coral UV-pigment IDs ?

[horge]

Just hoping there's someone out there who has access to ID's on the various pigments reputedly utilized by shallow water corals for protection against UV burn.

This first popped up in the SPS coloration thread, and we came up zip.

[Martyn]

Horge you have likely read this what do you make of it.
and also the MAAs.
Martyn.


Quote Craig Bingman
Coral Fluorescence: An Update
I suspect that all of the readers of this column have looked at a reef aquarium illuminated only with actinic lighting. Under such lighting conditions, many species of corals glow in gaudy fluorescent colors — blue, green, yellow, orange and even pink. Some of you may have wondered what on earth the corals are doing to make such a unique spectacle possible.
Several years ago, I speculated that at least some coral fluorescent proteins were probably homologous to the green fluorescent protein (GFP) from Aqueoria victoria (Bingman 1995). A recent research report in Nature Biotechnology (1999) has confirmed this suspicion. Mikhail Matz and coworkers have cloned a total of six fluorescent proteins from five different non-bioluminescent cnidarians, and found that all are homologous to GPF from A. victoria. Even more interesting, from an aquarium perspective, is that all five of these organisms are found in tropical coral reef environments.
It may be useful to give a brief background of this field of research in order to appreciate the current results. About five years ago, a protein from a bioluminescent jellyfish, Aqueoria victoria was cloned. It was shown to be useful as a marker of gene expression in other organisms. Researchers found that if they transformed experimental animals with a gene of interest that was fused with DNA encoding for GFP, expression of that gene could be easily tracked, especially in transparent organisms, simply by illuminating them with ultraviolet-A (UV-A) or blue light. The tissues where that modified gene was expressed literally glowed in the dark. This technique has been very useful in understanding tissue-specific gene expression in developing animals.
One of the most remarkable things about GFP is that it makes a fluorescent chromophore from part of its own amino acid backbone. It starts out as a fairly normal-looking protein, with only normal amino acids, and cyclizes a couple of its amino acids to make this special fluorescent molecule. The fluorescent molecule remains attached to the rest of the protein, buried at its site of formation in the core of the molecule.
So, my original speculation was that if a jellyfish “knew� how to perform this chemistry, it was likely that other related organisms — the corals from tropical waters we keep in our aquariums — might also possess the genetic acumen allowing them to make similar fluorescent molecules. Matz and coworkers identified a sequence in GFP that they thought would be conserved in all related molecules, and used a technique called “polymerase chain reaction� to amplify molecules with this sequence to facilitate their molecular cloning. They succeeded in isolating two fluorescent proteins from a Zoanthus sp. and one each from Anemonia majano, Discosoma striata, Discosoma sp. ("red") and Clavularia sp.
Two important characteristics of fluorescent substances are their absorption maxima and their emission maxima. The absorption maxima for these proteins ranges from 456 to 558 nanometers (nm) and their emission maxima from 485 to 583 nm. Some have speculated that fluorescent proteins in reef corals may shift the wavelength of light impinging on the coral to a wavelength more suited for use in photosynthesis. In this regard, these six proteins would be moderately capable of that, having quantum efficiencies (the probability that an absorbed photon will give rise to a fluorescent photon) of between 0.63 to 0.24. In contrast, the GFP from A. victoria, the bioluminescent jellyfish, has a quantum yield of 0.80. So, these proteins are less efficient at transforming light to other wavelengths than the jellyfish protein is.
In terms of UV protection, the fluorescent spectra for these new proteins typically show very little absorption over the UV-A band. One shows a small peak at 335 nm in the UV-B band, but this absorption feature is dwarfed by the absorption features for all these proteins in the visible region. So it would seem that these recombinant proteins make rather lousy sunscreen as well, at least in the states studied.
So, I would have to say that, at present, their biological role is still a bit of a mystery. Of course, I have ideas about what that is, but I’ll save that for a future report.



Walter C Dunlap
Australian Institute of Marine Science
and
J Malcolm Shick
Department of Biological Sciences, University of Maine




Synopsis
Shallow-water environments of tropical coral reefs are characterised by high levels of ultraviolet-A (UVA, 320-400 nm) and ultraviolet-B (UVB, 280-320 nm) radiation. This is due to the thinness of the earth’s ozone layer near the equator and to the UV-transparency of tropical ocean waters. All shallow-water marine organisms have natural features and behaviors that can reduce exposure to UV radiation and limit the amount of photodamage to functional biomolecules and organelles.

Shallow-water coral reef


In marine organisms this protection often includes the production of natural UV-absorbing compounds ("sunscreens") and related antioxidants. This protection is phylogenically widespread among marine organisms occurs in all shallow-water habitats at the global scale.



Photobiology
The clear waters surrounding tropical coral reefs are typically oligotrophic (nutrient-poor), yet these reefs sustain high productivity by supporting dense populations of marine organisms. This paradox is resolved by many coral reef invertebrates by accommodating unicellular, endosymbiotic algae (Symbiodinium spp., commonly referred to "zooxanthellae") within their tissues. This photoautotrophic symbiosis allows for a beneficial exchange of nutrients between the algae and animal host.

Elkhorn coral


Organic carbon produced by the algal partner is released to the host for nutrition while inorganic metabolic wastes are recycled to fertilise algal photosynthesis.
Such algal-animal symbioses are especially common in the shallow-water environment of coral reefs where they are typically exposed to high levels of visible and ultraviolet radiation. In clear water, UV radiation can penetrate to ecologically significant depths (<20 m) and this presents an evolutionary challenge to symbiosis. Since algal symbionts reside within coral tissues, overlying tissues of the animal host must be transparent to facilitate the penetration of the visible wavelengths of light required for algal photosynthesis.

Coral symbiotic Zooxanthellae algae


The photosynthetic requirement for symbiosis thus precludes the morphological development of a protective covering (hair, scales and feathers in higher vertebrates) to intercept potentially harmful wavelengths of UV radiation from reaching vulnerable biochemical sites in both partners. This problem is exacerbated by the release of photosynthetic oxygen within the host tissues which, in combination with high light intensities, is a potential cause of photooxidative stress to the symbiosis.
Having evolved at low latitudes where exposure to high UV intensities are part of the normal environment, it is not unexpected that corals have developed an efficient defence against the potential damage of long-term solar irradiation. Shibata in 1969 was first to discover the presence of a UV-absorbing substance ("S-320") in the aqueous extracts of shallow-water corals. Concentrations of S-320 in corals were subsequently observed to vary inversely with depth, presumably in compensation to the ambient levels of solar UV radiation penetrating to their habitat.

Photomicrograph of a coral polyp showing
clusters of endosymbiotic algae


This correlation may explain the observation by Siebeck that corals collected from depths of 1-2 m were found to be more resistant to artificial near-UV exposure than corals of the same species growing at depths greater than 5-6 m.
S-320 has since been identified in corals to be comprised of a family of mycosporine-like amino acids (MAAs) having absorption maxima in the range 310-360 nm and were originally discovered as common metabolites found in a diverse range of marine organisms. The parent class of mycosporines was prior discovered as fungal metabolites occurring in the developing mycelium associated with light-induced sporulation. Their reproductive function has later been postulated to provide UV protection to fungal during propagation by atmospheric transport spores while exposed to direct solar irradiation.
The deleterious effects of solar UV radiation in the coral reef environment was first demonstrated by Jokiel in 1980 by observing that cryptic reef epifauna were killed when relocated and acutely exposed to ambient levels of UV radiation in shallow water.


Typical MAAs found in marine organisms


From these observations Jokiel suggested that the community structure of coral reefs was profoundly affected by the relative UV tolerances of their constituent species. Jokiel and York were later to provide physiological evidence that enhanced UV exposure reduces skeletal growth in a reef-building coral, Pocillopora damicornis, and reduces the photosynthetic capacity of its isolated endosymbiotic zooxanthellae measured in vitro. However, analyses of MAAs in coral tissues reveal that concentrations are predominantly associated with the coral tissue, and this protective barrier would account for why photosynthesis by symbionts in hospite (within the host) under equivalent conditions is largely unaffected by UV exposure under prevailing environmental conditions.
MAAs are assumed to be produced by the algal partner in coral symbiosis since biosynthesis involves the shikimic acid pathway, a biochemical route unavailable to invertebrate synthesis. The major distribution of MAAs in coral symbiosis, however, resides within the animal tissues suggesting that the algal partner provides UV protection to the whole of the symbiosis via MAA translocation. A protective function for these compounds is inferred by their efficient UVA- and UVB-absorbing properties, together with the often observed correlation between MAA concentrations and natural or experimental levels of UV exposure. It has also been determined that oxy-carbonyl mycosporines (e.g. mycosporine-glycine), but not the imino-carbonyl MAAs, may also function as a physiological antioxidant [see Commercial activities], whereas the oxidative robustness of imino-mycosporines is in keeping with their primary function as a stable sunscreen to provide long-term UV protection.


Sources and functions of MAAs
MAAs are nature’s sunscreen in the living marine environment. They are typically found in high concentrations in algal producers, but are also common in higher invertebrates and are often found together with several biochemically related gadusols having strong antioxidant properties [see A novel antioxidant derived from seaweed]. MAAs have been identified in taxonomically diverse marine organisms, including a heterotrophic bacterium, cyanobacteria, microalgae (phytoplankton) and macroalgae (seaweeds).

Crown-of-Thorns starfish feeding
on a coral colony


Within non-symbiotic marine invertebrates MAAs have been identified in echinodems (starfish and sea urchins), a mussel, a sea hare, brine shrimp and an ascidian. MAAs are also found in the eyes, skin and reproductive tissues of tropical and temperate fishes. They are common in microalgal-invertebrate symbioses on coral reefs and elsewhere, including: sponges, scleractinians (hard corals) including their eggs and mucus, sea anemones, octocorals (soft corals), a zoanthid, a jellyfish, tridacnid clams and ascidians. MAAs occur in organisms from tropical and subtropical coral reefs to the Antarctic Ocean where they may protect benthic species and the planktonic food web of the southern oceans from extreme fluctuations of UV exposure under the "ozone hole" caused by the depletion stratospheric ozone [see Antarctic research].
Assuming that invertebrates are unable to synthesize MAAs, dietary accumulation of MAAs by non-symbiotic reef consumers may be a significant pathway for UV protection. This pathway was hypothesized on observing high concentrations of MAAs (mostly pathythine) in the dermal tissues of the Crown-of-Thorn Starfish feeding on the MAA-rich tissues of corals.

A coral reef holothurian


MAAs are also trophically acquired by holothurians (sea cucumbers) from ingesting sediment epiflora whereby algal MAAs are relocation to epidermal tissues and gonads suggesting a photoprotective function against topical UV exposure and during reproduction; MAAs are also present in the enteric contents and gut tissues consistent with a dietary origin. This trophic pathway of MAA accumulation has been confirmed in sea urchins by controlled feeding experiments, and the photoprotective function of MAAs was clearly demonstrated by the improved reproductive success of fertilized eggs rich in MAAs after exposure to simulated levels of full solar radiation.



Commercial sunscreen development
Shallow-water corals produce large quantities of MAAs and their apparent ability to withstand long-term, environmental UV exposure suggests utilisation of the unique properties of their natural UV-absorbing chromophore for human and industrial sunscreen applications [see Sunscreen research].

The next wave of sunscreens
is surfacing now


Reviews (with references)
Dunlap WC and Shick JM (1998). Ultraviolet radiation-absorbing mycosporine-like amino acids in coral reef organisms: a biochemical and environmental perspective. J. Phycol. 34: 418-430 (1998).
Dunlap WC, Chalker BE, Banderanayake WM and WuWon, JJ (1998). Nature’s sunscreen from the Great Barrier Reef, Australia. Int. J. Cosmet. Sci. 20: 41-51.

Current areas of collaborative research
Acclimatization and affects of UV light on coral reproduction and recruitment.
Photooxidative stress and coral bleaching via destruction of photosynthetic pigments and/or loss of endosymbionts.
Trophic transfer of MAAs in the phytoplankton-zooplankton food-chain and validation of the concept of UV-induced trophic co-adaptation.
Affects of environmental change on coral reef and other marine organisms.