Lighting the Reef Aquarium – Spectrum or Intensity?

It seems that avid reef aquaria
hobbyists are constantly in search of a better lighting system.
Perhaps one is motivated by a desire for more rapid coral growth,
or simply an aquarium that is more pleasing to the eye. The
question “What is the best lamp?” is often asked.
Although the question is valid, the “best lamp” means
different things among hobbyists. Is an aesthetically pleasing
aquarium the goal, or is the promotion of photosynthesis the
ultimate objective? The former is purely subjective. Finding an
answer to the latter is quantifiable, but requires some rather
sophisticated equipment. This article suggests an answer to the
photosynthesis issue based on results of experiments conducted
with a newly available research instrument.

Those factors promoting photosynthesis must be given serious
attention since most tropical corals of interest to hobbyists
contain symbiotic zooxanthellae algal cells. Of these factors,
lighting is of primary importance. Debates have raged over which
parameter, light intensity or spectral quality is more important.
Both, of course, play a part in promoting photosynthesis in
zooxanthellae. Light intensity or Photosynthetic Photon Flux
Density (or PPFD, simply the number of light particles – photons
– falling upon a given surface) must meet zooxanthellaes’
minimal requirements or the algal cells eventually die. If
spectral quality is not correct, photosynthesis is not promoted
and zooxanthellae become “starved” for proper light and
will soon perish.

Estimating the spectral requirements of zooxanthellae is not
particularly easy. Zooxanthellae contain various photosynthetic
pigments, including chlorophyll A, chlorophyll C2, the
carotenoid peridinin and perhaps others (all of which may be in
varying proportions due to the photoadaptive capabilities of
zooxanthellae). However, researchers have established the quality
of light absorbed by these pigments and we can safely assume
certain wavelengths are required.

Figure 1 demonstrates the light energy harvested by
zooxanthellae isolated from the stony coral Favia, in a
chart called an “action spectrum.” An action spectrum
describes the relative effectiveness of energy at different
wavelengths in producing particular biochemical or biological
responses (such as oxygen evolution, carbon uptake, electron
transport rate, etc., during photosynthesis).

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Graph

Figure 1. An “Action Spectrum” for
zooxanthellae. See text. After Muscatine, 1980.

To believe that blue (430-480 nm) and red (600- 700 nm)
wavelengths are required is only partially true. As Figure 1
demonstrates, a wide range of wavelengths are absorbed by
chlorophylls A and C2; however, peridinin and perhaps
other photopigments, effectively harvest light energy outside of
the range normally associated with photosynthesis.

Researchers have addressed light quality and its effects on
zooxanthellae and coral growth. Perhaps the most interesting is a
paper by Kinzie et al. (1984); they presented evidence that
corals grown more rapidly under blue and white light of the same
intensities (~12% of solar Photosynthetically Active Radiation –
PAR, ~250 µMols·m2·sec, or 10,000 lux) than under
“green” or “red” light of equal intensities.
These scientists used clear or colored acrylic filters and
natural sunlight. The blue filter transmitted wavelengths of ~
400 to 500 nm and the clear filter (transmission quality not
shown in the paper) likely was a fair representation of sunlight
(although most acrylics attenuate all wavelengths but tend to
decrease violet and blue disproportionately). “Blue”
light is suggested to have some rather “magical”
properties – it has been noted to increase rates of protein
synthesis in some algae, as well as cause shifts in
photosynthetic pigment concentrations in zooxanthellae. Blue
light has also been reported to increase rates of photosynthesis
(Kinzie and Hunter, 1987). Are spectral characteristics of
“blue” metal halide lamps sufficient to promote
photosynthesis more efficiently in zooxanthellae of captive
corals?

Unfortunately, the spectral qualities of light transmitted by
these researchers’ filters only faintly resemble those of
lights used over aquaria. It is a leap of faith to apply the
results obtained under filtered sunlight to artificial light
sources, which have spectral spikes. However, this has not
stopped many from interpreting that higher Kelvin lamps are best
for promoting photosynthesis in corals.

In an excellent series of articles, Joshi and Morgan (1998;
1999) presented spectral qualities of many metal halide lamps
commonly sold in the pet industry, but stopped short of making
recommendations to hobbyists. So, the question remains – are
there major advantages to zooxanthellae/corals when using certain
lamps, or is there only aesthetic appeal? Do common lamps with
output weighted in the violet/blue regions of the spectrum and
readily available to hobbyists actually increase the rates of
photosynthesis?

Two lamps were chosen for use in an experiment designed to
determine if differing spectral qualities do indeed make a
difference in photosynthesis rates. The first lamp is a Philips
175-watt 4,000° K metal halide lamp (usually available for less
than $20 in major home improvement centers). The second lamp is
an Aquarium Lighting Systems 175-watt 12,000°K
“Sunburst” metal halide lamp. Spectral signatures of
these lamps were determined with an Ocean Optics spectrometer.
Spectral compositions were estimated by use of a LiCor quantum
meter and glass cut-off filters. Use of these filters provides
reasonable estimates of violet and blue wavelengths (400-465 nm)
and red wavelengths (600-700 nm). These filters transmit few
wavelengths in the yellow and orange portion of the spectrum.
Considering that metal halide lamps have spectral spikes at 575
and 577 nm (due to the element mercury contained within the arc
tubes), the percentages of blue radiation shown in the pie charts
are slightly overstated (See Figures 2 – 5). However, the
Sunburst 12,000°K lamp is the “bluest;” the Philips
lamp less so.

Graph

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Figure 2. Spectral signature of Philips
4,000°K metal halide lamp.

Graph

Figure 3.

Graph

Figure 4. Spectral Signature of Aquarium
Lighting Systems 12,000°K “Sunburst” metal halide
lamp.

Graph

Figure 5.

There are many ways to estimate the effectiveness of light on
corals. If one has the time and patience, simple observations of
growth (along with rigorous control of other factors) may
suffice. A more sophisticated approach is one using a
respirometer and delta oxygen evolution as the metric in judging
rates of photosynthesis. Preliminary results suggested there is
no benefit to photosynthesis when using a 20,000°K metal halide
lamp as opposed to the use of an inexpensive halide lamp (Riddle
and Amussen, 1999). However, respirometry is an inexact science,
fraught with all the drawbacks of experiments conducted in small,
sealed experimental chambers.

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A new technique is now available – that of Pulsed Amplitude
Modulation (PAM) fluorometry. This experiment employed a Mini-PAM
meter, manufactured by Walz GmbH, Germany. This method is
non-intrusive and is gaining acceptance as the preferred method
of measuring rates of photosynthesis (Beer et al., 1998). The
Mini-PAM measures the fluorescence yield of the chlorophyll A
molecules in the photosystem of zooxanthellae in response to
changes in illumination. Chlorophyll fluorescence is assumed to
arise from reradiation of absorbed light energy from Photosystem
II (PS II) antenna pigments (including chlorophyll A, chlorophyll
C2, and peridinin).

Fluorescence and the photochemical reactions of photosynthesis
are competing processes in the dissipation of absorbed light
energy. Energy absorbed by antenna pigments is generally assumed
to have three primary pathways for dissipation (see Figure 6).
First, it can be reradiated (fluoresced); second, it can be
dissipated as heat or, third, it can be transferred to the
reaction center of PS II. Once in the reaction center, this
energy is available for use in photochemistry. Reduction
-oxidation potential of the primary quinone acceptor (QA) governs
what happens next. If the Qa is oxidized (the reaction center is
said to be “open”), a photochemical reaction will occur
and eventually lead to oxygen evolution and carbon fixation, the
events that we associate with photosynthesis. However, if the QA
is reduced (the reaction center is “closed”) the energy
cannot be used in photochemistry. Therefore the chances of
thermal dissipation and fluorescence will increase.

Thus, the magnitude of the fluorescence signal depends mainly
on the amount of light energy absorbed (which itself depends on
the spectral quality and intensity of the illumination source and
the quantity and absorption spectra of the photosynthetic
pigments present in the cells) and the fraction of reaction
centers that are open.

The Mini-PAM exploits the relationship between photochemistry
and fluorescence, and how it changes under different illumination
conditions, to estimate the capacity of photosynthetic cells to
photosynthesize (i.e., the fraction of reaction centers that are
open). Essentially the Mini-PAM estimates the fraction of
reaction centers that are open by comparing the magnitude of the
fluorescence signal under ambient illumination (e.g., different
lamps or sunlight) and the magnitude of the fluorescence signal
following a saturating flash of light that temporarily overwhelms
PS II and closes all the reaction centers.

Over time scales of several seconds/minutes we can assume that
the pigment content of the cells does not change. Thus, if one
were to measure the changes in fluorescence emitted by
chlorophyll at a given spot on a coral induced by different
amounts of light, an estimation of photochemical efficiency can
be estimated. If one were to measure the fluorescence of this
same spot when exposed to different spectral qualities (but at
given light intensities), an estimation can be made of light
sources’ ability to promote photosynthesis and hence
comparisons can be made. In essence, this meter indirectly
measures changes in the electron transport rate of PS II under
different illumination conditions.

Graph

Figure 6. Assumed potential pathways for
energy in Photosystem II. See text. After Olaizola and
Yamamoto, 1994.

In order to determine if light quality makes a difference in
the rate of photosynthesis, the quantity of light must be
equivalent in each portion of the trials. A jig, holding the
lamps, allowed quick and easy adjustments of light intensity to
predetermined levels. The submersible probe of an Apogee
Instruments quantum meter was placed immediately next to the
coral used in the experiments and measured light energy, more
specifically Photosynthetically Active Radiation (PAR).

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The coral chosen for the experiment is one popular with
hobbyists – the “Mushroom” coral (Fungia
scutaria
). This coral is usually maintained in a 300-gallon
system with flow-through of natural seawater. Natural sunlight is
the light source and is attenuated with shade cloth. Maximum
light intensity, as measured at noon and at the coral’s
surface is about 17% of natural sunlight’s visible energy –
~350 µMols·m2·sec, or ~17,500 lux. The relatively flat shape of
the Fungia specimen allowed use of a shallow water basin
and easy positioning of the PAM probe.

Results and Discussion

The Mini-PAM meter has the ability to collect and store much
information, including time and date, minimum and maximum
chlorophyll fluorescence, photosynthetically active radiation,
yield, etc. Data obtained during the experiment was downloaded
and analyzed.

Photosynthesis boils down to a flow of electrons, therefore
the Electron Transport Rate (ETR) is indicative of the rate of
photosynthesis. Calculation of the ETR is straightforward and is
as follows: Quantum Yield (Y) multiplied by photosynthetic photon
flux density (PPFD) absorbed by the Photosystem = ETR (Ilan and
Beer, 1999). We did not conduct measurements of absorbed PPFD but
we made the assumption that the pigment content of the
coral’s zooxanthellae did not change during the experiment.
Thus we report the “relative” ETR (not the absolute
ETR). We also made the assumption that the ETR is zero during
total darkness.

A portion of the information gathered is summarized in Figure
7, which shows the relative ETR measured under three difference
PPFD intensities for each lamp (46, 85 and 127 mMols m-2 s-1
light intensities that are quite common at the bottom of
aquaria where Fungia specimens are often placed).

Graph

Figure 7. Comparison of Relative Electron
Transport Rate in zooxanthellae under two different lamps at
different PPFDs.

This experiment’s results suggest information potentially
valuable for hobbyists – that rates of photosynthesis were
essentially the same under these two distinctly different light
sources. Other than aesthetic value, there appears to be no
advantage, photosynthetically speaking, in using high Kelvin
lamps.

The implication of these results should be of interest to
hobbyists; it suggests that lamp selection (with due regard to
lamp intensity) may be based on appeal, whether that is price or
the “look” it gives to a tank, without fear of
hindering photosynthesis. Economy-minded hobbyists and coral
farmers may find this especially useful. It appears that light
intensity and relatively simple light measurements alone
adequately judge lamp efficiencies within the context of
zooxanthellae photosynthesis. This should not be construed to
mean that all light sources are adequate for reef
aquaria use.

The spectral signatures obtained with the spectrometer
demonstrate that these two metal halide lamps are full spectrum
(though the 12,000° K lamp output is skewed towards the blue
portion of the spectrum) and most resembles the “white
light” category defined by Kinzie et al. (1984). Results
garnered with the PAM meter suggest these two lamps are more or
less equally efficient in the promotion of photosynthesis when
PPFD values are the same.

It is inappropriate to claim that there are no major
differences among the plethora of lamps available and their
abilities to promote photosynthesis. Certainly the depreciation
of overall lamp light output (PPFD) should be considered and
readers are encouraged to review the works of Joshi and Morgan
(1998; 1999, 2000) and others. Future experiments involving
spectral quality and its effects should include more data points,
different lamps and perhaps different coral species. Clearly,
more work is required before we have an answer to the “best
lamp” question. For now, it appears that spectral quality
might be subordinate to lamp intensity.

Special thanks go to Alexander Diffley for assistance in
collection of the data.

References

  1. Beer, S., M. Ilan, A. Eshel, A. Weil and I. Brickner, 1998.
    Use of pulse amplitude modulated (PAM) fluorometry for in
    situ measurements of photosynthesis in two Red Sea faviid
    corals.
    Mar. Biol., 131(4): 607-612.
  2. Ilan, M. and S. Beer, 1999. A new technique for
    non-intrusive in situ measurements of symbiotic
    photosynthesis.
    Coral Reefs 18:
    1:74.
  3. Joshi, S. and D. Morgan, 1998. Spectral analysis of
    metal halide lamps used in the reef aquarium hobby, Part
    I.
    Aquarium Frontiers Online.
    November.
  4. Joshi, S. and D. Morgan, 1999. Spectral analysis of
    metal halide lamps used in the reef aquarium hobby, Part
    II.
    Aquarium Frontiers Online.
    January.
  5. Joshi, S., 2001. An analysis of recent metal halide lamps:
    Shedding some light on new reef tank illumination. Marine Fish
    and Reef USA, 2002 Annual. Fancy Publications, Irvine, Ca. pp.
    56-69.
  6. Kinzie, R.A. and T. Hunter, 1987. Effect of light quality
    on photosynthesis of the reef coral Montipora
    verrucosa. Mar. Biol., 94:95-109.
  7. Kinzie, R.A., P.L. Jokiel and R. York, 1984. Effects of
    light of altered spectral composition on coral zooxanthellae
    associations and on zooxanthellae in vitro. Mar. Biol.,
    78:239-248.
  8. Muscatine, L., 1980. Productivity of zooxanthellae. In:
    Falkowski, P.G. (ed). Primary Productivity in the Sea.
    Plenum Press, New York. Pp. 381-402.
  9. Olaizola, M. and H. Yamamoto, 1994. Short-term response of
    the diadinoxanthin cycle and fluorescence yield to high
    radiation in Chaetoceros muelleri (Bacillariophyceae).
    J. Phycol. 30: 606-612.
  10. Riddle, D. and A. Amussen, 1999. Spectrum or intensity?
    Proc. 1st Int. Conf. on Marine Ornamentals,
    Kailua-Kona, Hawaii. Page 70
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Dana Riddle
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