The effects of mild a.gificial UV conditions (4.7 W m−2 UV-A, 320–400 nm and 0.20 W m−2 UV-B, 280–320 nm) plus PAR (25–30 µmol photons m−2 s−1) at 4–6°C on photosynthesis and ultrastructure of the aeroterrestrial green macroalga Prasiola crispa (Lightfoot) Kützing from the Arctic were investigated. Specimens were collected from an aeroterrestrial rocky cliff in Kongsfjorden (Ny-Ålesund, Spitsbergen, Norway, 78°55.5′N, 11°56.0′E). Exposure for 6 h and 24 h to PAR + UV-A + UV-B led to a decrease in the optimum quantum yield (PS II efficiency, Fv /Fm) to 73.7% and 42.0% of the control value, respectively. In addition, the relative electron transport rate as a function of the photon fluence rate (PI curve) was determined. This showed that the α value declined in concert with Fv /Fm in UV-exposed samples. In parallel to the fluorescence measurements, tissue samples were prepared for transmission electron microscopy. Whereas 6 h of UV treatment did not lead to significant alterations in the ultrastructure compared to control samples, 24 h of UV treatment caused slight defects. Occasionally, thylakoid membranes showed dilatations and the number of plastoglobuli appeared reduced. Formation of a large number of cytoplasmic globules with a diameter in the range of 0.4–0.8 µm and a medium electron-dense contrast was observed. The data presented demonstrate that P. crispa is rather insensitive to experimental UV treatment and thus seems to be well adapted to cope with increased UV irradiation expected to occur as a consequence of decreased stratospheric ozone concentrations in the northern hemisphere.
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