Oxidative stress causes coral bleaching during exposure to elevated temperatures

 Elevated temperatures and solar ultraviolet (UV) radiation have been implicated as recent causes for the loss of symbiotic algae (i.e., bleaching) in corals and other invertebrates with photoautotrophic symbionts. One hypothesized mechanism of coral bleaching involves the production of reduced oxygen intermediates, or toxic oxygen, in the dinoflagellate symbionts and host tissues that subsequently causes cellular damage and expulsion of symbionts. Measurements of photosynthesis in the Caribbean coral Agaricia tenuifolia, taken during temperature-induced stress and exposure to full solar radiation, showed a decrease in photosynthetic performance followed by bleaching. Exposure of corals to exogenous antioxidants that scavenge reactive oxygen species during temperature-induced stress improves maximum photosynthetic capacity to rates indistinguishable from corals measured at the ambient temperature of their site of collection. Additionally, these antioxidants prevent the coral from “ bleaching ” and affect the mechanism of symbiont loss from the coral host. These observations confirm a role for oxidative stress, whether caused by elevated temperatures or exposure to UV radiation, in the bleaching phenomenon. Accepted: 18 October 1996     

The impact of coral bleaching on the pigment profile of the symbiotic alga, Symbiodinium

Bleaching of corals by loss of symbiotic dinoflagellate algae and/or photosynthetic pigments is commonly triggered by elevated temperatures coupled with high irradiance, and is a first-order threat to coral reef communities. In this study, a high-resolution high-performance liquid chromatography method integrated with mass spectrometry was applied to obtain the first definitive identification of chlorophyll and carotenoid pigments of three clades of symbiotic dinoflagellate algae (Symbiodinium) in corals, and their response to experimentally elevated temperature and irradiance. The carotenoids peridinin, dinoxanthin, diadinoxanthin (Dn), diatoxanthin (Dt) and beta-carotene were detected, together with chlorophylls a and c2, and phaeophytin a, in all three algal clades in unstressed corals. On exposure to elevated temperature and irradiance, three coral species (Montastrea franksi and Favia fragum with clade B algae, and Montastrea cavernosa with clade C) bleached by loss of 50-80% of their algal cells, with no significant impact to chlorophyll a or c2, or peridinin in retained algal cells. One species (Agaricia sp. with clade C) showed no significant reduction in algal cells at elevated temperature and irradiance, but lost substantial amounts of chlorophyll a and carotenoid pigments, presumably through photo-oxidative processes. Two coral species (Porites astreoides and Porites porites both bearing clade A algae) did not bleach. The impact of elevated temperature and irradiance on the levels of the photoprotective xanthophylls (Dn + Dt) and beta-carotene varied among the corals, both in pool size and xanthophyll cycling, and was not correlated to coral bleaching resistance.     

Regulation and control of intracellular algae (= zooxanthellae) in hard corals

To examine algal (= zooxanthellae) regulation and control, and the factors determining algal densities in hard corals, the zooxanthellae mitotic index and release rates were regularly determined in branch tips from a colony of a staghorn coral, Acropora formosa, recovering from a coral ''bleaching'' event (the stress-related dissociation of the coral–algal symbiosis). Mathematical models based upon density-dependent decreases in the algal division frequency and increases in algal release rates during the post-bleaching recovery period accurately predict the observed recovery period (ca. 20 weeks). The models suggest that (i) the colony recovered its algal population from the division of the remaining zooxanthellae, and (ii) the continual loss of zooxanthellae significantly slowed the recovery of the coral. Possible reasons for the ''paradoxical'' loss of healthy zooxanthellae from the bleached coral are discussed in terms of endodermal processes occurring in the recovering coral and the redistribution of newly formed zooxanthellae to aposymbiotic host cells. At a steady-state algal density of 2.1 x 10⁶ zooxanthellae cm^-2 at the end of the recovery period, the zooxanthellae would have to form a double layer of cells in the coral tissues, consistent with microscopic observations. Neighbouring colonies of A. formosa with inherently higher algal densities possess proportionately smaller zooxanthellae. Results suggest that space availability and the size of the algal symbionts determines the algal densities in the coral colonies. The large increases in the algal densities reported in corals exposed to elevated nutrient concentrations (i.e between a two- and five-fold increase in the algal standing stock) are not consistent with this theory. We suggest that increases of this magnitude are a product of the experimental conditions: reasons for this statement are discussed. We propose that the stability of the coral–algal symbiosis under non-stress conditions, and the constancy of zooxanthellae densities in corals reported across growth form, depth and geographic range, are related to space availability limiting algal densities. However, at these densities, zooxanthellae have attributes consistent with nutrient limitation.     

Mitotic index and size of symbiotic algae in Caribbean Reef corals

Size and frequency of division were determined for zooxanthellae from nine scleractinian coral species collected in February, 1983 at Discovery Bay, Jamaica, from four depths over a 51 m bathymetric range. Mean diameters of zooxanthellae ranged from 6.4 to 12.6 m. Small zooxanthellae were found in Madracis mirabilis, Eusmilia fastigiata and Dendrogyra cylindrus whereas larger cells were seen in Agaricia agaricites, Porites astreoides, Montastrea cavernosa and Acropora cervicornis. Zooxanthellae division was not phased over a diel cycle. The percentage of zooxanthellae in a paired stage of cytokinesis (mitotic index or MI) was highly variable and ranged from 1.1% to 14.1%. Values measured in E. fastigiata and D. cylindrus were greater than in the other corals. MI was higher in branch tips of A. cervicornis than in branch bases. Daily average MI was negatively correlated with mean cell diameter and for a majority of the coral species increased with habitat depth. MI values were used to estimate specific growth rates and generation times for zooxanthellae in vivo for comparison with other dinoflagellates.     

Symbiosis regulation in a facultatively symbiotic temperate coral: zooxanthellae division and expulsion

Zooxanthellae mitotic index (MI) and expulsion rates were measured in the facultatively symbiotic scleractinian Astrangia poculata during winter and summer off the southern New England coast, USA. While MI was significantly higher in summer than in winter, mean expulsion rates were comparable between seasons. Corals therefore appear to allow increases in symbiont density when symbiosis is advantageous during the warm season, followed by a net reduction during the cold season when zooxanthellae may draw resources from the coral. Given previous reports that photosynthesis in A. poculata symbionts does not occur below approximately 6°C, considerable zooxanthellae division at 3°C and in darkness suggests that zooxanthellae are heterotrophic at low seasonal temperatures. Finally, examination of expulsion as a function of zooxanthellae density revealed that corals with very low zooxanthellae densities export a significantly greater proportion of their symbionts, apparently allowing them to persist in a stable azooxanthellate state.     

Recovery from a near-lethal exposure to ultraviolet-C radiation in a scleractinian coral

Hermatypic (reef building) corals live in an environment characterized by high ambient levels of photosynthetically active radiation (PAR) and ultraviolet radiation (UVR). Photoadaptive mechanisms have evolved to protect the sensitive cell structures of the host coral and their photosynthetic, endosymbiotic zooxanthellae. Environmental stressors may destabilize the coral-zooxanthellae system resulting in the expulsion of zooxanthellae and/or loss of photosynthetic pigment within zooxanthellae, causing a condition known as bleaching. It is estimated that 1% of the world’s coral population is lost yearly, partly due to bleaching. Despite intensive research efforts, a single unified mechanism cannot explain this phenomenon. Although UVA and UVB cellular damage is well documented, UVC damage is rarely reported due to its almost complete absorption in the stratosphere. A small scale coral propagation system at the University of Maine was accidentally exposed to 15.5 h of UVC radiation (253.7 nm) from a G15T8 germicidal lamp, resulting in a cumulative surface irradiance of 8.39 × 10⁴ J m⁻². An experiment was designed to monitor the progression of UVC induced damage. Branch sections from affected scleractinian corals, Acropora yongei and Acropora formosa were submitted to histopathology to provide an historical record of tissue response. The death of gastrodermal cells and necrosis resulted in the release of intracellular zooxanthellae into the gastrovascular canals. Zooxanthellae were also injured as evidenced by pale coloration, increased vacuolization and loss of membrane integrity. The recovery of damaged coral tissue likely proceeds by re-epithelialization and zooxanthellae repopulation of gastrodermal cells by adjacent healthy tissue.     

Bleaching of Hermatypic Corals

In this paper, I review data on the magnitude and extent of reef coral bleaching events and consider modern hypotheses on the mechanisms of this natural phenomenon and experimental data lying at their basis. Four possible mechanisms of color loss by hermatypic corals have been confirmed experimentally: bacterial infection, change of zooxanthellae type in the polyps to improve the heat resistance of the photosynthetic function of coral to elevated seawater temperature, intoxication of zooxanthellae by animal metabolic wastes at high temperature and light levels, and thermal and photodestruction of the animal and algal cells. The heating effect of photosynthetic active radiation on the zooxanthellar cells in coral polyps was verified theoretically. The calculations showed that in the natural environment, the additional light-induced heating of zooxanthellae is not above 0.01°C and that it cannot cause disruption of the animal and zooxanthellae symbiosis.     

Scleractinian coral recruitment patterns at Salt River submarine canyon,St. Croix,U.S. Virgin Islands

Scleractinian coral recruitment patterns were studied at depths of 9, 18, 27 and 37 m on the east and west walls of Salt River submarine canyon, St. Croix, U.S. Virgin Islands, by censusing coral juveniles which settled on experimental settling plates placed on the reef for 3–26 months as well as coral juveniles within quadrats on the reef. The most common species in the juvenile population within quadrats were Agaricia agaricites, Porites astreoides, Madracis decactis, Stephanocoenia michelinii, and A. lamarcki. The only species settling on settling plates were Agaricia spp., Madracis decactis, Porites spp., Stephanocoenia michelinii and Favia fragum. A total of 271 corals settled on 342 plates, with 51% of the juveniles on the east wall and 49% on the west wall. Of these 34% settled on horizontal surfaces and 66% on vertical surfaces. Based on results from quadrats, Agaricia agaricites and Porites astreoides had high recruitment rates relative to their abundance on the reef. In contrast, Agaricia lamarcki, Montastraea annularis, M. cavernosa and Siderastrea siderea had high amounts of cover compared to their abundance as juveniles within quadrats. The mean number of juveniles per m² within quadrats ranged from 3 to 42. In general, there was a decrease in the mean number of juveniles and the number of species with depth. Total number of juveniles on settling plates was highest at 18 m on both walls. The largest number within quadrats was at 18 m on the east wall, followed by 9 m and 18 m on the west wall. High rates of coral recruitment tended to be associated with low algal biomass and relatively high grazing pressure by urchins and fishes.     

Highly stable symbioses among western Atlantic brooding corals

The reproductive mode of corals largely determines how zooxanthellae (Symbiodinium spp.) are acquired. Typically, broadcast spawning corals obtain symbionts from the surrounding environment, whereas most brooders transfer symbionts from maternal parent to offspring. Brooding corals are therefore predicted to harbor stable communities of Symbiodinium. This study documents the associations between Symbiodinium spp. and brooding corals in response to seasonal environmental fluctuations. Between March 2002 and December 2005, endosymbiont identity was determined seasonally from replicate colonies (n = 6) of three brooding species, Agaricia agaricites, Porites astreoides and Siderastrea radians, from shallow environments (1–4 m) of the Florida Keys and Bahamas. Symbionts were identified via denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer 2 (ITS2) region. No change was detected in the Symbiodinium communities harbored within these brooding colonies. Additionally, no change in symbiosis was observed through a moderate bleaching event, thereby demonstrating that some bleached corals recover without changing symbionts.     

Exaptation in corals to high seawater temperatures: Low concentrations of apoptotic and necrotic cells in host coral tissue under bleaching conditions

Scleractinian corals are known to suffer bleaching or loss of their symbiotic zooxanthellae under conditions of elevated seawater temperatures often associated with climate change (i.e. global warming). This can occur on a massive scale and has caused the decimation of reefs on a global basis. During the bleaching process, the expelled zooxanthellae suffer cell damage from heat stress, characterized by irreversible ultrastructural and physiological changes which are symptomatic of cell degeneration and death (called apoptosis) or necrosis. A question that remains unanswered, however, is whether the coral hosts themselves are sensitive to seawater temperatures, and, if so, to what degree? In a controlled experiment, we exposed corals Acropora hyacinthus (Dana, 1846) and Porites solida (Forskål, 1775) with their symbiotic zooxanthellae (Symbiodinium sp.) to temperatures of 28 °C (control), 30 °C, 32 °C, and 34 °C for 48 h and also to 36 °C for 12 h. We assessed coral and zooxanthellar cells in-situ for symptoms of apoptosis and necrosis using transmission electron microscopy (TEM), fluorescent microscopy (FM), and flow cytometry (FC). We found that the coral host cells in-situ exhibited, for the most part, little or no mortality from increased seawater temperatures. Damage to the coral hosts only occurred under conditions of prolonged exposure (≥ 12 h) at high temperatures (34 °C), or at exceptionally high temperatures (e.g. 36 °C). On the other hand, we found high levels of apoptosis and necrosis in the zooxanthellae in-situ under all treatment conditions of elevated seawater temperatures. We found that during bleaching, the host cells are not experiencing much mortality - but the zooxanthellae, even while still within the host, are. The host corals exhibit exaptation to accommodate temperatures as high as ≥ 34 °C. Temperature stress within these highly specific and coevolved symbiotic systems is derived not from host sensitivity to temperature, but from the symbiont's sensitivity and the loss of the coral's endosymbiotic partners.     

Photo-acclimation of the hermatypic coral Stylophora pistillata while subjected to either starvation or food provisioning

This study investigated the photo-acclimation capacity of the coral Stylophora pistillata (Esper). Outer branches of coral colonies, taken from 2 m, were subjected to 90, 20, or 3% of incident surface photosynthetic active radiation (PAR(0)), or kept in total darkness. The corals were maintained either in filtered seawater (i.e., under starvation), or in seawater that had daily additions of zooplankton (rotifers). The experiments were maintained for 31 days. Zooxanthellae population densities and chlorophyll concentrations increased in S. pistillata fragments subjected to 20 and 3% PAR(0). The zooxanthellae densities decreased after 6 days in corals kept in total darkness, although chlorophyll concentrations remained higher. Corals that were fed and subjected to 90% PAR(0) showed lower degrading zooxanthellae frequencies, higher photosynthetic and respiration rates, and higher chlorophyll concentrations than corals in the same light regime under starvation. Complete acclimation to dim (20% PAR(0)) and low (3% PAR(0)) light was only apparent for corals fed with zooplankton. Changes in zooxanthellae population densities occurred through differential rates of zooxanthellae division and degradation.     

Calcification and associated physiological parameters during a stress event in the scleractinian coral Stylophora pistillata

High calcification rates observed in reef coral organisms are due to the symbiotic relationship established between scleractinian corals and their photosynthetic dinoflagellates, commonly called zooxanthellae. Zooxanthellae are known to enhance calcification in the light, a process referred as "light-enhanced calcification". The disruption of the relationship between corals and their zooxanthellae leads to bleaching. Bleaching is one of the major causes of the present decline of coral reefs related to climate change and anthropogenic activities. In our aquaria, corals experienced a chemical pollution leading to bleaching and ending with the death of corals. During the time course of this bleaching event, we measured multiple parameters and could evidence four major consecutive steps: 1) at month 1 (January 2005), the stress affected primarily the photosystem II machinery of zooxanthellae resulting in an immediate decrease of photosystem II efficiency, 2) at month 2, the stress affected the photosynthetic production of O2 by zooxanthellae and the rate of light calcification, 3) at month 3, there was a decrease in both light and dark calcification rates, the appearance of the first oxidative damage in the zooxanthellae, the disruption of symbiosis, 4) and finally the death of corals at month 6.     

Symbiont diversity in scleractinian corals from tropical reefs and subtropical non-reef communities in Taiwan

We examined zooxanthellae diversity in scleractinian corals from southern Taiwan and the Penghu Archipelago, a tropical coral reef and a subtropical non-reefal community, respectively. Zooxanthellae diversity was investigated in 52 species of scleractinian corals from 26 genera and 13 families, using restriction fragment length polymorphism (RFLP), and phylogenetic analyses of the nuclear small-subunit ribosomal DNA (nssrDNA) and large-subunit ribosomal DNA (nlsrDNA). RFLP and phylogenetic analyses of nuclear-encoded ribosomal RNA genes showed that Symbiodinium clade C was the dominant zooxanthellae in scleractinian corals in the seas around Taiwan; Symbiodinium clade D was also found in some species. Both Symbiodinium clade C and D were found in colonies of seven species of scleractinian corals. Symbiodinium clade D was associated with corals that inhabit either shallow water or the reef edge in deep water, supporting the hypothesis that Symbiodinium clade D is a relatively stress-tolerant zooxanthellae found in marginal habitats.Communicated by Biological Editor H.R. Lasker     

Recovery of the coral Montastrea annularis in the Florida Keys after the 1987 Caribbean “bleaching event”

Many reef-building corals and other cnidarians lost photosynthetic pigments and symbiotic algae (zooxanthellae) during the coral bleaching event in the Caribbean in 1987. The Florida Reef Tract included some of the first documented cases, with widespread bleaching of the massive coral Montastrea annularis beginning in late August. Phototransects at Carysfort Reef showed discoloration of >90% of colonies of this species in March 1988 compared to 0% in July 1986; however no mortality was observed between 1986 and 1988. Samples of corals collected in February and June 1988 had zooxanthellae densities ranging from 0.1 in the most lightly colored corals, to 1.6x10⁶ cells/cm² in the darker corals. Minimum densities increased to 0.5x10⁶ cells/cm² by August 1989. Chlorophyll-a content of zooxanthellae and zooxanthellar mitotic indices were significantly higher in corals with lower densities of zooxanthellae, suggesting that zooxanthellar at low densities may be more nutrientsufficient than those in unbleached corals. Ash-free dry weight of coral tissue was positively correlated with zooxanthellae density at all sample times and was significantly lower in June 1988 compared to August 1989. Proteins and lipids per cm² were significantly higher in August 1989 than in February or June, 1988. Although recovery of zooxanthellae density and coral pigmentation to normal levels may occur in less than one year, regrowth of tissue biomass and energy stores lost during the period of low symbiont densities may take significantly longer.     

Spatio-Temporal Analyses of Symbiodinium Physiology of the Coral Pocillopora verrucosa along Large-Scale Nutrient and Temperature Gradients in the Red Sea

Algal symbionts (zooxanthellae, genus Symbiodinium) of scleractinian corals respond strongly to temperature, nutrient and light changes. These factors vary greatly along the north-south gradient in the Red Sea and include conditions, which are outside of those typically considered optimal for coral growth. Nevertheless, coral communities thrive throughout the Red Sea, suggesting that zooxanthellae have successfully acclimatized or adapted to the harsh conditions they experience particularly in the south (high temperatures and high nutrient supply). As such, the Red Sea is a region, which may help to better understand how zooxanthellae and their coral hosts successfully acclimatize or adapt to environmental change (e.g. increased temperatures and localized eutrophication). To gain further insight into the physiology of coral symbionts in the Red Sea, we examined the abundance of dominant Symbiodinium types associated with the coral Pocillopora verrucosa, and measured Symbiodinium physiological characteristics (i.e. photosynthetic processes, cell density, pigmentation, and protein composition) along the latitudinal gradient of the Red Sea in summer and winter. Despite the strong environmental gradients from north to south, our results demonstrate that Symbiodinium microadriaticum (type A1) was the predominant species in P. verrucosa along the latitudinal gradient. Furthermore, measured physiological characteristics were found to vary more with prevailing seasonal environmental conditions than with region-specific differences, although the measured environmental parameters displayed much higher spatial than temporal variability. We conclude that our findings might present the result of long-term acclimatization or adaptation of S. microadriaticum to regionally specific conditions within the Red Sea. Of additional note, high nutrients in the South correlated with high zooxanthellae density indicating a compensation for a temperature-driven loss of photosynthetic performance, which may prove promising for the resilience of these corals under increase of temperature increase and eutrophication.     

Experimental responses to elevated water temperature in genotypes of the reef coral<Emphasis Type="Italic"> Pocillopora damicornis</Emphasis> from upwelling and non-upwelling environments in Panama

The authors investigated the response to experimentally elevated water temperature in genotypes of Pocillopora damicornis from three coral reefs in the upwelling Gulf of Panama and four coral reefs in the non-upwelling Gulf of Chiriquí, Panamanian Pacific. Sea-surface temperature in the Gulf of Panama declines below 20 °C during seasonal upwelling, while in the thermally stable Gulf of Chiriquí, the temperature ranges from 27 to 29 °C. Genotypes of P. damicornis from the seven locations were determined by allozyme electrophoresis. The most abundant genotype at each location was selected for a thermal tolerance experiment where corals were exposed to water temperature of 30 °C (1 °C above ambient) for 43 days. Four site coral genotypes can be uniquely differentiated by the GPI locus, two by the LGG-2 locus, and two by a combination of the MDH-1, LGG-2, and LTY-3 loci. A visual assessment of the coral condition after exposure to an elevated temperature showed that corals from localities in the non-upwelling environment retained a normal to slightly pale appearance, while corals from the upwelling environment bleached and their polyps were mostly retracted. A two-way ANOVA confirmed that corals were significantly affected by water temperature and locality. The zooxanthellae were also significantly affected by the interaction of elevated temperature and locality of the corals. Mean zooxanthellae density decreased by 25 and 55%, respectively, in experimentally heated corals from the non-upwelling and upwelling environments. Low concentrations of photosynthetic pigments per live area of the corals were the norm in corals under elevated temperature. The mean concentration of chlorophyll a per live area of the corals was reduced by 17 and 49%, respectively, in heated corals from the non-upwelling and upwelling sites. Coral genotypes from the upwelling Gulf of Panama demonstrated higher vulnerability to thermal stress than coral genotypes from the non-upwelling Gulf of Chiriquí. However, the latter showed greater differences in their responses. Thus, even at small geographic scales, corals can display different levels of tolerance to thermal stress. The difference in thermal tolerance between corals from upwelling and non-upwelling environments is concomitant with greater genetic differences in experimental corals from the thermally stable Gulf of Chiriquí compared with corals from the upwelling Gulf of Panama.Communicated by K.S. Sealey     

Zooxanthellae Harvested by Ciliates Associated with Brown Band Syndrome of Corals Remain Photosynthetically Competent

Brown band syndrome is a new coral affliction characterized by a local accumulation of yet-unidentified ciliates migrating as a band along the branches of coral colonies. In the current study, morphologically intact zooxanthellae (= Symbiodinium) were observed in great numbers inside the ciliates (>50 dinoflagellates per ciliate). Microscale oxygen measurements and variable chlorophyll a fluorescence analysis along with microscopic observations demonstrated that zooxanthellae within the ciliates are photosynthetically competent and do not become compromised during the progression of the brown band zone. Zooxanthellae showed similar trends in light acclimation in a comparison of rapid light curve and steady-state light curve measures of variable chlorophyll a fluorescence. Extended light exposure of steady-state light curves resulted in higher quantum yields of photosystem II. The brown band tissue exhibited higher photosynthetically active radiation absorptivity, indicating more efficient light absorption due to a higher density of zooxanthellae in the ciliate-dominated zone. This caused relatively higher gross photosynthesis rates in the zone with zooxanthella-containing ciliates compared to healthy coral tissue. The observation of photosynthetically active intracellular zooxanthellae in the ciliates suggests that the latter can benefit from photosynthates produced by ingested zooxanthellae and from photosynthetic oxygen production that alleviates diffusion limitation of oxic respiration in the densely populated brown band tissue. It remains to be shown whether the zooxanthellae form a stable symbiotic association with the ciliate or are engulfed incidentally during grazing on coral tissue and then maintained as active inside the ciliate for a period before being digested and replaced by new zooxanthellae.     

Net uptake of dissolved free amino acids by four scleractinian corals

High pressure liquid chromatography was employed to provide the first definitive proof of the net uptake of dissolved free amino acids (DFFA) at nanomolar levels by four scleractinian corals (Montastrea annularis, Madracis mirabilis, Agaricia fragilìs, and Favia fragum). During 2 h incubations all species exhibited simultaneous net uptake of eight amino acids. For M. annularis and F. fragum uptake of some dissolved amino acids occurred at concentrations lower than those found in reef waters. Microbial activity or adsorption of DFAA to exposed coral skeletons during these experiments did not appear to be important. Although it seems unlikely that DFAA uptake can provide a significant energy source for corals under ambient condìtions, it may be important in the acquisition and retention of nitrogen by these animals.     

Remote monitoring of chlorophyll fluorescence in two reef corals during the 2005 bleaching event at Lee Stocking Island, Bahamas

Zooxanthellae fluorescence was measured in situ, remotely, and in near real-time with a pulse amplitude modulated (PAM) fluorometer for a colony of Siderastrea siderea and Agaricia tenuifolia at Lee Stocking Island, Bahamas during the Caribbean-wide 2005 bleaching event. These colonies displayed evidence of photosystem II (PS II) inactivation coincident with thermal stress and seasonally high doses of solar radiation. Hurricane-associated declines in temperature and light appear to have facilitated the recovery of maximum quantum yield of PS II within these two colonies, although both corals responded differently to individual storms. PAM fluorometry, coupled with long-term measurement of in situ light and temperature, provides much more detail of coral photobiology on a seasonal time scale and during possible bleaching conditions than sporadic, subjective, and qualitative observations. S. siderea displayed evidence of PS II inactivation over a month prior to the issuing of a satellite-based, sea surface temperature (SST) bleaching alert by the National Oceanic and Atmospheric Administration (NOAA). In fact, recovery had already begun in S. siderea when the bleaching alert was issued. Fluorescence data for A. tenuifolia were difficult to interpret because the shaded parts of a colony were monitored and thus did not perfectly coincide with thermal stress and seasonally high doses of solar radiation as in S. siderea. These results further emphasize the limitations of solely monitoring SST (satellite or in situ) as a bleaching indicator without considering the physiological status of coral-zooxanthellae symbioses. Communicated by Environment Editor Prof. Rob van Woesik     

PHOTOSYNTHESIS IN SOME ALCYONACEAN CORALS

Burkholder , Paul R., and Lillian M. Burkholder . (Brooklyn Botanic Garden, Brooklyn, New York.) Photosynthesis in some Alcyonacean corals. Amer. Jour. Bot. 47(10) : 866–872. Illus. 1960.—Quantitative production of oxygen in bottles, and amounts of chlorophyll and carotenoid pigments were studied in selected species of Alcyonacean corals, the stony coral Porites, porites, and turtle grass, Thalassia testudinum, in Puerto Rico. Photosynthetic production exceeds the consumption of oxygen in combined respiration of the corals and their contained algal zooxanthellae, both in short-term light experiments and over 24 hr. of natural, alternating day and night. Observations of oxygen production in 8 species of corals, exposed to full sunlight in 1 ft. of water, gave assimilation values of 0.9 mg. O₂ (equivalent to 0.27 mg. C fixed) per mg. chlorophyll a per hour. A maximum assimilation value of 1.56 was observed in Antillogorgia turgida. From data on the increased oxygen it was calculated that from 0.33 to 1.65 mg. residual fixed carbon per g. of fresh coral were accumulated in Porites porites and Antillogorgia turgida during a day and night period of 24 hr. Photosynthesis of turtle grass in the same environment was more than 3 times the rate of these corals. The zooxanthellae contain carotenoids in abundance, along with chlorophylls a and c. On a unit of chlorophyll a basis, the photosynthetic systems of corals appear to be less active in reduction of carbon dioxide in bottles than some other photosynthetic systems of the sea, such as turtle grass and phytoplankton observed under optimum conditions.