Diurnal changes in the photochemical efficiency of the symbiotic dinoflagellates (Dinophyceae) of corals: photoprotection, photoinactivation and the relationship to coral bleaching

The photochemical efficiency of symbiotic dinoflagellates within the tissues of two reef‐building corals in response to normal and excess irradiance at water temperatures < 30 °C were investigated using pulse amplitude modulated (PAM) chlorophyll fluorescence techniques. Dark‐adapted F_v/F_m showed clear diurnal changes, decreasing to a low at solar noon and increasing in the afternoon. However, F_v/F_m also drifted downwards at night or in prolonged darkness, and increased rapidly during the early morning twilight. This parameter also increased when the oxygen concentration of the water holding the corals was increased. Such changes have not been described previously, and most probably reflect state transitions associated with PQ pool reduction via chlororespiration. These unusual characteristics may be a feature of an endosymbiotic environment, reflective of the well‐documented night‐time tissue hypoxia that occurs in corals. F_v/F_m decreased to 0·25 in response to full sunlight in shade‐acclimated (shade) colonies of Stylophora pistillata, which is considerably lower than in light‐acclimated (sun) colonies. In sun colonies, the reversible decrease in F_v/F_m was caused by a lowering of F_m and F_o suggesting photoprotection and no lasting damage. The decrease in F_v/F_m, however, was caused by a decrease in F_m and an increase in F_o in shade colonies suggesting photoinactivation and long‐term cumulative photoinhibition. Shade colonies rapidly lost their symbiotic algae (bleached) during exposure to full sunlight. This study is consistent with the hypothesis that excess light leads to chronic damage of symbiotic dinoflagellates and their eventual removal from reef‐building corals. It is significant that this can occur with high light conditions alone.     

Nutritional status and metabolism of the coral Stylophora subseriata along a eutrophication gradient in Spermonde Archipelago (Indonesia)

Coral responses to degrading water quality are highly variable between species and depend on their trophic plasticity, acclimatization potential, and stress resistance. To assess the nutritional status and metabolism of the common scleractinian coral, Stylophora subseriata, in situ experiments were carried along a eutrophication gradient in Spermonde Archipelago, Indonesia. Coral fragments were incubated in light and dark chambers to measure photosynthesis, respiration, and calcification in a number of shallow reefs along the gradient. Chlorophyll a (chl a), protein content, maximum quantum yield (F _v/F _m), and effective quantum yield (Φ PS II) were measured on the zooxanthellae, in addition to host tissue protein content and biomass. Photosynthetic rates were 2.5-fold higher near-shore than mid-shelf due to higher areal zooxanthellae and chl a concentrations and a higher photochemical efficiency (Φ PS II). A 2- and 3-fold increase in areal host tissue protein and biomass was found, indicating a higher nutritional supply in coastal waters. Dark respiration, however, showed no corresponding changes. There was a weak correlation between calcification and photosynthesis (Pearson r = 0.386) and a lack of metabolic stress, as indicated by constant respiration and F _v/F _m and the “clean” and healthy appearance of the colonies in spite of high turbidity in near-shore waters. The latter suggests that part of the energetic gains through increased auto- and heterotrophy were spent on metabolic expenditures, e.g., mucus production. While coastal pollution is always deleterious to the reef ecosystem as a whole, our results show that the effect on corals may not always be negative. Thus, S. subseriata may be one of the few examples of corals actually profiting from land-based sources of pollution.     

Differential susceptibility to oxidative stress of two scleractinian corals: antioxidant functioning of mycosporine-glycine

This study examined the importance of mycosporine-glycine (Myc-Gly) as a functional antioxidant in the thermal-stress susceptibility of two scleractinian corals, Platygyra ryukyuensis and Stylophora pistillata. Photochemical efficiency of PSII (F_v/F_m), activity of antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), and composition and abundance of mycosporine-like amino acids (MAAs) in the coral tissue and in symbiotic zooxanthellae were analyzed during 12-h exposure to high temperature (33 °C). After 6- and 12-h exposures at 33 °C, S. pistillata showed a significantly more pronounced decline in F_v/F_m compared to P. ryukyuensis. A 6-h exposure at 33 °C induced a significant increase in the activities of SOD and CAT in both host and zooxanthellae components of S. pistillata while in P. ryukyuensis a significant increase was observed only in the CAT activity of zooxanthellae. After 12-h exposure, the SOD activity of P. ryukyuensis was unaffected in the coral tissue but slightly increased in zooxanthellae, whereas the CAT activity in the coral tissue showed a 2.5-fold increase. The total activity of antioxidant enzymes was significantly higher in S. pistillata than in P. ryukyuensis, suggesting that P. ryukyuensis is less sensitive to oxidative stress than S. pistillata. This differential susceptibility of the corals is consistent with a 20-fold higher initial concentration of Myc-Gly in P. ryukyuensis compared to S. pistillata. In the coral tissue and zooxanthellae of both species investigated, the first 6 h of exposure to thermal stress induced a pronounced reduction in the abundance of Myc-Gly but not in other MAAs. When exposure was prolonged to 12 h, the Myc-Gly pool continued to decrease in P. ryukyuensis and was completely depleted in S. pistillata. The delay in the onset of oxidative stress in P. ryukyuensis and the dramatic increase in the activities of the antioxidant enzymes in S. pistillata, which contains low concentrations of Myc-Gly suggest that Myc-Gly provides rapid protection against oxidative stress before the antioxidant enzymes are induced. These findings strongly suggest that Myc-Gly is functioning as a biological antioxidant in the coral tissue and zooxanthellae and demonstrate its importance in the survival of reef-building corals under thermal stress.     

Comparison of stress susceptibility of in hospite and isolated zooxanthellae among five coral species

Coral species in a similar habitat often show different bleaching susceptibilities. It is not understood which partner of coral-zooxanthellae complexes is responsible for differential stress susceptibility. Stress susceptibilities of in hospite and isolated zooxanthellae from five species of corals collected from shallow water in Okinawa were compared. To estimate stress susceptibility, we measured the maximum quantum yields (F_v/F_m) of in hospite and isolated zooxanthellae after 3-h exposure to either 28 or 34 °C at various light intensities and their recovery after 12 h under dim light at 26 °C. Significant reduction in photochemical efficiency (F_v/F_m) of photosystem II (PSII) was observed in in hospite zooxanthellae exposed to high light intensity (1000 μmol quanta m⁻² s⁻¹), while PSII activity of isolated zooxanthellae decreased significantly even at a lower light intensity (70 μmol quanta m⁻² s⁻¹). The recovery of the PSII activity after 12 h was incomplete in both in hospite and isolated zooxanthellae, indicating the presence of chronic photoinhibition. The stress susceptibility of isolated zooxanthellae was more variable among species than in hospite zooxanthellae. The order of stress susceptibility among the five coral species was different between in hospite and isolated zooxanthellae. The present results suggest that the host plays a significant role in determining bleaching susceptibility of corals, though zooxanthellae from different host have different stress susceptibilities.     

Contrasting effects of high‐intensity photosynthetically active radiation on two bloom‐forming dinoflagellates

While light limitation can inhibit bloom formation in dinoflagellates, the potential for high‐intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well‐studied. We measured the effects of high‐intensity PAR on the bloom‐forming dinoflagellates Alexandrium fundyense and Heterocapsa rotundata. Various physiological parameters (photosynthetic efficiency F_v/F_m, cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll‐a content) were measured before and after exposure to high‐intensity natural sunlight in short‐term light stress experiments. In addition, photosynthesis‐irradiance (P‐E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species‐specific responses to high PAR. While high light decreased F_v/F_m in both species, A. fundyense showed little additional evidence of light stress in short‐term experiments, although increased membrane permeability and intracellular DMSP indicated a response to handling. P‐E responses further indicated a high light‐adapted species with Chl‐a inversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per‐cell photosynthesis rates suggest a trade‐off between photoprotection and C fixation in high light‐acclimated cells. Heterocapsa rotundata cells, in contrast, swelled in response to high light and sometimes lysed in short‐term experiments, releasing DMSP. P‐E responses confirmed a low light‐adapted species with high photosynthetic efficiencies associated with trade‐offs in the form of substantial photoinhibition and a lack of plasticity in Chl‐a content. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species‐specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.     

Photophysiological stress in scleractinian corals in response to short-term sedimentation

Effects of short-term sedimentation on common coastal coral species were investigated in laboratory and field experiments on the Great Barrier Reef (GBR) using pulse-amplitude modulated (PAM) chlorophyll fluorometry. In the laboratory, changes in maximal quantum yields of photosystem II (F_v/F_m) in Montipora peltiformis were examined in response to the amount of sedimentation (79-234 mg cm⁻²) and duration of exposure (0-36 h). In control colonies, F_v/F_m ranged from 0.67 to 0.71, and did not show any temporal trend, while maximum yields of sediment-covered fragments declined steadily and reached levels below 0.1 in most colonies after 36 h coverage. Maximal quantum yield in M. peltiformis declined linearly in relation to both the amount of sediment deposited per unit surface area and the duration of exposure. Zooxanthellae densities and chlorophyll concentrations per unit area of sediment-treated corals decreased in the same manner, however, their responses were not quite as strong as the changes in F_v/F_m. Within the ranges measured, sedimentation stress of colonies exposed to large amounts of sediment for short periods of time was similar to that exposed to low amounts of sediments for prolonged periods of time. Colonies were recovered from short-term, or low-level, sedimentation within <36 h, whereas long-term exposure, or high levels of sedimentation, killed exposed colony parts. Field experiments comparing susceptibilities of common coastal coral species towards sedimentation showed significant reductions in effective quantum yields (ΔF/F_m′) in 9 out of 12 common coastal species after 22 h of exposure. Three out of twelve investigated species were not affected by the experimental application of sediments (Galaxea fascicularis, Fungia crassa, and Pectinia lactuca). Our results suggest that anthropogenic sediment deposition can negatively affect the photosynthetic activity of zooxanthellae and thus the viability of corals. However, the results also showed the ability of corals to compartmentalise sedimentation stress, as the photosynthetic activity only from tissues directly underneath the sediment declined, whereas that of adjacent clean tissues did not change measurably.     

Antioxidant plasticity and thermal sensitivity in four types of Symbiodinium sp.

Warmer than average summer sea surface temperature is one of the main drivers for coral bleaching, which describes the loss of endosymbiotic dinoflagellates (genus: Symbiodinium) in reef‐building corals. Past research has established that oxidative stress in the symbiont plays an important part in the bleaching cascade. Corals hosting different genotypes of Symbiodinium may have varying thermal bleaching thresholds, but changes in the symbiont's antioxidant system that may accompany these differences have received less attention. This study shows that constitutive activity and up‐regulation of different parts of the antioxidant network under thermal stress differs between four Symbiodinium types in culture and that thermal susceptibility can be linked to glutathione redox homeostasis. In Symbiodinium B1, C1 and E, declining maximum quantum yield of PSII (F_v/F_m) and death at 33°C were generally associated with elevated superoxide dismutase (SOD) activity and a more oxidized glutathione pool. Symbiodinium F1 exhibited no decline in F_v/F_m or growth, but showed proportionally larger increases in ascorbate peroxidase (APX) activity and glutathione content (GSx), while maintaining GSx in a reduced state. Depressed growth in Symbiodinium B1 at a sublethal temperature of 29°C was associated with transiently increased APX activity and glutathione pool size, and an overall increase in glutathione reductase (GR) activity. The collapse of GR activity at 33°C, together with increased SOD, APX and glutathione S‐transferase activity, contributed to a strong oxidation of the glutathione pool with subsequent death. Integrating responses of multiple components of the antioxidant network highlights the importance of antioxidant plasticity in explaining type‐specific temperature responses in Symbiodinium.     

Different photochemical responses of phytoplankters from the large shallow Taihu Lake of subtropical China in relation to light and mixing

The maximum quantum yield of photosystem II was estimated from variable chlorophyll a fluorescence in samples of phytoplankton collected from the Taihu Lake in China to determine the responses of different phytoplankters to irradiance and vertical mixing. Meteorological and environmental variables were also monitored synchronously. The maximum quantum yield of three phytoplankton groups: cyanobacteria, chlorophytes, and diatoms/dinoflagellates, showed a similar diurnal change pattern. F _v/F _m decreased with a significant depth-dependent variation as irradiance increased during the morning and increased as irradiance declined in the afternoon. Furthermore, the rates of F _v/F _m depression were dependent upon the photon flux density, whereas the rates of recovery of F _v/F _m were dependent upon the historical photon density. Moreover, photoinhibition affected the instantaneous growth rates of phytoplankton. Although at noon cyanobacteria had a higher photoinhibition value (up to 41%) than chlorophytes (32%) and diatoms/dinoflagellates (34%) at the surface, no significant difference in diurnal growth rates among the three phytoplankton groups were observed indicating that cyanobacteria could photoacclimate better than chlorophytes and diatoms/dinoflagellates. In addition, cyanobacteria had a higher nonphotochemical quenching value than chlorophytes and diatoms/dinoflagellates at the surface at noon, which indicated that cyanobacteria were better at dissipating excess energy. The ratios of enclosed bottle samples F _v/F _m to free lake samples F _v/F _m showed different responses for the three phytoplankton groups to irradiance and vertical mixing when wind speed was approximately constant at about 3.0 m s⁻¹. When wind speed was lower than 3.0 m s⁻¹, cyanobacteria accumulated mainly at the surface and 0.3 m, because of their positive buoyancy, where diurnal growth rates of phytoplankton were relatively higher than those at 0.6 m and 0.9 m. Chlorophytes were homogenized completely by vertical mixing, while diatoms/dinoflagellates avoided active high irradiance by moving downward at noon, and then upward again when irradiance decreased. These results explain the dominance of cyanobacteria in Taihu Lake. Handling editor: L. Naselli-Flores     

In situ measured seasonal variations in F v/F m of two common Red Sea corals

Pulse amplitude modulated (PAM) fluorometry has been suggested as a tool for estimating environmental stresses on corals. However, information regarding natural changes in maximal quantum yields (F _v/F _m) of corals during “normal” (i.e. non-bleaching) years has been limited. In this study, seasonal variations in F _v/F _m for Stylophora pistillata and Favia favus, measured in situ, correlated with seasonal changes in solar irradiance but not in sea temperature. Interactions between sea temperature and irradiance were further studied by growing these corals and Pocillopora damicornis under controlled conditions. Exposure to high light with normal or high temperatures resulted in lower F _v/F _m values than exposure to low light at both temperatures. Thus, high irradiances may cause decreased F _v/F _m values in corals at least as much as, if not more than, high temperatures. Such seasonal variations should be taken into account when using PAM fluorometry as a diagnostic tool for predicting coral bleaching.     

Genome‐wide association study identifies variation of glucosidase being linked to natural variation of the maximal quantum yield of photosystem II

The maximum quantum yield of photosystem II (as reflected by variable to maximum chlorophyll a fluorescence, F_v/F_m) is regarded as one of the most important photosynthetic parameters. The genetic basis underlying natural variation in F_v/F_m, which shows low level of variations in plants under non‐stress conditions, is not easy to be exploited using the conventional gene cloning approaches. Thus, in order to answer this question, we have followed another strategy: we used genome‐wide association study (GWAS) and transgenic analysis in a rice mini‐core collection. We report here that four single‐nucleotide polymorphisms, located in the promoter region of β‐glucosidase 5 (BGlu‐5), are associated with observed variation in F_v/F_m. Indeed, our transgenic analysis showed a good correlation between BGlu‐5 and F_v/F_m. Thus, our work demonstrates the feasibility of using GWAS to study natural variation in F_v/F_m, suggesting that cis‐element polymorphism, affecting the BGlu‐5 expression level, may, indirectly, contribute to F_v/F_m variation in rice through the gibberellin signaling pathway. Further research is needed to understand the mechanism of our novel observation.     

Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals : evidence for photoinhibition and photoprotection

 Diurnal patterns of photoinhibition have been identified in seven species of shallow water reef corals from the Andaman Sea, off the west coast of Thailand, using pulse amplitude fluorometry. Photochemical efficiency (F_v/F_m) and quantum yield (ΔF/F_m∑) of symbiotic dinoflagellates within the corals declined after dawn to reach a minimum between midday and early afternoon, recovering to former dawn levels by early evening. Parallel studies on the xanthophylls diadinoxanthin (Dn) and diatoxanthin (Dt), and their inter-conversion, also revealed a strong diurnal pattern as well as inverse correlations between the xanthophyll ratio Dt/(Dn+Dt) and F_v/F_m and ΔF/F_m′. These findings suggest a photoprotective function for these pigments. Accepted: 18 March 1999     

EFFECT OF ELEVATED TEMPERATURE,DARKNESS, AND HYDROGEN PEROXIDE TREATMENT ON OXIDATIVE STRESS AND CELL DEATH IN THE BLOOM‐FORMING TOXIC CYANOBACTERIUM MICROCYSTIS AERUGINOSA1

This study assessed the implication of oxidative stress in the mortality of cells of Microcystis aeruginosa Kütz. Cultures grown at 25°C were exposed to 32°C, darkness, and hydrogen peroxide (0.5 mM) for 96 h. The cellular abundance, chl a concentration and content, maximum photochemical efficiency of PSII (F_v/F_m ratio), intracellular oxidative stress (determined with dihydrorhodamine 123 [DHR]), cell mortality (revealed by SYTOX‐labeling of DNA), and activation of caspase 3–like proteins were assessed every 24 h. The presence of DNA degradation in cells of M. aeruginosa was also assessed using a terminal deoxynucletidyl transferase‐mediated dUTP nick end labeling (TUNEL) assay at 96 h. Transferring cultures from 25°C to 32°C was generally beneficial to the cells. The cellular abundance and chl a concentration increased, and the mortality remained low (except for a transient burst at 72 h) as did the oxidative stress. In darkness, cells did not divide, and the F_v/F_m continuously decreased with time. The slow increase in intracellular oxidative stress coincided with the activation of caspase 3–like proteins and a 15% and 17% increase in mortality and TUNEL‐positive cells, respectively. Exposure to hydrogen peroxide had the most detrimental effect on cells as growth ceased and the F_v/F_m declined to near zero in less than 24 h. The 2‐fold increase in oxidative stress matched the activation of caspase 3–like proteins and a 40% and 37% increase in mortality and TUNEL‐positive cells, respectively. These results demonstrate the implication of oxidative stress in the stress response and mortality of M. aeruginosa.     

Temperature-induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae

The early effects of heat stress on the photosynthesis of symbiotic dinoflagellates (zooxanthellae) within the tissues of a reef-building coral were examined using pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photorespirometry. Exposure of Stylophora pistillata to 33 and 34 °C for 4 h resulted in (1) the development of strong non-photochemical quenching (qN) of the chlorophyll fluorescence signal, (2) marked decreases in photosynthetic oxygen evolution, and (3) decreases in optimal quantum yield (F_v/F_m) of photosystem II (PSII). Quantum yield decreased to a greater extent on the illuminated surfaces of coral branches than on lower (shaded) surfaces, and also when high irradiance intensities were combined with elevated temperature (33 °C as opposed to 28 °C). qN collapsed in heat-stressed samples when quenching analysis was conducted in the absence of oxygen. Collectively, these observations are interpreted as the initiation of photoprotective dissipation of excess absorbed energy as heat (qN) and O₂-dependent electron flow through the Mehler-Ascorbate-Peroxidase cycle (MAP-cycle) following the point at which the rate of light-driven electron transport exceeds the capacity of the Calvin cycle. A model for coral bleaching is proposed whereby the primary site of heat damage in S. pistillata is carboxylation within the Calvin cycle, as has been observed during heat damage in higher plants. Damage to PSII and a reduction in F_v/F_m (i.e. photoinhibition) are secondary effects following the overwhelming of photoprotective mechanisms by light. This secondary factor increases the effect of the primary variable, temperature. Potential restrictions of electron flow in heat-stressed zooxanthellae are discussed with respect to Calvin cycle enzymes and the unusual status of the dinoflagellate Rubisco. Significant features of our model are that (1) damage to PSII is not the initial step in the sequence of heat stress in zooxanthellae, and (2) light plays a key secondary role in the initiation of the bleaching phenomena.     

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.     

Black leaf-clips increased minimum fluorescence emission in clipped leaves exposed to high solar radiation during dark adaptation

Tomato and pepper leaves were clipped with black leaf clips for dark adaptation under solar radiation in the late spring or early summer 2010 in southern Italy. The leaves showed highly variable maximum PSII quantum yield (F_v/F_m = 0.026−0.802) using a continuous-excitation fluorometer Pocket PEA. These results were confirmed using the modulated fluorometer FMS1 on tomato leaves in mid summer, with F_v/F_m as low as 0.222 ± 0.277 due to nearly equal minimum (F_o) and maximum (F_m) fluorescence emission. A significant clip effect on F_v/F_m occurred after only 12 (tomato) or 25 (pepper) min. Increasing the leaf temperature from 25 to 50°C reportedly induced an F_o increase and Fm decrease so that F_v/F_m approached zero. The hypothesis that black leaf clips overheated under intense solar irradiance was verified by shrouding the clipped leaves with aluminum foil. In clipped leaves of pepper, F_v/F_m with the black clip/Pocket-PEA was 0.769 ± 0.025 (shrouded) and as low as 0.271 ± 0.163 (nonshrouded), the latter showing a double F_o and 32% lower F_m. An 8% clip effect on F_v/F_m was observed with the white clip/FMS1. To avoid the clip effect in high irradiance environments, F_v/F_m measurements with black clip/Pocket PEA system required leaf dark adaptation with radiation-reflecting shrouds. It would be useful if manufacturing companies could develop better radiation-reflecting leaf clips for the Pocket PEA fluorometer.     

EFFECTS OF MODERATE HEAT STRESS AND DISSOLVED INORGANIC CARBON CONCENTRATION ON PHOTOSYNTHESIS AND RESPIRATION OF SYMBIODINIUM SP. (DINOPHYCEAE) IN CULTURE AND IN SYMBIOSIS1

The influence of temperature and inorganic carbon (C_i) concentration on photosynthesis was examined in whole corals and samples of cultured symbiotic dinoflagellates (Symbiodinium sp.) using combined measurements from a membrane inlet mass spectrometer and chl a fluorometer. In whole corals, O₂ production at 26°C was significantly limited at C_i concentrations below ambient seawater (～2.2 mM). Further additions of C_i up to ～10 mM caused no further stimulation of oxygenic photosynthesis. Following exposure to 30°C (2 d), net oxygen production decreased significantly in whole corals, as a result of reduced production of photosynthetically derived oxygen rather than increased host consumption. Whole corals maintained a rate of oxygen evolution around eight times lower than cultured Symbiodinium sp. at inorganic carbon concentrations <2 mM, but cultures displayed greater levels of photoinhibition following heat treatment (30°C, 2 d). Whole corals and cultured zooxanthellae differed considerably in their responses to C_i concentration and moderate heat stress, demonstrating that cultured Symbiodinium make an incongruous model for those in hospite. Reduced net oxygen evolution, in whole corals, under conditions of low C_i (<2 mM) has been interpreted in terms of possible sink limitation leading to increased nonphotochemical energy dissipation. The advantages of combined measurement of net gas exchange and fluorometry offered by this method are discussed.     

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.     

Quantification of total and particulate dimethylsulfoniopropionate (DMSP) in five Bermudian coral species across a depth gradient

The symbiotic dinoflagellate microalgae of corals (Symbiodinium spp.) contain high concentrations of dimethylsulfoniopropionate (DMSP), a multifunctional metabolite commonly found in many species of marine algae and dinoflagellates. A photoprotective antioxidant function for DMSP and its breakdown products has often been inferred in algae, but its role(s) in the coral–algal symbiosis remains elusive. To examine potential correlations between environmental and physiological parameters and DMSP, total DMSP (DMSP_t, from the host coral and zooxanthellae), particulate DMSP (DMSP_p, from the zooxanthellae only), coral surface area, and total protein, as well as zooxanthellae density, chlorophyll concentration, cell volume and genotype (i.e., clade) were measured in five coral species from the Diploria-Montastraea-Porites species complex in Bermuda along a depth gradient of 4, 12, 18, and 24 m. DMSP_t concentrations were consistently greater than DMSP_p concentrations in all species suggesting the possible translocation of DMSP from symbiont to host. D. labyrinthiformis was notably different from the other corals examined, showing DMSP_p and DMSP_t increases (per coral surface area or tissue biomass) with increasing water depth. However, overall, there were no consistent depth-related patterns in DMSP_p and DMSP_t concentrations. Further research, investigating dimethylsulfide (DMS), dimethylsulfoxide, and acrylate levels and DMSP-lyase activity in correlation with other biomarker endpoints that have been shown to be depth (i.e., temperature and light) responsive are needed to substantiate the significance of these findings.     

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.     

USE OF PULSE‐AMPLITUDE‐MODULATED FLUORESCENCE TO ASSESS THE PHYSIOLOGICAL STATUS OF CLADOPHORA SP. ALONG A WATER QUALITY GRADIENT1

This study investigated the application of pulse‐amplitude‐modulated (PAM) fluorometry as a rapid assessment of benthic macroalgal physiological status. Maximum quantum efficiency (F_v/F_m), dark–light induction curves, and rapid fluorescence light‐response curves (RLC) were measured on the filamentous macroalgal Cladophora sp. from Lake Ontario on 5 d at 16 sites spanning a gradient of light and nutrient supply. For Cladophora sp. growing in situ, light limitation was assessed by comparing average daily irradiance with the light utilization efficiency parameter (α) derived from RLCs. In this study, there was a nonlinear relationship between F_v/F_m and the degree of P limitation in macroalgae. However, only light‐saturated Cladophora sp. showed a significant positive linear relationship between F_v/F_m and P nutrient status. The absence of this relationship among light‐limited algae indicates that their photosynthetic rate would be stimulated by increased water clarity, and not by increased P supply. PAM fluorescence measures were successfully able to identify light‐saturated macroalgae and, among these, assess the degree to which they were nutrient limited. These results enable us to test hypotheses arising from numeric models predicting the impact of changes in light penetration and nutrient supply on benthic primary production.