UV-A screening in Cladophora sp. lowers internal UV-A availability and photoreactivation as compared to non-UV screening in Ulva intestinalis

Abstract

In the Baltic Sea, two co-occurring green macroalgae Cladophora sp. and Ulva intestinalis grow in the upper eulittoral. Due to regular and high sunlight exposure in their habitat, both species need resistance mechanisms to protect themselves against ultraviolet-B (UV-B)-induced DNA damage. While Cladophora sp. possesses efficient screening of UV-B and ultraviolet-A (UV-A) radiation, U. intestinalis was recently shown to have higher DNA repair by UVA-driven photoreactivation than Cladophora sp. [F. Pescheck and W. Bilger, Mar. Biol., 2018, 165, 132]. In the present study, the hypothesis that the screening of UV-A radiation limits internal UV-A availability for photoreactivation in Cladophora sp. was tested. Both species had identical and much lower fractions of damaged DNA when sampled in situ under direct sunlight as expected based on a photophysical prediction. To quantify the effect of UV-A screening spectrally and physiologically, in vivo UV screening spectra were determined and the UV-A photon flux dependency of photoreactivation was investigated for both species. Identical intrinsic photoreactivation rates were revealed by the applied correction for internal UV-A photon flux density and under irradiation with visible radiation which is not screened by the UV absorbing compounds in Cladophora sp. Natural sunlight was weighted with in vivo action spectra for DNA damage induction and light-dependent repair. The resulting spectrum was further corrected for the apparent UV screening spectra of both species to calculate the species-specific internal ratios of DNA damaging and photoreactivating photons. This photophysical modelling improves the understanding of UV damage and tolerance mechanisms in the two co-occurring green macroalgae under solar irradiation.