Dietary acquisition of photoprotective compounds (mycosporine-like amino acids, carotenoids) and acclimation to ultraviolet radiation in a freshwater copepod

We experimentally tested the hypothesis that accumulations of dietary compounds such as carotenoids or UVabsorbing mycosporine-like amino acids (MAAs) protect against natural levels of ultraviolet radiation (UVR). A calanoid copepod, Leptodiaptomus minutus, was collected from a relatively UV-transparent lake in Pennsylvania where levels of copepod MAAs and carotenoids vary during the year (MAAs high/carotenoids low in summer). Animals raised in the laboratory under different diet/UVR treatments accumulated MAAs from an MAA-producing dinoflagellate but not from a cryptomonad that lacks them. The acquisition efficiency increased under exposure to UVR-supplemented photosynthetically active radiation (PAR, 400–700 nm), yielding MAA concentrations up to 0.7% dry weight compared with only 0.3% under unsupplemented PAR. Proportions of individual MAAs differed between the animals and their diet. Shorter wavelength absorbing palythine and shinorine (lmax 320 and 334 nm, respectively) were disproportionately accumulated over usujirene and palythene (lmax ca. 359 nm). Carotenoids accumulated under UVR exposure (to 1% dry weight) when dietary MAAs were not available. Tolerance of ultraviolet-B (UV-B) radiation was assessed as LE50s (UV exposure giving 50% mortality after 5 d) following 12-h acute exposure to artificial UV-B radiation. LE50s increased 2.5-fold for UV-acclimated, MAA-rich animals, but only 1.5-fold for UV-acclimated, carotenoid-rich animals. Compared with carotenoids, MAAs offer this copepod a more effective photoprotection strategy, potentially as important as photorepair of DNA damage, to promote tolerance of natural levels of UV-B radiation. Zooplankton living in surface waters potentially encounter harmful levels of ultraviolet radiation (UVR), especially UVB (Williamson et al. 1994). Assessing the effect on natural populations, however, requires complex integration of spectral sensitivity, incident radiation, and active or passive movements within the depth-gradient of UVR intensity (Browman et al. 2000). UVR often is rapidly attenuated with depth in freshwaters (Morris et al. 1995). Many organisms physically avoid harmful intensities, including migrating copepods (Alonso et al. 2004) and other zooplankton. Moreover, zooplankton and other organisms possess biochemical defenses against UVR; these variously intercept UV photons, neutralize oxidizing photoproducts, or repair damage to DNA and other cell constituents (Mitchell and Karentz 1993; Banaszak 2003; Hessen 2003). Some zooplankton are more tolerant of UVR than others, for example, copepods com1 Corresponding author ([email protected]).