[Coral-List] CO2 rise and bleaching

Martin P ê cheux martin-pecheux at wanadoo.fr
Mon Oct 6 07:54:14 EDT 2003

Dear all,

Here is my new article, just published. I email it because the subject is
important, but it was published in a small French review. Commentaries or
critics are welcome.

Marine Life, 2002 (sic), 12, 1-2, 63-68.

Martin Pêcheux
IUFM Sciences de la Vie et de la Terre, Université de Nice,
3 Allée des Effes, 94260 Fresnes,  France.
E-mail : martin-pecheux at wanadoo.fr
(Supplementary Information : http://perso.wanadoo.fr/martin-pecheux)(soon)

Reef mass "bleaching" is a very serious threat to this major Earth
ecosystem. Global Warming is generally invoked. The direct involvement of
CO2 rise in the reef mass bleaching has not yet been fully addressed. Here I
present experimental results of the synergistic effects of light,
temperature and CO2 on the symbiotic sea anemone Anemonia viridis.
Photosynthetic state was measured by chlorophyll fluorescence fast kinetic
rise, allowing determination of the classical stress indicator, the
so-called Fv/Fm ratio. Evidence is provided here showing that CO2 acts very
much like temperature. Present CO2 rise has an effect equivalent to a
warming of 0.4°C, more probably 1.2°C. Hence CO2 rise appears as a main
cause of reef bleaching. Given expected future CO2 levels, reefs must be
considered in very great danger.
 Key-words: CO2, bleaching, coral reef, photosystem II, chlorophyll
fluorescence, Anemonia.

Unexplained reef mass bleaching first appeared in the early 80's (see review
in Williams, Bunkley-Williams, 1990; Smith, Buddemeier, 1992; Goreau, Hayes,
1994; Glynn, 1996; Brown, 1997; Hoegh-Guldberg, 1999; Pêcheux, subm./online
soon/on request). This world-wide phenomenon has been increasing in
frequency and magnitude since then. Bleaching affects all reefal
photosynthetic symbioses, corals being the most spectacular, with
consecutive but variable mortality. Although certainly global, its causes
are not completely understood yet. The Global Warming of 0.5°C (Houghton et
al., 1996) when mass bleaching began is generally invoked as bleaching
occurs preferentially during summers with "above average" temperature, but
evidence of new maxima all over the tropics is still needed, and good
counter-examples exist, notably large foraminifers (Williams et al.,1997).
Bleaching is due to the expulsion of the symbionts and/or the loss of their
pigments, hence the name. The fact that it affects a great diversity of
photosynthetic symbioses (corals, sea anemones and other cnidarians,
mollusks, sponges, foraminifers, ascidians in association with either
dinoflagellates, diatoms, chlorophytes or cyanobacteria) points to some
fundamental limitations of photosynthesis, the two main causes being
photoinhibition and photorespiration. The rise of CO2 from 280 ppm in
preindustrial time to 360 ppm today (Houghton et al., 1996) is a global
change affecting all shallow seawaters (Brewer et al., 1997, Peng et al.,
1998). Photosynthetic symbionts are particularly limited in uptake of
inorganic carbon from the seawater medium. But, in contrast to other
phototrophs, in corals (Allemand et al., 1998) and large foraminifers (Kuile
et al.,, 1989), photosynthesis decreases as CO2 rises, surely because of
weakening of HCO3- pumping by the host which use pH gradient, and thus
promotes photorespiration. It is also well known that photosystem II (PS II)
photochemistry is strongly modulated by HCO3- at the QA-QB site (Diner et
al., 1991; Govindjee, van Rensen, 1993). Analysis of the literature
indicates that photoinhibition is very important in bleaching : involvement
of light (clear sky and calm, transparent waters during events, preferential
occurrence on upper sides), xanthophylls ratio change in situ (Ambarsali et
al., 1997), and, in warming bleaching experiments, early thylakoid
disruption (Salih et al., 1996) and fall of Fv/Fm (Iglesias-Prieto et
al.,1992; Fitt, Warner, 1995; Jones et al.,1998; Iglesias-Prieto, 1997;
Tsimilli-Michael et al., 1998).
It is also already known that, without any other stress, corals reduce
calcification with lowering of calcium carbonate saturation  (Gattuso et
al., 1999, Kleypas et al., 1999), like calcifying marine plankton (Riebesell
et al., 2000). Bleaching affects also non-calcifying organisms, as sea
anemones and sponges. Marine microalgae are indeed directly sensitive to CO2
level (Tortell et al., 1997; Goldman, 1999).
A small environmental change is responsible for bleaching during maximum
summer stresses. Experiments with coral and large foraminifers showed that
elevated CO2 is a bleaching factor (Pêcheux, , 1994, 1996; in preparation).
In order to quantify the effect of CO2 in synergy with light and
temperature, the photochemical state was measured on tentacles of a
symbiotic sea anemone after systematic incubation of one hour under four
light levels, five temperatures and four CO2 levels.

Six clones of Anemonia viridis (Forskål, 1775) (non-purple tips morph) were
collected in May 1998 at half meter depth in the Villefranche-sur-mer Bay,
SouthEastern France. Temperature was about 15.7°C, the summer maximum is
29-30°C. Before experiments, they were kept in culture in a 100 litres
aquarium for one to three weeks at 25°C, 100 µE.m-2.s-1 Life-Glo fluorescent
tube, 12 h light:12 h dark and at 8.25±0.05 pH, fed twice a week with
various meats. They are still healthy after one year of culture.
Experimental protocol
Experiments were run in a pseudo-random order of light and temperature.
Tentacles, 1 to 2 cm long, were cut and put individually in 16 tubes of 5 ml
seawater at 25°C, 8.25 pH. This preparation was done under dim light (‰10µ
E.m-2.s-1) and took 5 minutes at most. It was seen in preliminary
experiments that cut tentacles in tubes stay healthy (i.e. no strong decline
of Fv/Fm, and before any microscopic signs of deliquescence) for at least 4
days in culture condition, or for two days under stress (32°C, 500
µE.m-2.s-1). They were kept in the dark for 25 minutes, with control
measurements of chlorophyll fluorescence during the last 10 minutes.
Seawaters were then replaced by pCO2-controlled pHs of 7.80, 8.10, 8.40 or
8.70±0.02 ones, with 4 replicates per pH treatment. Extremes are like those
observed exceptionally in reef lagoons (see Pêcheux, subm.). These decanted,
unfiltered seawater mediums were the same from the beginning to the end of
the experiments, kept in CO2-tight plastic 1.5 litre bottles in dark,
adjusted when needed before runs from small drifts (generally less than 0.03
pH) with addition of a same stock of 5.0 or 9.5 pH seawaters. Different pHs
were obtained by either adding commercial 95% CO2 gas or by bubbling in
closed circulation with air bathed through a commercial NaOH-Ca(OH)2
solution. pH was measured with a Bioblock 3301 model, with 0.01 precision,
0.02 accuracy, calibrated with Radiometer Copenhagen 7.00 and 9.16 IUPAC NBS
buffers. The NBS pHs of 8.70, 8.40, 8.10, 7.80 used in the experiment
correspond to CO2 levels of respectively about 85, 230, 570 and 1270 ppm at
25°C, given NBS pH/seawater Hansson pH conversion, a measured salinity of
38.75 S (sea gravimeter, temperature and Dikson, Goyet (1994) equations), an
alkalinity of 2603 µeq./kg (using the standard salinity-alkalinity
Mediterranean relationship) and DOE (1994) equations.
Samples were incubated at a temperature of either 25, 28, 30, 32 or 34°C
(±0.2°C maximum range, measured with a mercury ASTM ERTGO thermometer, 0.1°C
accuracy) and light (measured with a Sekonic M38) of either 50, 160, 500 or
1600 µE.m-2.s-1 (except at 28°C-1600 µE.m-2.s-1) for 70 minutes. Two 10
minutes cycles of chlorophyll fluorescence measurements were performed, one
after 50 minutes, with 10±1 seconds of predarkening, the second beginning at
1 hour with 1-minute predarkening. Results with ten seconds predarkening are
essentially similar to the others. Without changing the seawater medium,
further measurement was made after 30 minutes in darkness at 25°C.
Chlorophyll fluorescence measurements
Measurements of the polyphasic rise of chlorophyll fluorescence were done
with a Plant Efficiency Analyser (Hansatech Instruments Ltd, King's Lynn
Norfolk, PE 4NE, UK) at 12 bits precision, with recording every 10 µs the
first 2 ms, then every ms till 1 s, and every 100 ms then after. Maximum
LED's excitation light intensity was used, around 6000 µE.m-2.s-1, peak at
650 nm. Preliminary experiments had shown that in culture condition, a
standard one minute predarkening allows full opening of PS II traps, without
starting the dark adaptation. As in corals and large foraminifers, the
maximum fluorescence level (Fm) is often reached after few seconds, up to 5
seconds, a characteristic of reef symbioses (Tsimilli-Michael et al., 1998).
The fast kinetics of chlorophyll fluorescence rise was analysed according to
established model (Strasser et al., 1995; Tsimilli-Michael et al., 1998).
Here I focus only  on the primary photosynthetic efficiency parameter Fv/Fm
(Baker, 1996), calculated from the ratio of the variable (Fv) to maximum
(Fm) chlorophyll a fluorescence, and corresponding to the ratio of exciton
trapped by the PS II per photon absorbed by chlorophyll antenna, measured
here in light-adapted state.

First, the influence of light is preponderant (Fig. 1). After one hour of
incubation in light, Fv/Fm displays a classical down regulation. It is
remarkably well correlated with the logarithm of light level (Fv/Fm=-0.352
log light+1.254, r2=0.908, p<0.0001, n=304).  As in land plants (Epron,
1997; Spunda et al., 1998), temperature and CO2 effects are superimposed on
this light trend.  Under dim (50 µE.m-2.s-1) light, Fv/Fm rises slightly at
first with temperature, then declines above 30°C ("Monday morning effect")
(Fig. 1a). It is well known that weak light acts in an antagonist manner to
high temperature (Havaux, Strasser, 1990). Under mid-level (160 and 500
µE.m-2.s-1) light, Fv/Fm increases somewhat linearly with temperature. This
trend is very weak under full, noon (1600 µE.m-2.s-1) light, as values are
very low ("Sunday afternoon effect"). The influence of CO2, as recorded
through pH, has exactly the same pattern as does temperature, although with
more variance  (Fig. 1b).
The parallelism between temperature and CO2 effects allows one to establish
an equivalence between them, by comparing their Fv/Fm slopes of regression.
For example, at 160 µE.m-2.s-1, acidification by CO2 of one pH unit has the
same effect than 3.7°C warming. The rise of atmospheric CO2 from 280 ppm to
360 ppm (Houghton et al., 1996) has induced a world-wide acidification of
surface seawater, of -0.0853 pH unit (22% increase of proton concentration)
from 8.315 down to 8.229 for a mean reef seawater (25°C, 36S, 2380 µEq.
Alk./kg, and Dickson, Goyet (1994) equations). This acidification
corresponds in this case to an equivalent warming of Teq=0.315°C. Moreover,
this equivalence increases with light level (Table 1). As mass bleaching
occurs when the sky is clear, the value for full sun is taken as the
reference value (Teq‰0.4°C). A similar calculation leads to the equivalence
of the CO2 increase with a 4.9% light change. This value of Teq is probably
underestimated. As it can be clearly seen at 500 µE.m-2.s-1, the CO2 level
has an effect essentially within the natural range of pH 8.1 and 8.4 (560
and 230 ppm CO2). Using only values at 8.1-8.4 pH yields Teq‰1.2°C.

The above data establish that, first, in a light-adapted state, PS II
efficiency is enhanced by temperature and CO2 over a range relevant for mass
bleaching. It suggests that bleaching originates in greater photochemistry
sensu lato instead of photoinhibition. But this is a short term effect :
reef symbiotic systems kept at high temperature are overwhelmed within a few
hours or days and Fv/Fm begins to decrease instead. Relaxation in darkness
at 25°C for 30 minutes after the incubation showed a decreasing trend with
temperature, as in corals (Fitt, Warner, 1995; Warner et al., 1996; Jones et
al., 1998), and, with longer delay (about a day), also with pH (Pêcheux,
subm.). This "boom then bust" effect can arise with secondary
photoinhibition, following from an imbalance between photochemistry and dark
reactions (Jones et al., 1998). Alternatively, the Fv/Fm increase with
temperature and CO2 might be due to a reduction of the protective
non-photochemical quenching, already invoked in bleaching (Warner et al.,
Secondly, and despite much research on the effect  of CO2 on marine
ecosystem (Bazzaz 1990; Raven et al., 1994; Kleypass et al., 1999; Riebesell
et al., 2000), it was unexpected that CO2 has a so strong effect, and
moreover that it was so similar to temperature. In fact, this points to some
identical mechanism of action. I speculate that the temperature effect is
mediated by internal CO2, perhaps at the QA-QB PS II site. Whatever the
precise mechanisms of coral reef host/symbiont rupture, the line of
arguments that CO2 is a bleaching factor is based on the following : a)
small temperature increase induces bleaching, as it is well known both in
the laboratory and in the field ; b) as summarised above, it originates in
photosynthetic mechanisms, the best indicator of which is Fv/Fm ; and c) as
shown in this paper, CO2 has the same effect on Fv/Fm as does temperature.
Indeed, in long term bleaching experiments with corals and large
foraminifers, small increases of CO2, as does most environmental changes,
favour bleaching, although in a complex way. Similar values of Teq (0.6 to
1.2°C) were found, often with greater sensitivity between 8.1 and 8.4 pH,
when Fv/Fm is around the triggering level for bleaching (0.275±0.050) (in
The CO2 level in the future decades, 500 to 700 ppm, will induce a seawater
acidification of -0.20 to -0.32 pH unit, equivalent, according to these
data, to a stress of 2.8°C to 4.5°C, in addition to Global Warming. This is
a tremendous change. Given observed damage to reefs (Wilkinson, 1998), 90%
to 99% of photosynthetic symbiotic organisms may die, and reefs as they are
known today will disappear. Thus, strong mitigation of anthropogenic CO2
increase is urgent.

I thank R. J. Strasser for material support and strong help ; and Govindjee,
M. Havaux and J. A. Raven for critical readings of the manuscript. This work
was supported by French RMI n°224397K.

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 Table 1  : Temperature equivalence of stress due to actual CO2. For each
light levels, temperature equivalence of CO2 rise stress, calculated by the
ratio of the two slopes of regression of Fv/Fm versus pH and versus
temperature, multiplied by the mean actual acidification of -0.0853 pH unit.
Due to reversed trends ("Monday morning effect"), only data without 32-34°C
and pH 7.8 were used at 50 µE.m-2.s-1, and without 34°C and pH 7.8 at 160
µE.m-2.s-1. Although confidence intervals are very great for each data,
temperature equivalence increases significantly with the logarithm of light
: Teq=0.0889 log light+0.112, r2=0.973, p=0.014.

Light    n    slope pH    slope T    Teq (°C)
50    32    -0.02667    0.00893    0.255
160    48    -0.06542    0.01771    0.315
500    80    -0.05467    0.01291    0.361
1600    64    -0.00583    0.00128    0.388

 Fig. 1 : For each light level, Fv/Fm ± SE as a function of (a), temperature
(CO2/pH conditions pooled, n=16 each point) and (b), CO2/pH (temperature
conditions pooled, n=20 each point). With increasing temperature, at 50
µE.m-2.s-1, Fv/Fm increases till 30°C, then decreases  (both p<0.0001). At
160 µE.m-2.s-1, it increases till 32°C (p<0.0011) then it decreases slightly
(p=0.44). At 500 µE.m-2.s-1 there is only an upward trend (p<0.0001). At
1600 µE.m-2.s-1, the positive trend is non significant (p=0.38). With
acidification by CO2 (note inverted pH scale), at 50 µE.m-2.s-1, Fv/Fm
increases with CO2, from 8.7 down to 8.1 pH (p=0.016 ; 0.0035 with values
corrected from temperature deviations from mean in order to reduce
dispersion) then decreases at pH 7.8 although not significantly (p=0.72 ;
0.57). It increases at 160µE.m-2.s-1 (p=0.019 ; 0.0015) but not down to pH
7.8 (p=0.83 ; 0.57). At 500 µE.m-2.s-1, there is an overall increase
(p=0.023 ; 0.0049), but mostly between 8.4-8.1 pH (p=0.014 ; 0.0036) not
8.7-8.4 pH (p=0.47 ; 0.30) nor 8.1-7.8 pH (p=0.97 ; 0.97). At
1600µE.m-2.s-1, the positive trend is non significant (p=0.69 ; 0.68).

Received August 2000; accepted July 2001.

Data Fig. 1
Light    T/pH    Fv/Fm    SE
50    25    .6583    .0072
50    28    .6856    .0041
50    30    .6979    .0033
50    32    .6515    .0153
50    34    .6152    .0127
160    25    .4039    .0121
160    28    .4819    .0145
160    30    .4940    .0138
160    32    .5257    .0138
160    34    .5134    .0077
500    25    .2149    .0134
500    28    .2386    .0161
500    30    .2867    .0202
500    32    .2889    .0127
500    34    .3330    .0120
1600    25    .1377    .0074
1600    28    -    -
1600    30    .1535    .0107
1600    32    .1558    .0101
1600    34    .1457    .0105
50    8.7    .6367    .0101
50    8.4    .6674    .0056
50    8.1    .6734    .0066
50    7.8    .6693    .0159
160    8.7    .4567    .0143
160    8.4    .4793    .0177
160    8.1    .4977    .0133
160    7.8    .5015    .0112
500    8.7    .2578    .0184
500    8.4    .2419    .0123
500    8.1    .2953    .0168
500    7.8    .2947    .0143
1600    8.7    .1439    .0053
1600    8.4    .1479    .0096
1600    8.1    .1527    .0114
1600    7.8    .1482    .0079

For memory,from the 2nd European Regional meeting on Coral Reefs,
Luxembourg, September 1994, 255, CO2 rise and coral reef bleaching:
"(...) We conduct an experiment which indicates that CO2 is an important
factor of bleaching, at least in synergy with high temperature and light. 15
tips of the coral Stylophora pistillata  were subjected to summer conditions
(raises of temperature from 22.5 to 24°C and light from 75 to 550µE/m2.s)
under three levels of CO2: normal 360ppm, ‰1000ppm and ‰5000ppm CO2 for two
days before transfer back to normal conditions. Corals in the control
aquarium appeared always healthy. At 1000ppm CO2, there was strong reduction
of photosynthesis, and visible paling and closure of polyps for two weeks.
In the high pCO2 condition, the tips showed the first sign of bleaching
after 24 hours, then were bleached after 40 hours, and died all within 2
weeks. (...)" This is a clear evidence that CO2 is a bleaching factor. The
CO2 changes used here were very strong, but the reaction was very rapid.

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