Effects of solar UV-B radiation on canopy structure of Ulva communities from southern Spain

Abstract

Within the sheltered creeks of Cádiz bay, Ulva thalli form extended mat-like canopies. The effect of solar ultraviolet radiation on photosynthetic activity, the composition of photosynthetic and xanthophyll cycle pigments, and the amount of RubisCO, chaperonin 60 (CPN 60), and the induction of DNA damage in Ulva aff. rotundata Bliding from southern Spain was assessed in the field. Samples collected from the natural community were covered by screening filters, generating different radiation conditions. During daily cycles, individual thalli showed photoinhibitory effects of the natural solar radiation. This inhibition was even more pronounced in samples only exposed to photosynthetically active radiation (PAR). Strongly increased heat dissipation in these samples indicated the activity of regulatory mechanisms involved in dynamic photoinhibition. Adverse effects of UV-B radiation on photosynthesis were only observed in combination with high levels of PAR, indicating the synergistic effects of the two wavelength ranges. In samples exposed either to PAR+UV-A or to UV-B+UV-A without PAR, no inhibition of photosynthetic quantum yield was found in the course of the day. At the natural site, the top layer of the mat-like canopies is generally completely bleached. Artificially designed Ulva canopies exhibited fast bleaching of the top layer under the natural solar radiation conditions, while this was not observed in canopies either shielded from UV or from PAR. The bleached first layer of the canopies acts as a selective UV-B filter, and thus prevents subcanopy thalli from exposure to harmful radiation. This was confirmed by the differences in photosynthetic activity, pigment composition, and the concentration of RubisCO in thalli with different positions within the canopy. In addition, the induction of the stress protein CPN 60 under UV exposure and the low accumulation of DNA damage indicate the presence of physiological protection mechanisms against harmful UV-B. A mechanism of UV-B-induced inhibition of photosynthesis under field conditions is proposed.

Isolate-specific effects of ultraviolet radiation on photosynthesis, growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium Microcoleus chthonoplastes

“…rotundata Bliding from Southern Spain. At the natural site, the top layer of these mat-like canopies is generally completely bleached, but acts as a selective UV-B Wlter, and thus prevents subcanopy thalli from exposure to harmful radiation (Bischof et al 2002). Cells and tissue of brown algae that contain UV-absorbing compounds have the potential to protect any underlying living cells from the harmful eVect of high levels of UVR.…”

Oxidative stress and enzymatic scavenging of superoxide radicals induced by solar UV-B radiation in <i>Ulva</i> canopies from southern Spain

“…Previous studies have shown the importance of canopy arrangement for maintaining high production rates of algal assemblages within these highly eutrophic systems (Pérez-Lloréns et al, 1996;Hernández et al, 1997). A recent study on the effects of solar UV radiation on Ulva canopies proposed canopy formation as an ecological strategy for shielding productive material from harmful UV-B radiation (Bischof et al, 2002b): the bleached top layers of mats, which exhibit marked absorption of solar radiation only in the UV-B range of the spectrum, shield the subcanopy layers. In the same study it was also proposed that under field conditions the combination of high irradiance of photosynthetically active radiation (PAR) and UV-B radiation results in the loss of photosynthetic proteins and pigments, probably due to a high production of reactive oxygen species (ROS) in photosynthesis.…”

Ecophysiological performance of an urban strain of the aeroterrestrial green algaKlebsormidiumsp. (Klebsormidiales, Klebsormidiophyceae)

“…the top layer of the mat-like canopies is generally completely bleached thereby providing photoprotection for subcanopy thalli (Bischof et al, 2002).…”