Karentz et al. [1991] [Karentz Cleaver Mitchell 1991 JPHYCO] studied twelve species of Antarctic diatoms for cell survival characteristics and molecular responses to UV-B radiation and determined the average exposure for cell death. Their studies, which did not simulate natural sunlight conditions and were not intended to do so, showed that: 1) dose responses of population survival to UV exposure varied considerably among species, and there were significant differences as a function of wavelengths available or absent for photorepair; 2) a general relationship was evident between the surface area/volume ratios of cells and the amount of damage induced by UV exposure. Smaller cells, with larger ratios, sustained greater amounts of damage per unit of DNA. However, when they studied growth of diatom species under natural irradiance in the Antarctic [Karentz tolerance 1994], they found that the growth rates were not enhanced when UV was excluded.
The results of Helbling et al. [Helbling Villafane Holm-Hansen 1994], interpreting the distributions and characteristics of Antarctic phytoplankton, did not provide conclusive results with respect to the relative influence of UVR on different size components of phytoplankton photosynthesis. They show that small flagellates are strongly inhibited by UVR but also suggest that these phytoplankton are able to acquire resistance to UV by photoadaptive processes. Further their results suggest that diatoms, most of which were in the larger microplankton size range, were relatively resistant to UVR, consistent with the culture experiments of Karentz [Karentz tolerance 1994]. Since it has long been recognized that differential influence of UV radiation with respect to size of phytoplankton could have major implications for lower food web processes [elsayed 1988], this issue of differential sensitivity to UVR possibly selecting for size remains an important unresolved question.
Phytoplankton have evolved a variety of protective mechanisms associated with high solar radiation in general and high UV exposure in particular. One mechanism is the synthesis of UV-absorbing compounds. Chalker and Dunlap [Chalker Dunlap 1990] summarize a substantial body of literature dealing with UV-B and UV-A absorbing compounds in marine macroalgae. They point out that UV-B absorbing compounds (especially mycosporine-like amino acids, MAAs) have been found in many marine organisms, are frequently related to environmental levels of UV radiation [Dunlap Chalker Oliver 1986, Sivalingham Ikawa Nisizawa 1974], and hence have been proposed as a physiological adaptation to UV exposure. Vernet [1990] [Vernet 1990 UV radiation antarctic] showed that Antarctic phytoplankton exposed to ambient levels of UV radiation seem to have the ability to synthesize potentially protective UV-absorbing compounds, and that they may have the capacity to utilize some of the UV radiation in photosynthesis through pigments that absorb below 400nm. El Sayed et al. [1990] [ElSayed Stephens Bidigare Ondrusek 1990] show changes in photosynthetic pigmentation with elevated UV-B. Carreto et al. [Carreto Carignan Daleo 1990] in laboratory studies have demonstrated that MAA synthesis is stimulated by UVA, adding further evidence for potential physiological adaptation. Karentz et al. [1991a] surveyed 57 species (1 fish, 48 invertebrates, and 8 algae) from the vicinity of Palmer Station (Anvers Island, Antarctic Peninsula) for the presence of MAAs. They found that the majority of species examined had absorbance peaks in the range from 315 to 335nm and they identified eight MAAs. They suggest that this widespread occurrence of MAAs found in Antarctic marine organisms may provide some degree of natural biochemical protection from UV exposure during spring ozone depletion. Bidigare et al. [Bidigare Ondrusek Kang 1992] provided further direct chemical confirmation of MAAs in marine phytoplankton from the Southern Ocean. Their work during Icecolors'90 was undertaken to measure directly the effects of ozone diminution and UV radiation on Southern Ocean phytoplankton. Along a north-south transect across the marginal ice zone (MIZ), they found concentrations of diadinoxanthin (a photoprotective carotenoid found in Phaeocystis spp. and diatoms) highest in surface waters and decreasing with increasing depth suggesting photoprotective adaptation to potentially harmful irradiance.
Vernet and co-workers [Vernet Brody Holm-Hansen Mitchell 1994], studying the response of Antarctic phytoplankton to UVR, found results supporting the hypothesis that UV-absorbing compounds are photoprotective. Making use of the ratio of in vivo pigment specific phytoplankton absorption at 330nm and 675 nm, these workers found this ratio to be a good index of the relative absorption of UV with respect to photosynthetic pigment absorption. Their data suggest that synthesis of UV-absorbing compounds may occur as a response to phytoplankton assemblages exposed to relatively high surface irradiance because of shallow mixed depths or relatively clear waters, thus providing photoadaptive protection to enhanced levels of UVR. These findings are in general agreement with the analysis of Helbling et al. (1994).
All of the above observations lend credibility to the hypothesis that
physiological adaptation to UV exposure is possible in at
least some species of phytoplankton.
On the other hand, Cullen and Neale (1994) point out that there is little
direct evidence indicating the biological functions of UV-absorbing
compounds [Karentz 1993] and then go on to discuss recent results of
protection by UV-absorbing compounds in terrestrial plants.
While there may be analogies to mechanisms in terrestrial plants, these
authors point out that in microalgae, UV absorbing compounds operate over much
shorter pathlengths and hence may only provide partial protection to UV-B.
It is evident that a wide variety of mechanisms are available for the
acclimation of microalgae of UV-B [Marchant 1994, Vincent Roy 1993].