Quenching mechanisms

Exposure to UVR is known to result in a variety of negative effects; however, the interaction of UVR with photosensitizing molecules, some organic molecules and oxygen results in the production of toxic photoproducts including reactive oxygen species (ROS) both intracellular^ and in the external environment. Toxic photoproducts have the ability to cause more damage than the UVR exposure itself. Toxic photoproducts are neutralized by various agents including antioxidants such as ascorbate, quenchers such as carotenoids and various scavenging enzymes, the levels of which are up-regulated by the presence of UVR [123-125]. Exposure to UVR can result in an increase in the production of photoreactive species on the one hand and, on the other hand, result in the induction of enzymes to neutralize these species.

10.3.1 Antioxidants

Antioxidants, quenching molecules or radical scavenging enzymes such as superoxide dismutase (SOD), ascorbate peroxidase and catalase react with and neutralize the effects of the highly toxic ROS or hydroxyl radicals (HO) pro duced by the photooxidation of cell structures by UVR (see Chapter 8). Studies have shown that there is a positive relationship between the concentration of these antioxidant enzymes with PAR and UVR. In the temperate sea anemone Anthopleura elegantissima there was higher activity of SOD in UVR-exposed than in UVR-shielded individuals [126]. Symbiotic algae from the tropical anemone Aiptasia pallida exhibited higher antioxidant concentrations in UVR-exposed versus UVR-shielded anemones [123] and the effects are reversible by reciprocal transplantation of individuals taken from high light and low light conditions [124]. In the symbiotic dinoflagellate Symbiodinium microadriaticum it was found that SOD concentration is directly related to oxygen tension when compared in cultured cells grown under hypoxic, normoxic and hyperoxic conditions [127]. The levels of these scavenging enzymes are increased by temperature, PAR and UVR in symbiotic algae of the zoanthid Palythoa cari-baeorum [125] and their concentration decreases with increasing depth in the coral Acropora microphthalma [82].

Flavonoids possess free-radical scavenging activity [128] and the MAA my-cosporine-glycine also exhibits moderate antioxidant activity [103]. These compounds may be additional sources of protection against photooxidative stress due to the high concentrations of oxygen in photosynthetic symbioses. Imino-type MAAs such as palythine and shinorine had no detectable antioxidant activity [103].

10.3.2 Carotenoids and the xanthophyll cycle

Carotenoids, perhaps best known as photosynthetic accessory pigments, are also radical-trapping antioxidants, which scavenge oxygen radicals, neutralize the singlet excited state of oxygen OO2), quench the triplet state of chlorophyll a, which occurs under excess light exposure, and inhibit lipid peroxidation. Increased concentrations of carotenoids were found in natural populations of the surface-blooming cyanobacterium Microcystis aeruginosa [129] and after exposure to artificial UV-B in cultures of the marine phytoplankter Tetraselmis sp. [128] and attributed to the photoprotective function of these pigments. In eukaryotes, some carotenoids are also part of the xanthophyll cycle that quenches excess photochemical energy by dissipating it as heat and thereby limiting photoinhibition [131]. The advantage of this cycle is that it has a very rapid response time to high photon flux density. In seagrasses, the xanthophyll cycle consists of a series of light-dependent reactions involving three oxygenated derivatives of carotenoids: violaxanthin, antheraxanthin and zeaxanthin [132]. During high light exposure, violaxanthin is converted into antheraxanthin and subsequently into zeaxanthin, which then accumulates, until there is a sufficient decrease in light exposure for reconversion into violaxanthin [133]. While important in protection against high photon flux density in seagrasses, it was found that the conversion into zeaxanthin was not sufficient to protect against UVR in Halodule ovalis [132]. Ascorbate, apart from its function as an antioxidant, also functions to reduce violaxanthin to antheraxanthin and zeaxan-

thin, leading to thermal dissipation of excess energy [133].

In many species of microalgae, such as dinoflagellates, diatoms and prym-nesiophytes, the xanthophyll cycle pigments found in seagrasses are absent and, instead, the reaction involved is the reversible conversion of diadinoxanthin (Dn) into diatoxanthin (Dt) by de-epoxidation. High concentrations of Dn were found in natural populations of the red-tide dinoflagellate Prorocentrum micans and a possible role in photoprotection under high illumination was proposed [87] although interconversion between Dn and Dt was not examined. Under exposure to high photon flux densities, the free-living dinoflagellate Alexandrium excavatum accumulated Dt, which was reverted back into Dn under dark conditions with no new pigment synthesis involved [134]. Following exposure to artificial UVR and PAR over a 40 day period of cultures of the common coastal diatom Thalassiosira weissflogii, Dt was the only pigment to show a distinct increase [66]. This study also showed that the induction of the xanthophyll cycle coincides with increased photoinhibition (as inferred from decreased photochemical capacity) and decreased growth rates whereas when photoinhibition decreased, so did the concentration of Dt suggesting a photoprotective role by the xanthophyll cycle. Recovery from photoinhibition did not require the prior accumulation of MAAs but rather induction of the xanthophyll cycle and once the cells had recovered from photoinhibition, MAAs accumulated and the concentration of xanthophyll cycle pigments decreased [66]. The xanthophyll cycle also functions in dinoflagellates symbiotic with corals [135] with the interconversion of Dn and Dt showing a strong diurnal pattern [136]. In some microalgae, the xanthophyll cycle involves the conversion between violaxanthin and zeaxanthin [136]. The xanthophyll cycle pigments barely absorb in the UVR and therefore their role in preventing damage by UVR may not be directly as photoprotectants but indirectly as quenchers of toxic photoproducts.

Transparency is a common adaptation to lake and oceanic environments. As a camouflage, transparency is complicated by the presence of UVR because the presence of UVR-absorbing compounds decreases UV transparency and may reveal some organisms to predators and prey with UV vision [137]. However, in general, pigmented species are less sensitive to solar radiation than unpigmented species [60]. Planktonic crustaceans exhibit marked colouring [138]. For example, the calanoid copepod Acanthodiaptomus denticornis has both translucent and red-coloured morphs, whose coloring is due to carotenoid pigments derived from their algal food source. The red colored morphs were shown to have lower mortality than translucent morphs when exposed to UVR [139].

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