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Fluorescent Pigments in Corals are Photoprotective


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All reef-forming corals depend on the photosynthesis performed by their algal symbiont, and such corals are therefore restricted to the photic zone. The intensity of light in this zone declines over several orders of magnitude--from high and damaging levels at the surface to extreme shade conditions at the lower limit. The ability of corals to tolerate this range implies effective mechanisms for light acclimation and adaptation. Here we show that the fluorescent pigments (FPs) of corals provide a photobiological system for regulating the light environment of coral host tissue. Previous studies have suggested that under low light, FPs may enhance light availability. We now report that in excessive sunlight FPs are photoprotective; they achieve this by dissipating excess energy at wavelengths of low photosynthetic activity, as well as by reflecting of visible and infrared light by FP-containing chromatophores. We also show that FPs enhance the resistance to mass bleaching of corals during periods of heat stress, which has implications for the effect of environmental stress on the diversity of reef-building corals, such as enhanced survival of a broad range of corals allowing maintenance of habitat diversity.
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Fluorescent pigments in corals are photoprotective.
Anya Salih*,, Anthony Larkum*, Guy Cox, Michael Kühl and Ove Hoegh-Guldberg
* School of Biological Sciences, A08, The University of Sydney, NSW, Australia.
† Electron Microscope Unit, F09, The University of Sydney, NSW, Australia.
‡ Marine Biological Laboratory, University of Copenhagen, Denmark.
§ Current address: Centre for Marine Studies, University of Queensland, St Lucia, QLD 4072, Australia
All reef-forming corals depend on the photosynthesis of their dinoflagellate algal
symbiont and are therefore restricted to the photic zone. In this zone light intensity
declines over several decades of intensity from high and damaging levels
(accompanied by UV radiation) at the surface to extreme shade conditions at the
lower limit1. The ability of corals to tolerate this range implies effective mechanisms
for light acclimation and adaptation2. Here we show that the fluorescent pigments3-9
(FPs) of corals provide a photobiological system for regulating the light environment
of coral host tissue. Previous studies have suggested that under low light, FPs may
enhance light availability4,5. We now present evidence that in excessive sunlight, FPs
are photoprotective through dissipation of excess energy at wavelengths of low
photosynthetic activity, as well as by reflection of visible and infra-red light by FP-
containing chromatophores. We also show that FPs enhance the resistance to mass
bleaching of corals during periods of heat stress, which has implications for the
impact of environmental stress on the diversity of reef-building corals.
The bright colours of corals (Scleractinia) and other Anthozoa are due to pigments of
animal-host origin3, many of which are intensely fluorescent under UVA and blue light, with
emission maxima at 420-620 nm4-9 (Fig. 1). In many corals distinct morphs are found which
differ greatly in the concentration of FPs. FPs are part of a group of coral pigments for which
the generic term "pocilloporins" has been proposed9,11. It includes both brightly coloured, low
fluorescence forms and highly fluorescent forms9 described here. Both types of pigments are
partially homologous to Green Fluorescent Protein (GFP)8,9, first found in the luminescent
jelly-fish Aequorea10 and widely used in cell biology. While the function of GFP in a
luminescent system is known, the role of similar FPs in non-luminescent anthozoans has
hitherto been unclear6,11,12.
We surveyed the distribution of fluorescent corals on the Great Barrier Reef (GBR)
(see methods) and found that 124 spp of 56 genera in 16 sampled families contained
fluorescent morphs, often found growing side by side with non-fluorescent morphs. Colour
polymorphism is typical of corals12,13, but many FPs are invisible in daylight, therefore this
widespread abundance was not previously known. The highest numbers of fluorescent morphs
were recorded at the shallowest sites; thus 97% of sampled reef flat corals at Heron Island,
southern GBR, contained medium or high FP concentrations. Moreover, their relative
concentration was significantly higher in sun-exposed as compared to their shaded colony
parts (P<0.001, T-test). We therefore explored an early suggestion3 that fluorescent pigments
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in shallow water corals might function in photoprotection, by comparing fluorescent and non-
fluorescent morphs.
We found that the emission maxima of FPs ranged from blue to green to red (Fig 1),
which is consistent with studies done on the isolated FP proteins of corals9. The shorter
wavelength FPs were more abundant. Microscopically we identified 2 broad groups: FPs
bound within 0.2-8µm fluorescent pigment granules (FPGs) as reported previously3,5,7 and
inter- or intracellular FPs, not enclosed in granules (CFPs). Significantly, the majority of corals
contained multiple FPG and CFP types. Correspondences between emission and excitation
maxima of FPs which occur in close association (Fig 1, e-f) suggest that energy
transformation to longer, non-photosynthetically active wavelengths might in some cases be a
sequential process, with the fluorescence of one pigment exciting another, as expected from
spectra of isolated proteins8,9. We demonstrated that this process could occur by comparing
fluorescence of green FPGs (excitation max. 482.5 nm) alone and mixed with blue FPGs
(excitation max. 382.5 nm). Only weak green fluorescence is seen under 330-380nm
excitation; addition of blue FPGs emitting at 480nm enhanced green fluorescence intensity 4 to
7-fold. (The effect was strongly dependent on distance, with maximal enhancement when blue
and green granules were less than 10µm apart.) The final energy spill would then (depending
on the pigments involved) lie between the two major peaks of the coral photosynthetic action
spectrum and hence be relatively inactive in photosynthesis (Fig 1f). This would be the inverse
counterpart of the process of light transfer to photosynthesis which others have proposed in
light limited habitats4-5 (Fig 1 g)
High light causes photodamage and photoinhibition14 in coral symbionts15-19. We
hypothesized that FPs may reduce the susceptibility to photoinhibition of fluorescent corals by
filtering out damaging UVA and excessive PAR. We compared the degree of day-time
photoinhibition in a polymorphic intertidal species, Acropora palifera, by exposing replicate
sub-colonies made from green fluorescent, brown medium fluorescent and beige non-
fluorescent mother colonies to full sunlight and monitored photosynthesis by chlorophyll
fluorescence analysis with a pulse amplitude modulation (PAM) fluorometer19,20. As
expected15,16, corals showed pronounced photoinhibition during periods of peak irradiance;
non-fluorescent morphs, however, were significantly more photoinhibited and recovered to
pre-inhibition rates slower than fluorescent morphs (P<0.001, ANOVA) (Fig. 2a). Similar
measurements with other polymorphic species (Acropora nobilis, Pocillopora damicornis,
Goniastrea retiformis) also indicated that FPs are correlated with reduced photoinhibition.
Since high solar radiation is a factor in the widely observed mass bleaching of
corals17,21,22, FPs might affect susceptibility to bleaching. Bleaching occurs as a consequence
of damage to dinoflagellate photosynthesis caused by combined effects of thermal stress and
sunlight18,19,22; consequently the dinoflagellates either degrade or are expelled from the host.
During the severe 1998 GBR mass bleaching event, we sampled 21 common coral species
affected by bleaching to varying degrees and found a significant correlation (r2 = 0.9471;
P<0.0001) between bleaching resistance (i.e., high tissue dinoflagellate biomass) and the
concentration of FPs within the tissue (Fig. 2b).
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Light scattering is an important factor linked to the sun screening function of
FPs. We measured spectral reflective properties of coral tissues with fiber-optic
microprobes23,24 positioned over specific parts of single coral polyps. The highly reflective bare
coral skeleton was used as a reflection standard (100%). FPs greatly modified the surface light
environment not only by their emissions but also by light scattering and reflectance, which was
higher in areas with high FPG concentrations (Fig. 3a). White pigmented regions of tissues,
formed by dense layers of FP chromatophores, had 60-100% reflectivity (Fig. 3, a-b). The
most pigmented, and most reflective, parts of colonies: (i) branch tips and colony edges and
(ii) the oral disk/cone and tentacle tips, which on polyp retraction form a sun-screening polyp
‘plug’7 (Fig. 4c), correspond to known areas of highest cell division and areas immediately
above reproductive organs, respectively. This distribution points to a photoprotective role of
FPs in screening sensitive coral tissues as well as symbionts.
Our observations also indicate that corals actively vary the areal density of
pigment chromatophores via polyp expansion/contraction. During expansion more light
penetrates into the tissues through the gaps between FPGs. Under high light, polyp contraction
leads to denser concentration of tissue FPGs and cytoplasmic FPs, forming a thicker and a
quasi continuous FP layer, acting as an effective sunscreen (Fig. 3, b-c) by light scattering and
by radiant fluorescence energy transfer from shorter to longer wavelengths. We also found
that FP-containing polyps of shade-adapted and high light-adapted corals exhibited
differences in spectral reflectivity. Shade-adapted polyps absorbed most of the incident light,
in line with previous observations25, while high light-adapted polyps were generally 20-100%
more reflective (Fig. 3b).
What are the causes of such different tissue optical properties of shade- and
high light-adapted corals? The 3-D cellular localization of FPs in corals showed a clear
difference in the distribution of FPs relative to the layers of endosymbionts7. In high light-
acclimated corals, FPs are localized above the endosymbionts (Fig. 4 a-c) and are in a
position to screen them from excess sunlight. In shade-adapted corals from light-limited
habitats, FPs are localized endodermally, among or below the layers of endosymbionts (Fig.
4d), consistent with their proposed function of light enhancement for photosynthesis via
wavelength-transformation and back-scattering4,5.
The results presented above all suggest that FPs reduce the photoinhibitory
effect of high levels of solar radiation, which in conjunction with thermal stress leads to
bleaching. These findings improve our understanding of the causes of observed inter- and
intraspecific variability in bleaching17,26 and may provide an insight into how changing global
climatic conditions will influence the species diversity and rate of change of coral reef
In conclusion, our study provides a new and more complete understanding of
the role of coral FPs, which appear to be involved in regulation of the internal light
microenvironment of coral tissues. The evidence presented here indicates that the role of FPs
in photoprotection in shallow water, hitherto neglected, is at least as significant as the function
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of light capture in deep water previously assigned to them. Dinoflagellate photosynthesis is
vulnerable to both UV27,28 and high levels of PAR16-19. While accessory pigments in
dinoflagellates can dissipate excess PAR as heat29, FPs can dissipate excess light energy via
fluorescence and light scattering. FPs may also supplement UV-screening by mycosporin-like
amino acids (MAAs)30 since some FPs can transform absorbed UVA radiation to longer non-
actinic wavelengths via fluorescence. By screening chlorophylls and peridinin from high levels
of solar radiation and by absorbing UVA, FPs thereby decrease the likelihood of irreversible
photoinhibition, photooxidation and subsequent coral bleaching. By changing their optical
properties with the help of these GFP-like pigments, coral polyps are able to optimise the
photosynthetic activity of their tissues for the better survival of the organism.
Survey sites, sampling and manipulations: Surveys of fluorescent corals were made at the
inter-tidal lagoon, reef flat and inner and outer slope of Heron Island (23o26’S, 151o55’E) and
One Tree Island (OTI) (23o30’S, 152o06’E), and at several GBR mid-shelf-reefs (1-20m
depths). Corals were sampled by chiselling pieces from replicate colonies (n = 3-6). Each
sample was broken in two, and, subsequently, one subsample was frozen and the other was
chemically fixed as described previously7 for microscopy. Polymorphic Acropora palifera
colonies used in photoinhibition experiment were collected from the OTI lagoon from ~1m
depth. Three colonies of each colour morph were broken into replicate sub-colonies and fixed
in horizontal position in flowing seawater (27-28oC) with one side exposed to sunlight and the
other shaded. Controls were kept at 50-80µmol photons m-2s-1. During the March, 1998
bleaching event samples were taken at Coats (17o28’S, 146o30’E) and Cayley (18o30’S,
147oE) mid-shelf reefs from 1-6m depths. Replicate samples (n=3-6) were taken from species
selected by susceptibility to bleaching: 9 bleached (including non fluorescent Acropora nobilis
morph;); 5 partially bleached (including fluorescent A. nobilis); 7 unbleached.
Microscopy: Frozen, glutaradehyde-fixed and live coral samples were analysed by
fluorescence widefield and confocal (CLSM) microscopy as described previously7.
Fluorescence characteristics of FPs were not substantially affected by glutaraldehyde fixation.
Confocal imaging used 488nm excitation, with detection at 520-550 nm (FPs) and >585nm
(chlorophyll). 3-D reconstruction from optical sections was done with VoxelView Ultra 2.1.2
(Vital Images, USA). A fluorescence microscope fitted with cooled CCD camera (PCO
Sensicam) was used to test energy transfer from blue to green FPGs, both extracted from
Plesiastrea versipora. Intensity (in the green) was measured as the ratio of emission excited
at 330-380nm to the emission excited at 450-490nm, thereby compensating for differences
between granules. Relative FP concentrations in light- and shade-samples as well as in post-
bleaching samples were measured, semi-quantitatively, by CLSM as the fluorescence intensity
per µm2 of imaged coral surface (replicate 3-6 colonies / specimen). Zooxanthellae were
extracted from samples, their biomass cm-2 of coral surface was determined microscopically
and correlated to the relative concentration of FPs in surface tissue as determined by CLSM.
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Coral surface areas were measured as described previously18.
Spectroscopy: Fluorescence excitation and emission spectra of FPGs isolated by
homogenization and repeated centrifugation of coral tissues in phosphate buffer (0.1M, pH
7.2) were determined by a Perkin Elmer Luminescence Spectrometer S50B. All spectra were
normalized to their peaks. Reflectance spectra were measured on live corals in seawater by a
tapered (40µm tip) fibre-optic field radiance microprobe23 positioned ~100µm above the coral
surface. The microprobe was connected to a fibre-optic diode-array spectrometer
(Hamamatsu PMA-11) with a 300-800nm spectral range. A micromanipulator was used to
position the microprobe tip above specific single polyp regions, as viewed under a dissection
microscope. Samples were illuminated by a UV-VIS metal halide light source via a 1 mm
quartz fibre equipped with a collimator at the output end. Spectra of reflected light from the
corals were normalized to the spectrum of reflected light from a reflectance standard in order
to obtain reflectance spectra corrected for the spectral composition of incident light
(normalized reflectance). Data was subsequently expressed as percentages of surface
downwelling radiance reflected from cleaned coral skeleton (relative reflectance).
Active fluorescence measurements: Throughout the day (06:00, 09:00, 12:00, 14:00,
18:00) photoinhibition of light-exposed and shaded portions of sub-colonies (n=3 per morph),
and shaded controls, was measured as decrease in the maximal potential quantum yield
(Fv/Fm) of PSII by a pulse amplitude modulation fluorometer (DIVING-PAM)19,20 after 30
min dark-adaption. Photosynthetically active radiation (400-700nm) during the experiment
was measured by LI-190SA quantum sensor.
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We acknowledge the Great Barrier Reef Marine Park Authority, especially J. Oliver, R. Berkelmans, M.
Russell and U. Engelhart for financial and logistic support. This work was also supported by an Australian
Research Council (ARC) SPIRT PhD award supporting A. Salih and ARC grants to O. Hoegh-Guldberg
and AWD Larkum. M. Kühl acknowledges the financial support of the Danish Natural Science Research
Council. We thank the staff of Heron and One Tree Island research stations for assistance during
Correspondence and requests for materials should be addressed to: Anya Salih,
Electron Microscope Unit FO9, University of Sydney, NSW 2006, Australia
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Figure 1 Main types of fluorescent pigments in coral polyps found in blue, green,
yellow and red combinations with overlapping excitation and emission spectra. a-
b, Mainly blue, in Acropora nobilis. c-d, Mainly green, in Pocillopora damicornis.
e-f, Emissions of outer blue/green and underlying yellow FPs in 'sun' Porites
cylindrica. Coral photosynthetic action spectrum24 (red line) shows that much of
the energy is emitted at wavelengths not usable in photosynthesis. g. Sub-surface
red FPs in green Montipora digitata. Arrow, red FPs in mesenterial filaments.
Scale bars 0.5mm
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Figure 2 Photoinhibition and bleaching responses of corals. a, Maximal potential
quantum yield (Fv/Fm) of dinoflagellates in green highly fluorescent (GF), brown
medium fluorescent (BF) and non fluorescent (NF) Acropora palifera. Results are
means ± S.E. for 3 sub-colonies x morph x 2 tanks x time interval. b,
Dinoflagellates cm-2 and relative concentration of fluorescent pigments µm-2 of
sampled corals: ¡ - bleached, s - part-bleached, l - unbleached. Inset - enlarged
section of graph marked in square.
Figure 3 Apparent reflectance. a, Plesiastrea versipora - tissues lacking FPs (1);
tissue with blue FPs overlying skeletal ridge (2); contracted tentacle with green
FPGs (3); blue FPGs in expanded (4) and contracted (5) oral disc; dense blue
FPGs in white oral disc of shallow-water Platygyra daedalea (6). b, Intertidal
yellow Porites cylindrica - 'shade' expanded (7); contracted (8); and 'sun'
expanded (11), contracted (12) tentacles; edge of polyp calyx (9); septal skeleton
with thin tissue (10). Dips in spectra are due to absorption by photosynthetic
pigments. Spectral peaks due to FPs emission and reflection.
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Figure 4 Reconstruction of serial confocal sections through tissues of 'sun' and
'shade' corals. a, FPs in chromatophores above endosymbionts in polyp tentacle
of 'sun' intertidal Goniopora tenuidens. b, FPG-filled chromatophores with long
extensions enveloping endosymbionts. Bars - 50µm and 10µm, respectively. c,
Retracted tentacles with dense FPGs form a 'plug' over polyp. Bar - 1 mm. d,
Endodermal FPs below dinoflagellates in 'shade' Lobophyllia corymbosa. Bar -
50µm. FPs shown in green/yellow, symbiotic dinoflagellates in red. Arrows - FP
chromatophores. Letter captions: Ec - ectoderm (epidermis), M - mesogloea; En -
endoderm (gastrodermis)
... When ocean water becomes too warm for corals, some individuals produce a brightly colored ''chemical sunscreen'' to try to protect themselves against fatally high-water temperatures and sun exposure (Bollati et al., 2020;Gittins et al., 2015;Ramesh et al., 2020;Salih et al., 2000;Smith et al., 2013). Some research has verified that this phenomenon is a final line of defense before the coral bleaches to white and dies (Bollati et al., 2020;Roth and Deheyn, 2013). ...
... A photoprotective function of green fluorescent protein (GFP)-like pigments has been considered and are divided into two major groups: the fluorescent proteins (FPs), in charge for cyan to red hues and the non-fluorescent chromoproteins (CPs), which produce pink, purple and blue pigmentation (Dove et al., 2001;Roth et al., 2010). It is proposed that these pigments act through screening (Bollati et al., 2020;Salih et al., 2000), a process whereby the light received by the photosynthetic pigments has passed through a layer containing the photoprotective pigments, i.e. convert short energetic wavelengths into longer, less harmful wavelengths (Ben-Zvi et al., 2022;Roth et al., 2010;Salih et al., 2000). There is evidence that under high irradiance and temperature stress, symbiont capacity for photoprotection can be exceeded and photodamage can occur (Iglesias-Prieto et al., 1992;Warner et al., 1999). ...
... A photoprotective function of green fluorescent protein (GFP)-like pigments has been considered and are divided into two major groups: the fluorescent proteins (FPs), in charge for cyan to red hues and the non-fluorescent chromoproteins (CPs), which produce pink, purple and blue pigmentation (Dove et al., 2001;Roth et al., 2010). It is proposed that these pigments act through screening (Bollati et al., 2020;Salih et al., 2000), a process whereby the light received by the photosynthetic pigments has passed through a layer containing the photoprotective pigments, i.e. convert short energetic wavelengths into longer, less harmful wavelengths (Ben-Zvi et al., 2022;Roth et al., 2010;Salih et al., 2000). There is evidence that under high irradiance and temperature stress, symbiont capacity for photoprotection can be exceeded and photodamage can occur (Iglesias-Prieto et al., 1992;Warner et al., 1999). ...
The health of the coral species Siderastrea stellata was investigated as an indicator of climate changes impacts at the Fernando de Noronha Marine National Park, southwestern Atlantic Ocean. Chlorophyll a maximum quantum yield and Rapid Light Curves (RLCs) were produced using a red-light pulse amplitude modulated fluorometer, Mini-PAM in S. stellata colonies. We collected genetic material from the same colonies in order to identify Symbiodineaceae hosted in each of them, considering that the colonies showed very different pigmentations between them. Our findings showed that colonies with pink pigmentation may be associated with higher temperatures, while indicating a high saturation point (Ek) and consequent greater efficiency in the dissipation of radiant energy. Our genetic analysis also demonstrated a high fidelity in association with Cladocopium spp. predominantly. Despite this, we hypothesized that this association may be the result of changes in populations of Breviolum spp. due to stressful events.
... An example is Siderastrea siderea that turns pale blue or lavender (Fig. 3g), presumably due to bacterial or native intracellular pigments within tissues that are uncovered subsequent to loss of endosymbionts. 137,138 Bleaching can be diffuse, focal, or multifocal. In cases where lesions are white, bleaching can be distinguished from tissue loss by careful examination to ensure presence of intact polyps (Fig. 3f). ...
... Endogenous pigments of corals vary widely by species, many having roles in photoprotection or improving light availability for photosynthesis. Increased cellular expression or deposition of melanin, 100 carotenoids, 76 and fluorescent pigments 101,137 have all been documented in pigmented lesions of corals. In some cases, bleached corals display vibrant color. ...
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Stony corals (Scleractinia) are in the Phylum Cnidaria (cnidae referring to various types of stinging cells). They may be solitary or colonial, but all secrete an external, supporting aragonite skeleton. Large, colonial members of this phylum are responsible for the accretion of coral reefs in tropical and subtropical waters that form the foundations of the most biodiverse marine ecosystems. Coral reefs worldwide, but particularly in the Caribbean, are experiencing unprecedented levels of disease, resulting in reef degradation. Most coral diseases remain poorly described and lack clear case definitions, while the etiologies and pathogenesis are even more elusive. This introductory guide is focused on reef-building corals and describes basic gross and microscopic lesions in these corals in order to serve as an invitation to other veterinary pathologists to play a critical role in defining and advancing the field of coral pathology.
... By contrast, fluorescent corals, which have active surfaces and unique fluorescence effects, have attracted the attention of researchers [18][19][20]. They contain fluorescent proteins that protect themselves and emit faint light in a variety of colors, including red, yellow, green, blue, etc. in the deep-sea environment [21]. These weak lights can provide energy for the photosynthesis of photosynthetic microorganisms and effectively inhibit their adhesion, thus achieving antifouling effects [22]. ...
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Marine microbial adhesion is the fundamental cause of large-scale biological fouling. Low surface energy coatings can prevent marine installations from biofouling; nevertheless, their static antifouling abilities are limited in the absence of shear forces produced by seawater. Novel waterborne antifouling coatings inspired by fluorescent coral were reported in this paper. Waterproof long afterglow phosphors (WLAP) were introduced into waterborne silicone elastomers by the physical blending method. The composite coatings store energy during the day, and the various colors of light emitted at night affect the regular physiological activities of marine bacteria. Due to the synergistic effect of fouling-release and fluorescence antifouling, the WLAP/polydimethylsiloxane (PDMS) composite coating showed excellent antifouling abilities. The antibacterial performance of coatings was tested under simulated day-night alternation, continuous light, and constant dark conditions, respectively. The results illustrated that the antibacterial performance of composite coatings under simulated day-night alternation conditions was significantly better than that under continuous light or darkness. The weak lights emitted by the coating can effectively inhibit the adhesion of bacteria. C-SB/PDMS showed the best antibacterial effect, with a bacterial adhesion rate (BAR) of only 3.7%. Constant strong light also affects the normal physiological behavior of bacteria, and the weak light of coatings was covered. The antibacterial ability of coatings primarily relied on their surface properties under continuous dark conditions. The fluorescent effect played a vital role in the synergetic antifouling mechanism. This study enhanced the static antifouling abilities of coatings and provided a new direction for environmentally friendly and long-acting marine antifouling coatings.
... Coral have thus adapted to optimize their light capture mechanisms to enhance the symbiotic relationship in response to varying light quantities (i.e., intensity) and qualities (i.e., spectrum) (Hoogenboom et al., 2008;Iluz and Dubinsky, 2015;Kahng et al., 2019). For example, corals are well adapted to the harsh irradiance conditions experienced in shallow-water coral reefs (Wangpraseurt et al., 2014) through a range of structural and physiological adaptations, including the modulation of skeletal architecture and host tissue thickness (Kramer et al., 2022a;Wangpraseurt et al., 2012), as well as the synthesis of photoprotective animal host proteins that modulate light capture and photosynthesis (Lyndby et al., 2016;Salih et al., 2000). Typically, corals exposed to high-light conditions have a greater ability to cope with excess light, whereas corals residing in low-light environments exhibit highly efficient photosynthetic performance (Einbinder et al., 2016;Kramer et al., 2022b;Martinez et al., 2020). ...
Urbanization and infrastructure development have changed the night-time light regime of many coastal marine habitats. Consequently, Artificial Light at Night (ALAN) is becoming a global ecological concern, particularly in nearshore coral reef ecosystems. However, the effects of ALAN on coral architecture and their optical properties are unexplored. Here, we conducted a long-term ex situ experiment (30 months from settlement) on juvenile Stylophora pistillata corals grown under ALAN conditions using light-emitting diodes (LEDs) and fluorescent lamps, mimicking light-polluted habitats. We found that corals exposed to ALAN exhibited altered skeletal morphology that subsequently resulted in reduced light capture capacity, while also gaining better structural and optical modifications to increased light levels than their ambient-light counterparts. Additionally, light-polluted corals developed a more porous skeleton compared to the control corals. We suggest that ALAN induces light stress in corals, leading to a decrease in the solar energy available for photosynthesis during daytime illumination.
... Studies on corals (Scleractinia) and tardigrades (Tardigrada) have determined that photoluminescence may act as a photoprotective shield (Salih et al. 2000;Suma et al. 2020). ...
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Photoluminescence (encompassing both fluorescence and phosphorescence) is the absorption and re-emission of light, usually converting photons from lower to higher wavelengths. Since this phenomenon occurs vividly in some, but not all, mammals, the question emerges of whether fur photoluminescence is optically meaningful for those species that possess it. Despite sporadic accounts of photoluminescent mammal species in the literature, there have been no dedicated studies of the prevalence of this trait in any region of Australia. The photoluminescent characteristics of fur have never been examined for most mammal species worldwide. Only a handful of fur luminophores (fluorophores and/or phosphors) have been identified to date, with more suspected to be present in fur. The nature of photoluminescence in fur is also little understood, but has been noted as brighter in live and recently dead animals, with recent museum-based studies flagging, but not accounting for, the chemical changes that fur undergoes in different conditions. Since its detailed documentation in European rabbits (Oryctolagus cuniculus) more than 100 years ago, most studies have assumed that photoluminescence is a dormant by-product of some unknown physiological function. However, potential visual functions have recently been hypothesised because of a resurgence of interest coupled with colour photographs of mammals photoluminescing. In this thesis, I studied photoluminescence in Australian mammals from the Wet Tropics of Far North Queensland. I addressed gaps in the literature associated with prevalence, the luminophores responsible, retention of photochemical properties, and the function of photoluminescence in the field. Firstly, I investigated how prevalent the phenomenon of photoluminescence is among mammals of the Wet Tropics, Australia, using fresh roadkill animals and frozen specimens from three collections. Although only a subset of Wet Tropics mammal diversity was studied here, I present the most comprehensive account to date of the occurrence of fur photoluminescence across taxa using fresh roadkill animals. Ninety-five per cent of mammals displayed at least a subtle photoluminescence in the fur at some wavelengths. Forty-two per cent of marsupial species and 29% of placental species displayed noticeably bright photoluminescence. Both monotreme species exhibited subtle photoluminescence. There appeared to be no pattern associated with specific diet or lifestyle factors based on species life history characteristics. My findings suggest that photoluminescence is more common than previously known, and that the biochemical basis of fur photoluminescence may be common among mammals. Secondly, I collected fur samples from seven of these Wet Tropics mammal species to extract and identify the luminophores contributing to photoluminescence. I used high-performance liquid chromatography and liquid chromatography/electrospray ionisation mass spectrometry to identify these luminophores. For two species of bandicoot (the long-nosed bandicoot (Perameles nasuta) and the northern brown bandicoot (Isoodon macrourus)), the northern quoll (Dasyurus hallucatus) and the platypus (Ornithorhynchus anatinus), the work presented here is the first attempt to isolate luminophores from the fur in these genera. I found evidence that supported the presence of coproporphyrin and protoporphyrin, and molecules matching the monoisotopic masses of uroporphyrin and heptacarboxylporphyrin, in the species studied here. These porphyrins had already been identified in the pelage of other mammal species, and exist in a range of organisms from bacteria to birds. Several other photoluminescent molecules extracted from the fur remain to be identified. Thirdly, I investigated the lability of pink fur photoluminescence in response to light exposure, to ascertain whether observed intraspecies differences can be taken at face value, or whether they may be confounded by environmental conditions. I also tested the effects of wet preservation on both pink and blue fur photoluminescence. I conducted photobleaching experiments using northern brown bandicoot and long-nosed bandicoot pelts and found that pink photoluminescence noticeably fades in as little as two minutes of full sun exposure. These experiments have important implications for researchers working with porphyrin-based photoluminescence. Wet preservation in ethanol nearly extinguished the photoluminescence of both laboratory (Norway) rat (Rattus norvegicus) and bandicoot fur, but initial fixation in formalin partially preserved photoluminescence at a low level. These findings flag the probability of false negatives in studies based solely on museum specimens. Finally, I investigated the plausibility of a visual function for fur photoluminescence by placing photoluminescent and non-photoluminescent models in the field and assessing the behavioural responses of wild animals to these models over a six-month period. I used remote cameras to observe behaviour under both full moon and new moon cycles to determine whether photoluminescence could be triggered by natural nocturnal lighting conditions. I found that wild nocturnal animals did not show a preference for either model, suggesting either that natural moonlight was not sufficient to stimulate photoluminescence, that wild nocturnal vertebrates were unable to detect photoluminescence in natural conditions, or that these animals do not use this visual property of fur when making behavioural decisions.
... In contrast, coral taxa with massive and encrusting morphologies (e.g., Lobophyllia and Porites) appear to be more tolerant, with lower incidence of bleaching and mortality (Fisk and Done, 1985;Glynn, 1993;Loya et al., 2001;Baird and Marshall, 2002;Pratchett et al., 2013;Hughes et al., 2017;Harrison et al., 2018; but see Guest et al., 2016). Differences in bleaching responses among taxa have been linked to a variety of traits related to both the coral host (e.g., densities of fluorescent proteins: Salih et al., 2000;mass transfer rates: van Woesik et al., 2012; respiration rates and colony integration: Baird and Marshall, 2002;reviewed in Wooldridge, 2014) and their photosynthetic symbionts (e.g., clade type: Rowan et al., 1997; symbiont plasticity: Grottoli et al., 2014; also see Baker, 2004). Regardless of the mechanism, differential bleaching susceptibility among coral taxa in the Great Barrier Reef Marine Park (GBRMP) has resulted in dramatically altered coral communities (Johns et al., 2014;Hughes et al., 2018b;Hughes et al., 2019a;, with potential long-term consequences for ecosystem function (Richardson et al., 2018;McWilliam et al., 2020). ...
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Climate-induced coral bleaching represents the foremost threat to coral assemblages globally, however bleaching susceptibility varies among and within coral taxa. We compared bleaching susceptibility among 10 coral morpho-taxa and two colony size classes relative to reef-scale bleaching severity at 33 reefs across the Great Barrier Reef and Coral Sea Marine Parks in February-March 2020. Colony size and bleaching severity caused the hierarchy of bleaching susceptibility among taxa to change considerably. Notably, massive Porites shifted from being among the least likely taxa to exhibit bleaching, to among the most susceptible as overall bleaching severity increased. Juvenile corals (≤5 cm diameter) were generally more resistant to bleaching, except for Montipora and Pocillopora colonies, which were more likely to bleach than adults (>5 cm). These findings suggest that colony size and reef-scale bleaching severity are important determinants of bleaching susceptibility among taxa and provide insights into possible shifts in the structure of coral assemblages caused by bleaching events.
The jellyfish Cassiopea largely cover their carbon demand via photosynthates produced by microalgal endosymbionts, but how holobiont morphology and tissue optical properties affect the light microclimate and symbiont photosynthesis in Cassiopea remain unexplored. Here, we use optical coherence tomography (OCT) to study the morphology of Cassiopea medusae at high spatial resolution. We include detailed 3D reconstructions of external micromorphology, and show the spatial distribution of endosymbionts and white granules in the bell tissue. Furthermore, we use OCT data to extract inherent optical properties from light-scattering white granules in Cassiopea, and show that granules enhance local light-availability for symbionts in close proximity. Individual granules had a scattering coefficient of µs = 200–300 cm⁻¹, and scattering anisotropy factor of g = 0.7, while large tissue-regions filled with white granules had a lower µs = 40–100 cm⁻¹, and g = 0.8–0.9. We combined OCT information with isotopic labelling experiments to investigate the effect of enhanced light-availability in whitish tissue regions. Endosymbionts located in whitish tissue exhibited significantly higher carbon fixation compared to symbionts in anastomosing tissue (i.e. tissue without light-scattering white granules). Our findings support previous suggestions that white granules in Cassiopea play an important role in the host modulation of the light-microenvironment.
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This research article presents a novel method for achieving tunable colours (blue to red) from a single organic fluorophore. Emission tuning is associated with different self-assembled structures and aggregates size controlled by reaction time.
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Corals have the ability to synthesize various pigments, responsible for their characteristic vivid coloration. Most coral host pigments are green fluorescent protein (GFP)-like pigments exhibiting diverse spectral properties covering almost the entire visible spectrum, with pigments fluorescing from cyan to red. The type of pigment a coral can synthesize varies inter- and intraspecifically. However, the precise role of host pigments in coral biology has not been fully elucidated. Host pigments have the ability to modify local light fields and could thus contribute to optimizing the light exposure of the photosymbionts. Such fine-tuning of the light microenvironment could enable the holobiont to adapt to broader environmental conditions. Putative mechanisms include energy transfer between host pigments, as well as modulation of their scattering properties via tissue plasticity and granule formation that affect the distribution and organization of host pigments in coral tissue. These mechanisms can enable either photoprotection or photoenhancement depending on the coral’s environment. In this review, we summarize and discuss current knowledge about the link between host pigments and symbiont photosynthesis in reef-building corals, and discuss limitations and challenges of experimental investigation of this connection.
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Two new PAM fluorometers (pulse amplitude modulated) were used in an investigation of photosynthetic performance of Prochloron resident as a symbiont in the ascidian Lissoclinum patella, growing in a coral reef of Heron Island on the Great Barrier Reef. With a new DIVING-PAM in situ measurements of effective PSII quantum yield (δF/Fm′) as a function of quantum flux density (rapid light curves) were carried out in 2.5 m depth in the reef and in a seawater tank. Photosynthetic electron transport rates were measured on in hospite Prochloron both in situ and in collected material. Both light-limited and light-saturated yields were exceptionally high. Maximal yields (Fv/Fm) were ˜0.83. A new TEACHING-PAM was employed for analysing dark-light induction and light-dark relaxation kinetics in collected samples with Prochloron in hospite. Considerable variability in kinetic responses was observed which was found to be at least in part due to differences in O2 concentration. It is suggested that endogenous reductants feed electrons into the intersystem transport chain, which normally is reoxidized by O2 (chlororespiration), and that in the dark, the reduction level of PSII acceptors is increased due to a decline in O2 concentration. The pattern of fluorescence responses differed markedly from those found in cyanobacteria and provides new insights into light-harvesting responses of a photosynthetic prokaryote with a membrane bound light-harvesting system, as contrasted with an extrinsic light-harvesting system.
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This paper* presents a series of detailed paleogeographical analyses of the Caribbean region, beginning with the opening of the Caribbean basin in the Middle Jurassic and running to the end of the Middle Miocene. Three intervals within the Cenozoic are given special treatment: Eocene-Oligocene transition (35-33 Ma), Late Oligocene (27-25 Ma), and early Middle Miocene (16-14 Ma). While land mammals and other terrestrial vertebrates may have occupied landmasses in the Caribbean basin at any time, according to the interpretation presented here the existing Greater Antillean islands, as islands, are no older than Middle Eocene. Earlier islands must have existed, but it is not likely that they remained as such (i.e., as subaerial entities) due to repeated transgressions, subsidence, and (not incidentally) the K/T bolide impact and associated mega-tsunamis. Accordingly, we infer that the on-island lineages forming the existing (i.e., Quaternary) Antillean fauna must all be younger than Middle Eocene. The fossil record, although still very poor, is consistent with the observation that most land mammal lineages entered the Greater Antilles around the Eocene-Oligocene transition. Western Laurasia (North America) and western Gondwana (South America) were physically connected as continental areas until the mid-Jurassic, ca. 170 Ma. Terrestrial connections between these continental areas since then can only have occurred via landbridges. In the Cretaceous, three major uplift events, recorded as regional unconformities, may have produced intercontinental landbridges involving the Cretaceous Antillean island arc. The Late Campanian/Early Maastrichtian uplift event is the one most likely to have resulted in a landbridge, as it would have been coeval with uplift of the dying Cretaceous arc. However, evidence is too limited for any certainty on this point. The existing landbridge (Panamanian isthmus) was completed in the Pliocene; evidence for a precursor bridge late in the Middle Miocene is ambiguous. We marshal extensive geological evidence to show that, during the Eocene-Oligocene transition, the developing northern Greater Antilles and northwestern South America were briefly connected by a "landspan" (i.e., a subaerial connection between a continent and one or more off-shelf islands) centered on the emergent Aves Ridge. This structure (Greater Antilles + Aves Ridge) is dubbed GAARlandia. The massive uplift event that apparently permitted these connections was spent by 32 Ma; a general subsidence followed, ending the GAARlandia landspan phase. Thereafter, Caribbean neotectonism resulted in the subdivision of existing land areas. The GAARlandia hypothesis has great significance for understanding the history of the Antillean biota. Typically, the historical biogeography of the Greater Antilles is discussed in terms of whether the fauna was largely shaped by strict dispersal or strict continent-island vicariance. The GAARlandia hypothesis involves elements of both. Continent-island vicariance sensu Rosen appears to be excludable for any time period since the mid-Jurassic. Even if vicariance occurred at that time, its relevance for understanding the origin of the modern Antillean biota is minimal. Hedges and co-workers have strongly espoused over-water dispersal as the major and perhaps only method of vertebrate faunal formation in the Caribbean region. However, surface-current dispersal of propagules is inadequate as an explanation of observed distribution patterns of terrestrial faunas in the Greater Antilles. Even though there is a general tendency for Caribbean surface currents to flow northward with respect to the South American coastline, experimental evidence indicates that the final depositional sites of passively floating objects is highly unpredictable. Crucially, prior to the Pliocene, regional paleoceanography was such that current-flow patterns from major rivers would have delivered South American waifs to the Central American coast, not to the Greater or Lesser Antilles. Since at least three (capromyid rodents, pitheciine primates, and megalonychid sloths) and possibly four (nesophontid insectivores) lineages of Antillean mammals were already on one or more of the Greater Antilles by the Early Miocene, Hedges' inference as to the primacy of over-water dispersal appears to be at odds with the facts. By contrast, the landspan model is consistent with most aspects of Antillean land-mammal biogeography as currently known; whether it is consistent with the biogeography of other groups remains to be seen.
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The Caribbean region has been a center for debate about processes that gave rise to current species distributions. This dialogue is of particular interest to herpetologists, because much of the terrestrial vertebrate fauna is composed of amphibians and reptiles. Some workers have examined patterns of evolution and distribution of these organisms and concluded that widespread dispersal is the primary process explaining current biogeography; others have examined the same data and concluded that vicariance associated with complex tectonic movements is the primary biogeographic process. In this essay, we review Caribbean biogeography, focusing on a recent study of albumin immunological data. These data were interpreted as demonstrating that lineages on the Greater Antilles were too recent in origin, relative to the mainland as well as among islands, to be explained by vicariance. A novel hypothesis was presented, drawn from recent geological evidence of an extraterrestrial bolide impacting on the northern coast of the Yucatan at the Cretaceous-Tertiary boundary. The location, timing, and magnitude of the impact as well as concomitant tsunamis were used to explain why recent dispersal appears to explain the origin of current herpetological lineages in the Caribbean. We re-examine the geological and immunological data that were used to generate the bolide hypothesis. Additionally, we use Brooks Parsimony Analysis to analyze phylogenetic patterns of Caribbean taxa. From these data, we conclude that (1) estimates of timing of Caribbean tectonic events are poorly constrained, (2) considerable immunological data are of sufficient age to conform to the predicted timing of vicariant events associated with the Greater Antilles, (3) dispersal events between the mainland and the Greater Antilles as well as among the Greater Antillean islands can be documented with immunological evidence provided that assumptions of evolutionary rates and direction of travel are examined carefully, (4) consistent patterns of phylogenetic relations of Caribbean taxa suggest a common history for many taxa, and (5) the pattern exhibited in conclusions 2 and 4 are sufficient to invoke vicariance as an important force in shaping current diversity within the Caribbean.
This updated and expanded A New Island Biogeography of the Sea of Cortés, first published nearly 20 years ago, integrates new and broader studies encompassing more taxa and more complete island coverage. The present synthesis provides a basis for further research and exploration in upcoming years of the biologically fascinating Sea of Cortés region. The Gulf region is increasingly being exploited, for its natural resources by way of marine fisheries, and for its stunning natural beauty by way of a burgeoning tourism industry. Further, the region's human population is increasing apace. It is appropriate, therefore, that this volume discusses these evolving circumstances, and the efforts of the Mexican government to regulate and manage them. The new Biogeography includes a section on the conservation issues in the Sea of Cortés, past accomplishments and conservation needs as yet outstanding. This book should be of strong interest to conservation biologists, ecologists, and evolutionary biologists more generally.
Number of individuals of diurnal orb-weaving spiders declined exponentially with distance from a presumed source and typically increased with island area. When these effects were taken into account, number of individuals of all species combined on islands without lizards were c10 times higher than those on islands with lizards. Number of individuals of certain species showed no relation to area on lizard-inhabited islands. Species numbers increased with area and maximum vegetation height and decreased with distance. When these effects were taken into account, islands without lizards averaged 50% more species than those with lizards. Extinction rate was strongly related to population size for all spider species. Populations going extinct over a year's time constituted in total only 3-4% of the total number of individuals from all populations whether going extinct or not. Absolute extinction rate was positively related to species number and distance and was negatively related to area. Absolute immigration rate was positively related to area and was negatively related to species number and distance. Absolute turnover thus showed no strong relation to any variable. For the same archipelago, relative turnover in orb spiders was 34-59% yr-1 and in lizards was only 1% yr-1. -from Authors
Diamond, J. M. (Physiology Department, University of California Medical Center, Los Angeles, California 90024) 1977. Continental and insular speciation in Pacific land birds. Syst. Zool. 26:263-268. —Three modes of allopatric speciation can be distinguished, depending on whether the isolating geographic barrier is within a single land mass (“continental speciation”), between islands of the same archipelago, or between different archipelagoes (“insular speciation”). The contributions of these three modes to speciation in Pacific land birds are analyzed. Continental speciation in birds has occurred in no Pacific land mass smaller than Australia, New Guinea, and possibly New Zealand; intraarchipelagal speciation has occurred only on six of the most remote archipelagoes; and inter-archipelagal speciation has produced most of the sympatric bird species pairs from the Bismarcks to Samoa. The frequency of each mode depends on area and isolation of the island, and on mobility and perhaps population density of the taxa involved. What is an “island” to some taxa may be a “continent” to others. For example, New Caledonia behaves as a continent to higher plants, insects, and lizards, but not to birds or ferns. [Speciation; Pacific land birds.]