Feeding sustains photosynthetic quantum yield of a scleractinian coral during thermal stress

Thermal resistance of the coral-zooxanthellae symbiosis has been associated with chronic photoinhibition, increased antioxidant activity and protein repair involving high demands of nitrogen and energy. While the relative importance of heterotrophy as a source of nutrients and energy for cnidarian hosts, and as a means of nitrogen acquisition for their zooxanthellae, is well documented, the effect of feeding on the thermal sensitivity of the symbiotic association has been so far overlooked. Here we examine the effect of zooplankton feeding versus starvation on the bleaching susceptibility and photosynthetic activity of photosystem II (PSII) of zooxanthellae in the scleractinian coral Stylophora pistillata in response to thermal stress (daily temperature rises of 2-3 degrees C) over 10 days, employing pulse-amplitude-modulated chlorophyll fluorometry. Fed and starved corals displayed a decrease in daily maximum potential quantum yield (F (v)/F (m)) of PSII, effective quantum yield (F/F (m)') and relative electron transport rates over the course of 10 days. However after 10 days of exposure to elevated temperature, F (v)/F (m) of fed corals was still 50-70% higher than F (v)/F (m) of starved corals. Starved corals showed strong signs of chronic photoinhibition, which was reflected in a significant decline in nocturnal recovery rates of PSII relative to fed corals. This was paralleled by the progressive inability to dissipate excess excitation energy via non-photochemical quenching (NPQ). After 10 days, NPQ of starved corals had decreased by about 80% relative to fed corals. Feeding treatment had no significant effect on chlorophyll a and c (2) concentrations and zooxanthellae densities, but the mitotic indices were significantly lower in starved than in fed corals. Collectively the results indicate that exogenous food may reduce the photophysiological damage of zooxanthellae that typically leads to bleaching and could therefore play an important role in mediating the thermal resistance of some corals.     

Nitrogen and photosynthetic function of hermatypic corals. Oxygen exchange of Stylophora pistillata coral under artificial feeding

The change of Stylophora pistillata coral photosynthetic function (oxygen exchange and biomass of symbionts) under starvation and food enrichment was studied to understand the role of heterotrophy in nitrogen supplements of zooxanthellae. The starvation caused the decrease of frequency of zooxanthellae cells division in 7-10 times. The number of degraded algae cells increased in same proportion and, as a result, the density of zooxanthellae in corals decreased about two times during one-two weeks. Under starvation corals kept their photosynthetic capacity at the level of corals in situ by means of enhancing the zooxanthellae gross photosynthesis. The respiration rate of coral had tendency to increase and the dry mass of polyp tissue to decrease. Under artificial feeding which was following starvation the zooxanthellae density increased in 1.5-2 times, and particular food caused more intensive accumulation of zooxanthellae comparing to dissolved inorganic ammonium. The feeding regime did not affect dry mass of polyp tissue and chlorophyll content as well as respiration and gross productivity of the corals. The conclusion about high effectiveness of particular feeding for supplying symbiotic algae with nitrogen was made and trophic status of zooxanthellae in hospite was determined as unlimited by nitrogen.     

The effect of heterotrophy on photosynthesis and tissue composition of two scleractinian corals under elevated temperature

Exogenous food can increase protein levels of coral host tissue, zooxanthellae densities, chlorophyll (chl) concentrations and rates of photosynthesis and is thought to play an important role in the resilience of bleached corals. There is however no information about the effect of heterotrophy on the bleaching susceptibility of corals under elevated temperature conditions. This study investigates potential interactions between food availability, basal metabolic functions (photosynthesis and respiration), energy status (lipid concentrations), total protein concentrations and the bleaching susceptibility (loss of chl and/or zooxanthellae) of the scleractinian corals Stylophora pistillata (Esper) and Galaxea fascicularis (Linnaeus) in response to elevated temperature (daily temperature rises of 3-4 °C) over 15 days. Feeding experiments were carried out in which the corals were either fed daily with zooplankton or starved. Compared to fed corals, starvation of both species resulted in a significant decrease in daily photosynthetic oxygen evolution over time. Gross (Pg) and net (Pn) photosynthetic production of starved corals of both species between 10:00-11:00 hrs had declined by ~50% at day 15 while there were no marked changes in Pg and Pn of fed corals. After 15 days, starved S. pistillata contained significantly lower zooxanthellae densities, lipid and protein concentrations than fed corals. Starved G. fascicularis also displayed a decrease in zooxantllae densities which was accompanied by a significant decline in algal chl concentrations. Contrary to S. pistillata, feeding treatment had no effect on the lipid concentrations of G. fascicularis. Total protein concentrations however were significantly lower in straved than in fed G. fascicularis. Furthermore, starvation resulted in a significant decrease in respiration of S. pistillata during the last four days of the experiment while treatment had no effect on the respiration rates of G. fascicularis. Overall the oxygen consumption of S. pistillata of both treatments was about 39-67% higher than the respiration of G. fascicularis indicating that low metabolic rates may have allowed starved G. fascicularis to conserve energy reserves over the course of the experiment. The combined results reveal a strong positive relationship between food availability, sustained photosynthetic activity and reduced loss in pigmentation of both species under elevated temperature conditions.     

Viruses: agents of coral disease?

The potential role of viruses in coral disease has only recently begun to receive attention. Here we describe our attempts to determine whether viruses are present in thermally stressed corals Pavona danai, Acropora formosa and Stylophora pistillata and zoanthids Zoanthus sp., and their zooxanthellae. Heat-shocked P. danai, A. formosa and Zoanthus sp. all produced numerous virus-like particles (VLPs) that were evident in the animal tissue, zooxanthellae and the surrounding seawater; VLPs were also seen around heat-shocked freshly isolated zooxanthellae (FIZ) from P. danai and S. pistillata. The most commonly seen VLPs were tail-less, hexagonal and about 40 to 50 nm in diameter, though a diverse range of other VLP morphotypes (e.g. rounded, rod-shaped, droplet-shaped, filamentous) were also present around corals. When VLPs around heat-shocked FIZ from S. pistillata were added to non-stressed FIZ from this coral, they resulted in cell lysis, suggesting that an infectious agent was present; however, analysis with transmission electron microscopy provided no clear evidence of viral infection. The release of diverse VLPs was again apparent when flow cytometry was used to enumerate release by heat-stressed A. formosa nubbins. Our data support the infection of reef corals by viruses, though we cannot yet determine the precise origin (i.e. coral, zooxanthellae and/or surface microbes) of the VLPs seen. Furthermore, genome sequence data are required to establish the presence of viruses unequivocally.     

Effects of temperature and UV radiation increases on the photosynthetic efficiency in four scleractinian coral species

Experiments were performed on coral species containing clade A (Stylophora pistillata, Montipora aequituberculata) or clade C (Acropora sp., Pavona cactus) zooxanthellae. The photosynthetic efficiency (F(v)/F(m)) of the corals was first assessed during a short-term increase in temperature (from 27 degrees C to 29 degrees C, 32 degrees C, and 34 degrees C) and acute exposure to UV radiation (20.5 W m(-2) UVA and 1.2 W m(-2) UVB) alone or in combination. Increasing temperature to 34 degrees C significantly decreased the F(v)/F(m) in S. pistillata and M. aequituberculata. Increased UV radiation alone significantly decreased the F(v)/F(m) of all coral species, even at 27 degrees C. There was a combined effect of temperature and UV radiation, which reduced F(v)/F(m) in all corals by 25% to 40%. During a long-term exposure to UV radiation (17 days) the F(v)/F(m) was significantly reduced after 3 days' exposure in all species, which did not recover their initial values, even after 17 days. By this time, all corals had synthesized mycosporine-like amino acids (MAAs). The concentration and diversity of MAAs differed among species, being higher for corals containing clade A zooxanthellae. Prolonged exposure to UV radiation at the nonstressful temperature of 27 degrees C conferred protection against independent, thermally induced photoinhibition in all four species.     

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.     

The effects of feeding frequency and symbiosis with zooxanthellae on the biochemical composition of Astrangia Danae Milne Edwards & Haime 1849

The coral Astrangia danae Milne Edwards & Haime 1849 occurs naturally with and without symbiotic algae and thus may have two sources of nourishment: (1) particles captured by the coral polyps, and (2) photosynthetic products translocated from their zooxanthellae. Symbiotic colonies may have both sources, and nonsymbiotic ones certainly have only the former. The relative importance of these two food sources was studied in the laboratory by examining the tissues of corals fed with frozen brine shrimp. Stock corals were fed once per week. Two to three weeks prior to each experiment, selected corals were placed on one of three feeding schedules: starved (S), fed once per week (1/wk), and fed three times per week (3/wk). The coral tissues were analyzed for protein, lipid, carbohydrate, and zooxanthellae content. Increased feeding frequency (1/wk → 3/wk) resulted in an increased tissue biomass and lipid to protein (L/P) ratio; starvation (1/wk → S) caused a decrease in these parameters. Symbiosis with zooxanthellae had an effect similar to increased feeding frequency in that the S and 1/wk symbiotic corals had a higher L/P ratio than comparable nonsymbiotic ones. There were no significant differences in L/P ratios between the 3/wk symbiotic and nonsymbiotic corals. Freshly collected colonies had a tissue composition most similar to the laboratory animals fed 3/wk. This result is consistent with the hypothesis that ingestion of solid food is the major nutritional source for A. danae in Narragansett Bay, Rhode Island, but our experiments suggest that the algae can have an important effect on tissue L/P ratios during times of food scarcity.     

Application of chlorophyll fluorescence technique in the study of coral symbiotic zooxanthellae micro-ecology

Mutualistic relationship between coral polyps and their symbiotic zooxanthellae living within their tissues are the most essential features of a coral reef ecosystem. In this symbiotic system, the coral polyps provide a protected habitat, carbon dioxide and nutrients needed for photosynthesis to zooxanthellae; in turn, the symbiotic zooxanthellae provide food as products of photosynthesis to coral polyps. The Photosynthesis of zooxanthellae is therefore an important process of this symbiotic system as well as the development of the whole coral reef ecosystem. The recent application of chlorophyll fluorescence technique in the study of the zooxanthellae’s photosynthesis has greatly improved our understanding on the micro-ecology of corals and the symbiotic zooxanthellae. This paper summarizes the recent progress as the following aspects: (1) The ecological characteristics of the photosynthesis of symbiotic zooxanthellae, such as the diurnal and seasonal changes in the photochemical efficiency of the zooxanthellae, and the relationship between zooxanthellae photosynthesis and the world-wide coral bleaching. (2) The mechanism of corals acclimating to the changes of irradiance via spatial and temporal photoacclimations, including the corals’ photobiology; zooxanthella size, pigmentation, location and clade, and the relationship between light extremes and the corals’ metabolism and calcification. (3) The understanding of the response of zooxanthellae to various environmental stresses, such as long-term changes in the chlorophyll fluorescence of bleached and recovering corals; the tolerance of corals to thermal bleaching; the changes to photosystem II of symbiotic zooxanthellae after heat stress and bleaching. Due to the above findings, the chlorophyll fluorescence values of those coral species sensitive to environmental changes have been utilized as indicators of coral health as well as the status of coral reef ecosystems. In summary, the chlorophyll fluorescence technique has great potential in the understanding, monitoring, protecting and managing coral reefs.     

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.     

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.     

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.     

Experimental assessment of the feeding effort of three scleractinian coral species during a thermal stress: Effect on the rates of photosynthesis

This study aimed at investigating changes in feeding rates of three scleractinian coral species (Stylophora pistillata, Turbinaria reniformis and Galaxea fascicularis) between control (26 °C) and short-term stress conditions (31 °C), and to assess the effect of feeding on the photosynthetic efficiency of the corals. Feeding rates varied according to the feeding effort of the corals, itself depending on the environmental conditions. Indeed, S. pistillata significantly decreased its feeding rates at 31 °C, while rates of T. reniformis and G. fascicularis were increased between 26 and 31 °C. Independently of the feeding rates, food supply helped in preventing damage to the photosynthetic apparatus of the zooxanthellae. Indeed, starved corals from the three species showed significant decrease in both the electron transport rates and in the photosynthetic rates, following a loss in the amount of chlorophyll and experiencing photoinhibition of the photosystem II. However, no bleaching was observed in heated fed corals, with no decrease in their photosynthetic efficiency or performance.     

Endolithic algae: an alternative source of photoassimilates during coral bleaching

Recent reports of worldwide coral bleaching events leading to devastating coral mortality have caused alarm among scientists and resource managers. Differential survival of coral species through bleaching events has been widely documented. We suggest that among the possible factors contributing to survival of coral species during such events are endolithic algae harboured in their skeleton, providing an alternative source of energy. We studied the dynamics of photosynthetic pigment concentrations and biomass of endoliths in the skeleton of the encrusting coral Oculina patagonica throughout a bleaching event. During repeated summer bleaching events these endolithic algae receive increased photosynthetically active radiation, increase markedly in biomass, and produce increasing amounts of photoassimilates, which are translocated to the coral. Chlorophyll concentrations and biomass of endoliths were 4.6 +/- 1.57 and 1570 +/- 427 microg cm(-2) respectively, in skeletons of relatively healthy colonies (0-40% bleaching) but up to 14.8 +/- 2.5 and 4036 +/- 764 microg cm(-2) endolith chlorophyll and biomass respectively, in skeletons of bleached colonies (greater than 40% bleaching). The translocation dynamics of (14)C-labelled photoassimilates from the endoliths to bleached coral tissue showed significantly higher 14C activity of the endoliths harboured within the skeletons of bleached corals than that of the endoliths in non-bleached corals. This alternative source of energy may be vital for the survivorship of O. patagonica, allowing gradual recruitment of zooxanthellae and subsequent recovery during the following winter.     

Coral photobiology: new light on old views

The relationship between reef-building corals and light-harvesting pigments of zooxanthellae (Symbiodinium sp.) has been acknowledged for decades. The photosynthetic activity of the algal endocellular symbionts may provide up to 90% of the energy needed for the coral holobiont. This relationship limits the bathymetric distribution of coral reefs to the upper 100 m of tropical shorelines. However, even corals growing under high light intensities have to supplement the photosynthates translocated from the algae by predation on nutrient-rich zooplankton. New information has revealed how the fate of carbon acquired through photosynthesis differs from that secured by predation, whose rates are controlled by light-induced tentacular extension. The Goreau paradigm of “light-enhanced calcification” is being reevaluated, based on evidence that blue light stimulates coral calcification independently from photosynthesis rates. Furthermore, under dim light, calcification rates were stoichiometrically uncoupled from photosynthesis. The rates of photosynthesis of the zooxanthellae exhibit a clear endogenous rhythmicity maintained by light patterns. This daily pattern is concomitant with a periodicity of all the antioxidant protective mechanisms that wax and wane to meet the concomitant fluctuation in oxygen evolution. The phases of the moon are involved in the triggering of coral reproduction and control the spectacular annual mass-spawning events taking place in several reefs. The intensity and directionality of the underwater light field affect the architecture of coral colonies, leading to an optimization of the exposure of the zooxanthellae to light. We present a summary of major gaps in our understanding of the relationship between light and corals as a roadmap for future research.     

Acclimation of the Hermatypic Coral Stylophora pistillata to Bright Light

Photoacclimation dynamics to bright light was studied in the symbiotic reef-building coral Stylophora pistillata. Coral colonies were collected from shallow shaded sites (2 m, 40–20% PAR_s) from a fringing reef at Sesoko Island, Okinawa, Japan. Outer branches were broken off from the colonies and placed in an outdoor aquarium until the start of the experiments. After maintenance of the branches in an aquarium under a light intensity of 30% PAR_s for 30 days (experiment 1) or for 90 days (experiment 2), the samples were exposed to 95% PAR_s for 120 days in the same aquarium. The population density of zooxanthellae, chlorophyll concentration, locations of zooxanthellae, proliferating zooxanthellae frequency (PZF), and degrading zooxanthellae frequency (DZF) were examined. It was shown that after acclimation of coral branches to bright light, the population density of zooxanthellae, chlorophyll concentration calculated per 1000 polyps, and chlorophyll concentration in zooxanthellae decreased. The size of zooxanthellae significantly decreased. A decrease in the population density of zooxanthellae was detected by the eighth day of acclimation, and stabilization in the density of the symbionts occurred in the period from the 40th to the 60th day of the experiment. The chlorophyll concentration in zooxanthellae significantly decreased by the second day of exposure to bright light and stabilized by the fourth day. The PZF level sharply dropped on the second day, while the DZF level sharply increased and was higher than the PZF level for 40 days of exposure to bright light. We conclude, therefore, that the population density of zooxanthellae is regulated by the rates of two processes: cell division and the cell degradation.     

Bio-optical modeling of photosynthetic pigments in corals

The spectral reflectance of coral is inherently related to the amounts of photosynthetic pigments present in the zooxanthellae. There are no studies, however, showing that the suite of major photosynthetic pigments can be predicted from optical reflectance spectra. In this study, we measured cm-scale in vivo and in situ spectral reflectance for several colonies of the massive corals Porites lobata and Porites lutea, two colonies of the branching coral Porites compressa, and one colony of the encrusting coral Montipora flabellata in Kaneohe Bay, Oahu, Hawaii. For each reflectance spectrum, we collected a tissue sample and utilized high-performance liquid chromatography to quantify six major photosynthetic pigments, located in the zooxanthellae. We used multivariate multiple regression analysis with cross-validation to build and test an empirical linear model for predicting pigment concentrations from optical reflectance spectra. The model accurately predicted concentrations of chlorophyll a, chlorophyll c ₂, peridinin, diadinoxanthin, diatoxanthin and β-carotene, with correlation coefficients of 0.997, 0.941, 0.995, 0.996, 0.980 and 0.984, respectively. The relationship between predicted and actual concentrations was 1:1 for each pigment, except chlorophyll c ₂. This simple empirical model demonstrates the potential for routine, rapid, non-invasive monitoring of coral-zooxanthellae status, and ultimately for remote sensing of reef biogeochemical processes.     

Heterotrophy in Tropical Scleractinian Corals

The dual character of corals, that they are both auto- and heterotrophs, was recognized early in the twentieth Century. It is generally accepted that the symbiotic association between corals and their endosymbiotic algae (called zooxanthellae) is fundamental to the development of coral reefs in oligotrophic tropical oceans because zooxanthellae transfer the major part of their photosynthates to the coral host (autotrophic nutrition). However, numerous studies have confirmed that many species of corals are also active heterotrophs, ingesting organisms ranging from bacteria to mesozooplankton. Heterotrophy accounts for between 0 and 66% of the fixed carbon incorporated into coral skeletons and can meet from 15 to 35% of daily metabolic requirements in healthy corals and up to 100% in bleached corals. Apart from this carbon input, feeding is likely to be important to most scleractinian corals, since nitrogen, phosphorus, and other nutrients that cannot be supplied from photosynthesis by the coral's symbiotic algae must come from zooplankton capture, particulate matter or dissolved compounds. A recent study showed that during bleaching events some coral species, by increasing their feeding rates, are able to maintain and restore energy reserves.
This review assesses the importance and effects of heterotrophy in tropical scleractinian corals. We first provide background information on the different food sources (from dissolved organic matter to meso- and macrozooplankton). We then consider the nutritional inputs of feeding. Finally, we review feeding effects on the different physiological parameters of corals (tissue composition, photosynthesis and skeletal growth).     

Relationship of Vibrio species infection and elevated temperatures to yellow blotch/band disease in Caribbean corals

The bacterial and temperature factors leading to yellow blotch/band disease (YBD), which affects the major reef-building Caribbean corals Montastrea spp., have been investigated. Groups of bacteria isolated from affected corals and inoculated onto healthy corals caused disease signs similar to those of YBD. The 16S rRNA genes from these bacteria were sequenced and found to correspond to four Vibrio spp. Elevating the water temperature notably increased the rate of spread of YBD on inoculated corals and induced greater coral mortality. YBD-infected corals held at elevated water temperatures had 50% lower zooxanthella densities, 80% lower division rates, and a 75% decrease in chlorophyll a and c2 pigments compared with controls. Histological sections indicated that the algal pyrenoid was fragmented into separate segments, along with a reconfiguration and swelling of the zooxanthellae, as well as vacuolization. YBD does not appear to produce the same physiological response formerly observed in corals undergoing temperature-related bleaching. Evidence indicates that YBD affects primarily the symbiotic algae rather than coral tissue.     

Identity and diversity of coral endosymbionts (zooxanthellae) from three Palauan reefs with contrasting bleaching, temperature and shading histories

The potential of corals to associate with more temperature-tolerant strains of algae (zooxanthellae, Symbiodinium) can have important implications for the future of coral reefs in an era of global climate change. In this study, the genetic identity and diversity of zooxanthellae was investigated at three reefs with contrasting histories of bleaching mortality, water temperature and shading, in the Republic of Palau (Micronesia). Single-stranded conformation polymorphism and sequence analysis of the ribosomal DNA internal transcribed spacer (ITS)1 region was used for genotyping. A chronically warm but partly shaded coral reef in a marine lake that is hydrographically well connected to the surrounding waters harboured only two single-stranded conformation polymorphism profiles (i.e. zooxanthella communities). It consisted only of Symbiodinium D in all 13 nonporitid species and two Porites species investigated, with the remaining five Porites harbouring C*. Despite the high temperature in this lake (> 0.5 degrees above ambient), this reef did not suffer coral mortality during the (1998) bleaching event, however, no bleaching-sensitive coral families and genera occur in the coral community. This setting contrasts strongly with two other reefs with generally lower temperatures, in which 10 and 12 zooxanthella communities with moderate to low proportions of clade D zooxanthellae were found. The data indicate that whole coral assemblages, when growing in elevated seawater temperatures and at reduced irradiance, can be composed of colonies associated with the more thermo-tolerant clade D zooxanthellae. Future increases in seawater temperature might, therefore, result in an increasing prevalence of Symbiodinium phylotype D in scleractinian corals, possibly associated with a loss of diversity in both zooxanthellae and corals.