Species specific differences and cell size aspects

Large differences in vulnerability for DNA damage induction have been reported for aquatic organisms. Joux et al. [68] showed a high variability in UV-B responses in marine bacterial isolates as determined by survival. In contrast, CPD accumulation in the absence of repair was similar for four of five of the isolates tested. Photoreactivation kinetics were shown to be more likely candidates determining UV vulnerability. All species exhibited photoreactivation, especially under UV-A. It was concluded that UV-B may affect the microbial community structure in marine surface waters. This was supported recently in microcosm experiments carried out the Gulf of Mexico and the Southern Ocean [109].

Phytoplankton groups and species also differ in their vulnerability for UVR-induced damage. Diatoms, green algae and cyanobacteria are thought to be most resistant to UVR, followed by prymnesiophytes and other flagellates [110]. Long-term in situ experiments showed shifts in species composition in favor of more UVR resistant organisms (i.e. diatoms) in marine Antarctic phytoplankton assemblages [4,111] and a fresh water phytoplankton community [3]. Karentz et al. [9] described differences in DNA damage levels and cell survival related to differences in cell size for a variety of diatom species. It was suggested that UV-B-induced DNA damage occurs more frequently in small cells than in larger cells, due to a lower DNA screening efficiency by cell components or UVR screening compounds such as MAAs. Sommaruga and Buma [112] demonstrated large species-specific differences in DNA damage accumulation in aquatic phagotrophic protists, with representatives of the Kinetoplastida (bodonids) being the most vulnerable. This high vulnerability was thought to be related with the high AT content of these organisms [112]. Wiencke et al. [113] investigated CPD induction in zoospores of brown seaweeds, showing species-specific variability in CPD induction in these zoospores when exposed to artificial light. It was also assumed that DNA damage in zoospores might be higher when compared with induction rates in sporophytes. The occurrence of DNA damage in the zoospores of these seaweeds could affect the survival and selective adaptation in benthic macroalgae. Alterations in the genomic information at this early developmental stage could have important consequences for the later development of sporophytes and gametophytes [113]. Van de Poll et al. [73] demonstrated large species specific differences in UV-B vulnerability in a range of marine red macrophytes when grown under identical artificial UV-B conditions. Littoral species were highly UV-B resistant and DNA damage accumulation was negligible. In contrast, some sublittoral species showed high CPD accumulation rates due to low repair capacity. They concluded that DNA repair pathways play a major role in determining the UV sensitivity of red macrophytes. In addition, structural differences in UV tolerance between the tested species appeared to reflect their natural habitat in the water column (littoral, sub-littoral) [73].

It is generally believed that small organisms are more susceptible to DNA damage induction than larger cells. Garcia-Pichel [114] calculated that cells <2 ^m cannot efficiently use UVR-absorbing compounds as sunscreens. In apparent support of this, Joux et al. [68] showed that CPD accumulation in four of five marine bacterial isolates was similar to damage accumulation in a DNA solution. Smaller organisms, therefore, would have less ability to protect themselves from UV-B induced damage. Karentz et al. [9] found a positive trend between the surface-to-volume ratio and photoproduct induction in cultures of Antarctic phytoplankton showing that smaller cells accumulated DNA damage faster than larger cells (Figure 1). If this is generally applicable, open ocean plankton would be highly vulnerable to DNA damage induction. Marine bacteria and tiny phototrophic plankton, such as prochlorophytes and Synechococ-cus spp., form the majority of the organisms in (oligotrophic) tropical marine waters, comprising up to 99 % of total particulate DNA [ 115,116]. As found in a comparative UV-B vulnerability study, however, small open ocean phytoplankton were not per definition more vulnerable to CPD accumulation than larger phytoplankton cells from other areas (see section 9.7). On the other hand, many studies have shown that size fractions, dominated by bacteria, showed higher CPD numbers as compared with larger size fractions from the same water sample [11,94] (see also section 9.7). For example, in Antarctic marine assemblages, CPD accumulation occurred at a much higher rate in the bacterial fraction as compared with the phytoplankton fraction > 10 pim (Figure 10). The same was found for a pelagic community in Bahia Bustamante, Argentina [96]. On the other hand, no significant differences between smaller and larger size fractions were found for other locations. Although Lyons et al. [52] generally detected less DNA damage in the >0.8 pim size fraction as compared to the small size fraction, on occasion, they reported higher DNA damage levels in the >0.8 Hm size fraction collected in the water column from a reef area. Also, Jeffrey et al. occasionally measured more damage in the >0.8 /im fraction in the Gulf of Mexico ([11,87] and unpublished results). More recently, Maedor et al. [108] found lower levels of DNA damage in bacterioplankton than larger plankton in incubations conducted at Palmer Station. Laurion and Vincent [117] demonstrated in subarctic lakes that cyanobacteria-dominated picophytoplankton were more resistant to UV-B as could be expected from relationships based on cell size. However, in this study photosynthesis measurements were used for vulnerability assessments, which may not be comparable with vulnerability for DNA damage induction. Helbling et al. [53] argued that in general small cells

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Figure 10. Plankton incubation experiments at Rothera Station (Antarctica), 1998. January 21, 30 and February 1st: clear days; January 27th: some clouds. Gray bars: incident, biologically effective UY-B (CPD Mb-1) measured with a DNA dosimeter; black bars: accumulated damage in the small size fraction (0.2 to 2 /¿m) (CPD Mb-1); white bars: accumulated damage in the large size fractions (>10 fxm). Samples were incubated from 9.00 until ~ 19.00. Error bars represent standard deviations of the mean of at least two measurements.

21 Jan 27 Jan 30 Jan

1 Feb

Figure 10. Plankton incubation experiments at Rothera Station (Antarctica), 1998. January 21, 30 and February 1st: clear days; January 27th: some clouds. Gray bars: incident, biologically effective UY-B (CPD Mb-1) measured with a DNA dosimeter; black bars: accumulated damage in the small size fraction (0.2 to 2 /¿m) (CPD Mb-1); white bars: accumulated damage in the large size fractions (>10 fxm). Samples were incubated from 9.00 until ~ 19.00. Error bars represent standard deviations of the mean of at least two measurements.

are more vulnerable for DNA damage induction whereas they are more resistant to damage to the photosynthetic apparatus.

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