Changes in Phenolic Compounds and Cellular Ultrastructure of Arctic and Antarctic Strains of Zygnema (Zygnematophyceae, Streptophyta) after Exposure to Experimentally Enhanced UV to PAR Ratio

Microbial Ecology, Jan 2013

Ultraviolet (UV) radiation has become an important stress factor in polar regions due to anthropogenically induced ozone depletion. Although extensive research has been conducted on adaptations of polar organisms to this stress factor, few studies have focused on semi-terrestrial algae so far, in spite of their apparent vulnerability. This study investigates the effect of UV on two semi-terrestrial arctic strains (B, G) and one Antarctic strain (E) of the green alga Zygnema, isolated from Arctic and Antarctic habitats. Isolates of Zygnema were exposed to experimentally enhanced UV A and B (predominant UV A) to photosynthetic active radiation (PAR) ratio. The pigment content, photosynthetic performance and ultrastructure were studied by means of high-performance liquid chromatography (HPLC), chlorophyll a fluorescence and transmission electron microscopy (TEM). In addition, phylogenetic relationships of the investigated strains were characterised using rbcL sequences, which determined that the Antarctic isolate (E) and one of the Arctic isolates (B) were closely related, while G is a distinct lineage. The production of protective phenolic compounds was confirmed in all of the tested strains by HPLC analysis for both controls and UV-exposed samples. Moreover, in strain E, the content of phenolics increased significantly (p = 0.001) after UV treatment. Simultaneously, the maximum quantum yield of photosystem II photochemistry significantly decreased in UV-exposed strains E and G (p < 0.001), showing a clear stress response. The phenolics were most probably stored at the cell periphery in vacuoles and cytoplasmic bodies that appear as electron-dense particles when observed by TEM after high-pressure freeze fixation. While two strains reacted moderately on UV exposure in their ultrastructure, in strain G, damage was found in chloroplasts and mitochondria. Plastidal pigments and xanthophyll cycle pigments were investigated by HPLC analysis; UV A- and UV B-exposed samples had a higher deepoxidation state as controls, particularly evident in strain B. The results indicate that phenolics are involved in UV protection of Zygnema and also revealed different responses to UV stress across the three strains, suggesting that other protection mechanisms may be involved in these organisms.

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Changes in Phenolic Compounds and Cellular Ultrastructure of Arctic and Antarctic Strains of Zygnema (Zygnematophyceae, Streptophyta) after Exposure to Experimentally Enhanced UV to PAR Ratio

Martina Pichrtov 0 1 2 4 Daniel Remias 0 1 2 4 Louise A. Lewis 0 1 2 4 Andreas Holzinger 0 1 2 4 0 D. Remias Pharmacognosy, Institute of Pharmacy, University of Innsbruck , Innrain 80-82, 6020 Innsbruck, Austria 1 M. Pichrtov Institute of Botany, Academy of Sciences of the Czech Republic, Dukelsk 135 , 37982 Tebo, Czech Republic 2 M. Pichrtov Department of Botany, Faculty of Science, Charles University in Prague , Bentsk 2, 12801 Prague 2, Czech Republic 3 ) Functional Plant Biology, Institute of Botany, University of Innsbruck , Sternwartestr. 15, 6020 Innsbruck, Austria 4 L. A. Lewis Department of Ecology and Evolutionary Biology, University of Connecticut , Storrs, CT 06269-3043, USA Ultraviolet (UV) radiation has become an important stress factor in polar regions due to anthropogenically induced ozone depletion. Although extensive research has been conducted on adaptations of polar organisms to this stress factor, few studies have focused on semi-terrestrial algae so far, in spite of their apparent vulnerability. This study investigates the effect of UV on two semi-terrestrial arctic strains (B, G) and one Antarctic strain (E) of the green alga Zygnema, isolated from Arctic and Antarctic habitats. Isolates of Zygnema were exposed to experimentally enhanced UV A and B (predominant UV A) to photosynthetic active radiation (PAR) ratio. The - pigment content, photosynthetic performance and ultrastructure were studied by means of high-performance liquid chromatography (HPLC), chlorophyll a fluorescence and transmission electron microscopy (TEM). In addition, phylogenetic relationships of the investigated strains were characterised using rbcL sequences, which determined that the Antarctic isolate (E) and one of the Arctic isolates (B) were closely related, while G is a distinct lineage. The production of protective phenolic compounds was confirmed in all of the tested strains by HPLC analysis for both controls and UVexposed samples. Moreover, in strain E, the content of phenolics increased significantly (p0 0.001) after UV treatment. Simultaneously, the maximum quantum yield of photosystem II photochemistry significantly decreased in UV-exposed strains E and G (p<0.001), showing a clear stress response. The phenolics were most probably stored at the cell periphery in vacuoles and cytoplasmic bodies that appear as electrondense particles when observed by TEM after high-pressure freeze fixation. While two strains reacted moderately on UV exposure in their ultrastructure, in strain G, damage was found in chloroplasts and mitochondria. Plastidal pigments and xanthophyll cycle pigments were investigated by HPLC analysis; UVA- and UV B-exposed samples had a higher deepoxidation state as controls, particularly evident in strain B. The results indicate that phenolics are involved in UV protection of Zygnema and also revealed different responses to UV stress across the three strains, suggesting that other protection mechanisms may be involved in these organisms. Polar regions are characterised by extreme climatic conditions. Organisms living there have to possess adaptations that enable them to survive in such a harsh environment. Numerous abiotic stress factors have been connected with polar climate, including low temperature, drought, nutrient limitation and periodic freezethaw cycles during the summer [74]. Among those extreme abiotic factors, solar ultraviolet (UV) radiation seems to be not usually considered a major stress factor in polar regions [23, 24]. Solar elevation decreases towards higher latitudes, and therefore, irradiation is lower in polar regions than in temperate and tropical zones. Moreover, solar rays travel a longer path through the atmosphere in getting to the poles, resulting in a greater proportion of shortwave radiation being absorbed and scattered [23]. Over the last few decades and in light of anthropogenically induced ozone depletion, there has been greater interest in the biological effects of UV radiation on polar organisms [34, 49, 75]. It has been hypothesised that certain polar organisms may be unable to adapt to an increasing UV environment. Moreover, predictions for future scenarios allow speculations about an increase of UV radiation reaching the Earths surface particularly in polar regions. These effects are expected to be further enhanced due to climate change [10]. Many damaging effects caused by UV irradiation have been described, of which the dominant targets are DNA and the photosynthetic apparatus, and secondary effects are caused by the production of reactive oxygen species (ROS) [e.g. 14, 19, 6267, 76]. Photoautotrophic organisms may be especially threatened by increases in UV radiation because solar radiation is essential for their growth and survival. In polar regions, eukaryotic algae are significant primary producers with the ability to inhabit and even dominate practically all habitats [17]. Recently, extensive research has been performed on the UV resistance (...truncated)


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Martina Pichrtová, Daniel Remias, Louise A. Lewis, Andreas Holzinger. Changes in Phenolic Compounds and Cellular Ultrastructure of Arctic and Antarctic Strains of Zygnema (Zygnematophyceae, Streptophyta) after Exposure to Experimentally Enhanced UV to PAR Ratio, Microbial Ecology, 2013, pp. 68-83, Volume 65, Issue 1, DOI: 10.1007/s00248-012-0096-9