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Effects of Heat Stress on Phytopigments of Zooxanthellae (Symbiodinium spp.) Symbiotic with the Corals Acropora hyacinthus, Porites solida, and Favites complanata

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The question of whether global warming affects phytopigments within Symbiodinium spp. of coral is central to understanding the process of coral bleaching. In this study, corals Acropora hyacinthus, Porites solida, and Favites complanata were exposed to elevated temperatures (28, 30, 32, and 34 o C) for 48 h and the responses of Symbiodinium chl a, chl c, total carotenoids, total phaeophytins, and fucoxanthins were examined. In A. hyacinthus, the phytopigment concentrations at 28 and 30 o C were significantly different from 32 and 34 o C, implying a step-function response initiated between 30 and 32 o C. In P. solida, all phytopigments varied significantly between the temperatures, except in the case of total carotenoids which had no significant response. In F. complanata, all phytopigments decreased linearly as temperature increased. Our results indicate that decreases in Symbiodinium phytopigments in response to heat stress may be a need to adapt while their host coral may already be adapted (or exapted).
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Effects of Heat Stress on Phytopigments of Zooxanthellae
(Symbiodinium spp.) Symbiotic with the Corals Acropora hyacinthus,
Porites solida, and Favites complanata
Kevin B. Strychar (Corresponding author)
Texas A & M University – Corpus Christi
Department of Life Sciences, Corpus Christi, Texas 78412, USA
Tel: 1-361-825-5883 E-mail: kevin.strychar@tamucc.edu
Paul W. Sammarco
Louisiana Universities Marine Consortium (LUMCON)
8124 Hwy. 56, Chauvin, LA 70344, USA
Tel: 1-985-851-2876 E-mail: psammarco@lumcon.edu
Received: October 31, 2011 Accepted: November 16, 2011 Published: January 1, 2012
doi:10.5539/ijb.v4n1p3 URL: http://dx.doi.org/10.5539/ijb.v4n1p3
Abstract
The question of whether global warming affects phytopigments within Symbiodinium spp. of coral is central to
understanding the process of coral bleaching. In this study, corals Acropora hyacinthus, Porites solida, and
Favites complanata were exposed to elevated temperatures (28, 30, 32, and 34oC) for 48 h and the responses of
Symbiodinium chl a, chl c, total carotenoids, total phaeophytins, and fucoxanthins were examined. In A.
hyacinthus, the phytopigment concentrations at 28 and 30oC were significantly different from 32 and 34oC,
implying a step-function response initiated between 30 and 32oC. In P. solida, all phytopigments varied
significantly between the temperatures, except in the case of total carotenoids which had no significant response.
In F. complanata, all phytopigments decreased linearly as temperature increased. Our results indicate that
decreases in Symbiodinium phytopigments in response to heat stress may be a need to adapt while their host
coral may already be adapted (or exapted).
Keywords: Carotenoids, Chlorophyll a (chl a), Chlorophyll c (chl c), Coral bleaching, Fucoxanthins, Global
warming, Phytopigments, Phaeopigments
1. Introduction
In scleractinian corals containing endosymbiotic zooxanthellae, loss of coloration (or bleaching) is due to the
loss of the dinoflagellates (Symbiodinium spp.) and their associated pigments (Venn et al., 2006). Some corals
contain light-absorbing pigments (Shibata, 1969), including fluorescent chromoproteins (Salih et al., 2000, 2006;
Dove et al., 2006; Riddle, 2009). In this paper, however, only the phyopigments of the endosymbiotic
zooxanthellae of hermatypic corals will be considered.
The loss of phytopigments results in decreased photosynthesis and reduced production of symbiont
photosynthate, which can in turn contribute to death of the host (see Salih et al., 2000). Phytopigments also
provide the host with ultraviolet radiation (UVR) protection and color as a physical trait (Salih et al., 2000, 2006;
Dove et al., 2006; Banaszak & Lesser, 2009). Studies examining the effects of elevated temperature on the
phytopigments of Symbiodinium cells in scleractinian corals are equivocal. Flores-Ramírez and Liñán-Cabello
(2007), for example, have shown increased levels of chlorophyll a (chl a) and carotenoid pigments in the
zooxanthellae of Pocillopora capitata as temperatures increased from 22 to 28°C. Ralph et al. (2001) have
reported that photosynthesis in expelled zooxanthellae from Cyphastrea serailia and Pocillopora damicornis is
fully functional, although they do not report the zooxanthellar clade(s) in the holobiont coral. Some studies,
however, indicate that phytopigment concentrations decrease as temperatures increase (Jokiel & Coles, 1974;
Kleppel et al., 1989; Jones et al., 1998; Fitt et al., 2000). Others offer evidence that chlorophyll concentrations in
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zooxanthellae remain unchanged under elevated seawater temperature conditions (Hoegh-Guldberg and Smith
1989a, 1989b). Thus, previous experimentation has not yielded clear results regarding whether phytopigment
concentrations of Symbiodinium increase, decrease, or remain stable during temperature increases.
Jokiel and Coles (1974) and Coles and Jokiel (1977) demonstrated that certain Indo-Pacific corals vary in their
sensitivity to increasing seawater temperatures. Warner et al. (1999), Baker et al. (2004), Baker (2003, 2004),
and Lajeunesse et al. (2004) have also shown that some species of Symbiodinium exhibit optimum growth and
phytopigment synthesis rates between 26 to 32°C. Other species function best at 26°C, poorly at 30°C, and cease
to function at all at 32°C (Warner et al., 1999). More recently, Venn et al. (2006) documented a 50-80% loss of
symbiont cells in Montastrea cavernosa at 32°C, but observed no significant pigment loss (chl a and peridinin)
concentrations in retained cells. These authors also reported, however, a substantial loss of chl a and carotenoids
at 32°C but no loss of Agaricia sp. symbiont cells. The maximum temperatures at which a symbiont and its
associated pigments can function, then, defines the upper limit of the range within which the host coral can
receive nutrients from their symbionts. The question arises as to whether the pigments involved in
photosynthesis play a key role for the survival or mortality of Symbiodinium and the host under conditions of
increased temperatures (Fitt et al., 1993; Apprill et al., 2007). Hoegh-Guldberg and Smith (1989b) observed
increases in pigment concentrations within Seriatopora hystrix during bleaching on the Great Barrier Reef,
despite decreases in Symbiodinium populations. By contrast, Dove et al. (2006) have reported decreased pigment
concentrations without a significant loss of zooxanthellae.
Although chl a is the primary phytopigment responsible for photosynthesis in zooxanthellae, secondary pigments
such as chlorophyll c (chl c) and carotenoids absorb light energy that chl a does not (Govindjee and Govindjee,
1974; Chalker and Dunlap, 1982). Carotenoids have two primary functions in photosynthesis: 1) the capture of
light energy and transfer of that energy to chlorophyl molecules to be used for photochemical reactions (Ston and
Kosakowska, 2000); and 2) photo-protection of reaction centers, cells, tissues, and pigment-protein antennae
(Demming et al., 1987; Gilmore et al., 1995; Demming-Adams et al., 1996; Porra et al., 1997; Mobley and
Gleason, 2003). They operate within the PS-I system and consist predominantly of carotenes. Those operating
within the PS-II system primarily contain xanthophylls (Govindjee, 1975). Symbiodinium spp. are dinoflagellates
which belong to the Peridiniales. This order contains a variety of xanthophylls. Symbiodinium is known to
contain a relatively small amount of the accessory pigment fucoxanthin, representing one of the phytopigments
we considered here. Zhang et al. (2005) have found through genetic analyses that the taxonomy of Symbiodinium
is not clear as to whether it falls within the peridin lineage or some other. Phaeophorbides and phaeophytins are
the principal degradation products of chl a. Such phaeopigments, when present, indicate algal senescence, decay,
or autolysis (Govindjee, 1975).
Phytopigments are known to be susceptible to elevated temperatures (e.g., Thebud & Santarius, 1982), and PS-II
appears to be the more sensitive of the two photosynthetic systems (Becker et al., 1990; Havaux, 1993). It is not
yet known whether heat stress causes a decrease in the functional ability of phytopigments in Symbiodinium,
causing degeneration of the carotenoids and thus disrupting the PS-I and -II systems (Warner et al., 1996). There
has also been some recent interest in characterizing degradation of pigment systems due to temperature stress in
the symbionts of scleractinian corals, particularly with respect to chl a (Fang et al., 1995; Brown et al., 1999;
Mobley and Gleason, 2003; Smith et al., 2005; Hill and Ralph, 2006; Liñán-Cabello et al., 2006; Flores-Ramírez
& Liñán-Cabello, 2007; Cooper & Fabricius, 2011). Information is needed on the mechanism by which heat
stress affects multiple pigments other than chl a, in order to increase our understanding of susceptibility of
zooxanthellae to bleaching. In addition, this information is needed from several species of corals drawn from
different families within the Scleractinia, in order to determine whether the responses of phytopigments to heat
stress vary inter-specifically or not.
In this study, we examine the effects of heat stress on various phytopigments of Symbiodinium within three
scleractinian coral hosts - Acropora hyacinthus, Porites solida, and Favites complanata – representing the
families Acroporidae, Poritidae, and Faviidae. We do this in order to gain a deeper understanding of the
bleaching mechanism under varying temperatures, and to elucidate both species-specific and
phytopigment-specific responses.
2. Materials and Methods
2.1 Experimental design
The experiment followed a two-way, balanced, mixed model, 3-level nested, balanced, orthogonal design (see
Sokal & Rohlf, 1981; McKillup, 2006). The first factor (fixed) was scleractinian corals, of which there were
three species – Acropora hyacinthus, Porites solida, and Favites complanata, derived from three different
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families – the Acroporidae, the Poritidae, and the Faviidae. Replications were represented by trials of the
experiment, of which there were five (5) in temporal sequence in contiguous 48-h periods, nested within species.
This trial duration was chosen because this is the minimum amount of time required to induce bleaching in
corals when exposed to increased temperatures (Strychar et al., 2004b; 2005).
The second factor (fixed) was the experimental temperatures of 28, 30, 32, and 34oC. Two replicate tanks were
nested within each temperature treatment. Two (2) incubation chambers were used for each temperature
treatment, one in each tank. Each one of those chambers received two (2) coral fragments. The total number of
replicates per temperature treatment within any given trial was 10. The total number of coral samples used per
species was 80 (4 temperatures × 2 replicates/tank × 5 trials/temperature), each ~100 g in weight. A total of 20
coral fragments were exposed to each experimental treatment/temperature. Data on phytopigment concentrations
from the two corals in a given chamber per trial were averaged. Thus, an experimental replicate is defined by the
number of incubation chambers, not the number of corals in each chamber; (i.e., “n” = 10 not 20). This provided
a more conservative approach to data analysis. It should be noted that any given coral fragment was only used in
the experiment once.
2.2 Experimental set-up
Corals were collected from the Capricorn Bunker Reefs of Barren Island (23°10'S, 151°55'E) and Outer Rocks
(23°4'S, 151°57.2'E), Queensland, Australia. Coral fragments were collected by SCUBA divers at 7-10 m using
pliers or a hammer and chisel. Collected fragments were placed separately into plastic containers and sealed at
depth, and transported via divers to the surface. The plastic containers were placed into temperature-controlled
coolers containing seawater, held on the dive boat. The holding containers were then opened to allow water
exchange to occur; 75% of the sea water was replaced every two hours. The corals were then transported to the
laboratory at Central Queensland University (Australia) and placed in a 200 L holding tank containing 1
µm-filtered seawater, derived from near-shore waters in the region. All corals were subjected to a 12:12 h
day:night light regime (150 µE m-2s-1).
The control temperature used in these experiments was 28°C, the mean maximum weekly sea surface
temperature (SST) from 1981 to 2001 (Reynolds and Smith, 1994). Coral fragments that were acclimated to this
temperature for > 4 months exhibited no symptoms of bleaching stress. In addition, skeletal areas on the coral
fragments that had been damaged during collection healed over during this acclimation period.
The experimental variable-temperature tanks have been described in detail in Strychar et al. (2004b), but will be
briefly described here. Sea water was collected from the coast (~30 km from where the corals were collected),
transported, and then pumped into an enclosed 4,000 L storage tank. The sea water was then pumped through 10
µm and 1 µm filters at a constant flow rate of 20 ml min-1, through Tygon tubing to a series of plastic incubation
containers. Each incubation chamber contained an incurrent and excurrent pipe that regulated filtered seawater
levels to a volume of 1.5 L. Water within each holding container was mixed with an octagon-shaped Teflon
“spin-wedge” (Crown Scientific Ltd.) stir bar, spun by a magnetic stirrer. Salinity concentrations were monitored
every 12 h with a TPS WP-84 conductivity-salinity meter. Temperatures within each holding chamber were
maintained using 125W Jäger (Aquacenta Ltd.) aquarium heaters.
Corals were subjected to seawater heated to 30, 32, or 34°C and compared to a control at 28°C. Temperatures
within each chamber were monitored every 15 min with submersible digital data-loggers (Tiny-tags®; Hastings
Data Loggers) placed at the bottom of each holding container. Mercury-filled thermometers, accurate to 0.1°C,
were also used to help identify water temperature fluctuations in each holding container. If the water temperature
fluctuated more than 0.5°C, the aquarium heaters were re-adjusted. The rates of temperature increase and the
duration of exposure used in this experiment are probably faster and shorter than those that might occur naturally
prior to, and during, a bleaching event in the field. Thus, any change in zooxanthellar phytopigment
concentrations oberved here might be considered an underestimate of that which might be observed over longer
periods of time.
Lighting for all experiments consisted of two 10,000K and one 7,500K actinic fluorescent lights, mimicking
diurnal patterns of 12 h light: 12 h dark and a level of irradiance consistent with both the holding tank and the
conditions at depth in the field (~150 µE m-2s-1). Light levels were monitored every 6 h with a Li-Cor 250 Quantum
photometer.
2.3 Phytopigment analysis of symbiont Symbiodinium spp. collected from each coral
Symbiont pigments included the following five phytopigments: chlorophyll a (chl a), chlorophyll c (chl c), total
carotenoids, total phaeophytins, and fucoxanthins. Approximately 5 ml of coral tissue were water-picked off of the
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coral skeletons, which greatly reduced if not eliminated contamination by Ostreobium sp. and other endolithic
algae (Agustin, 2007). The collected tissue slurry was re-suspended in filtered seawater to create a 10 ml sample
(final volume), and homogenized with an Ultra Turrax T-25 at low speed; homogenization enhances the rupture of
algal cells and increases the extraction efficiency of acetone (Sartory & Grobbelaar, 1984). The tissue homogenate
was then filtered through Whatman GF/C glass fibre filters. Marker et al. (1980) reported that Whatman GF/C
filters are better than membrane filters because they efficiently retain chlorophyll. Each filter was cut into small
pieces, resuspended into 10 mL of 90% acetone, and homogenized for 1 min at low speed (Jones, 1997). Acetone
was used as the extraction solvent, since Mantoura and Llewellyn (1983) found that methanol solvents lead to
derivative products from chl a. Extracts of phytopigments were kept in the dark at -20°C for a minimum of 24 h
following the methods Lenz and Fritsche (1980). This was done in order to reduce pigment degradation due to the
death and decomposition of zooxanthellar cells. Thawed extracts were then centrifuged at 1000×g for 5 min to
remove filter fibers and algal debris. Phytopigment absorbances were read on a Perkin Elmer UV/VIS Lambda Bio
20 spectrophotometer.
The methods used here to extract the pigments followed Jeffrey and Humphrey (1975), American Public Health
Association (APHA, 1989), and those of Leletka and Popova (2005). No attempt was made to describe or quantify
pigment breakdown. We focused entirely on changes in Symbiodinium pigment concentrations, standardized by
number of zooxanthellar cells present in situ, in response to elevated temperature. All phytopigment
concentrations isolated from symbiont cells are expressed in terms of picograms per cell (pg cell-1), following
Hoegh-Guldberg and Smith (1989), and Jones (1997); changes in cell densities with respect to experimental
temperature changes may be found in Strychar et al., 2004a. Data corresponding to each experimentally tested
coral was collected at the beginning and at the conclusion of each experiment to help determine the total number of
Symbiodinium present. At each sampling interval, the number of Symbiodinium present was determined following
methods described in Strychar et al. (2004a). Briefly, the number of in situ live + dead + mitotic Symbiodinium
cells were enumerated using a hemocytometer (replicate counts, n=10) and a light microscope, and expressed per
unit volume of extracted coral tissue (1 g in 1 mL of filtered sea-water). For comparative purposes, tissue was also
removed from other replicate corals that were not used in the experiments.
We recognize that HPLC (high pressure liquid chromatography) is the best method to examine pigment
concentrations; however, we neither had access to nor the resources to use this instrument. Latasa et al. (1996)
report that absorption techniques using spectrophotometric analyses produce historically accurate and comparative
data when pigments are stored in the dark and at -20°C. Comparative analyses of three separate labs using HPLC
vs. spectrophotometry in their study yielded generally comparable pigment concentration data. The
spectrophotometry data was more precise than the HPLC data, in that the authors observed a range variation of ± 6%
between the different labs using this technique vs. HPLC data, which exhibited a ±10% to ±20% range of
variability. Brotas et al. (2007) concluded that chlorophyll spectrophotometric analyses are reliable, but that HPLC
is indispensable when available and provides greater accuracy and a higher level of resolution. Here, the statistical
comparisons between treatments were significantly different, indicating that the observed responses were robust.
Samples were examined for phytopigment concentrations of chl a and chl c using light wavelengths of 664 nm and
630 nm, corrected by subtracting the absorbance value at 750 nm, as specified by Arar (1997). Following this
author’s recommendations, equations used for specific pigments are:
(1). chl a = 11.85 × [(Abs 664 nm – Abs 750 nm) –0.08 × (Abs 630 nm – Abs 750 nm)];
(2). chl c = 24.52 × [(Abs 630 nm – Abs 750 nm)– 1.67 × (Abs 664 nm – Abs 750 nm)];
(3). Carotenoid concentrations (C-car) C-car = 7.6 × [(Abs 480 nm – Abs 750 nm) – (1.49 × {Abs 510 nm –
Abs750 nm})];
(4). Phaeophytin concentrations = 26.7 × [1.7 × ((Absa 665 nm – Absa 750 nm) – (Absb 664 nm – Absb 750 nm)];
(5). Fucoxanthin concentrations = [Abs 470 nm – 1.239 (Abs 631 nm + Abs 581 nm – (0.3 × Abs 664 nm) –
(0.0275 × Abs 664 nm)]/141
Carotene is one of the pigments that passes its absorbed energy to chlorophyll when exposed to light. Here we
ask whether carotenoids as a whole respond differently than other pigment groups upon exposure to increased
seawater temperatures (Agustin, 2007). This approach of using carotenoids as a group to examine phytopigment
changes during the bleaching process has been successfully used by Liñán-Cabello et al. (2006) on corals from
the Pacific coast of Mexico. Analysis to determine phaeophytin concentrations was performed before and after
the addition of 3 drops of 1M HCl ml-1 of extract (Arar, 1997). In the above equation referring to phaeophytins,
Abs = absorbance, Absa = absorbance after acidification, and Absb = absorbance before acidification. Techniques
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used to examine fucoxanthin concentrations did not have a correction factor for carotenoid absorption, which
could potentially lead to a liberal estimate of fucoxanthins (Seely et al., 1972). Thus, these calculations were
corrected for carotenes by subtracting the total amount of carotenoids derived from equation 3.
2.4 Statistical analysis
Pigment data were tested for heteroscedasticity prior to analysis. Data found to be heteroscedastic were
log-transformed for normalization purposes and then analysed by ANOVA and linear and polynomial regression,
using SigmaPlot V 8.0, BiomStat V3.2, and SPSS. Percent data were arcsine-transformed prior to analysis for
purposes of normalization. Data found to be significantly different were further tested via T’, T-K, and GT2 a
posterior multiple comparisons of means analyses; these were used to identify specific treatments contributing to
the observed significance. Only significant linear and curvilinear regression lines are shown in figures. Details of
statistical results are presented in the text and in figure and table legends.
3. Results
3.1 Overall comparisons
An overall analysis of the chl a data via a 2-way nested ANOVA revealed that there was a highly significant
difference between endosymbiont phytopigment concentrations between coral species (p < 0.001, two-way nested
ANOVA) and between temperatures (p < 0.01). There was also a significant interaction between species and
temperature (p < 0.01), indicating that zooxanthellar phytopigments of different coral species did not respond
similarly to temperature changes in all cases. There was also a significant difference between tanks (p < 0.05).
3.2 Symbiodinium phytopigment responses in Acropora hyacinthus
The absolute concentrations of all pigments were within the same range as those observed in Symbiodinium from
Acropora hyacinthus. In A. hyacinthus, Symbiodinium chl a concentrations dropped significantly as temperature
increased (p < 0.01, two-way ANOVA). Colonies held at 32 and 34°C had significantly lower Symbiodinium chl
a concentrations than those at lower temperatures, i.e. 28 and 30oC (p < 0.05, T’, T-K, and GT2 a a posteriori
multiple comparisons tests between means; Figure 1). This same general pattern occurred in chl c, fucoxanthins,
total carotenoids, and total phaeophytins (p < 0.001, two-way ANOVA; p < 0.05, a posterior multiple
comparisons tests); Figure 1). This difference was pronounced in all pigments tested, when comparing the 28oC
vs. 34oC treatments (p < 0.001, Tukey test). The decrease in total carotenoid concentration decreased in a
significantly linear manner in response to increasing seawater temperatures (p < 0.05, linear regression analysis).
3.3 Symbiodinium phytopigment responses in Porites solida
Concentrations of Symbiodinium chl a associated with Porites solida exhibited differences similar to those
observed in A. hyacinthus as seawater temperatures were increased (p < 0.001, ANOVA; Figure 2). That is,
responses at 28oC and 30oC were significantly different from responses at 32oC and 34oC (p < 0.05, a posteriori
tests – as above). In chl c and in the fucoxanthins, concentrations were significantly different between
temperature treatments (p < 0.001 and 0.05, respectively, ANOVA). The concentrations of chl c were
significantly lower under the 34oC treatment than under all other temperatures, including the control (p < 0.05, a
posteriori tests), resulting in a significant curvilinear relationship between chl c concentration and temperataure
despite the small number of X-values used in the analysis (p < 0.001, polynomial least squares regression
analysis; Figure 2). The peak in the curve occurred at 30oC. There was no significant difference between
zooxanthellar carotenoids under the different temperature treatments (p > 0.05, ANOVA); the variances of the
individual means were large enough to obscure any significant trends that might have been present.
Zooxanthellar phaeophytin concentrations were significantly different between treatments (p < 0.001, ANOVA).
Significant differences were detected between individual temperature treatments in both the fucoxanthins and
total phaeophytins (p < 0.05, a posteriori tests). Test results were complex enough, however, to not yield any
clear trends, due to high variances in the data.
Initial concentrations of all Symbiodinium pigments were lower in P. solida than in A. hyacinthus, as was the
overall intensity of negative responses to increases in seawater temperature. For example, in P. solida,
Symbiodinium chl c concentrations decreased from 0.4 pg cell-1 to 0.2 pg cell-1, or by 50%, while chl c
Symbiodinium concentrations in A. hyacinthus decreased from 0.7 pg cell-1 to 0.2 pg cell-1 or by 70%.
3.4 Symbiodinium phytopigment responses in Favites complanata
In Favites complanata, chl a, chl c, fucoxanthin, and carotenoid concentrations varied significantly between all
experimental temperatures (p < 0.05-0.01, ANOVA). In this coral species, however, Symbiodinium pigment
concentrations decreased significantly in a linear fashion in all zooxanthellar phytopigments as seawater
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temperatures increased (p < 0.05-0.01, linear regression analyses; Figure 3). Chl a of Symbiodinium in this coral
species decreased significantly and linearly from about 0.9 to 0.5 pg cell-1 as temperature increased (p < 0.05,
linear regression analysis).
A posteriori tests detected significant differences between experimental temperatures in Symbiodinium
concentrations of chl a, chl c, total fucoxanthins, and total carotenoids (p < 0.05, T’, T-K, and GT2 tests).
Specific differences between temperatures, however, were not clear; comparisons of individual temperatures
often overlapped in homogeneity in their groupings. Thus, they will not be discussed any further. Total
Symbiodinium phaeophytin concentrations in F. complanata were not different enough between temperature
treatments to be detected by ANOVA (p > 0.05). This was most likely due to high variance associated with the
individual means (Figure 3). The decrease in phaeophytin concentrations was clear enough, however, to be
detected by least squares linear regression (p < 0.05).
3.5 Overall comparisons
Considering comparisons of overall concentrations of zooxanthellar phytopigments, there was a highly
significant difference in total Symbiodinium pigment concentrations between experimental seawater
temperatures (p < 0.001, three-way ANOVA). In addition, significantly different concentrations were found
between the five sets of Symbiodinium pigments examined here (p < 0.01, three-way ANOVA). Specific changes
will be discussed below, but in general, a posteriori tests revealed that Symbiodinium from A. hyacinthus had
higher concentrations of pigments than Symbiodinium from either P. solida and F. complanata. Symbiodinium
Chl a concentrations were significantly higher in the coral A. hyancinthus than in P. solida at 28oC (p < 0.05,
Tukey tests). Symbiodinium chl c concentrations also varied significantly between these two coral species at
32oC (p < 0.05). Symbiodinium fucoxanthin concentrations were found to be significantly different between
these two coral species, but this time at 28oC (p < 0.01). Concentrations of Symbiodinium phaeophytins were
homogeneous between all coral species at all temperatures measured here (p > 0.05). Symbiodinium pigment
concentrations from F. complanata were also comparable to those observed from A. hyancinthus and Porites
solida (p > 0.05).
Overall patterns indicate that Symbiodinium from A. hyacinthus was the most sensitive to elevated seawater
temperatures with respect to changes in pigment concentration; that is, these algal symbionts exhibited the
highest proportional drop with respect to mean concentration of pigment (Figure 4). All Symbiodinium pigment
concentrations in this coral species decreased by 74-77% between 28 and 34°C. Symbiodinium derived from
Favites complanata was the next most sensitive, with its pigment concentrations dropping by 45-60%.
Symbiodinium from Porites solida was the least sensitive; its pigment concentrations dropped by 23-64%. These
values were highly dependent upon the particular symbiont pigment under consideration, with total phaeophytins
exhibiting the highest rate of reduction and total carotenoids the lowest.
4. Discussion
Changes in zooxanthellar phytopigment concentration in corals may be used as an indicator of bleaching and
stress within 48 h of their exposure to elevated seawater temperatures. These declines in concentration are most
likely conservative estimates of those that might occur in the field under natural conditions of prolonged
increased seawater temperatures. That is, the holobiont may be expected to be exposed for a much longer time
period. Kleppel et al. (1989) observed loss of symbiont pigments during a natural bleaching event off south-east
Florida, which accounted for a decrease of ~72% of chl c in the coral Montastraea annularis. Within 48 h in our
study, decreases in concentrations were observed in all symbiont pigments considered and in all three coral
species tested.
These species were derived from three different families of Scleractinia. This indicates a general response or
sensitivity among zooxanthellae that are symbiotic within the families Acroporidae, Poritidae and Faviidae from
the Great Barrier Reef region. Ralph et al. (2001) found that the zooxanthellae in Pocillopora damicornis are
highly resistant to seawater temperature increases, even after expelled. Hoegh-Guldberg and Smith (1989)
observed little to no changes in Symbiodinium chl a concentrations in Stylophora pistillata when exposed to
elevated temperatures of 27 to 32°C. Both of these species are pocilloporids. Unfortunately, we did not
investigate any pocilloporid species; therefore, no direct comparison may be made, although this implies that
endosymbionts of pocilloporids may be more temperature-tolerant than in our species. On the other hand, they
found similar results with Cyphastrea serailia, a faviid. This indicates that there is some variance in
phytopigment sensitivity to increased seawater temperatures within the Faviidae.
Our results also suggest that as symbiont photosynthetic ability decreases, the level of nutrients passed from
zooxanthellae to the host will also decrease (see Abrego et al., 2008). Li et al. (1984) and Jones et al. (1998)
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Published by Canadian Center of Science and Education 9
observed decreased carbon fixation when photosynthesis of their symbionts decreased. The levels of change in
Symbiodinium phytopigment concentration vary inter-specifically. The temperatures considered here are similar
to those proposed to occur under projected climate change conditions (Goenaga et al., 1989; Schlichter, 1990).
The change in Symbiodinium pigment concentrations observed in this experiment may have been due to: a)
zooxanthellae being lost from the host coral; or b) the pigments degrading within the symbiont cells, while algae
were being retained by the host.
The observed decline in symbiont pigment concentration at 32oC in our study suggests that elevated
temperatures (e.g. 32°C) cannot be tolerated by Symbiodinium, even for short periods of time. At 34°C, the
symbiont pigments probably became dysfunctional (Warner et al., 1999; Agustin et al., 2006) and began to
degrade (Demming-Adams et al., 1996; Hendry and Price, 1993). This agrees with the earlier results of Warner
et al. (1996) who observed that the greatest variation in Symbiodinium pigment concentration in Agaricia
agaricites and Siderastrea radians occurs when temperatures are increased from 30 to 32°C (experimental
temperature range: 30 to 36°C). Hoegh-Guldberg and Smith (1989) also reported changes in Symbiodinium
pigment concentrations in Seriatopora hystrix. These authors show symbiont chl a concentrations decreasing
from ~4.5 pg cell-1 at 27°C to 3 pg cell-1 at 30°C when exposed to such temperatures over 4 days, but the drop
was apparently not significant due to high variances in the data. Our changes, however, represent significant
drops in Symbiodinium chl a concentration at 32oC. We hypothesize that the symbiont photosynthetic pathway
probably remains intact, at least partially, at temperatures < 30°C.
One of the reasons why we may have observed interspecific variation in responses of the phytopigment
concentrations to increased seawater temperature is that the corals harbor different clades. Firstly, we have
demonstrated that these corals possess Symbiodinium clades b and c. Clade b only occurs in Acropora hyacinthus,
while clade c occurs in all of these species (Strychar et al., 2005; also see Sammarco & Strychar, 2009). It would
appear that differential responses of phytopigment concentrations observed between Favites complanata and
Porites solida may be due to varying complements or relative abundances of sub-clades of clade c. On the other
hand, the high pigment losses observed in A. hyacinthus may be due to the presence of clade b, which may in
turn be less tolerant of high temperatures than clade c. Thus, higher rates of pigment loss would have resulted.
This question remains open.
Responses of Symbiodinium phytopigment concentration to elevated seawater temperature varied greatly
between coral species and between specific symbiont pigments. In general, Symbiodinium total phaeophytins
appeared to be affected the most by elevated seawater temperatures, followed by chl c. Symbiodinium
carotenoids were, on the whole, the least affected by temperature. Reese et al. (1988), however, found that
carotenoids and chlorophyll concentrations in Symbiodinium from Montastraea annularis decreased by 81-97%
in bleached corals vs. non-bleached corals. The contrast between the levels of response in Symbiodinium
carotenoids in Reese et al.’s (1988) study vs. ours may be due to species-specific differences in endosymbiont
physiological tolerances. Hoegh-Guldberg and Smith (1989) suggest that such inter-specific differences in
symbiont response between different coral species may be due to an initial decline in both zooxanthellar pigment
content and density. Mobley and Gleason (2003) suggest that carotenoid content in symbiotic anthozoans may
increase by ingestion of carotenoid-rich zooplankton. It is also possible that the host may compensate for heat
stress by drawing more heavily upon its stored lipids, which are characteristically high in carotenoid content (see
Goodwin, 1980; Olsen & Owens, 1998). We speculate that, under low temperature stress (e.g. 28 to 30°C), the
presence of Symbiodinium carotenoids, such as -carotene, whether derived from symbiont production or from
ingestion by the host, may ameliorate some of the deleterious effects of heat stress (Demmig-Adams and Adams,
2000). Peñuelas and Munné-Bosch (2005) have shown that carotenoids, including -carotene, are antioxidants
that act by dissipating excess reactive oxygen species and excitation energy in heat-stressed cells.
Chlorophyl a and chl c are pivotal to both the PS-I and –II photosystems (De Martino et al., 2000). Our data
indicate that these two pigments were depleted in Symbiodinium under temperature stress. This supports the
findings of Shenkar et al. (2006) who found a significant negative correlation between chl a concentrations,
bacterial bleaching, and average seasonal SSTs, with a maximum observed drop of 92%. Declines in
concentrations of the other Symbiodinium pigments also impacts both of these photosystems, further inhibiting
production of sugars within the zooxanthellae (Tchernov et al., 2004; Smith et al., 2005; Brüssow, 2007). If
photosynthate production is compromised, then energy supplies may become limiting for both the endosymbiont
and host cells, increasing stress and inducing necrosis (Strychar et al., 2005; Sammarco & Strychar, 2009).
Acropora hyacinthus is known to be particularly susceptible to bleaching (Baird and Marshall, 2002; Strychar et
al., 2004b; Wooldridge et al., 2005). It has been experimentally demonstrated that Symbiodinium cells are lost
from this coral when temperatures are increased from 30 to 32°C (Marshall & Baird, 2000; Strychar et al.,
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10
2004b). In all pigments of Acropora hyacinthus, experimental increases in seawater temperature did not result in
any significant decreases in pigment concentration until 32oC. In fact, further a posteriori analyses revealed that
responses of these pigments to temperature were the same between 28 and 30oC and also between 32 and 34oC.
This implies that the major sensitivity in this holobiont is occurring between 30oC and 32oC, in concurrence with
previous studies, and could be described as a step-function. Unfortunately, the experimental temperatures used
here are insufficient to allow a detailed analysis; further studies are recommended.
It is not known precisely how much chl a is needed within each Symbiodinium cell to ensure survival. If one
assumes that Symbiodinium survival is dependent upon the mean total chl a concentration measured at 28oC and
30oC – i.e., 2.6 pg cell-1 (A. hyacinthus; Figure 1), then the reduction of chl a to ~0.8 pg cell-1 (average chl a
concentration calculated from 32 and 34°C treatments) probably resulted in a drop in efficiency of the
photochemical reactions within the symbiont. Thus, corals more heavily dependent on Symbiodinium as an
energy source may be more susceptible to bleaching than other corals. The host coral can, however, survive
severe bleaching events. Grottoli et al. (2006) have demonstrated that Montipora capitata can replenish its
energy reserves and biomass by increasing its food intake five-fold after temperature-induced bleaching and the
loss of its symbionts. Similarly, Ferrier-Pagés et al. (2010) studying heat-stressed Stylophora pistillata,
Turbinaria reniformis, and Galaxea fascicularis, showed that these coral did not experience a decrease in their
photosynthetic ability when fed. This helps to explain the results of Goreau and Goreau (1960) were able to keep
some bleached corals alive in the dark for years by feeding them Artemia nauplii (also see Goreau and Goreau,
1959; Goreau et al., 1971; Pecheux, 1997). They believe that the cumulative effects of both thermal stress and
starvation caused mortality. Leder et al. (1991) performed a similar experiment over a period of 10-12 mos with
like results.
It is possible that a reduced flux of photosynthate from the symbiont to the coral host might be deleterious to the
coral provoking the host’s response. Kuguru et al. (2010), for example, observed host-cellular responses in
corallimorpharians responding to zooxanthellae reducing their chlorophyll abundance as changes in irradiance
occured. Here, only data on pigment concentrations per algal cell have been presented. It is therefore difficult to
deduce the likely impact photosynthate changes have on the coral without having additional information on
zooxanthellar pigment per unit area, a proxy for photosynthesis. Acropora hyacinthus has a relatively large
standing stock of zooxanthellae (see Drew, 1972; Muscatine et al., 1989; Jones and Yellowlees, 1997) and was
the most sensitive to increased seawater temperatures of all scleractinian corals with respect to apoptotic and
necrotic responses as well as phytopigment concentrations. Yet Strychar (2002) and Strychar et al. (2004b) show
that the standing stock of Symbiodinium in this and the other coral species is not correlated with inter-specific
differences in heat tolerance. Hoegh-Guldberg and Smith (1989), working with Stylophora pistillata, concluded
that increased seawater temperatures caused bleaching and a decrease in Symbiodinium pigment concentrations;
these effects, however, were not associated with changes in the resident zooxanthellar population. This has also
been shown to be the case with Montipora monasteriata (Dove et al., 2006). Of the corals studied here, Porites
solida possessed zooxanthellae that were the most tolerant to elevated seawater temperatures, and the most
resistant to bleaching (Strychar et al., 2004b; 2005). Photo-protective pigments are also produced by the host
coral (Salih et al., 2000; 2006; Oswald et al., 2007; Riddle, 2007; Dove et al., 2008), and recent evidence
suggests that these are also affected by elevated seawater temperatures (Lesser and Farrell, 2004).
Corals that were less prone to bleaching possessed more stable Symbiodinium carotenoid concentrations, and this
conferred greater heat-stability on the PS-I and PS-II systems (Hill & Ralph, 2006; Liñán-Cabello et al., 2006;
Flores-Ramírez & Liñán-Cabello, 2007). Jones et al. (1998) concluded that damage to the PS-I system has a
greater impact on zooxanthellar photosynthesis than damage to the PS-II system. As was the case in Venn et al.’s
(2006) study, the primary Symbiodinium carotenoids - carotenes (predominant within PS-I) and xanthophylls
(predominant within PS-II; Govindjee, 1975) – could not be differentiated from each other here, due to
equipment limitations. We do recommend, however, that these two pigments be considered separately in future
studies to reveal more subtle changes due to temperature. These pigments may have separate functions in the
organisms. For example, Barlow et al. (2007) describe different roles for carotenoids in smaller vs. larger
plankton cells. In the former, the carotenoids function in photo-protection, while in the latter, they serve in
photosynthesis. Carotenoids generally function to dissipate heat energy within PS-II system (Demming et al.,
1987; Gilmore et al., 1995; Hill and Ralph, 2006).
There are many species and clades within the Symbiodinium taxon, and it is therefore likely that each symbiont
could have different temperature susceptibilities. Species-specific resistance of zooxanthellate corals to varying
levels of temperature stress may be the result of variability in heat-resistant Symbiodinium phytopigments. Corals
less prone to bleaching exhibited much smaller changes in Symbiodinium carotenoid pigment concentrations.
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Published by Canadian Center of Science and Education 11
This set of pigments is responsible for photosynthate production and protection of Photosystems I and II. If
zooxanthellar chl c and fucoxanthin pigments (both accessory pigments to chl a) become dysfunctional, the
Symbiodinium cells may not be harmed by elevated temperatures in the short-term. Reduced concentrations of
chl a, however, will harm it and therefore affect the symbiotic relationship.
5. Conclusion
It has been shown that the phytopigments of Symbiodinium are susceptible to disruption under conditions of
elevated seawater temperatures. In addition, this susceptibility occurs in three different Indo-Pacific species of
coral derived from three different families within the Scleractinia. Our results also suggest that a drop in
phytopigment concentration in response to elevated temperatures may well be a general phenomenon occurring
across the Scleractinia – at least on the Great Barrier Reef. This is consistent with results of parallel experiments
indicating that the symbiont zooxanthellae are much more susceptible to temperature stress than their host
counterparts (Sammarco & Strychar, 2009). It also supports earlier findings that most of the adaptation to
temperature in this symbiotic pairing is most likely occurring in the zooxanthellae, not the hosts, which are
already adapted (or exapted) to high seawater temperatures. Secondly, our results suggest that, because of the
observed variability between pigments within a species, and the interspecific variability observed between
holobionts, the character of the specific responses of a phytopigment or group of phytopigments to elevated
temperatures becomes difficult to predict.
Acknowledgements
We thank the Great Barrier Reef Marine Park Authority and the Zoological Society of New South Wales
(Australia) for grants awarded to KBS, and the Centre for Land and Water, Central Queensland University, for
additional assistance awarded to M. Coates, P.T. Scott, and T.J. Piva, supporting this project. Thanks to A.
Lirette for assistance with graphics and to G. Muller-Parker for comments on an earlier draft of the manuscript.
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Figure 1. Acropora hyacinthus
Effect of temperature on phytopigment concentrations in zooxanthellae in situ under varying experimental
temperatures: 28°C (control), 30°C, 32°C, and 34°C. Data presented as mean plus 95% confidence intervals. See
Materials and Methods for experimental design. nt = 10 readings/temperature. Data shown for chlorophyll a,
chlorophyll c, fucoxanthins, total carotenoids, and total phaeopigments. Significant differences between
temperatures in all phytopigments (p < 0.001, two-way nested ANOVA). Pigment concentrations for 28oC and
30oC were both significantly different from 32oC and 34oC in all pigments (p < 0.05, T’, T-K, and GT2 a
posteriori multiple comparisons techniques). Grouping of non-significant means indicated by horizontal line
over the points. Pytopigment concentrations (excluding fucoxanthins) decreased in a significantly linear manner
with temperature (e.g. carotenoids; p < 0.05, y = 5.6111 – 0.1593 x). Scaling on Y-axes standardized within
pigments and between graphs to facilitate comparisons between related figures.
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Figure 2. Porites solida
Effect of temperature on phytopigment concentrations in zooxanthellae in situ under varying experimental
temperatures – 28°C (control), 30°C, 32°C, and 34°C. Significant difference between all temperatures for all
pigments (p < 0.05 – 0.001, ANOVA), except for total carotenoids (p > 0.05). Significant differences between
temperatures in chl a concentrations (p < 0.001, ANOVA); significant differences between [28oC and 30oC] vs.
[32oC and 34oC] via T’, T-K, and GT2 a posteriori tests (p < 0.05). Chl c – significant difference between
temperatures (p < 0.001, two-way ANOVA), with a significant difference between [28°C, 30°C, and 32°C] vs.
34°C, a posteriori tests, p < 0.05); relationship best described by significant second degree polynomial
regression (p < 0.001, Y = -0.0486x2 + 2.8583x – 40.0014; fucoxanthins (p < 0.05, two-way ANOVA;
significant differences between individual means (p < 0.05, a posteriori tests – as above; see text for discussion).
Carotenoids (excluding fucoxanthins), no significant difference in phytopigment concentrations between
experimental temperatures (p > 0.05, two-way ANOVA). Total phaeophytins, significant difference between
experimental temperatures (p < 0.001, two-way ANOVA); significant differences between individual means (p <
0.05, a posteriori tests). See Figure 1 legend for additional details.
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Figure 3. Favites complanata
Effect of temperature on phytopigment concentrations in zooxanthellae in situ under varying experimental
temperatures – 28°C (control), 30°C, 32°C, and 34°C. Data presented as mean plus 95% confidence intervals.
Highly significant difference between all temperatures for chl-a, chl-c, fucoxanthins, and total carotenoids.
Significant negative relationship found between temperature and phytopigment concentration via least squares
linear regression in all phytopigments: Chl a (p < 0.01, ANOVA; p < 0.05, linear regression, Y = 15.142 – 0.361
x); chl c (p< 0.01, ANOVA; p < 0.01, linear regression, Y = 4.429 – 0.115 x); fucoxanthins, (p < 0.05, ANOVA;
p < 0.05, linear regression, Y = 2.261 – 0.054 x); carotenoids, excluding fucoxanthins, (p < 0.01, ANOVA; p <
0.05, linear regression, Y = 5.949 – 0.140); and phaeophytins (p < 0.05, linear regression, Y = 29.721 – 0.709 x).
See legend of Figure 1 for additional details.
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Published by Canadian Center of Science and Education 19
Figure 4. Species-specific proportional changes in pigment concentrations between three zooxanthellate
scleractinian corals, under experimentally elevated seawater temperatures
Data represent final average pigment concentrations after exposure to a maximum experimental temperature of
34oC for 48 h divided by average initial concentration at 28oC (control). Percent changes shown for chl a, chl c,
fucoxanthins, total carotenoids, and total phaeophytins. Data transformed by arcsine for purposes of
normalization. ni = 10. Note that the zooxanthellar pigments within Acropora hyacinthus decreased at
approximately the same rate, while chl c was depleted faster than all other pigments in Favites complanata. All
zooxanthellar pigments changed at different rates in Porites solida.
n
... Recent studies indicate that the hosts have higher temperature tolerances than their zooxanthellae. It is likely that it is the photosynthetic machinery which is temperature-sensitive in the zooxanthellae [43][44][45]. In the host cnidarians, however, it is not known whether this character represents an adaptation to higher seawater temperatures which had been experienced earlier in their evolutionary history, or an adaptation to another selective factor in its evolutionary history separate from temperature but now fortuitously playing a role in temperature-tolerance (exaptation). ...
... The host animal cells were clearly more tolerant to all experimental temperatures, indicating better thermal adaptation. This pattern is very similar to that observed for both the endosymbiotic cells and the host cells in scleractinian corals studied earlier [42][43][44][45][46][47]50]. ...
... AV-fluor was used to identify cells undergoing apoptosis [47]. The conjugates bind to phosphatidylserine in cells undergoing apoptosis and fluoresce green when excited with a blue light [44]. This mixture was added to aliquots containing 100 ml of Symbiodinium cell suspension (1.8610 5 cells) and separate aliquots containing host cells. ...
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Increases in Sea Surface Temperatures (SSTs) as a result of global warming have caused reef-building scleractinian corals to bleach worldwide, a result of the loss of obligate endosymbiotic zooxanthellae. Since the 1980's, bleaching severity and frequency has increased, in some cases causing mass mortality of corals. Earlier experiments have demonstrated that zooxanthellae in scleractinian corals from three families from the Great Barrier Reef, Australia (Faviidae, Poritidae, and Acroporidae) are more sensitive to heat stress than their hosts, exhibiting differential symptoms of programmed cell death - apoptosis and necrosis. Most zooxanthellar phylotypes are dying during expulsion upon release from the host. The host corals appear to be adapted or exapted to the heat increases. We attempt to determine whether this adaptation/exaptation occurs in octocorals by examining the heat-sensitivities of zooxanthellae and their host octocoral alcyonacean soft corals - Sarcophyton ehrenbergi (Alcyoniidae), Sinularia lochmodes (Alcyoniidae), and Xenia elongata (Xeniidae), species from two different families. The soft coral holobionts were subjected to experimental seawater temperatures of 28, 30, 32, 34, and 36°C for 48 hrs. Host and zooxanthellar cells were examined for viability, apoptosis, and necrosis (in hospite and expelled) using transmission electron microscopy (TEM), fluorescent microscopy (FM), and flow cytometry (FC). As experimental temperatures increased, zooxanthellae generally exhibited apoptotic and necrotic symptoms at lower temperatures than host cells and were expelled. Responses varied species-specifically. Soft coral hosts were adapted/exapted to higher seawater temperatures than their zooxanthellae. As with the scleractinians, the zooxanthellae appear to be the limiting factor for survival of the holobiont in the groups tested, in this region. These limits have now been shown to operate in six species within five families and two orders of the Cnidaria in the western Pacific. We hypothesize that this relationship may have taxonomic implications for other obligate zooxanthellate cnidarians subject to bleaching.
... These observations suggest that changes in the Symbiodinium complement can have adaptive value (Buddemeier and Fautin, 1993;Buddemeier et al., 2004). Further, these changes in the types of dinoflagellates may be viewed in the context of adaptation or acclimatizaton of symbionts (and/or holobionts) to environmental changes through modified algal division rates (Baghdasarian and Muscatine, 2000;Strychar et al., 2004), photosynthetic pigment concentrations (Strychar and Sammarco, 2012;Hoadley et al., 2016), and other cellular parameters to manage reactive oxygen species (Weis, 2008). ...
... However, chlorophyll loss in CS-73 was attributed to photobleaching rather than thermal bleaching (Hoegh-Guldberg, 1999), because Takahashi et al.'s light intensities were 5-fold higher than what we used in the present study (i.e., 200 vs. 40 mmol quanta m 22 s 21 ). Strychar and Sammarco (2012) also reported reduced chlorophyll concentrations in Symbiodinium as a result of high temperatures, but this effect was only observed at temperatures of at least 32 7C (to 34 7C); they found no effect at a temperature of ∼30 7C, which was close to the high temperature used in our current study. Another experiment reporting high temperature-induced declines in the chlorophyll content of Symbiodinium in reef corals also used a high temperature of 32 7C (Dove et al., 2006). ...
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This study tested the bleaching response of the Pacific coral Seriatopora caliendrum to short-term exposure to high temperature and elevated partial pressure of carbon dioxide (pCO2). Juvenile colonies collected from Nanwan Bay, Taiwan, were used in a factorial experimental design in which 2 temperatures (∼27.6 °C and ∼30.4 °C) and 2 pCO2 values (∼47.2 Pa and ∼90.7 Pa) were crossed to evaluate, over 12 days, the effects on the densities and physiology of the symbiotic dinoflagellates (Symbiodinium) in the corals. Thermal bleaching, as defined by a reduction of Symbiodinium densities at high temperature, was unaffected by high pCO2. The division, or mitotic index (MI), of Symbiodinium remaining in thermally bleached corals was about 35% lower than in control colonies, but they contained about 53% more chlorophyll. Bleaching was highly variable among colonies, but the differences were unrelated to MI or pigment content of Symbiodinium remaining in the coral host. At the end of the study, all of the corals contained clade C Symbiodinium (either C1d or C15), and the genetic variation of symbionts did not account for among-colony bleaching differences. These results showed that high temperature causes coral bleaching independent of pCO2, and underscores the potential role of the coral host in driving intraspecific variation in coral bleaching.
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Global increases in coral disease prevalence have been linked to ocean warming through changes in coral-associated bacterial communities, pathogen virulence and immune system function. However, the interactive effects of temperature and pathogens on the coral holobiont are poorly understood. Here, we assessed three compartments of the holobiont (host, Symbiodinium, bacterial community) of the coral Montipora aequituberculata challenged with the pathogen Vibrio coralliilyticus and the commensal bacterium Oceanospirillales sp. under ambient (27°C) and elevated (29.5°C and 32°C) seawater temperatures. Few visual signs of bleaching and disease development were apparent in any of the treatments, but responses were detected in the holobiont compartments. V. coralliilyticus acted synergistically and negatively impacted the photochemical efficiency of Symbiodinium at 32°C, while Oceanospirillales had no significant effect on photosynthetic efficiency. The coral, however, exhibited a minor response to the bacterial challenges, with the response towards V. coralliilyticus being significantly more pronounced, and involving the prophenoloxidase system and multiple immune system related genes. Elevated seawater temperatures did not induce shifts in the coral-associated bacterial community, but caused significant gene expression modulation in both Symbiodinium and the coral host. While Symbiodinium exhibited an anti-viral response and upregulated stress response genes, M. aequituberculata showed regulation of genes involved in stress and innate immune response processes, including immune and cytokine receptor signalling, the complement system, immune cell activation and phagocytosis, as well as molecular chaperones. These observations show that M. aequituberculata is capable of maintaining a stable bacterial community under elevated seawater temperatures, and thereby contributes to preventing disease development.
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Coral bleaching results from the dissociation of Symbiodinium and is primarily related to sea surface temperatures above mean yearly maximums. The numbers of live, dead, and mitotic Symbiodinium cells lost from three scleractinian corals from three different families (Acropora hyacinthus, Favites complanata, and Porites solida), which have not been studied previously in central Queensland (Australia), were compared at 28, 30, 32, and 34◦C. Specific expulsion rates, growth rates, and mitotic indices were compared for each host at each temperature. Porites solida was the most robust coral, A. hyacinthus bleached more readily at low temperatures and F. complanata showed levels of intermediate bleaching tolerance to elevated temperatures. However, the timing of Symbiodinium cell loss was similar between all corals tested. Mitotic indices and specific growth rates were found to be positively associated with increasing temperature; thus, symbiont reproduction increased despite elevated losses of Symbiodinium from the host. Because all corals in the present study were symbiotic with Symbiodinium from clade C, different levels of stress tolerance to temperature suggests that bleaching resistance is an attribute associated with the coral host and, to a lesser degree, the symbiont.
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The photosynthetic apparatus of plants is particularly susceptible to damage by exposure to elevated temperatures. Heat treatment of isolated chloroplasts revealed that Photosystem II is generally the most heat-labile component, suffering loss of oxygen evolution capacity (1,2) following brief (min) exposures to temperatures elevated, but still experienced by plants in nature (40–50°C). Heat treatment of intact plant tissue reveals similar alteration of the photosynthetic apparatus(3,4). In addition to loss of PSII function, membrane disorganization results in the disconnection of LHCP from PS II centers (5). Loss of chloride has been correlated to loss of O2-evolving capacity in heated PS II (6,7). Some loss of polypeptides from heated thylakcid membranes has been reported (8). In addition to the loss of water-splitting activity, a loss of PSII reaction center function has also been observed in tissue heated in vivo (3,4) and in vitro (1,2,9). The loss of PSII reaction center function upon heat treatment is generally less severe than the loss of water-oxidizing capacity.
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The quest begins in prehistoric times with religion and the exploration of the connection between food and sex. This leads to an investigation of the deep links between food and culture, exploring the basic question of "what is eating?" The second section embarks on a biochemistry-oriented journey tracing the path of a food molecule through the central carbon pathway until it is decomposed into CO2, H2O and ATP. The third section delves into the evolution of eating systems, beginning with the elements of the primordial soup through the birth of single cell organisms such as bacteria and archea. We then follow this evolution in the fourth section through higher developed organisms: from the first organisms in the ocean to the ones on land. The next two sections explore the stories of food from an ecological, then behavioral viewpoint, leading the reader from animals to early hominids, and into human history. The final section takes apart an anthropocentric view of the world by presenting man as prey for the oldest predators: microbes. The text closes with an agronomical outlook on how to feed the billions. The goal of The Quest for Food is to catalyze discussions between scientists working in food science, and those in biological and biomedical research. © 2007 Springer Science+Business Media, LLC. All rights reserved.
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Mass coral reef bleaching and mortality as a result of prolonged seawater warming following the 1997–1998 El Niño-Southern Oscillation forced a change in conservation priorities in assessing threats to the health of coral reefs worldwide. By some estimates, approximately one sixth of the world’s coral reefs was destroyed over a single 9-month period during the 1997–1998 bleaching event (Wilkinson 2000). Most of this destruction occurred in the Indian Ocean, where prolonged elevations of sea surface temperature were maintained by prevailing currents that pooled warm water in the western Indo-Pacific. In most cases, coral reef destruction equated to a dramatic reduction in live coral cover on these reefs (e.g., McClanahan 2000; Loch et al. 2002), but it is noteworthy that even the most severely affected reefs maintained significant pockets of live coral scattered throughout their original distributions. Moreover, many coral reef ecosystems that suffered extensive bleaching (e.g., parts of the Caribbean and Great Barrier Reef) did not experience significant eventual mortality (Wilkinson 2002). Consequently, coral reef recovery has in many places been more rapid than initially expected, particularly in the western Pacific. Although 1997–1998 clearly represents an annus horribilis for many coral reefs worldwide, the destruction it witnessed may not be as irreversible or as cumulative as originally thought. How resistant and/or resilient were reef corals (and coral reefs) to this event? How might resistance and resilience change over time in response to rising temperatures and recurrent bleaching episodes? To what extent can we expect the destruction and recovery patterns of 1997–1998 to be common features of reef bleaching and mortality in the years to come?