Differential tolerance of UV radiation between Chaoborus species and role of photoprotective compounds

Abstract

Previous studies with larvae of the American species Chaoborus punctipennis suggest that this genus is highly sensitive to solar ultraviolet radiation (UVR). Here, we first tested the UV sensitivity of late-instar larvae of two European Chaoborus species collected from a fishless shallow lake and second assessed their strategies to minimize UV damage. Exposure of late-instar larvae of C. pallidus to artificial UVR caused 50% mortality at a dose of 63.9 J cm -2 . In contrast, no mortality was observed in C. obscuripes at the same or even at a higher dose (85.1 J cm -2 ). Several UV-absorbing compounds identified as mycosporine-like amino acids (MAAs) were found by high-performance liquid chromatography analysis in C. obscuripes [mean total MAAs: 0.142 μg mg -1 dry weight (DW)], but not in C. pallidus. Larvae of C. flavicans also had the same suite of MAAs (shinorine, mycosporine-glycine, palythine, usujirene and polythene) and similar MAAs concentration (0.186 μg mg -1 DW) as C. obscuripes, whereas C. crystallinus had very low concentrations (0.008 μg mg -1 DW) and only two MAAs. The same suite of MAAs found in larvae of C. obscuripes was also detected in pupae, adult midges and eggs of this species. Whereas C. obscuripes was able to accumulate MAAs through the diet, the origin of these secondary metabolites in the lake remains unclear. Our results indicate that sensitivity to UVR in Chaoborus is species-specific and suggest that resistance in C. obscuripes is associated with the presence of photoprotective compounds. Populations of UV-sensitive Chaoborus species may be at disadvantage in water bodies becoming more transparent to UVR as consequence of climate change.