Most of our knowledge on the interaction between UVR and mutualistic associations is based on studies on algal-invertebrate symbiosis, particularly on scleractinian corals and their endosymbiotic dinoflagellates, the so-called zooxanthellae from the genus Symbiodinium. Recently, the scientific literature on this topic has been extensively reviewed [33,62,63]. Here, I will only briefly highlight the most important aspects regarding potential population changes in this association as affected by UVR and review the information for other symbiotic relationships.
Bleaching or discoloration in corals has increased dramatically in tropical areas over the past 20 years. This phenomenon is the result of the expulsion or loss of endosymbionts or at least their pigments. Although not necessarily lethal to the coral, widespread bleaching may cause massive death of coral reefs [64,65]. The role of solar UVR as responsible for coral bleaching remains controversial . However, independently of the factor(s) that may cause bleaching, expulsion of endosymbiotic zooxanthellae will expose them directly to the potential negative effects of UVR. Several studies have shown that UVR inhibits the growth of different species of Symbiodinium when isolated from the host, although species-specific differences in sensitivity have also been observed . Furthermore, UVR severely depresses photosynthetic rates in freshly isolated zooxanthellae from corals or other reef organisms, but this effect is small or absent in hospite. For example, in Prochloron sp., a prokaryotic microalgal symbiont of a colonial tropical ascidia, photosynthesis was strongly inhibited in isolation but not in the host . The different sensitivity between isolated and in hospite forms appears to be related to protection given by the host through the accumulation in their tissue of sunscreens such as mycosporine-like amino acids (see Chapter 10).
Symbiosis between algae and protists is widespread, for example, among marine and freshwater planktonic ciliates and large foraminifera. In some cases, the whole cell of different groups of algae lives as endosymbionts in the host, while other species preserve only the plastids. Although the latter type of association is not a "true symbiosis", functionally, plastids represent a source of photosynthetic products for the host, similarly to true algal endosymbionts . Surprisingly, effects of UVR on algal-protozoan symbiotic associations have hardly been studied despite their important role in food webs, particularly as primary producers in oligotrophic systems. Martin-Webb  performed UV-exclusion experiments with natural ciliate assemblages including Mesodinium rubrum and Laboea strobila collected from a shallow area (Georg Bank, 40-42 °N) on the continental shelf, NW Atlantic. The haptorid ciliate M. rubrum contains a cryptophyte symbiont and is an important primary producer in coastal areas , while the large-sized L. strobila is a conspicuous plastidic oligotrichid in temperate waters . Results from these experiments indicated a lower UV-B sensitivity in symbiotic than other ciliates from this coastal area .
Large foraminifera from several families have endosymbionts represented by chlorophytes, rhodophytes, dinoflagellates, or pennate diatoms. The type of algal symbionts appears to influence the optimal depth occupied by some species of foraminifera . Yet, bleaching, particularly of the reef-dwelling Amphis-tegina gibbosa, has been observed in populations of subtropical waters like the Florida Keys . Indirect evidence suggests that, similar to bleaching in corals, UVR may be contributing to this phenomenon [72,73]. Thus, for example, A. gibbosa shows a seasonal bleaching cycle with maxima during the summer solstices, preceding maximum summer water temperature by ca. two months. Moreover, bleaching in this species is observed in remote areas, where pollution is unlikely to be a contributing factor [72,73]. Interestingly, affected A. gibbosa is highly predated by the foraminifer Floresina amphiphaga and also often found infested by cyanobacteria. These observations were never recorded before the detection of bleaching in this species .
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