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You are here: Home Volume VI June 2007 Feature Article: Coral Coloration, Part 5: Non-fluorescent Chromoproteins (CP-480 to CP-562)

Feature Article: Coral Coloration, Part 5: Non-fluorescent Chromoproteins (CP-480 to CP-562)

By Dana Riddle Posted Jun 14, 2007 08:00 PM Pomacanthus Publications, Inc.
Contrary to popular belief, not all coral colors are fluorescent.

The first four installments of this series dealt with fluorescent proteins found within various anthozoans. An understanding of fluorescence is needed before we examine another phenomenon – that of reflective pigments within coral tissues, which in turn is a prerequisite to understanding spectral data collected from various corals (presented later in this series).

Fig1.jpg

Figure 1. Contrary to popular belief, not all coral colors are fluorescent. This Pocillopora meandrina contains a highly reflective non-fluorescent chromoprotein.

There is considerably less attention paid to these pigments by scientists. Since they only reflect light (and do not fluoresce and hence are not particularly useful in biomedical research), these pigments have received the attention of far fewer researchers. However, these colored pigments are of interest to those reef aquaria hobbyists wishing to exhibit vibrantly colored corals within reef aquaria.

It is a common mistake to contribute all vivid coloration of corals to fluorescence – many corals’ beautiful blues, purples, pinks or reds due to the presence of chromoproteins.

Chromoproteins

Chromoproteins (from the Greek words ‘khroma’ (color) and ‘prote’ (primary)), for our purposes, are organic pigment-containing compounds that simply absorb and reflect radiation – they are normally not fluorescent or only weakly fluorescent. Their coloration is generally purple-blue (Labas et al., 2002), although violet, blue or pink/red chromoprotein pigments are known to exist. Generally, these pigments absorb light in the bandwidth of ~560- ~590nm (though there are exceptions). See Table 2.

Different terminology is used when discussing non-fluorescent chromoproteins. These are some terms with which we should be familiar:

Absorb: To retain light without reflection or transmission.

Absorption: The process in which incident radiation is retained without reflection or transmission on passing through a medium.

Absorbance: The ability of a solution or layer of a substance to absorb radiation. Expressed mathematically (for those who want to know) as the negative common logarithm of the transmittance of the substance and abbreviated as ‘Abs’.

FWHM = Full Width at Half Maximum. The spectral width (in nanometers) at the level that is 50% of the value at the peak.

Pocilloporan – Chromoproteins originally isolated from Pocillopora stony corals.

Reflect: To ‘bounce back’ light without alteration of wavelength.

Reflection: The process in which incident radiation is returned without alternation of wavelength, in others words; reflected light is not absorbed or fluoresced.

Reflectance: The ratio of the total amount of radiation (such as light) reflected by a surface to the total amount of radiation incident to the surface.

In the cases of non-fluorescent chromoproteins, that light energy that is not absorbed by a substance (coral tissue, host pigments, photopigments) is reflected, and apparent color is a combination of absorbed and reflected light. One of the most apparent examples of apparent color is that of green leaves containing chlorophylls. Since chlorophylls absorb red and blue light, the majority of the light reflected is green. Corals containing zooxanthellae are often brown, since the zooxanthellae contain chlorophylls and peridinin which absorb blue, green and red light, thus making them appear brown. Corals containing chromoproteins will usually appear purple-blue, but can be almost any color of the rainbow.

Color Definitions

Defining color by bandwidth would seem a straightforward proposition. However, few references agree. For clarification, see Table 1 for a listing of bandwidth and associated colors used in this article.

Table 1. Color and associated wavelengths.
Color Wavelength (nm)
Violet 400 - 430
Blue 431 - 480
Green-Blue 481 - 490
Blue-Green 491 - 510
Green 511 - 530
Yellow-Green 531 - 570
Yellow 571 - 580
Orange 581 - 600
Red 601 - 700
Table 2. A concise listing of chromoproteins and hosts.

Host Color Reference
CP-480 Pocillopora Red Shibata, 1969
CP-500 Acropora Yellow Shibata, 1969
CP-560 Pocillopora damicornis Pink Dove et al., 1995
CP-560 Seriatopora hystrix Pink Dove et al., 1995
CP-560 Stylophora pistillata Pink Dove et al., 1995
CP-562 Anemonia sculata Reddish Wiedenmann et al., 2000
CP-568 Anemonia sculata Violet Labas et al., 2002
CP-571 Condylactis gigantea Red Labas et al., 2002
CP-571 Condylactis passiflora Red Labas et al., 2002
CP-575 Anemonia sculata Purple Verkusha & Lukyanov, 2004
CP-578 Heteractis crisperi Violet/Purple Labas et al., 2002
CP-578 Acropora digitifera Violet/Purple Dove et al., 1995
CP-580 Goniopora tenuidens Purple/Red Martynov et al., 2003
CP-586 Acropora Violet Shibata, 1969
CP-588 Rhizostoma anemone Blue Bulina et al., 2004
CP-588 Cassiopea xamachana Blue Bulina et al., 2004
CP-588 Acropora formosa Blue Dove et al., 1995
CP-590 Montipora efflorescens Blue Nienhaus et al., 2005
CP-595 Anemonia sculata 'Kindling' Protein Bulina et al., 2004
CP-597 Anemonia equina Blue Shkrob et al., 2005
CP-610 Cnidopis japonica Blue Chan et al., 2006
Table 3. An early summary of absorption bandwidths of water-soluble chromoproteins extracted from various corals. This work still has relevance. From Kawaguti, 1944.
Pink, Red and Purple Pigments
Coral 1st Band 1st Max Abs 2nd Band 2nd Max Abs 3rd Band
Fungia repunda 552-583 * 521-535 * 502-510
Pocillopora eydouxi 552-583 * 521-535 * 502-510
Pocillora damicornis 552-583 * 521-535 * 502-510
Stylophora sp. 552-583 * 521-535 * 502-510
Acropora grandis 552-583 * 521-535 * 502-510
Acropora pacifica 552-583 * 521-535 * 502-510
Acropora variaviris 552-583 * 521-535 * 502-510
Acropora hyacynthus 552-583 * 521-535 * 502-510
Porites sp. 552-583 * 521-535 * 502-510
Pocillopora acuta 552-583 * 521-535 * 502-510
Hydnophora microconus 552-583 * 521-535 * 502-510
Echinopora lamellosa 552-583 * 521-535 * 502-510
Goniopora sp. 568-600 ~588nm 527-537 ~533nm *
Porites (arboreal) 566-596 ~581 530 * *
Porites (massive) 566-596 ~581 530 * *
Acropora echinata 615 583nm * * *
Acropora sp. 567-605 583nm 531-541 * *
Porites sp. 567-605 583nm 531-541 * *
Favites sp. 567-605 583nm 531-541 * *
Montipora ramosa 567-605 583nm 531-541 * *
Montipora verrucosa 567-605 583nm 531-541 * *
Goniopora sp. 567-605 583nm 531-541 * *
Echinopora lamellosa 567-605 583nm 531-541 * *
Acropora leptocyanthus 567-605 589nm * * *
Porites sp. 557-608 584nm * * *
Acropora formosa 567-615 583nm 531-541 537nm *
Acropora echinata 567-615 583nm 531-541 537nm *
Porites sp. 557-608 584nm * * *

CP-480

  • Maximum Absorption (wavelength): Actually in the violet range, with a shoulder absorption at 480nm.
  • Full Width at Half Maximum (FWHM): Not determined
  • FWHM Bandwidth: Not determined
  • Apparent Color: Red
  • Shibata (1969) reports this pigment was isolated from a red Pocillopora specimen. Figure 2 demonstrates the absorption of a water-soluble pigment extracted from this stony coral.
Fig2.JPG

Figure 2. This pigment appears red since it absorbs mostly blue and green light. From Shibata, 1969.

Fig3.JPG

Figure 3. Many Pocillopora specimens (such as this P. damicornis) exhibit an attractive reddish coloration due to the presence of at least one chromoprotein.

CP-500

  • Maximum Absorption: 400nm (violet)
  • Full Width at Half Maximum (FWHM): Not determined
  • FWHM Bandwidth: Not determined
  • Apparent Color: Yellow

CP-500 is reportedly found in a yellow Acropora specimen from Australia’s GBR (Shibata, 1969). See Figure 4 for absorption of a water-soluble pigment that appears yellow.

Fig4.JPG

Figure 4. A chromoprotein found in a yellow Acropora (from Shibata, 1969).

CP-560

Chromoproteins with absorption maxima are found in a variety of corals including (but likely not limited to) Pocillopora damicornis, Stylophora pistillata and Acropora species.

Pocillopora damicornis

  • Maximum Absorption (Pocillopora damicornis): 560nm (yellow-green)
  • Full Width at Half Maximum (FWHM): 80nm
  • FWHM Bandwidth: 520-600nm
  • Apparent Color: Pink
  • See Figure 5.
Fig5.JPG

Figure 5. Absorbance of chromoprotein 560 from a pink Pocillopora damicornis. It is little wonder the coral appears pink – blue and green wavelengths are strongly absorbed in comparison to red wavelengths.

Stylophora pistillata

  • After Dove et al., 1995.
  • Maximum Absorption (Stylophora pistillata): Actually at 400nm (violet), but listed as 560nm (yellow-green).
  • Full Width at Half Maximum (FWHM): Not determined
  • FWHM Bandwidth: Not determined
  • Color: Red
  • See Figures 6 and 7.
Fig6.JPG

Figure 6. Absorption spectrum of a reddish chromoprotein from Stylophora pistillata. After Dove et al., 1995.

Fig7.jpg

Figure 7. This Stylophora (pistillata?) specimen likely contains CP560.

Acropora sp.

  • Maximum Absorption (Acropora sp.): 400nm (violet) in the visible range.
  • Full Width at Half Maximum (FWHM): Not determined
  • FWHM Bandwidth: Not determined
  • Color: Red
  • Shibata (1969) reports that this pigment (CP-560) is found along with another chromoprotein (CP-590), and appears red. See Figure 8.
Fig8.JPG

Figure 8. CP-560 and CP-590 in combination can appear red in color. From Shibata, 1969.

CP-562 – A “Kindling” Chromoprotein

  • Maximum Absorption: 562nm (yellow-green)
  • Full Width at Half Maximum (FWHM): 60nm
  • FWHM Bandwidth: 520-580nm
  • Apparent Color: Reddish

This reddish chromoprotein is found in the anemone Anemonia sculata var.rufescens (specifically referred to asasCP-562). See Figure 9. This is an interesting pigment – ‘green’ light (540-560nm – ‘yellow-green light according to our definition) transforms (‘kindles’) non-fluorescent CP-562 (which appears red) to a fluorescent state with an emission maxima of 595nm (which appears as red fluorescence due to the emission bandwidth). This pigment, along with CP-580, blows a neat hole in our tidy concepts of fluorescent and non-fluorescent pigments – these can be both! See http://www.advancedaquarist.com/2006/11/aafeature2 for further details.

Fig9.JPG

Figure 9. Absorption spectrum of CP-562 (after Wiedenmann et al., 2002).

In Closing

We have reviewed the absorbance of several non-fluorescent proteins found in anthozoans. The reader will have noticed that these chromoproteins appear red or pink in color. Next time, we’ll continue our examination of chromoproteins and review those responsible for purple, blue and violet coloration. We’ll also introduce the concept of reflectance, and what it tells us about not only color, but the health of the coral host and its symbiotic zooxanthellae.

I’m in the process of updating the reference list and will include that at the end of this series. Those interested in discussing coral pigments can reach me at RiddleLabs@aol.com

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