Short Take: Variable Rates of Photosynthesis of Zooxanthellae within the Lobe Coral (Porites species): An Explanation for ‘Mottled’ Bleaching?

TitlePhoto.jpg

Porites coral colonies are very
common in waters along the Kailua-Kona, Hawaii coast.

Hobbyists tend to think of corals as having homogenous rates
of photosynthesis, as witnessed by categorical statements such
as “SPS corals need a lot of light.” These sorts of comments
are generally based on tried and true observations, and are
often valid. They have led the hobby to many successes. As
our understanding of corals gains breadth and depth, we begin
to recognize the details of those animals we maintain in
captivity. These finer points lend clues to ultimately solving
some of the remaining puzzles of maintaining corals in
artificial environments.

This article will present information concerning variable
rates of photosynthesis across the surface of a small Lobe
Coral (tentatively identified as Porites lobata) and
offer discussion of the implications.

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Procedure

Corals were legally collected (under permit by the Hawaii
DAR) from the Puako area of the Big Island of Hawaii. A
modified 25mm core bit removed plugs from Porites
colonies (See Figure 1), which were ultimately used in
experiments determining competitiveness of corals and turf
algae. A few ‘plugs’ (maintained as ‘reserves’ for the main
experiment) were held within a 100-gallon RubberMaid trough at
the Natural Energy Laboratory of Hawaii (NELHA) in Kailua-Kona,
Hawaii. Water chemistry and temperature stability are of
little concern, since natural, running seawater is available
for constant flow-through.

These coral plugs were exposed to maximum amounts of
unfiltered sunlight, and light intensity at the coral colonies
peaked at ~2,000 µmol·m2·sec
(~100,000 lux) at noon on cloudless days. These colonies were
allowed to photoacclimate for several weeks before this
experiment began.

figure1.jpg

Figure 1: The Porites ‘plug’ used in the
experiment.

After acclimation, several coral samples were transported to
a darkened, air-conditioned laboratory. A coral specimen was
then transferred to a 38-liter aquarium filled with natural
seawater. A small air pump provided aeration and water
motion.

A pulsed amplitude modulation (PAM) Chlorophyll Fluorometer
(PAM-210, Heinz Walz GmbH, Effeltrich, Germany) was used to
measure chlorophyll fluorescence and ultimately calculated
Photosynthetic Yield, which was used to estimate Relative
Electron Transport Rates within zooxanthellae
photosystems.

The 1.5mm fiber optic sensor of the fluorometer delivered
actinic light (pre-programmed at an intensity of 391
µmol·m2·sec) to the zooxanthellae in
order to stimulate photosynthesis; this probe also gathered and
provided chlorophyll fluorescence data to a microprocessor that
calculated Photosynthetic Yields. The fiber optic sensor was
held at an approximate angle of 60º in relationship to the
coral surface and Yield measurements were taken every ~2mm, for
a total of 41 measurements.

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Relative Electron Transport Rates (rETR) were calculated
using the formula: rETR=Photosynthetic Yield X PAR
(µmol·m2·sec), which is appropriate
for corals (Ralph et. al., 2002). Results were entered into an
Excel worksheet and rETR was charted.

Results

The results demonstrate that rates of photosynthesis can
substantially differ across fine scales of surface area (See
Figure 2
).

figure2.jpg

Figure 2: Relative Electron Transport Rate across
the surface of the coral shown in Figure 1.

The units in the legend are
µmol·electrons·m2·sec
which indicate flow of electrons from Photosystems II to
Photosystems I within the symbiotic zooxanthellae.

Discussion

Rates of photosynthesis are not homogenous across the
surface area of even small coral colonies. Although the chart
is rather crude, there are clearly ‘hot spots’ of
photosynthetic activity. We see variations in rETR of ~33%,
even after photo-adaptation of zooxanthellae to high intensity
sunlight.

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There are several possibilities to explain the differences
in rates of photosynthesis:

  1. At least some coral species have profoundly different
    rates of photosynthesis in polyp tissues and the coenosarc
    (connecting tissue between the polyps) – see Ralph et. al.,
    2002. This is likely not the cause of variations observed
    during this experiment. Measurements of Yield proceeded
    after a dark-adaptation period of 20 minutes, and the coral
    polyps remained retracted during the procedure.
  2. Surface geometry affected the measurements. This is
    unlikely, since this small specimen was relatively flat, and
    results were replicable.
  3. Concentrations of zooxanthellae can vary across a corals
    surface. We were not able to check this, although microscopic
    exams of other corals sometimes show variations in numbers of
    zooxanthellae over surface area. In a similar vein, it is
    possible that different zooxanthellae ‘clades’ or species,
    possessing different photosynthetic capabilities, inhabit
    coral surface areas at fine scales.
  4. Water motion affected the rate of photosynthesis. The
    importance of water motion should not be ignored, however,
    results from previous experiments suggest minimal water
    motion can disrupt areas of stagnant water (by thinning of
    the boundary layer) and allow diffusion of nutrients and
    micro-nutrients required for photosynthesis, thus making this
    theory unlikely. However, we cannot completely rule out the
    possibility that micro-areas of stagnation did limit
    photosynthesis. See Kolber et. al., 1998 for further
    details.

The exact reason(s) for variations of photosynthetic rates
remains a mystery, but the results obviously suggest differing
rates of activity even in small colonies. One has to wonder
how the rates vary over an entire adult colony of Porites
lobata
(which can be meters in diameter and have many
light- and dark- adapted areas).

Although bleaching of this colony was not observed, it is
possible that, under different circumstances, loss of symbiotic
algae or photopigments could occur locally. This could explain
some observations of ‘spotty’ or ‘mottled’ bleaching patterns
often reported by hobbyists. This possibility will be
investigated.

Note: There is some green fluorescence, generally around the
perimeter, of the ‘plug’ colony shown in Figure 1. Attempts
were to establish enhanced or reduced rates of photosynthesis
within this pigmented area did not meet with success, and no
link between coral tissue fluorescence and zooxanthellae
chlorophyll fluorescence could be established.

This method of gathering and plotting data, although rather
crude, does offer a relatively inexpensive alternative to
expensive fluorometers with the ability of plotting
fluorescence over area. With use of the PAM-210’s fiber optic
cable, it would be possible (given enough patience) to record
and chart the relative electron transport rate of every
polyp
on a coral’s surface.

In summary, the results and implications of this experiment
suggest photosynthesis is heterogeneous across the surface of a
coral, even in one exposed, and allowed to photoacclimate, to
high intensity light. As such, differing rates of
photosynthesis is not limited to just illuminated and
non-illuminated areas (as seen with shaded areas). Also, no
photoprotective link is established between green fluorescence
and rates of photosynthesis via PAM fluorometry (see Mazel, et
al., 2003) for comments about green fluorescence and its impact
on zooxanthellae photosynthesis.

References

  1. Kolber, Z., O. Prasil and P. Falkowski, 2000. Measurements
    of variable chlorophyll fluorescence using fast repetition rate
    techniques: Defining methodology and experimental protocols.
    Biochim. Biophys. Acta., 13 (67): 88-106.
  2. Mazel, C., M. Lesser, M. Gorbunov, T. Barry, J.
    Farrell, K. Wyman and P. Falkowski, 2003. Green fluorescent
    proteins in Caribbean corals. Limnol. Oceanogr., 48(1, part
    2), 402-411.
  3. Ralph, P.J., R. Gademann, A.W.D. Larkum and M. Kühl,
    2002. Spatial heterogeneity in active chlorophyll
    fluorescence and PSII activity of coral tissues. Marine
    Biology, 141: 639-646.
Category:
  Advanced Aquarist
Dana Riddle
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 Dana Riddle

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