[Coral-List] Coral fluorescence
mazel at psicorp.com
Mon Aug 5 09:43:49 EDT 2013
Some brief answers, far from complete, and pointers to publications below.
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On Aug 4, 2013, at 2:44 AM, Barbara Gratzer wrote:
> Dear all,
> Since I have not received any answers yet, I will try to resend the
> questions I came across lately:
> 1. I thought corals possessing colours have many GFP like proteins. Now I
> have discovered several acroporid finger-corals with purple colour that are
> not fluorescent. Do you know why? Does GFP like protein not necessarily
> mean they are fluorescent or does the colour derive from other proteins?
> And why do some colours not possess colouration but are highly fluorescent
> in contrast to others?
There are non-fluorescent varieties of 'green fluorescent protein'. They are of the same protein 'family' but have different amino acid sequences at the chromophore location such that they preferentially absorb some wavelengths of light (i.e., impart color) but do not fluoresce. See Lukyanov et al, 2000. Natural animal coloration can be determined by a nonfluorescent green fluorescent protein. J. Biol. Chem, 275:25879-25882. (one of my favorite titles of a paper)
I have always found this intriguing and suggestive. While fluorescence in corals is common, what is more common is the presence of proteins of that family, whether fluorescent or not.
You are correct that some corals that do not appear colorful can be quite fluorescent. This will have to do with distribution and amount of the protein and also the excitation spectrum. Also, when you say a coral does not appear to be fluorescent it is often due to the way you normally view it, under sunlight or broadband white light. As bright as fluorescence might appear when viewed in the dark with the right lights, it is a relatively weak effect and often cannot compete with reflected sunlight in determining apparent color. In the Caribbean, there is a variety of Montastraea cavernosa that often appears quite orange at depth. If you bring it up to the surface or even into shallow water it appears brown due to the increased ambient light at the wavelengths of the fluorescence. Where most fluorescence is green, it is competing against the ambient light at green and neighboring wavelengths, often making the presence of the fluorescence less obvious.
> 2. What's exactly the purpose of being fluorescent. I know the pigments
> causing fluorescence are thought to act either as protection from
> stress/sunlight or as enhancer for symbiodinium to increase photosynthesis,
> but is it just a coincidence that they are fluorescent at the same time? I
> dont understand the correlation.
'Exactly the purpose' is still unclear and under debate. That either/or dichotomy between protection from excess sunlight and assistance to photosynthesis is an oversimplification and does not encompass additional suggested roles for the protein. And one must certainly think independently about the function of the protein and the function of the fluorescence.
For example, the protein may play a role in managing internal chemistry - see Bou-Abdallah, F., N. D. Chasteen, and M. P. Lesser, 2006. Quenching of superoxide radicals by green fluorescent protein, Biochim. Biophys. Acta, 1760:1690-1695. In such a case the fluorescence may well be a non-functional property of a protein that is there for a perfectly good reason, but not to be fluorescent.
There have been some other suggestions for function that are directly related to fluorescence, such as a beacon - Hollingsworth, L. L., R. A. Kinzie, T. D. Lewis, D. A. Krupp, and J. C. Leong, 2005.Phototaxis of motile zooxanthellae to green light may facilitate symbiont capture by coral larvae. Coral Reefs, 24:523.
Fluorescence, by the way, is a horrible way to assist photosynthesis. Throwing off photons in random directions as fluorescence does and hoping they will be caught by chlorophyll is an inefficient way to go about it. Also, one often finds that the fluorescent proteins and the symbiotic algae are not co-located, so a poor arrangement for either photoprotection or photosynthesis enhancement. Another point concerning aiding photosynthesis - if a coral is brightly fluorescent those are a lot of photons that are NOT being used internally. Nature has a better way to transfer energy, and that is by resonance transfer as is used in the phycobiliproteins (phycoerythrin, phycocyanin, allophycocyanin) that capture energy and hand it off to chlorophyll with very high efficiency. Phycoerythrin is about the most fluorescent molecule in nature but when it is tied up in this transport chain there is very little fluorescence emitted.
There are also arguments (i.e., publications) that the presence of fluorescent proteins make corals (A) more resistant and (B) less resistant to bleaching. If you're not confused yet, you should be.
> 3. Whats the purpose of different fluorescent light levels? Most of the
> corals emmit green and yellow light and very few emmit red light. Is it
> caused by the proteins or does it have something to do with different
> photosynthetic active pigments?
There are a number of variations of the fluorescent proteins that result in different emission colors. Not due to different photosynthetic pigments. Chlorophyll always has a deep red fluorescence, not strong. As to purpose of the different colors in the proteins, not clear that there is any.
e.g., Kelmanson, I. V., and M. V. Matz, 2003. Molecular basis and evolutionary origins of color diversity in great star coral Montastraea cavernosa (Scleractinia: Faviida). Mol. Biol. Evol., 20:1125-1133. http://mbe.oxfordjournals.org/content/20/7/1125.full.pdf
> 4. Within our coral nursery I have come across several Acropora species
> (especially purple coloured) that lost their original colour after being
> transplanted and only had a bright brown colour after 6 month. Is that a
> process caused by stress? Whats the purpose?
Not necessarily stress. The expression of the fluorescent proteins is influenced by the light that corals are exposed to. Reef aquarists have known this for a long time. By changing the light levels on the specimens they can get them to 'color up'. For a scientific publication related to this, see D’Angelo, C., A. Denzel, A. Vogt, M. V. Matz, F. Oswald, A. Salih, G. U. Nienhaus, and J. Wiedenmann, 2008. Blue light regulation of host pigment in reef-building corals. Mar. Ecol. Prog. Ser., 364:97-106. Varying level of blue light caused varying level of expression of the fluorescent pigments.
Stress may also play a role -
Roth and Deheyn, 2013. Effects of cold stress and heat stress on coral fluorescence in reef-building corals. Scientific Reports, 3, article 1421. http://www.nature.com/srep/2013/130312/srep01421/full/srep01421.html
Zawada, David G., and Jules S. Jaffe, 2003. Changes in the fluorescence of the Caribbean coral Montastraea faveolata during heat-induced bleaching. Limnol. Oceanogr., 48:412-425. http://aslo.org/lo/toc/vol_48/issue_1_part_2/0412.pdf
> Thanks in advance for your answers!
> Warm regards,
> Barbara Gratzer MSc
> ~~~ Σ^^°> ~~~
> Resident Marine biologist
> Huvafen Fushi Resort
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