Screening mechanisms

Screening can eliminate or at least reduce exposure to UVR by absorbing or reflecting damaging wavelengths prior to reaching UVR-sensitive cellular components. Screening may consist of the production of physical barriers such as morphological or structural features that prevent damaging wavelengths from passing or by the production of chemical compounds that absorb UVR. Usually, screening mechanisms, both physical and chemical, serve more than one purpose and thereby decrease the energy required to filter out damaging UV wavelengths.

10.2.1 Physical screening

Morphological and structural features that constitute physical barriers serve other purposes in addition to protection from damaging UVR. For example, structural features such as shells and spines also provide protection from predators. Other structural features include the production of mucous, sporopollenin and multiple-layered cell walls.

10.2.1.1 Mucus

The presence of mucus may act in physically screening cells from damage by UVR. The prymnesiophyte Phaeocystis pouchetii has a colonial stage in which the cells are embedded in a mucus matrix that has a high concentration of UVR-absorbing compounds that are excreted by individual cells but function in protecting the entire colony [7]. Mucus such as is excreted by Fungia species is believed to provide protection against sediments, desiccation and might also decrease the damaging effects of UVR by the presence of MAAs in the mucus [8,9], although the UVR screening property of MAAs in the mucus is poor [10].

10.2.1.2 Sporopollenin

Sporopollenin is a biopolymer of variable composition found in some algal cell walls, plant pollen and spores, and may function as an antimicrobial agent and provide a rigid cell wall support in large-celled species [11]. The absorption spectrum of sporopollenin has no peaks but rather increases in optical density with decreasing wavelength in the UV region. In a study of 16 species of microalgae, it was shown that 8 species, which were highly tolerant to UVR, had substantial amounts of sporopollenin that occurs in the algal cell walls and absorbs UVR, whereas the other 8 species, which were highly susceptible to UVR, contained little or no sporopollenin [11]. In addition, it was found that the sporopollenin provides constant background protection, whereas MAAs, which are also present, are inducible but exhibit a lag time in their synthesis.

10.2.1.3 Multiple cell walls

When exposed to artificial UVR for four weeks in culture, the temperate, symbiotic dinoflagellate Symbiodinium californium developed multiple-layered cell walls and this phenomenon disappeared after the cells were returned to culture conditions in the absence of UVR [12]. These additional cell walls were suggested to protect the UVR-sensitive cellular components from damage by UVR wavelengths. Such multiple-layered cell wall production was not observed in hospite, therefore the host anemone, Anthopleura elegantissima, which contains high concentrations of MAAs may provide sufficient protection from UVR under natural conditions.

10.2.2 Chemical screening

There are a number of different types of UVR-absorbing compounds, most of which have functions apart from photoprotection against UVR damage. Myco-sporine-like amino acids are the most commonly encountered UVR-absorbing compounds in aquatic organisms; however, other compounds that absorb in the UV-A and UV-B region include scytonemin, 3-hydroxykynurenine, melanin, various secondary metabolites and fluorescent pigments.

10.2.2.1 Mycosporines

Mycosporines are a group of compounds, first identified in the mycelia of various species of fungi as the compound P310, and shown to be absent in colonies grown in darkness and closely associated with photosporogenesis [13,14]. P310 is a low molecular weight, water soluble compound with a strong absorption at 310 nm resulting from the linking of a cyclohexenone ring with the nitrogen substituent of an amino acid or amino alcohol and named mycosporine [15]. UVR has been shown to be important in inducing sporulation in a number of fungal groups [14] and mycosporine was hypothesized to act as a photoprotec-tant in fungal spores, which during dispersal by atmospheric transport are directly exposed to solar radiation [16]. Various mycosporines have been identified depending on the attached substituent with the only amino acids involved being serine, glutamine and glutamic acid or their corresponding amino alcohols, serinol, glutaminol and glutamicol, respectively, or the amino acid, alanine [17,18]. The cyclohexenone unit is derived from the shikimic acid pathway [19], which is the same pathway involved in the synthesis of higher plant photoprotec-tants such as flavonoids [20].

10.2.2.2 Mycosporine-like amino acids (MA As)

Mycosporine-like amino acids (MAAs) are imino carbonyl derivatives of mycosporines. Originally termed 'S-320,' due to the maximum wavelength of absorption at approximately 320 nm in extracts from 5 species of Acropora, one species of Pocillopora and a species of cyanobacterium [21], these compounds were later identified as MAAs in the staghorn coral Acropora formosa [22]. The compounds are made up of a cyclohexenone or, more commonly, a cyclohexenimine ring with an amino acid side group. For further information on the chemistry of MAAs and of mycosporines, consult reference [18], on MAAs in coral reef organisms consult reference [23] and on MAAs in dinoflagellates consult refer ence [24]. In groups other than fungi, the only known mycosporines are myco-sporine-glycine and mycosporine-taurine. All other identified compounds are based on the aminocyclohexenimine ring system and are collectively termed mycosporine-like amino acids [18,25]. In this chapter, to prevent confusion between mycosporines derived from fungi and those from aquatic organisms, all mycosporine or mycosporine-like compounds present in aquatic species will be referred to as MAAs.

Absorption spectra of MAAs follow a normal distribution and absorb over a width of approximately 20 nm and most marine and freshwater organisms contain a suite of MAAs thus extending the photoprotective potential across a broader spectrum. The maximum wavelength of absorption is referred to as xmax and of the 19 known MAA compounds, Amax ranges from 309 to 360 nm (Table 1). An additional compound, Gadusol, which absorbs maximally at 296 nm and is related to MAAs, has been found in fish eggs and the brine shrimp Artemia [36,42]. Differences in absorption by MAAs are determined by the substitution of different amino acids and alcohols or other amino acid functionalities to mycosporine-glycine [18]. MAAs are extracted using methanol as the sole or principal solvent and the extracts separated using reversed-phase, high-performance liquid chromatography (HPLC) with a UV detector. Quantification of MAAs uses a set of purified standards. Standards are not commercially available therefore some studies have resorted to using Amax and published retention times. As more compounds are being identified, particularly those with similar retention times and Amax to the more commonly encountered MAAs, there is an increasing risk of incorrectly identifying and quantifying MAAs without standards. Due to the wide variety of MAAs that can be present in an extract, spectrophotometric scans cannot be used to identify the MAAs present but are often used as a preliminary test to determine if MAAs are present or absent and in the absence of HPLC can be used to plot changes in relative absorbance.

Phyletic distribution of MAAs: MAAs have been described from a wide variety of habitats ranging from Antarctica [39] and the Arctic to temperate [43] and tropical oceans [22] as well as from tropical [44] to alpine [45] and high Arctic lakes including freshwater and terrestrial ecosystems [46,47]. MAAs occur in a wide variety of aquatic organisms, spanning phytoplankton, all of the major algal divisions, almost all invertebrate phyla, and vertebrates (Table 2). To date these compounds have been identified in most but not all phyla, but whether some of these truly do not contain MAAs or potentially that very few studies have been attempted on lesser-known phyla needs to be verified. Coelenterates contain the greatest number of MAAs (Table 2) partially because so many studies have been conducted on this phylum and because they are the group from which the majority of the MAAs have originally been identified (Table 1). Not all species within each division or phylum synthesize or accumulate MAAs. As an example, a study of 26 isolates of symbiotic dinoflagellates in culture, all exposed to the same condition of UVR and PAR, showed that 15 of the isolates synthesize MAAs and that there was no correlation between the depth of original collection and the capacity for synthesis of MAAs [62].

Table 1. Identified mycosporine-like amino acids, giving their full name, abbreviation used in Table 2, the absorption maximum (Amax), the coefficient of extinction (e for solvent used consult the appropriate reference; ND = not determined), the species from which the compound was first isolated and the reference for that identification

Table 1. Identified mycosporine-like amino acids, giving their full name, abbreviation used in Table 2, the absorption maximum (Amax), the coefficient of extinction (e for solvent used consult the appropriate reference; ND = not determined), the species from which the compound was first isolated and the reference for that identification

Full name of mycosporine-like amino acid

Abbreviation

e (dm3 mole1 cm ')

Species

Reference

Mycosporine-taurine

MT

309

ND

Anthopleura eleganstissima

26

Mycosporine-glycine

MG

310

28100

Palythoa tuberculosa

27

Palythine

PI

320

36200

Palythoa tuberculosa

28

Chondrus yendoi

29

Palythine-serine

PS

320

10500

Pocillopora eydouxi

30

Palythine-threonine-sulfate

PTS

321

ND

Stylophora pistillata

31

Palythine-serine-sulfate

PSS

321

ND

Stylophora pistillata

31

N-Methylmycosporine-serine or

Mycosporine-methylamine-serine

MS

325

16600

Pocillopora eydouxi

30

Asterina-330

AS

330

43800

Asterina pectinifera

32

Mycosporine-glutamic acid-glycine

MGG

330

43900

Dysidea herbacea

33

iV-Methylmycosporine-threonine or

Mycosporine-methylamine-threonine

MMT

330

33000

Stylophora pistillata

34

Mycosporine-2-glycine

M2G

331

ND

Anthopleura eleganstissima

26

Palythinol

PL

332

43500

Palythoa tuberculosa

35

Mycosporine-glycine-aspartic acid

MGA

332-334

ND

Artemia sp.

36

Porphyra-334 (mytilin B)

PO

334

42300

Porphyra teñera

37

Shinorine (mytilin A)

SH

334

ND

Chondrus yendoi

38

Mycosporine-glycine-valine

MV

335

ND

Not specified

39

Palythenic acid

PA

337

29000

Halocynthia roretzi

40

Usujirene

US

357

ND

Palmaria palmata

41

Palythene

PE

360

50000

Palythoa tuberculosa

UJ LtJ

Table 2. The phylogenetic distribution of UVR-absorbing compounds in marine species. + indicates that the compound has been positively identified by HPLC in at least one species in the phylum or division indicated, - indicates that an analysis by HPLC was performed but no evidence of the compound was found and a blank space indicates that no reference was found in the literature for positive identification by HPLC; ? indicates a tentative identification; abbreviations of compounds are given in Table 1, with the addition of GA (Gadusol) and the maximum absorbance, in nm, is given in brackets; data were compiled primarily from original identifications and studies involving surveys or more than one species; studies using artificial feeding, phytoplankton assemblages, partial characterization or not using HPLC analysis were not used; in addition to the references used in Table 1, the following references were consulted: [42,43,45,48-61]

Table 2. The phylogenetic distribution of UVR-absorbing compounds in marine species. + indicates that the compound has been positively identified by HPLC in at least one species in the phylum or division indicated, - indicates that an analysis by HPLC was performed but no evidence of the compound was found and a blank space indicates that no reference was found in the literature for positive identification by HPLC; ? indicates a tentative identification; abbreviations of compounds are given in Table 1, with the addition of GA (Gadusol) and the maximum absorbance, in nm, is given in brackets; data were compiled primarily from original identifications and studies involving surveys or more than one species; studies using artificial feeding, phytoplankton assemblages, partial characterization or not using HPLC analysis were not used; in addition to the references used in Table 1, the following references were consulted: [42,43,45,48-61]

GA

MT

MG

PI

PS

PTS

PSS

MS

AS

MGG

MMT

M2G

PL

MGA PO

SH

MV

PA

US

PE

SC

(286)

(309)

(310)

(320)

(320)

(321)

(321)

(325)

(330)

(330)

(330)

(331)

(332)

(332 - 334) (334)

(334)

(335)

(337)

(357)

(360)

(386)

Cyanobacteria

+

+

+

+

+

+

+ ?

+

Dinophyceae

+

+

+

+

+

+

+

+

+

+

+

Bacillariophyceae

+

+

-

-

+

+

-

+

Flagellates

+

+

+

Protozoa

-

-

-

-

-

-

-

Chlorophyta

+

+

+

+

+

+

-

-

Phaeophyta

+

+

+

+

+

+

-

-

Rhodophyta

+

+

+

+

+

+

-

+

+

Anthophyta

Porifera

+

+

+

+

+

+

+

+

+

+ ?

+

Ctenophora

-

-

-

-

-

-

-

-

Coelenterata

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Platyhelminthes

+

+

-

-

+

+

+

-

Nemertinea

+

+

-

-

+

+

+

-

Rotifera

+

+

+

+ ?

+

Chaetognatha

-

-

-

-

-

-

-

-

Annelida

+

+

+

+

+

+

+

+

Crustacea

+

+

+

+

+

+ +

+

+

+

+ ?

+

Mollusca

+

+

+

+

+

+

+

+

+

Bryozoa

+

+

-

+

+

+

+

Echinodermata

+

+

+

+

+

+

+

-

Protochordata

+

+

+

+

+

+

+

+

Chordata

+

+

+

+

+

+

+

+

Chordata (eyes)

+

+

+

+

Chordata (eggs)

+

Biosynthesis of MAAs: The biosynthesis of mycosporines and MAAs is via the first steps of the shikimic acid pathway [63], as evidenced by the reduction or cessation of MAA accumulation in the presence of glyphosate, an inhibitor of this pathway [64]. The synthesis of MAAs can be very rapid. In the dinoflagel-lates, Alexandrium excavatum and Prorocentrum micans synthesis occurs within hours of transfer from low (20 pmol quanta m_2.s_1) to high (250 pmo\ quanta m~2s_1) light [50], which is consistent with the rapid changes in light during vertical migration [65]. In most other species, synthesis or accumulation is slower and can be on the order of weeks as was found for the dinoflagellate Symbiodinium microadriaticum [53] and the diatom Thalassiosira weissflogii [66]. In the red alga, Porphyra umbilicalis, 72 hours is not sufficient to induce MAA synthesis even when exposed to UVR [67]. Some species synthesize and release MAAs into the surrounding medium as shown in symbiotic dinoflagel-lates [53,62] and in the bloom-forming dinoflagellate Lingulodinium (= Gonyaulax) polyedra [68]. It has been suggested that the release of MAAs into the water column by free-living dinoflagellates may contribute to the attenuation of UVR during bloom events [68],

The shikimate pathway is only known in bacteria, fungi, algae and plants and there is no evidence of this pathway in invertebrates or vertebrates. Animals must therefore obtain MAAs through their diet [39,52,53,69] or by translocation from symbionts [53]. In the association between the dinoflagellate Symbiodinium microadriaticum and the jellyfish Cassiopeia xamachana, three MAAs are synthesized by the symbionts and the same three are exported to the host [53]. The coral Stylophora pistillata contains 10 different MAAs including shinorine [64], whereas the dinoflagellate symbionts in culture only synthesize shinorine [62]. Whether the symbionts in hospite produce all 10 compounds or whether the host, or possibly bacteria are metabolically converting shinorine into other compounds is yet to be resolved. The same five MAAs, mycosporine-glycine, shinorine, Porphyra-334, palythenic acid and palythine, although in different proportions, were found in the pteropod predator Clione antarctica, and in its exclusive prey, the herbivorous pteropod Limacina helicina [69]. The phyto-plankton assemblage on which L. helicina feeds contains only shinorine and Porphyra-334. Although the increase in the number of MAAs may have been due to accumulation of the MAAs before the study began, there is also the possibility that there is biochemical or possibly bacterial conversion of MAAs in L. helicina [69]. In the association between the anemone Anthopleura elegantissima and the dinoflagellate Symbiodinium californium, MAAs were detected in host tissues but not in freshly isolated algae, the symbionts in culture nor in the culture medium, suggesting that the MAAs are derived from the diet of the anemone [53]. The same pattern may be found in the giant clam, Tridacna crocea [70]. Studies involving controlled diets of known MAA composition have shown that there is dietary accumulation of MAAs, in the green sea urchin, Strongylocentrotus droebachiensis [71], the medaka fish, Oryzias latipes [72], the sea hare, Aplysia dactylomela [73] and in the Antarctic krill, Euphausia superba [74]. In S. droebachiensis, using two controlled diets of the MAA-rich alga, Mastocarpus stellatus versus the M AA-deficient alga, Laminaria saccharina, the sea urchins fed

M. stellatus have a much higher MAA concentration in the ovaries than sea urchins fed L. saccharina [71]. The higher MAA content in the diet allows for transfer of photoprotectants to eggs when released into the water column and exposed to UVR [71]. Embryos from adults of S. droebachiensis fed L. saccharina had a slightly longer U VR-induced delay in the first division of the embryo than did embryos from adults fed M. stellatus or a combination of both algae [75]. MAAs in the embryos of S. droebachiensis adults fed an MAA-rich diet provided photoprotection against abnormalities induced by UV-B to at least the four-armed pluteus stage [76],

The survivorship of larvae from the coral, Agaricia agaricites originating from 3 m depth, which contained a 3-fold higher concentration of MAAs, was greater than for larvae originating from 24 m depth [77]. In contrast, the origin of larvae from the coral Pocillopora damicornis was not a factor in survival in an experiment comparing shallow (0.5 m) and deep (2-3 m) larvae exposed to UVR compared with UVR-shielded larvae. Rather, the effect of UVR appeared to be on settlement [78]. However, the depth ranges in the two studies were vastly different and the total MAA concentrations were much higher in larvae of A. agaricites than P. damicornis.

Photoprotective Function of MAAs: The photoprotective function of MAAs has mostly been inferred indirectly from their UVR absorption properties [21,22] and high molar extinction coefficients (Table 1) as well as a series of results showing that there is a direct relationship between exposure to UVR and MAA concentration. These include: (a) a number of observations in sessile species that shallow growing individuals have higher concentrations of "S-320" or MAAs than deeper growing conspecifics such as the corals Porites lobata [79], Acropora spp. [80], Montipora verrucosa [81], Acropora microphthalma [82], various Caribbean and Hawaiian species [55], Porites astreoides [83] and Montastraea faveolata [84]; (b) the presence of peaks in UV absorbance in surface bloom-forming species such as Gonyaulax tamarensis var excavata [85], Noctiluca miliaris [86], Prorocentrum micans and Gonyaulax polyedra [87], Phaeocystis pouchetii [7], Heterocapsa triquetra [88] and Akashiwo sanguínea ( = Gymnodinium sanguineum) [56]; (c) the decrease in concentration of these compounds when UVR is filtered out and compared with UVR treated samples such as in the pennate diatoms Psuedonitzschia sp. and Fragilariopsis cylindrus [89], the red algae Eucheuma striatum [90] and Chondrus crispus [91], the jellyfish Cassiopeia xamachana [53], the anemone Phyllodiscus semoni [51], the corals Pocillopora damicornis [2], Montipora verrucosa [92] and Porites compressa [93], and the octocoral Clavularia sp. [51]; (d) the increase in concentration of these compounds when sessile species are transplanted to shallower depths such as Montipora verrucosa [81] and Porites astreoides [94]; (e) the observation that exposed portions of benthic species have higher concentrations of MAAs than shaded portions. For example, the tops of individual colonies of Montastraea annularis contain higher concentrations of MAAs than the sides of the colonies whereas the bottoms of the colonies have the lowest concentrations of MAAs [95] and that holothurian epidermal tissue contains higher concentrations of MAAs than do internal organs [52]; (f) the observation that there is seasonal variation in MAA concentration peaking in summer as shown for the red algae Palmaria decipiens [96] and Bangia atropurpurea [97]; and (g) the greater concentration of MAAs in algae from low latitudes compared to high latitudes thus showing a relationship between natural solar UVR doses and the concentration of MAAs [43].

Direct evidence for a photoprotective role has been shown in Porphyra-334 using measurements of quantum yield of fluorescence, intersystem crossing and photolysis, which showed that this compound does not generate radicals that would cause cellular damage [98]. MAAs have been shown to act as direct, spectrally-specific photoprotectants in the surface-blooming, red tide dinoflagel-late Akashiwo sanguinea ( = Gymnodinium sanguineum) [56]. Cultures of A. sanguined grown in high light have a markedly lower sensitivity to UVR, as estimated by biological weighting functions (BWFs) and accumulate MAAs in higher concentrations when compared with low light grown cultures. The wavelength range of lowest sensitivity (325 to 355 nm) corresponds to the region of maximal absorbance by the MAAs. No significant differences in UV biological weight were found in Prorocentrum micans suggesting that MAAs provide incomplete protection in this species [99,100].

Several species have been shown to not modify "S-320" or MAA concentration in response to UVR, such as the zoanthid Zoanthus sociatus, in response to increased levels of UVR [101], the octocoral Clavularia sp. over a depth gradient [51], the coral Montastraea annularis on transplantation from 24 m to 12 m over 21 days [102] and the temperate anemone Anthopleura elegantissima in UVR-exposed versus UVR-shielded experiments [53]. Data such as these have been used to suggest that MAAs are not directly photoprotective but are rather a byproduct of other chemical reactions and that photoprotection is a secondary function. An alternative explanation is that, at least for all of the above studies that involve coelenterates, the MAAs are derived from diet and therefore rather than MAA concentration being dependent on the intensity of UVR is dependent on the concentration of MAAs in the food source.

Functions other than photoprotection that have been attributed to MAAs include antioxidant activity [103], regulation of reproduction [104] and as osmolytes [105]. The role of MAAs as osmolytes has been tested and refuted due to the small contribution that MAAs make in comparison to other osmolytes in reducing osmotic stress [106].

10.2.2.3 Scytonemin

Scytonemin occurs mostly in the extracellular, mucilaginous sheath surrounding cyanobacterial cells and occurs in every major taxonomic group of cyanobac-teria and is considered to be a photoprotective compound [107]. This pigment has a molecular weight of 544-546 Da, is yellow-brown, lipid soluble and is a dimeric structure of indolic and phenolic subunits whose synthetic pathway is poorly understood. The 2max of absorption is approximately 370 nm in vivo and 384 nm in acetone [107] and there is also a strong UVC-absorbing component peaking at approximately 250 nm that extends into the UV-B [108]. The synthesis of scytonemin is strongly induced on exposure to UV-A-blue and only weakly by UV-B [109]. In full-sun exposed habitats, the concentrations of scytonemin are high and in culture the rate of synthesis of scytonemin is increased by exposure to UV-A or to high photon flux [107-109].

10.2.2.4 3-Hydroxykynurenine

3-Hydroxykynurenine is a water soluble, low molecular weight, tryptophan derivative that occurs in the lens pigments of several species of marine and freshwater fish [110,111] and the cuttlefish, Sepia officinalis [57]. 3-Hydroxykynurenine most closely resembles screening compounds found in the lenses of primates. This compound absorbs in the UV-A region with a peak absorbance of 370 nm and may function in protecting the lens from UVR, increasing visual acuity by reducing glare, scatter and chromatic aberration (misfocusing of short wavelengths) and maximizing contrast as well as aiding in prey detection or possibly functioning as a stabilizing lens protein [111].

10.2.2.5 Melanin

Melanin absorbs at all UVR and PAR wavelengths and thus is beneficial as a sunscreen in non-photosynthetic organisms such as Arctic and Alpine cladocerans [60]. Melanin was shown to protect platyfish-swordtail hybrids of the genus Xiphophorus by lowering the number of dimers caused by exposure to UVR of 290, 302 or 313 nm [112] and thus acting as a UVR photoprotectant in the skin of fish. Melanin is also believed to have more than one function, including acting as a free radical scavenger and energy transducer.

10.2.2.6 Secondary metabolites

Some secondary metabolites, such as flavonoids, phlorotannins and tridentatol, appear to perform multiple ecologically important roles such as resistance to predators and pathogens, sequestration of heavy metals as well as absorption of UVR wavelengths.

Flavonoids: Flavonoids function in the protection of UVR-sensitive cellular components by specifically absorbing from 280 to 340 nm but allowing transmission of PAR to the chloroplasts so as to not diminish photosynthetic yield. Flavonoids are commonly found in the epidermis of leaves, acting to protect the underlying photosynthetic units [113] and are widely distributed in angiosperms including seagrasses and aquatic mosses. The phenylpropanoid pathway is stimulated by exposure to UVR, which results in an accumulation of flavonoids mainly in the upper epidermal cell layer in plants due to an increased transcription of a series of enzymes [114]. Phenolic compounds perform diverse roles in angiosperms including resistance to predators and to pathogens as well as recruitment of pollinators to flowers and the attraction of seed dispersal agents.

Phlorotannins: Phlorotannins are secondary metabolites analogous to the shikimate-derived condensed polyphenolics found in plants and brown algae and absorb strongly from 280 to 320 nm [115]. Exposure to UV-B was found to increase the concentration of phlorotannins in the brown alga Ascophyllum nodosum [116], That phlorotannins act in photoprotection may be the reason why MAAs are virtually absent in brown algae in comparison to red and green algae [39,55,117]. Protecting cells against UVR damage is not the only function of phlorotannins as they are also known to be involved in defense against herbivores, pathogens and heavy metals.

Tridentatols: Tridentatols A to D have been described in the hydroid Triden-tata marginata, which is associated with the pelagic Sargassum community [118]. Floating at the surface of the ocean results in exposure to high levels of UVR. The four tridentatol compounds have absorption maxima ranging from 313 to 342 nm, and have been hypothesized to function in photoprotection as a result of this strong absorption in the UV-A and UV-B regions. Tridentatol A has also been identified as a deterrent of predators, thereby serving a dual role [118].

10.2.2.7 Fluorescent pigments (FPs)

FPs, found in coral tissues, are host-deriVed pigment proteins, related to a single family of green fluorescent proteins (GFPs) that fluoresce on exposure to UVR and PAR and includes the pocilloporins [119]. FPs were found to dissipate UVR via absorption at 330 nm by fluorescence as green light (when excited at 380 nm) thus converting the damaging UVR to PAR [119, 120]. More recent work indicates that under high light conditions, pocilloporins function in protection of the photosynthetic machinery against UVR and PAR [119]. A large number of shallow-dwelling corals contain FPs, thereby reducing sensitivity to photo inhibition and bleaching in reef corals by thermal dissipation of excess photons through fluorescence and light scattering [121]. Under low light conditions, these pigments are hypothesized to enhance light capturing ability such as in the deep dwelling coral species, Leptoseris fragilis [122].

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