Abstract
Species of Zygnema form macroscopically visible mats in polar and temperate terrestrial habitats, where they are exposed to environmental stresses. Three previously characterized isolates (Arctic Zygnema sp. B, Antarctic Zygnema sp. C, and temperate Zygnema sp. S) were tested for their tolerance to experimental UV radiation. Samples of young vegetative cells (1 month old) and pre-akinetes (6 months old) were exposed to photosynthetically active radiation (PAR, 400–700 nm, 400 μmol photons m−2 s−1) in combination with experimental UV-A (315–400 nm, 5.7 W m−2, no UV-B), designated as PA, or UV-A (10.1 W m−2) + UV-B (280–315 nm, 1.0 W m−2), designated as PAB. The experimental period lasted for 74 h; the radiation period was 16 h PAR/UV-A per day, or with additional UV-B for 14 h per day. The effective quantum yield, generally lower in pre-akinetes, was mostly reduced during the UV treatment, and recovery was significantly higher in young vegetative cells vs. pre-akinetes during the experiment. Analysis of the deepoxidation state of the xanthophyll-cycle pigments revealed a statistically significant (p < 0.05) increase in Zygnema spp. C and S. The content of UV-absorbing phenolic compounds was significantly higher (p < 0.05) in young vegetative cells compared to pre-akinetes. In young vegetative Zygnema sp. S, these phenolic compounds significantly increased (p < 0.05) upon PA and PAB. Transmission electron microscopy showed an intact ultrastructure with massive starch accumulations at the pyrenoids under PA and PAB. A possible increase in electron-dense bodies in PAB-treated cells and the occurrence of cubic membranes in the chloroplasts are likely protection strategies. Metabolite profiling by non-targeted RP-UHPLC-qToF-MS allowed a clear separation of the strains, but could not detect changes due to the PA and PAB treatments. Six hundred seventeen distinct molecular masses were detected, of which around 200 could be annotated from databases. These results indicate that young vegetative cells can adapt better to the experimental UV-B stress than pre-akinetes.
Highlights
IntroductionThe effects of UV radiation on green algae have been studied extensively (reviewed by, e.g., Holzinger and Lütz 2006; Karsten and Holzinger 2014; Holzinger and Pichrtová 2016), mainly after the detection of stratospheric ozone holes over the polar regions, increasing UV-B radiation
The effects of UV radiation on green algae have been studied extensively, mainly after the detection of stratospheric ozone holes over the polar regions, increasing UV-B radiation
In Zygnematophycean green algae, unusual phenolic compounds with UV-absorbing capacities have been found in Spirogyra sp. and Zygnema sp. (e.g., Nishizawa et al 1985; Cannell et al 1988; Pichrtová et al 2013)
Summary
The effects of UV radiation on green algae have been studied extensively (reviewed by, e.g., Holzinger and Lütz 2006; Karsten and Holzinger 2014; Holzinger and Pichrtová 2016), mainly after the detection of stratospheric ozone holes over the polar regions, increasing UV-B radiation. This could lead to destructive effects on chloroplasts and DNA, which in turn would influence algal development and distribution. (e.g., Nishizawa et al 1985; Cannell et al 1988; Pichrtová et al 2013) These phenolic substances may absorb in the visible waveband, such as the red vacuolar pigment in Zygogonium ericetorum, a glycosylated derivative of gallic acid, complexed with ferric iron (e.g., Aigner et al 2013; Herburger et al 2016). Because Zygnematophyceae possess neither MAAs nor secondary carotenoids, we focused our investigations on phenolic compounds
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