Ecology of UV
UV introduction and basic optics
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UV Effects on Zooplankton
prepared by Gaby
Dee, and Janet
Fischer
Zooplankton, unlike phytoplankton, are not directly dependent on light energy. This may allow them a greater range of responses to minimize exposure to UV-B in surface waters of lakes. The presence of organs such as eyes, and swimming appendages increase the range of possible responses to UV-B and may also render certain life stages more sensitive to UV-B than others. The potential for directly sensing UV through vision and the capacity for extensive vertical migration in response to high levels of UV-B may allow zooplankton to finely regulate the dose of UV-B to which they are exposed. However, many of these responses may have costs, or tradeoffs (Siebeck et al. 1994). For example, increased pigmentation may change susceptibility to visual predators, and vertical migration may alter the food and predator regimes to which organisms are exposed.
In essence, zooplankton inhabiting the surface waters of clear, low DOC lakes, can potentially be exposed to high levels of UV-B radiation. These zooplankters can alter their behavior to avoid exposure, or they risk being exposed. If they are exposed, they may contain photoprotective compounds in their carapaces or tissues to reduce the damage caused by exposure, or their cells may acquire DNA damage. If their DNA is damaged, they can utilize such repair mechanisms as photoenzymatic repair (PER) or nucleotide excision repair (NER) to deal with any DNA damage present. And if all these mechanisms fail, there will be an impact on the survival, growth, and reproduction (ie. fitness) of these zooplankters (slide 1 modified from Zagarese and Williamson 1994).
The well known phenomenon of diel vertical migration (DVM) (Hutchinson 1967) offers the potential for behavioral avoidance of photodamage by pelagic zooplankton. The ability to detect, and thus respond to UV-B might be of vital importance to zooplankton if, due to ozone depletion or a change in DOC, a selective increase in these wavelengths occured. While several papers have suggested that zooplankton are able to detect a broad range of UV radiation that includes UV-A (e.g. Moore 1912), very little information is available specifically on detection of UV-B, the more damaging radiation.
Though historically, DVM has been attributed to predation pressures, recent studies have shown evidence that UV radiation does indeed influence the vertical migrations of zooplankton, specifically Daphnia species. One such study of Daphnia pulicaria occured in a lake in which the species has a very strong DVM. Leech and Williamson (2001) found that a large proportion of individuals which were placed in UV transparent acrylic columns in the epilimnion at noon on a sunny day migrated downwards, whereas those in the UV shielded acrylic columns tended to remain closer to the surface. Another study on several different species of Daphnia which differed in their photoprotective compounds found that the extent to which the daphnids responded to UV radiation was inversely linked to their pigmentation (Rhode et al. 2001). In this study, artificial lighting was used to simulate natural sunlight. Unpigmented Daphnia culcullata and D. pulex migrated to the bottom of the UV exposed Perspex columns, while all the groups in the control columns (no UV light) remained near the surface. Individuals of the species Daphnia rosea, a species containing carotenoid, were found in the upper part of the UV exposed columns, while D. pulex individuals which were melanized were found even higher than the other groups in the UV exposed columns (and only slightly lower in distribution than their control counterparts). Both these studies show that all other factors being equal, Daphnia can cue in on their UV environment and use behavioral avoidance to escape harmful radiation. This behavior seems to be enhanced when they lack photoprotective compounds.
Zooplankton may be either transparent, blending into their environment, or may contain photoprotective compounds, such as carotenoids in their antennae, setae, and carapace, as is illustrated below for a calanoid copepod.
Three major types of photoprotective compounds occur in planktonic crustaceans: carotenoid pigments, cuticular melanin, and mycosporine-like amino acids (MAA's). It has been known since the works of Brehm (1938) that planktonic cladocera and copepods in high mountain lakes exhibit marked coloring, the dominant colors being black and brown in cladocerans and reddish brown in copepods.
The role of carotenoids as photoprotective pigments in freshwater calanoid copepods has been well documented (Ringelberg et al. 1984, Hairston 1978, 1979, 1980). In general, red morphs tolerate higher light intensities, and therefore are able to inhabit the epilimnion.
The primary role of melanin is direct sun blocking, though melanin precursors may also serve as anti-oxidants (Blois 1988). The role of melanin in photodamage protection is well established (Hebert and Emery 1990) and seems to be a generic property of alpine and arctic populations under high light stress. Examples of a Daphnia containing high concentrations of melanin (left) and one containing very little melanin (right) are seen below (pics from University of Guelph Cladoceran site).
Mycosporine-like amino acid compounds protect against UV-B but in contrast to the previous two compounds, are not pigmented. In an environment where predation is heavy, bright or dark coloration due to carotenoids or cuticular melanin may increase vulnerablility of zooplankton to predators. However, individuals containing MAA's could be protected from UV without increasing their risk of being eaten by a visual predator. One exception to this is when visual predators have UV photoreceptors as is the case with many larval fish (Leech and Johnsen 2003).
More information on photoprotective compounds should be added to this site soon.
There has been a great deal of evidence illustrating the fact that many zooplankton species have the ability to repair the damage done to their DNA by UV radiation. One such mechanism is photoenzymatic repair (PER) in which the enzyme photolyase, in the presence of longer wavelength UV-A and visible light, reverses pyrimidine dimers (Sutherland 1981, Mitchell and Karentz 1993). Another mechanism is nucleotide excision repair (NER) or dark repair.
Several labs associated with the UV-Lakes Project have UV-B lamp phototrons (Williamson et al. 2001) (slide 2) . Using this instrument, we can manipulate the presence/absence of repair wavelengths and, therefore, determine whether or not an organism utilizes PER in dealing with DNA damage. After exposure in the phototron, we record survival of the zooplankton for a period of 5 days at 20° C. Our results indicate that genera such as Daphnia rely heavily on PER for survival, whereas Asplanchna girodi adults seem to utilize dark repair much more and have little to no PER (Grad et al. 2001).
One of the major goals of the UV-Lakes Project is to tease out the types and relative importance of DNA repair done by the different groups of zooplankton in freshwater lakes.
For a more complete discussion about DNA repair, please follow the link to the DNA Damage and Repair section in the Task Bar at the upper left.
Life History Stages and Reproduction
The ability of different life history stages to tolerate UV radiation has been found to vary. In daphnids, calanoid copepods, and Chaoborus instars, juveniles are less tolerant to UV than are adults (Lacuna and Uye, 2000, 2001, Leech and Williamson 2000). When testing daphinds and copepods on our UV lamp phototron we have also found that the juveniles tend to be much less tolerant than adults of the same species when exposed to sublethal UV doses. In fact, we tested four different life history stages of Daphnia pulicaria and found that 2-4 day old juveniles were the least tolerant, adults had the third highest tolerance, subitaneous eggs exposed while in the brood pouch has the second highest tolerance, and eggs inside ephippial cases had by far the highest tolerance to UV (unpublished data) (slide 3). When the rotifer Asplanchna girodi was tested, however, we found that juveniles were much more tolerant than were the adults (by about a factor of 2, Grad et al. 2003). We also found that juveniles exhibited significant photoenzymatic repair while the adults did not. This is an important distinction if the data is to be used for modeling purposes because reciprocity would hold for the adults (as we have previously demonstrated) but should not hold for the juveniles.
For a more complete discussion of reciprocity, please follow the link to the Reciprocity section in the Task Bar at the upper left.
Exposure to UV radiation can also affect the ability of zooplankton to reproduce. Grad et al. (2001) found that reproduction was reduced (by about a factor of 2) in Daphnia pulicaria and Asplanchna girodi exposed to sublethal levels of UV in the presence of photorepair radiation, while in the absence of photorepair radiation, the few offspring which were born died shortly afterwards. This study showed that in Asplanchna exposed to UV photorepair radiation was much more important to reproductive processes than to survival processes, indicating that the effectiveness of repair systems could differ systematically within an orgainsm. It should be noted that both daphnids and rotifers in these experiments were reproducing asexually.
Info and photographs of various cladocerans (University of Guelph)
General
information about zooplankton ecology and identification:
( The zooplankton project (SMSU))
Brehm, V. 1938. Die Fotfarbung von Hochgebirgsseeorganismen. Biol. Rev. 13: 307-318.
Grad, G., B.J. Burnett, and C.E. Williamson. 2003. Ultraviolet damage and photoreactivation: timing and age are everything. Photochemistry and Photobiology. 78: 225-227. PDF file.
Grad, G., Williamson, C.E. and Karapelou, D.M. 2001. Zooplankton survival and reproduction responses to damaging UV radiation: A test of reciprocity and photoenzymatic repair. Limnol. Oceanogr. 46: 584-591. PDF file.
Hairston, N.G. Jr. 1978. Carotenoid photoprotection in Diaptomus kenai. Verh. Internat. Verein. Limnol. 20: 2541-2545.
Hairston, N.G. Jr. 1979. The adaptive significance of color polymorphism in two species of Diaptomus (Copepoda). Limnol. Oceanogr. 24: 15-37.
Hairston, N.G. Jr. 1980. The vertical distribution of diaptomid copepods in relation to body pigmentation. In: Kerfoot, W.C. (Ed.): Evolution and Ecology in Zooplankton Communities. pp. 98-110, University Press, Hannover.
Hutchinson, G.E. 1967. A Treatise on Limnology. II. Introduction to lake biology and limnoplankton. 2nd ed. - John Wiley and Sons, New York.
Lacuna, D.G., and Uye, S. 2001. Influence of mid-ultraviolet (UVB) radiation on the physiology of the marine planktonic copepod Acartia omorii and the potential role of photoreactivation. J. Plank. Res. 23: 143-155.
Lacuna, D.G., and Uye, S. 2000. Effect of UVB radiation on the survival, feeding, and egg production of the brackish-water copepod, Sinocalanus tenellus, with notes on protoreactivation. Hydrobiologia. 434: 73-79.
Leech, D. M. and Johnsen, S. 2003. Behavioral responses - UVR avoidance and vision. UV effects in aquatic organisms and ecosystems. E. W. Helbling and H. E. Zagarese. Cambridge, U.K., Royal Society of Chemistry. 1: 455-481.
Leech, D.M., and Williamson, C.E. 2001. In situ exposure to ultraviolet radiation alters the depth distribution of Daphnia. Limnol. Oceanogr. 46: 416-420. PDF file.
Leech, D.M., and Williamson, C.E. 2000. Is tolerance to UV radiation in zooplankton related to body size, taxon, or lake transparency? Ecol. Appl. 10: 1530-1540. PDF file.
Mitchell, D.L., and Karentz, D. 1993. The induction and repair of DNA photodamage in the environment. In Young, A.R., Bjorn, L.O., Moan, J., and Nultsch, W. (Eds.): Environmental UV Photobiology. pp. 345-377, Plenum Press.
Moore, A.R. 1912. Concerning negative phototropism in Daphnia pulex. J. Exp. Zool. 13: 573-575.
Rhode, S.C., Pawlowski, M., and Tollrian, R. 2001. The impact of ultraviolet radiation on the vertical distribution of zooplankton of the genus Daphnia. Nature. 412: 69-72.
Ringelberg, J., Keyser, A.L., and Flik, B.J.G. 1984. The mortality effect of ultraviolet radiation in a translucent and in a red morph of Acanthodiaptomus denticornis (Crustacea, Copepoda) and its possible ecological rlevance. Hydrobiologia 112: 217-222.
Siebeck, O. Vail, T.L., Williamson, C.E., and others. 1994. Impact of UV-B radiation on zooplankton and fish in pelagic freshwater ecosystems. Arch. Hydrobiol. Beih 43: 101-114.
Sutherland, B.M. 1981. Photoreactivation. Bioscience. 31: 439-444.
Williamson, C.E., Neale, P.J., Grad, G., De Lange, H.J., and Hargreaves, B.R. 2001. Beneficial and detrimental effects of UV on aquatic organisms: Implications of spectral variation. Ecol. Appl. 11: 1843-1857. PDF file.
Zagarese, H.E. and Williamson, C.E. 1994. Modeling the impacts of UV-B radiation on ecological interactions in freshwater and marine ecosystems. Stratospheric ozone depletion/UV-B radiation in the biosphere. R. H. Biggs and M. E. B. Joyner. Berlin, Springer-Verlag. I 18: 315-328.
