroalgae have been widely studied, but knowledge concerning the response of communities of algal epiphytic bacteria to increased UV-B radiation and differences between male and female algae is still lacking. Via 16S rDNA high-throughput sequencing technology, changes in the epiphytic bacterial communities on male and female S. thunbergii under increased UV-B radiation were studied in the lab. Under different UV-B radiation intensities, although the α diversity and community composition of epiphytic bacteria changed little, the β diversity indicated that the community structure of bacteria on S. thunbergii was obviously clustered, and the relative abundance of dominant bacteria and indicator species changed considerably. There were unique bacteria in each experimental group, and the bacteria whose abundance obviously changed were members of groups related to environmental resistance or adaptability. The variation in the abundance of epiphytic bacteria was different in male and female S. thunbergii, and the bacteria whose abundance greatly changed were mainly related to algal growth and metabolism. The abundance of genes with predicted functions related to metabolism, genetic information processing, environmental adaptation and infectious diseases changed with increased UV-B radiation, and those variations differed between epiphytic bacteria on male and female S. thunbergii. This study found that the algal epiphytic bacteria were influenced by the increase in UV-B radiation and underwent certain adaptations through adjustments to community structure and function, and this response was also affected by the sex of the macroalgae. These results are expected to serve as experimental basis and provide reference for further understanding of the response of algae epiphytic bacteria to enhanced UV-B radiation caused by the thinning of the ozone layer and the resulting changes in the relationship between algae and bacteria, which may change the community of the marine ecosystem and affect important marine ecological process.

Ultraviolet screening by slug tissue and tight packing of plastids protect photosynthetic sea slugs from photoinhibition

One of the main mysteries regarding photosynthetic sea slugs is how the slug plastids handle photoinhibition, the constant light-induced damage to Photosystem II of photosynthesis. Recovery from photoinhibition involves proteins encoded by both the nuclear and plastid genomes, and slugs with plastids isolated from the algal nucleus are therefore expected to be incapable of constantly repairing the damage as the plastids inside the slugs grow old. We studied photoinhibition-related properties of the sea slug Elysia timida that ingests its plastids from the green alga Acetabularia acetabulum. Spectral analysis of both the slugs and the algae revealed that there are two ways the slugs use to avoid major photoinhibition of their plastids. Firstly, highly photoinhibitory UV radiation is screened by the slug tissue or mucus before it reaches the plastids. Secondly, the slugs pack the plastids tightly in their thick bodies, and therefore plastids in the outer layers protect the inner ones from photoinhibition. Both properties are expected to greatly improve the longevity of the plastids inside the slugs, as the plastids do not need to repair excessive amounts of damage.

Balancing Damage via Non-Photochemical Quenching, Phenolic Compounds and Photorespiration in Ulva prolifera Induced by Low-Dose and Short-Term UV-B Radiation

The Yellow Sea green tide (YSGT) is the world’s largest transregional macroalgal blooms, and the causative species Ulva prolifera (U. prolifera) suffers from ultraviolet-b radiation (UVBR) during the floating migration process. Previous study confirmed that U. prolifera displayed a wide variety of physiological responses characterized as acclimation to UVBR, while the response mechanisms against low-dose and short-term radiation (LDSTR) are not clear. A study with photosynthetically active radiation (PAR) and UVBR was designed: normal light (NL: 72 μmol photons m⁻² s⁻¹), NL+0.3 (UVBR: 0.3 W·m⁻²), and NL+1.6 (UVBR: 1.6 W·m⁻²). The results showed that high-dose UVBR inhibited photosynthesis in thalli, especially under long-term exposure, while a variety of physiological responses were observed under LDSTR. The inhibition of photosynthesis appeared to be ameliorated by the algae under LDSTR. Further analysis showed that U. prolifera achieved balancing damage by means of non-photochemical quenching (NPQ), accumulation of phenolic compounds coupled with the ASA-GSH cycle involved in the antioxidant process and enhanced photorespiratory metabolism under LDSTR. This study provides new insights into the balancing damage mechanisms of U. prolifera under LDSTR, enabling the thalli to adapt to the light conditions during the long duration and distance involved in floating migration.

Downsizing in plants-UV light induces pronounced morphological changes in the absence of stress

Ultraviolet (UV) light induces a stocky phenotype in many plant species. In this study, we investigate this effect with regard to specific UV wavebands (UV-A or UV-B) and the cause for this dwarfing. UV-A- or UV-B-enrichment of growth light both resulted in a smaller cucumber (Cucumis sativus L.) phenotype, exhibiting decreased stem and petiole lengths and leaf area (LA). Effects were larger in plants grown in UV-B- than in UV-A-enriched light. In plants grown in UV-A-enriched light, decreases in stem and petiole lengths were similar independent of tissue age. In the presence of UV-B radiation, stems and petioles were progressively shorter the younger the tissue. Also, plants grown under UV-A-enriched light significantly reallocated photosynthates from shoot to root and also had thicker leaves with decreased specific LA. Our data therefore imply different morphological plant regulatory mechanisms under UV-A and UV-B radiation. There was no evidence of stress in the UV-exposed plants, neither in photosynthetic parameters, total chlorophyll content, or in accumulation of damaged DNA (cyclobutane pyrimidine dimers). The abscisic acid content of the plants also was consistent with non-stress conditions. Parameters such as total leaf antioxidant activity, leaf adaxial epidermal flavonol content and foliar total UV-absorbing pigment levels revealed successful UV acclimation of the plants. Thus, the UV-induced dwarfing, which displayed different phenotypes depending on UV wavelengths, occurred in healthy cucumber plants, implying a regulatory adjustment as part of the UV acclimation processes involving UV-A and/or UV-B photoreceptors.

Different Photosynthetic Responses of Pyropia yezoensis to Ultraviolet Radiation Under Changing Temperature and Photosynthetic Active Radiation Regimes

Macroalgae play a crucial role in coastal marine ecosystems, but they are also subject to multiple challenges due to tidal and seasonal alterations. In this work, we investigated the photosynthetic response of Pyropia yezoensis to ultraviolet radiation (PAR: 400-700 nm; PAB: 280-700 nm) under changing temperatures (5, 10 and 15°C) and light intensities (200, 500 and 800 μmol photons m^-2  s^-1 ). Under low light intensity (200 μmol photons m^-2  s^-1 ), P. yezoensis showed the lowest sensitivity to ultraviolet radiation, regardless of temperature. However, higher temperatures inhibited the repair rates (r) and damage rates (k) of photosystem II (PSII) in P. yezoensis. However, under higher light intensities (500 and 800 μmol photons m^-2  s^-1 ), P. yezoensis showed higher sensitivity to UV radiation. Both r and the ratio of repair rate to damage rate (r:k) were significantly inhibited in P. yezoensis by PAB, regardless of temperature. In addition, higher temperatures significantly decreased the relative UV-inhibition rates, while an increased carbon fixation rate was found. Our study suggested that higher light intensities enhanced the sensitivity to UV radiation, while higher temperatures could relieve the stress caused by high light intensity and UV radiation.

Effects of Ultraviolet Radiation (UV-A+UV-B) on the Antioxidant Metabolism of the Red Macroalga Species Acanthophora spicifera (Rhodophyta, Ceramiales) From Different Salinity and Nutrient Conditions

Acanthophora spicifera (M.Vahl) Børgesen is a macroalga of great economic importance. This study evaluated the antioxidant responses of two algal populations of A. spicifera adapted to different abiotic conditions when exposed to ultraviolet-A+ultraviolet-B radiation (UV-A+UV-B). Experiments were performed using the water at two collection points for 7 days of acclimatization and 7 days of exposure to UVR (3 h per day), followed by metabolic analyses. At point 1, water of 30 ± 1 practical salinity unit (psu) had concentrations of 1.06 ± 0.27 mm NH 4 + , 8.47 ± 0.01 mm NO 3 - , 0.17 ± 0.01 mm PO 4 - 3 and pH 7.88. At point 2, water of 35 ± 1 psu had concentrations of 1.13 ± 0.05 mm NH 4 + , 3.73 ± 0.01 mm NO 3 - , 0.52 ± 0.01 mm PO 4 - 3 and pH 8.55. Chlorophyll a, phycobiliproteins, carotenoids, mycosporins, polyphenolics and antioxidant enzymes (catalase, superoxide dismutase and guaiacol peroxidase) were evaluated. The present study demonstrates that ultraviolet radiation triggers antioxidant activity in the A. spicifera. However, such activation resulted in greater responses in samples of the point 1, with lower salinity and highest concentration of nutrients.

Reprint of Efficient fungal UV-screening provides a remarkably high UV-B tolerance of photosystem II in lichen photobionts

Lichen photobionts in situ have an extremely UV-B tolerant photosystem II efficiency (Fv/Fm). We have quantified the UV-B-screening offered by the mycobiont and the photobiont separately. The foliose lichens Nephroma arcticum and Umbilicaria spodochroa with 1: intact or 2: removed cortices were exposed to 0.7 Wm^-2 UV-B_BE for 4 h. Intact thalli experienced no reduction in Fv/Fm, whereas cortex removal lowered Fv/Fm in exposed photobiont layers by 22% for U. spodochroa and by 14% for N. arcticum. We also gave this UV-B dose to algal cultures of Coccomyxa and Trebouxia, the photobiont genera of N. arcticum and U. spodochroa, respectively. UV-B caused a 56% reduction in Fv/Fm for Coccomyxa, and as much as 98% in Trebouxia. The fluorescence excitation ratio (FER) technique comparing the fluorescence from UV-B or UV-A-excitation light with blue green excitation light using a Xe-PAM fluorometer showed that these photobiont genera did not screen any UV-B or UV-A The FER technique with a Multiplex fluorometer estimated the UV-A screening of isolated algae to be 13-16%, whereas intact lichens screened 92-95% of the UV-A. In conclusion, the cortex of N. arcticum and U. spodochroa transmitted no UV-B and little UV-A to the photobiont layer beneath. Thereby, the upper lichen cortex forms an efficient fungal solar radiation screen providing a high UV-B tolerance for studied photobionts in situ. By contrast, isolated photobionts have no UV-B screening and thus depend on their fungal partners in nature.

High impact of seasonal temperature changes on acclimation of photoprotection and radiation-induced damage in field grown Arabidopsis thaliana

At temperate latitudes environmental factors such as irradiance, including ultraviolet-B radiation (UV-B, 280-315 nm), temperature and day length vary widely over the course of a year in a concerted way. In the present study physiological acclimation of photoprotection, growth and development of the model organism Arabidopsis thaliana were correlated to these strongly but gradually changing conditions in a one year field study. Plants were sown in the field avoiding any manipulation (and abrupt change) during their life. Developmental rate was strongly dependent on prevailing temperature. Moderate signs of light stress in form of photoinhibition at photosystem II were significantly related to solar irradiances while amount of DNA damage was low and not correlated to UV-B irradiance. Although all the markers were hypothesized to primarily react to radiation, multiple regression analysis showed at least a similarly strong influence of temperature as that of light. Especially for the classical UV screening compounds a positive correlation to UV-B radiation during the course of the year was absent, whereas there was a significant negative correlation between temperature and quercetin content. The sum of violaxanthin cycle pigments was correlated to both, irradiance and temperature, but with opposite sign. Epidermal UV-B transmittance was also much better related to air temperature than to UV-B irradiance. The data show that under natural conditions temperature has at least a similar importance for photoprotective acclimation and partially also for photosensitivity as solar irradiance.

UV-A screening inCladophorasp. lowers internal UV-A availability and photoreactivation as compared to non-UV screening inUlva intestinalis

UV-A screening reduces UV-A-driven photoreactivation of UV-B-induced DNA damage in the green macroalgaCladophorasp.

Acclimation of Arabidopsis thaliana to low temperature protects against damage of photosystem II caused by exposure to UV-B radiation at 9 °C

Various environmental variables interact with UV-B radiation (280-315 nm), among them temperature. In many plants epidermal UV screening is induced by low temperature even in the absence of UV irradiation. On the other hand, low temperature can aggravate damage caused by UV-B radiation. We investigated the interaction of UV-B radiation and low temperature in Arabidopsis thaliana (L.) Heynh. Exposure of plants grown at moderate temperature (21 °C) to UV-B radiation at 9 °C resulted in significantly higher damage of photosystem II (PS II) as compared to exposure at 21 °C. The higher damage at low temperature was related to slower recovery of maximal PS II quantum efficiency at this temperature. Epidermal UV-B transmittance was measured using a method based on chlorophyll fluorescence measurements. Acclimation to low temperature enhanced epidermal UV-B screening and improved the UV-B resistance considerably. Differences in the apparent UV-B sensitivity of PS II between plants grown in moderate or acclimated to cool temperatures were strongly diminished when damage was related to the UV-B radiation reaching the mesophyll (UV-B_int) as calculated from incident UV-B irradiance and epidermal UV-B transmittance. Evidence is presented that the remaining differences in sensitivity are caused by an increased rate of repair in plants acclimated to 9 °C. The data suggest that enhanced epidermal UV-B screening at low temperature functions to compensate for slower repair of UV-B damage at these temperatures. It is proposed that the UV-B irradiance reaching the mesophyll should be considered as an important parameter in experiments on UV-B resistance of plants.

Efficient fungal UV-screening provides a remarkably high UV-B tolerance of photosystem II in lichen photobionts

Lichen photobionts in situ have an extremely UV-B tolerant photosystem II efficiency (Fv/Fm). We have quantified the UV-B-screening offered by the mycobiont and the photobiont separately. The foliose lichens Nephroma arcticum and Umbilicaria spodochroa with 1: intact or 2: removed cortices were exposed to 0.7 Wm^-2 UV-B_BE for 4 h. Intact thalli experienced no reduction in Fv/Fm, whereas cortex removal lowered Fv/Fm in exposed photobiont layers by 22% for U. spodochroa and by 14% for N. arcticum. We also gave this UV-B dose to algal cultures of Coccomyxa and Trebouxia, the photobiont genera of N. arcticum and U. spodochroa, respectively. UV-B caused a 56% reduction in Fv/Fm for Coccomyxa, and as much as 98% in Trebouxia. The fluorescence excitation ratio (FER) technique comparing the fluorescence from UV-B or UV-A-excitation light with blue green excitation light using a Xe-PAM fluorometer showed that these photobiont genera did not screen any UV-B or UV-A The FER technique with a Multiplex fluorometer estimated the UV-A screening of isolated algae to be 13-16%, whereas intact lichens screened 92-95% of the UV-A. In conclusion, the cortex of N. arcticum and U. spodochroa transmitted no UV-B and little UV-A to the photobiont layer beneath. Thereby, the upper lichen cortex forms an efficient fungal solar radiation screen providing a high UV-B tolerance for studied photobionts in situ. By contrast, isolated photobionts have no UV-B screening and thus depend on their fungal partners in nature.

Effects of UVB radiation on grazing of two cladocerans from high-altitude Andean lakes

Climate change and water extraction may result in increased exposition of the biota to ultraviolet-B radiation (UVB) in high-altitude Andean lakes. Although exposition to lethal doses in these lakes is unlikely, sub-lethal UVB doses may have strong impacts in key compartments such as zooplankton. Here, we aimed at determining the effect of sub-lethal UVB doses on filtration rates of two cladoceran species (Daphnia pulicaria and Ceriodaphnia dubia). We firstly estimated the Incipient Limiting Concentration (ILC) and the Gut Passage Time (GPT) for both species. Thereafter we exposed clones of each species to four increasing UVB doses (treatments): i) DUV-0 (Control), ii) DUV-1 (0.02 MJ m²), iii) DUV-2 (0.03 MJ m²) and iv) DUV-3 (0.15 MJ m²); and estimated their filtration rates using fluorescent micro-spheres. Our results suggest that increasing sub-lethal doses of UVB radiation may strongly disturb the structure and functioning of high-altitude Andean lakes. Filtration rates of D. pulicaria were not affected by the lowest dose applied (DUV-1), but decreased by 50% in treatments DUV-2 and DUV-3. Filtration rates for C. dubia were reduced by more than 80% in treatments DUV-1 and DUV-2 and 100% of mortality occurred at the highest UVB dose applied (DUV-3).

Role of Vitamin D Metabolism and Activity on Carcinogenesis

The vitamin D endocrine system regulates a broad variety of independent biological processes, and its deficiency is associated with rickets, bone diseases, diabetes, cardiovascular diseases, and tuberculosis. Cellular and molecular studies have also shown that it is implicated in the suppression of cancer cell invasion, angiogenesis, and metastasis. Sunlight exposure and consequent increased circulating levels of vitamin D are associated with reduced occurrence and a reduced mortality in different histological types of cancer, including those resident in the skin, prostate, breast, colon, ovary, kidney, and bladder. The vitamin D receptor (VDR) as a steroid hormone superfamily of nuclear receptors is highly expressed in epithelial cells at risk for carcinogenesis, providing a direct molecular link by which vitamin D status impacts on carcinogenesis. Because VDR expression is retained in many human tumors, vitamin D status may be an important modulator of cancer progression in persons living with cancer. The aim of this review is to highlight the relationship between vitamin D, VDR, and cancer, summarizing several mechanisms proposed to explain the potential protective effect of vitamin D against the development and progression of cancer.