Photoprotective Compounds Research Articles

A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land

BackgroundThe timescale of prokaryote evolution has been difficult to reconstruct because of a limited fossil record and complexities associated with molecular clocks and deep divergences. However, the relatively large number of genome sequences currently available has provided a better opportunity to control for potential biases such as horizontal gene transfer and rate differences among lineages. We assembled a data set of sequences from 32 proteins (~7600 amino acids) common to 72 species and estimated phylogenetic relationships and divergence times with a local clock method.ResultsOur phylogenetic results support most of the currently recognized higher-level groupings of prokaryotes. Of particular interest is a well-supported group of three major lineages of eubacteria (Actinobacteria, Deinococcus, and Cyanobacteria) that we call Terrabacteria and associate with an early colonization of land. Divergence time estimates for the major groups of eubacteria are between 2.5–3.2 billion years ago (Ga) while those for archaebacteria are mostly between 3.1–4.1 Ga. The time estimates suggest a Hadean origin of life (prior to 4.1 Ga), an early origin of methanogenesis (3.8–4.1 Ga), an origin of anaerobic methanotrophy after 3.1 Ga, an origin of phototrophy prior to 3.2 Ga, an early colonization of land 2.8–3.1 Ga, and an origin of aerobic methanotrophy 2.5–2.8 Ga.ConclusionsOur early time estimates for methanogenesis support the consideration of methane, in addition to carbon dioxide, as a greenhouse gas responsible for the early warming of the Earths' surface. Our divergence times for the origin of anaerobic methanotrophy are compatible with highly depleted carbon isotopic values found in rocks dated 2.8–2.6 Ga. An early origin of phototrophy is consistent with the earliest bacterial mats and structures identified as stromatolites, but a 2.6 Ga origin of cyanobacteria suggests that those Archean structures, if biologically produced, were made by anoxygenic photosynthesizers. The resistance to desiccation of Terrabacteria and their elaboration of photoprotective compounds suggests that the common ancestor of this group inhabited land. If true, then oxygenic photosynthesis may owe its origin to terrestrial adaptations.

Surface avoidance by freshwater zooplankton: Field evidence on the role of ultraviolet radiation

The avoidance of surface waters by crustacean zooplankton is a common phenomenon both in marine and freshwaters. The contemporary paradigm interprets such behavior as an antipredator strategy. However, the phenomenon has also been reported in predator‐free environments, which suggests that other variables may contribute to this depth‐selection behavior. We investigated the possibility that ultraviolet radiation (UVR) could influence the avoidance of surface waters in three interconnected lakes differing in their transparency. Each lake was sampled 17 or 18 times. In the two most transparent lakes, the percentage of the population occurring in the surface layer decreased during spring and summer, but this pattern was not observed in the less transparent lake. Such differences in the patterns of surface‐water avoidance were unrelated to the abundance and vertical distribution of the dominant planktivores (early stages of galaxiid fishes) as well as to temperature and food availability. They were inversely related, however, to the average UVR levels within the surface layer. A UV avoidance strategy was predicted for the copepod Boeckella gracilipes, on account of its low UV tolerance and lack of photoprotective compounds. Our results support this prediction and strongly suggest that surface avoidance by freshwater Patagonian zooplankton is not just a byproduct benefit of an antipredator behavior but is a direct response to high UVR levels.

Antioxidant and Pigment Composition during Autumnal Leaf Senescence in Woody Deciduous Species Differing in their Ecological Traits

Abstract: Photoprotection mechanisms have been studied during autumnal senescence in sun and shade leaves of woody plants with different ecological characteristics and senescence patterns. Three of them belonging to the same family, Betulaceae: the shade‐intolerant and early successional species (Betula alba L.), the shade‐tolerant and late successional species (Corylus avellana L.), and an N‐fixing tree with low N resorption efficiency (Alnus glutinosa L.). The other two species: a shade‐intolerant (Populus tremula L.) and a shade‐tolerant (Cornus sanguinea L.), were chosen because of their ability to accumulate anthocyanins during autumnal leaf senescence. The study of plants with different ecological strategies allowed us to establish general trends in photoprotection mechanisms during autumnal senescence, when nutrient remobilisation occurs, but also during whole leaf ontogeny. We have not found a clear relationship between shade tolerance and the level of photoprotection; the main difference between both groups of species being the presence of α‐carotene in shade leaves of shade‐tolerant species. Preceding autumn, nitrogen resorption started in mid‐summer and occurred in parallel with a slight and continuous ascorbate, chlorophyll and carotenoid degradation. However, the ascorbate pool remained highly reduced and lipid oxidation did not increase at this time. Contrasting with ascorbate, α‐tocopherol accumulated progressively in all species. Only during the last stages of senescence was chlorophyll preferentially degraded with respect to carotenoids, leading to the yellowing of leaves, except in A. glutinosa in which a large retention of chlorophyll and N took place. Senescing leaves were characterised, except in C. sanguinea, by a relative increase in the proportion of de‐epoxidised xanthophylls: zeaxanthin, antheraxanthin and lutein. The light‐induced accumulation of anthocyanins in C. sanguinea could play an additional protective role, compensating for the low retention of de‐epoxidised xanthophylls. These different strategies among deciduous species are consistent with a role for photoprotective compounds in enhancing nitrogen remobilization and storage for the next growing season.

Betacarotene supplementation protects from photoaging-associated mitochondrial DNA mutation.

Mutations of mitochondrial DNA accumulate during normal aging and can be detected at elevated levels in skin prematurely aged by chronic exposure to ultraviolet (UV) light (photoaging). In normal human fibroblasts, we have previously demonstrated that mtDNA deletions are induced by repetitive exposure to sublethal doses of UVA radiation mediated through singlet oxygen. Betacarotene is a known quencher of ROS and singlet oxygen in particular, and it is widely applied in photoprotective compounds. Therefore we investigated whether in our in vitro system, betacarotene is capable of protecting from the induction of photoaging-associated mtDNA deletions. All-E (trans) betacarotene was tested at doses from 0.25 to 3.0 microM for uptake into cells as well as its protective capacity. Assessment of cellular uptake of all-E betacarotene measured by HPLC revealed a dose dependent increase of intracellular concentrations, as well as an increase in oxidative metabolites. UVA-exposure led to a decrease of all-E-betacarotene, its Z-isomers and oxidative metabolites. Assessment of mtDNA deletions by PCR revealed reduced levels of mtDNA mutagenesis in cells coincubated with betacarotene at concentrations of 0.5 microM and higher. Taken together, these results indicate that betacarotene (i) is taken up into the cell in a dose dependent manner, (ii) interacts with UVA radiation in the cell and (iii) shows protective properties from the induction of a photoaging-associated mtDNA mutation.

Impact‐induced microbial endolithic habitats

Abstract— Asteroid and comet impacts on Earth are commonly viewed as agents of ecosystem destruction, be it on local or global scales. However, for some microbial communities, impacts may represent an opportunity for habitat formation as some substrates are rendered more suitable for colonization when processed by impacts. We describe how heavily shocked gneissic crystalline basement rocks exposed at the Haughton impact structure, Devon Island, Nunavut, Arctic Canada, are hosts to endolithic photosynthetic microorganisms in significantly greater abundance than lesser‐shocked or unshocked gneisses. Two factors contribute to this enhancement: (a) increased porosity due to impact fracturing and differential mineral vaporization, and (b) increased translucence due to the selective vaporization of opaque mineral phases. Using biological ultraviolet radiation dosimetry, and by measuring the concentrations of photoprotective compounds, we demonstrate that a covering of 0.8 mm of shocked gneiss can provide substantial protection from ultraviolet radiation, reducing the inactivation of Bacillus subtilis spores by 2 orders of magnitude. The colonisation of the shocked habitat represents a potential mechanism for pioneer microorganisms to invade an impact structure in the earliest stages of post‐impact primary succession. The communities are analogous to the endolithic communities associated with sedimentary rocks in Antarctica, but because they occur in shocked crystalline rocks, they illustrate a mechanism for the creation of microbial habitats on planetary surfaces that do not have exposed sedimentary units. This might have been the case on early Earth. The data have implications for the microhabitats in which biological signatures might be sought on Mars.

Photoprotective Compounds in Rice‐Field Cyanobacteria

AbstractCyanobacteria are cosmopolitan in distribution, forming a prominent component of microbial populations in both aquatic as well as terrestrial ecosystems. They are the dominant microflora in rice‐fields, where they significantly contribute to fertility as a natural biofertilizer. The fluence rates of UV‐B radiation impinging on the natural habitats is of major concern since being photoautotrophic organisms, cyanobacteria depend on solar radiation as the primary source of energy. UV‐B radiation causes reduction in growth, survival, pigment and protein content, heterocyst frequency and fixation of carbon and nitrogen in cyanobacteria. However, a number of cyanobacteria synthesize water soluble colorless mycosporine‐like amino acids (MAAs) and the lipid soluble yellow‐brown colored sheath pigment, scytonemin to counteract the damaging effects of UV‐B. Anabaena sp., Nostoc commune, Scytonema sp. and Lyngbya sp. were isolated from rice‐fields in India and screened for the presence of photoprotective compounds. Spectroscopic and biochemical analyses revealed the presence of shinorine, a bisubstituted MAA containing both glycine and serine group with an absorption maximum at 334 nm in all cyanobacteria except Lyngbya sp. There was a circadian induction in the synthesis of this compound by UV‐B. Polychromatic action spectra for the induction of MAAs in Anabaena sp. and N. commune show the induction to be UV‐B dependent peaking at 290 nm. Another photoprotective compound, scytonemin, with an absorption maximum at 386 nm (also absorbs significantly at 300, 278, 252 and 212 nm) was detected in all cyanobacteria except Anabaena sp. In addition, two unidentified, water‐soluble, yellowish and brownish compounds with an absorption maximum at 315 nm were recorded only in Scytonema sp. The spectral characteristics of these compounds neither matches with MAAs nor with scytonemin and seems to be a novel compound acting as an effective UV screens. In conclusion, a particular cyanobacterium having photoprotective compounds may be a potent candidate as biofertilizers for crop plants.

Large variability in the concentration of mycosporine‐like amino acids among zooplankton from lakes located across an altitude gradient

The qualitative and quantitative composition of mycosporine‐like amino acids (MAAs), a family of intracellular UV‐absorbing compounds, were investigated in zooplankton from 15 lakes located in the Central Alps between 913 and 2,485 m above sea level. The lakes differed in their UV water transparency (1% attenuation depth, Z1%, at 320 nm: 1.1–25.6 m) and maximum depth (Zmax: 3–133 m), thus offering the possibility to test the influence of different UV exposure conditions of zooplankton on the concentration of MAAs. Seven distinct MAAs were detected, but shinorine (maximum absorption: 334 nm) was the predominant compound. In the copepods Cyclops abyssorum, C. abyssorum tatricus, and Acanthodiaptomus denticornis, the total MAA concentration ranged from 0.01 to 3.1% of the dry weight. In the rotifers Keratella cochlearis and Polyarthra dolichoptera, MAAs were also found; however, these compounds were undetectable in Asplanchna priodonta as well as in the cladocerans Daphnia hyalina, D. longispina, Bosmina longispina, and Chydorus sphaericus. The total concentration of MAAs in populations of Cyclops spp. and phytoplankton collected simultaneously was not associated (r2 = 0.09, P > 0.05), suggesting a different dynamic in the accumulation of these compounds. The variability in the concentration of MAAs, however, was related with the diffuse attenuation coefficient at 320 nm (r2 = 0.74, P < 0.001) and the fraction of the water column to which 1% of the surface irradiance at 320 nm (Z1% : Zmax) penetrated (r2 = 0.86, P < 0.001). These relationships suggest that the prevailing UV exposure condition in the lakes is a major determinant of the concentration of MAAs found in zooplankton. Our data support the hypothesis that MAAs, together with other photoprotective compounds, play a major role in minimizing the damaging effects of solar UV radiation in zooplankton from transparent lakes.

Induction of mycosporine-like amino acids (MAAs) in cyanobacteria by solar ultraviolet-B radiation

Three filamentous and heterocystous N 2-fixing cyanobacteria, Anabaena sp., Nostoc commune and Scytonema sp. were tested for the presence of ultraviolet-absorbing mycosporine-like amino acids (MAAs) and their induction by solar ultraviolet-B (UV-B) radiation. High performance liquid chromatographic (HPLC) studies revealed the presence of only one type of MAAs in all three cyanobacteria, that was identified as shinorine, a bisubstituted MAA containing both glycine and serine groups having an absorption maximum at 334 nm and a retention time of around 2.8 min. There was a circadian induction in the synthesis of MAAs when the cultures were exposed to mid-latitude solar radiation (Playa Unión, Rawson, Chubut, Patagonia, Argentina) for 3 days, 4–6th February, 2000. Solar radiation was measured by an ELDONET (European Light Dosimeter Network) filter radiometer permanently installed on the roof of the Estación de Fotobiologı́a Playa Unión (43°18′ S; 65°03′ W). The maximum irradiances were around 450–500, 45–50 and 1.0–1.2 W m −2 for PAR (photosynthetic active radiation), UV-A (ultraviolet-A) and UV-B (ultraviolet-B), respectively. PAR and UV-A had no significant impact on MAA induction while UV-B induced the synthesis of shinorine in all three cyanobacteria. Shinorine was found to be induced mostly during the light period. During the dark period the concentration stayed almost constant. In addition to shinorine, another unidentified, water-soluble, brownish compound with an absorption maximum at 315 nm was found to be induced by UV-B only in Scytonema sp. and released into the medium. This substance was neither found in Anabaena sp. nor in Nostoc commune. Judging from the results, the studied cyanobacteria may protect themselves from deleterious short wavelength radiation by their ability to synthesize photoprotective compounds in response to UV-B radiation.

Time and dose-related ultraviolet B damage in viable pig skin explants held in a newly developed organ culture system.

For facilitating photochemical and toxicological studies an ex vivo skin model was developed in our laboratory using skin from domestic pigs. The model comprised the use of a complete skin piece, including the dermis and stratum corneum, of bigger areas to make future topical applications easier. Fully differentiated skin explants (5 x 50 mm, thickness 5 mm) were irradiated with ultraviolet B (UVB; 1-10 kJ/m2; 6 W/m2). Directly thereafter they were brought in culture (Dulbeccos modified Eagles medium containing hydrocortisone; air/liquid interface) for a maximum of 144 h. In nonirradiated skin explants, signs of tissue degeneration were observed after 48 h in culture (hematoxylin and eosin, light microscope). However, keratinocytes, isolated enzymatically (thermolysin and trypsin) at different time intervals in culture from nonirradiated skin explants showed negligible loss in viability (trypan blue exclusion) and increased apoptosis (terminal deoxynucleotidyl transferase-mediated deoxy uridine triphosphatase nick end labeling assay) for up to 72 h. Explants irradiated with a single dose of UVB showed a clear and reproducible dose- and time-dependent tissue degeneration, loss in keratinocyte viability and increase in apoptosis compared with nonirradiated explants at the same time interval. In conclusion, the presently designed ex vivo pig skin model can be a useful and cheap tool for future investigations of short-term UV-induced effects in combination with phototoxic and photoprotective compounds.

Photoprotective compounds in cyanobacteria, phytoplankton and macroalgae — a database

A database on photoprotective compounds in cyanobacteria, phytoplankton and macroalgae has been developed. It contains information on photoprotective compounds such as mycosporine-like amino acids (MAAs), scytonemin and other not yet identified compounds reported in aquatic organisms, their habitat, the collection site and date and the reference. Further information on the absorption maxima and extinction coefficients of different photoprotective compounds as well as experimental procedures are provided (see http://www.biologie.uni-erlangen.de/botanik1/index.html). The database answers the urgent need for a library on these substances and provides scientists in the field with the necessary information to identify and quantify screening pigments in different aquatic organisms from various growing sites and different collection dates.

Living in the intertidal zone - seasonal effects on heterosides and sun-screen compounds in the red alga Bangia atropurpurea (Bangiales).

The seasonal patterns of daylength and ultraviolet radiation (UVB and UVA) at Williamstown, Victoria, Australia were measured (October 1995-May 1996) and are considered in relation to levels of heterosides (soluble sugar compounds: D-isofloridoside, floridoside and L-isofloridoside) and sun-screen compounds (mycosporine-like amino acids: MAAs) in the intertidal red alga Bangia atropurpurea. UVB peaked in December-January at 2.2-2.4 W m(-2) and UVA also peaked at 70 W m(-2) in the same period. Total heteroside concentrations were highest (1230-1900 mmol kg(-1) dry weight) during November-December with floridoside and D-isofloridoside being 90% of the total. In late February through mid-April total heteroside contents were lower (315-905 mmol kg(-1) dry weight) with L-floridoside being as much as 34% of the total indicating a seasonal effect. Total MAAs varied from 3.4 to 7.1 mg g(-1) dry weight (mean 4.9 mg g(-1) dry weight) with both highest and lowest levels occurring in February. Porphyra-334 constituted 83 to 97% of this total with asterina-330, palythine and palythinol being 3-17%. Although maximum MAA concentrations did not show any significant parallel with the peak UV values, the quantitative data point to Bangia cells over the course of the seasons always loaded up with these photoprotective compounds.

The transiently pubescent young leaves of plane (Platanus orientalis) are deficient in photodissipative capacity

We have analyzed reflectance changes and carotenoid composition of young and mature leaves of Platanus orientalis L. in order to test the hypothesis that the transient occurrence of highly absorptive and reflective leaf hairs of young leaves (M. Ntefidou and Y. Manetas 1996, Aust. J. Plant Physiol. 23: 535–538) may be correlated to a weakly developed photodissipative capacity in the chloroplast. Compared to mature leaves, young leaves showed negligible reflectance changes at 530 nm upon sudden illumination, possibly indicating a limited production of zeaxanthin. In addition, actual pigment analysis confirmed lower pools of xanthophyll cycle components and reduced capacity for violaxanthin photoconversion in young leaves. Accordingly, the epoxidation state at saturating photon fluence rates was particularly high. A notable feature of xanthophyll cycle interconversions in young leaves was the inability to drive the system to complete de‐epoxidation, as antheraxanthin in the light was always higher than zeaxanthin. Among the rest of the carotenoids, the levels of β‐carotene were particularly low. Moreover, most of the photosynthetic pigments were considerably bleached when young leaves were exposed to high light. The above results strongly suggest that young leaves possess a limited photodissipative capacity and therefore, the presence of leaf hairs affords protection against excess light. When the leaf has matured and presumably the concentrations of photoprotective compounds are adequate, the loss of hairs is not of consequence. In fact, their presence on mature leaves may reduce the photosynthetically active radiation to non‐saturating levels for photosynthesis.

UV-B-induced damage and photoreactivation in three species of Boeckella (Copepoda, Calanoida)

Solar ultraviolet (UV) radiation poses a threat to most living organisms. Aquatic organisms have evolved three basic mechanisms to cope with harmful levels of radiation. Two mechanisms, avoid- ance (e.g. vertical migration) and photoprotection (e.g. production of photoprotective compounds that act as filters, antioxidants, etc.), serve to minimize the dose of UV radiation that reaches the organism's vital structures (DNA, membranes, etc.). The third mechanism, repair (e.g. dark repair mechanisms, such as nucleotide excision repair; or photoreactivation mechanisms, such as photoenzymatic repair), serves to repair the damage following UV exposure. Here, we compare the vulnerability to UV-B radi- ation of three copepod species (Boeckella brevicaudata, Boeckella gibbosa, and Boeckella gracilipes) that occur in lakes that differ in UV-B penetration and depth. Our aim was to gain insight into the sig- nificance of each of the three mechanisms in different UV-B environments. Results from a 3-day 'in situ' incubation in ultra-oligotrophic Lake Toncek showed that B.gracilipes is highly vulnerable to UV- B and UV-A radiation. In contrast, virtually no mortality was observed in B.gibbosa and B.brevicau- data during the same period. In order to discriminate the contribution of photoprotecti on and photoreactivation, the three species were subsequently exposed in the laboratory to an artificial source of UV-B radiation, both in the presence and absence of visible radiation (recovery radiation). The photoprotection potential (i.e. resistance to UV-B in the absence of recovery radiation) of B.gracilipes and B.gibbosa was lower than that of B.brevicaudata. On the other hand, photoreactivation (higher resistance to UV-B in the presence of recovery radiation) was observed in B.brevicaudata and B.gibbosa, but not in B.gracilipes. To cope with damaging UV-B levels in nature, B.gracilipes depends exclusively on the attenuation by the external media (i.e. avoidance). Although B.gibbosa tends to avoid the surface waters of lakes, it also occurs in shallow transparent pools. Most likely its ability to survive in these shallow, high UV environments is due to its photoreactivation potential. Finally, despite its occurrence in highly turbid lakes, B.brevicaudata seems extremely well suited to cope with UV-B radiation thanks to a combination of photoreactivation and photoprotection.