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

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.

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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.

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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.
ColorWavelength (nm)
Violet400 – 430
Blue431 – 480
Green-Blue481 – 490
Blue-Green491 – 510
Green511 – 530
Yellow-Green531 – 570
Yellow571 – 580
Orange581 – 600
Red601 – 700
Table 2. A concise listing of chromoproteins and hosts.
HostColorReference
CP-480PocilloporaRedShibata, 1969
CP-500AcroporaYellowShibata, 1969
CP-560Pocillopora damicornisPinkDove et al., 1995
CP-560Seriatopora hystrixPinkDove et al., 1995
CP-560Stylophora pistillataPinkDove et al., 1995
CP-562Anemonia sculataReddishWiedenmann et al., 2000
CP-568Anemonia sculataVioletLabas et al., 2002
CP-571Condylactis giganteaRedLabas et al., 2002
CP-571Condylactis passifloraRedLabas et al., 2002
CP-575Anemonia sculataPurpleVerkusha & Lukyanov, 2004
CP-578Heteractis crisperiViolet/PurpleLabas et al., 2002
CP-578Acropora digitiferaViolet/PurpleDove et al., 1995
CP-580Goniopora tenuidensPurple/RedMartynov et al., 2003
CP-586AcroporaVioletShibata, 1969
CP-588Rhizostoma anemoneBlueBulina et al., 2004
CP-588Cassiopea xamachanaBlueBulina et al., 2004
CP-588Acropora formosaBlueDove et al., 1995
CP-590Montipora efflorescensBlueNienhaus et al., 2005
CP-595Anemonia sculata‘Kindling’ ProteinBulina et al., 2004
CP-597Anemonia equinaBlueShkrob et al., 2005
CP-610Cnidopis japonicaBlueChan 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
Coral1st Band1st Max Abs2nd Band2nd Max Abs3rd Band
Fungia repunda552-583*521-535*502-510
Pocillopora eydouxi552-583*521-535*502-510
Pocillora damicornis552-583*521-535*502-510
Stylophora sp.552-583*521-535*502-510
Acropora grandis552-583*521-535*502-510
Acropora pacifica552-583*521-535*502-510
Acropora variaviris552-583*521-535*502-510
Acropora hyacynthus552-583*521-535*502-510
Porites sp.552-583*521-535*502-510
Pocillopora acuta552-583*521-535*502-510
Hydnophora microconus552-583*521-535*502-510
Echinopora lamellosa552-583*521-535*502-510
Goniopora sp.568-600~588nm527-537~533nm*
Porites (arboreal)566-596~581530**
Porites (massive)566-596~581530**
Acropora echinata615583nm***
Acropora sp.567-605583nm531-541**
Porites sp.567-605583nm531-541**
Favites sp.567-605583nm531-541**
Montipora ramosa567-605583nm531-541**
Montipora verrucosa567-605583nm531-541**
Goniopora sp.567-605583nm531-541**
Echinopora lamellosa567-605583nm531-541**
Acropora leptocyanthus567-605589nm***
Porites sp.557-608584nm***
Acropora formosa567-615583nm531-541537nm*
Acropora echinata567-615583nm531-541537nm*
Porites sp.557-608584nm***

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

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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

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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

Category:
  Advanced Aquarist
Dana Riddle
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 Dana Riddle

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