Compensation of lack of UV screening by cellular tolerance in green macroalgae (Ulvophyceae) from the upper eulittoral

Abstract

Living side by side in the upper eulittoral of the Baltic different species of green macroalgae can be assumed to require similar resistance against solar ultraviolet-B radiation (UVB, 280–315 nm). Avoidance of UVB absorption by UV-screening pigments acts as a fundamental UVB resistance mechanism in the majority of phototrophs, including green macroalgae from the order Cladophorales. Contrastingly, other orders of green macroalgae, like the Ulvales, Ulotrichales, or Bryopsidales, lack UV screening. Field grown thalli of coexisting species representative for all four orders were exposed to experimental UVB radiation and photosystem II (PSII) and DNA damage and its repair were assessed. UV-screening Cladophora sp. showed only half as much UVB-induced damage in comparison with the non-screening species Acrosiphonia sp., Bryopsis hypnoides, and Ulva intestinalis. However, intrinsic UVB sensitivity of PSII and DNA was very similar in all species. We hypothesized that the non-screening species would compensate the lack of protection by increased repair rates. UVA-driven CPD removal was more than twice as fast in non-UV screening as in screening species. Recovery of PSII was very efficient in Acrosiphonia sp. and U. intestinalis but not in B. hypnoides or Cladophora sp.. We conclude that DNA and PSII repair are important cellular tolerance mechanisms which compensate for the lack of UV-screening compounds in the green macroalgae Acrosiphonia sp. and U. intestinalis. Bryopsis hypnoides turned out to be more sensitive than the other species and may avoid UVB damage by growing in greater depth.