Lighting
the Reef Aquarium - Spectrum or Intensity?
By
Dana Riddle and Miguel Olaizola
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.
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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).
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.
Figure
2. Spectral signature of Philips 4,000°K metal halide
lamp.
Figure
3.
Figure
4. Spectral Signature of Aquarium Lighting Systems 12,000°K
"Sunburst" metal halide lamp.
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.
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.
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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.
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).
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 127mMols m-2 s-1 - light
intensities that are quite common at the bottom of aquaria where Fungia specimens
are often placed).
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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.
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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
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.
Ilan, M. and S. Beer, 1999. A new
technique for non-intrusive in situ measurements of symbiotic photosynthesis.Coral
Reefs 18: 1:74.
Joshi, S. and D. Morgan, 1998. Spectral
analysis of metal halide lamps used in the reef aquarium hobby, Part I. Aquarium
Frontiers Online. November.
Joshi, S. and D. Morgan, 1999. Spectral
analysis of metal halide lamps used in the reef aquarium hobby, Part II.Aquarium
Frontiers Online. January.
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.
Kinzie, R.A. and T. Hunter, 1987. Effect of
light quality on photosynthesis of the reef coral Montiporaverrucosa. Mar.
Biol., 94:95-109.
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.
Muscatine, L., 1980. Productivity of
zooxanthellae. In: Falkowski, P.G. (ed). Primary Productivity in the Sea. Plenum
Press, New York. Pp. 381-402.
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.
Riddle, D. and A. Amussen, 1999. Spectrum or
intensity? Proc. 1st Int. Conf. on Marine Ornamentals, Kailua-Kona, Hawaii.
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