A coral polyp model of photosynthesis, respiration and calcification incorporating a transcellular ion transport mechanism

A numerical simulation model of coral polyp photosynthesis, respiration and calcification was developed. The model is constructed with three components (ambient seawater, coelenteron and calcifying fluid), and incorporates photosynthesis, respiration and calcification processes with transcellular ion transport by Ca-ATPase activity and passive transmembrane CO₂ transport and diffusion. The model calculates dissolved inorganic carbon and total alkalinity in the ambient seawater, coelenteron and calcifying fluid, dissolved oxygen (DO) in the seawater and coelenteron and stored organic carbon (CH₂O). To reconstruct the drastic variation between light and dark respiration, respiration rate dependency on DO in the coelenteron is incorporated. The calcification rate depends on the aragonite saturation state in the calcifying fluid (Ωa _cal). Our simulation result was a good approximation of “light-enhanced calcification.” In our model, the mechanism is expressed as follows: (1) DO in the coelenteron is increased by photosynthesis, (2) respiration is stimulated by increased DO in the light (or respiration is limited by DO depletion in the dark), then (3) calcification increases due to Ca-ATPase, which is driven by the energy generated by respiration. The model simulation results were effective in reproducing the basic responses of the internal CO₂ system and DO. The daily calcification rate, the gross photosynthetic rate and the respiration rate under a high-flow condition increased compared to those under the zero-flow condition, but the net photosynthetic rate decreased. The calculated calcification rate responses to variations in the ambient aragonite saturation state (Ωa _amb) were nonlinear, and the responses agreed with experimental results of previous studies. Our model predicted that in response to ocean acidification (1) coral calcification will decrease, but will remain at a higher value until Ωa _amb decreases to 1, by maintaining a higher Ωa _cal due to the transcellular ion transport mechanism and (2) the net photosynthetic rate will increase.     

Patterns of distribution of calcite crystals in soft corals sclerites

The gross morphology of soft coral surface sclerites has been studied for taxonomic purposes for over a century. In contrast, sclerites located deep in the core of colonies have not received attention. Some soft coral groups develop massive colonies, in these organisms tissue depth can limit light penetration and circulation of internal fluids affecting the physiology of coral tissues and their symbiotic algae; such conditions have the potential to create contrasting calcifying conditions. To test this idea, we analyzed the crystal structure of sclerites extracted from different colony regions in selected specimens of zooxanthellate and azooxanthellate soft corals with different colony morphologies, these were: Sarcophyton mililatensis, Sinularia capillosa, Sinularia flexibilis, Dendronephthya sp. and Ceeceenus levis. We found that the crystals that constitute polyp sclerites differ from those forming stalk sclerites. We also observed different crystals in sclerites located at various depths in the stalk including signs of sclerite breakdown in the stalk core region. These results indicate different modes of calcification within each colonial organism analyzed and illustrate the complexity of organisms usually regarded as repetitive morphological and functional units. Our study indicates that soft corals are ideal material to study natural gradients of calcification conditions. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

A simple temperature-based model predicts the upper latitudinal limit of the temperate coral Astrangia poculata

A few hardy ahermatypic scleractinian corals occur in shallow waters well outside of the tropics, but little is known concerning their distribution limits at high latitudes. Using field data on the growth of Astrangia poculata over an annual period near its northern range limit in Rhode Island, USA, we tested the hypothesis that the distribution of this coral is limited by low temperature. A simple model based on satellite sea surface temperature and field growth data at monthly temporal resolution was used to estimate annual net coral growth north and south of the known range limit of A. poculata. Annual net coral growth was the result of new polyp budding above ~10 °C minus polyp loss below ~10 °C, which is caused by a state of torpor that leads to overgrowth by encroaching and settling organisms. The model accurately predicted A. poculata’s range limit around Cape Cod, Massachusetts, predicting no net growth northward as a result of corals’ inability to counteract polyp loss during winter with sufficient polyp budding during summer. The model also indicated that the range limit of A. poculata coincides with a decline in the benefit of associating with symbiotic dinoflagellates (Symbiodinium B2/S. psygmophilum), suggesting that symbiosis may become a liability under colder temperatures. While we cannot exclude the potential role of other coral life history traits or environmental factors in setting A. poculata’s northern range limit, our analysis suggests that low temperature constrains the growth and persistence of adult corals and would preclude coral growth northward of Cape Cod.     

A corrosive concoction: The combined effects of ocean warming and acidification on the early growth of a stony coral are multiplicative

Survival of coral planulae, and the successful settlement and healthy growth of primary polyps are critical for the dispersal of scleractinian corals and hence the recovery of degraded coral reefs. It is therefore important to explore how the warmer and more acidic oceanic conditions predicted for the future could affect these processes. This study used controlled culture to investigate the effects of a 1 °C increase in temperature and a 0.2-0.25 unit decrease in pH on the settlement and survival of planulae and the growth of primary polyps in the Tropical Eastern Pacific coral Porites panamensis. We found that primary polyp growth was reduced only marginally by more acidic seawater but the combined effect of high temperature and lowered pH caused a significant reduction in growth of primary polyps by almost a third. Elevated temperature was found to significantly reduce the amount of zooxanthellae in primary polyps, and when combined with lowered pH resulted in a significant reduction in biomass of primary polyps. However, survival and settlement of planula larvae were unaffected by increased temperature, lowered acidity or the combination of both. These results indicate that in future scenarios of increased temperature and oceanic acidity coral planulae will be able to disperse and settle successfully but primary polyp growth may be hampered. The recovery of reefs may therefore be impeded by global change even if local stressors are curbed and sufficient sources of planulae are available.     

Outcomes of Infections of Sea Anemone Aiptasia pallida with Vibrio spp. Pathogenic to Corals

Incidents of coral disease are on the rise. However, in the absence of a surrogate animal host, understanding of the interactions between coral pathogens and their hosts remains relatively limited, compared to other pathosystems of similar global importance. A tropical sea anemone, Aiptasia pallida, has been investigated as a surrogate model to study certain aspects of coral biology. Therefore, to test whether the utility of this surrogate model can be extended to study coral diseases, in the present study, we tested its susceptibility to common coral pathogens (Vibrio coralliilyticus and Vibrio shiloi) as well as polymicrobial consortia recovered from the Caribbean Yellow Band Disease (CYBD) lesions. A. pallida was susceptible to each of the tested pathogens. A. pallida responded to the pathogens with darkening of the tissues (associated with an increased melanization) and retraction of tentacles, followed by complete disintegration of polyp tissues. Loss of zooxanthellae was not observed; however, the disease progression pattern is consistent with the behavior of necrotizing pathogens. Virulence of some coral pathogens in Aiptasia was paralleled with their glycosidase activities.     

Habitat Selectivity and Reliance on Live Corals for Indo-Pacific Hawkfishes (Family: Cirrhitidae)

Hawkfishes (family: Cirrhitidae) are small conspicuous reef predators that commonly perch on, or shelter within, the branches of coral colonies. This study examined habitat associations of hawkfishes, and explicitly tested whether hawkfishes associate with specific types of live coral. Live coral use and habitat selectivity of hawkfishes was explored at six locations from Chagos in the central Indian Ocean extending east to Fiji in the Pacific Ocean. A total of 529 hawkfishes from seven species were recorded across all locations with 63% of individuals observed perching on, or sheltering within, live coral colonies. Five species (all except Cirrhitus pinnulatus and Cirrhitichthys oxycephalus) associated with live coral habitats. Cirrhitichthys falco selected for species of Pocillopora while Paracirrhites arcatus and P. forsteri selected for both Pocillopora and Acropora, revealing that these habitats are used disproportionately more than expected based on the local cover of these coral genera. Habitat selection was consistent across geographic locations, and species of Pocillopora were the most frequently used and most consistently selected even though this coral genus never comprised more than 6% of the total coral cover at any of the locations. Across locations, Paracirrhites arcatus and P. forsteri were the most abundant species and variation in their abundance corresponded with local patterns of live coral cover and abundance of Pocilloporid corals, respectively. These findings demonstrate the link between small predatory fishes and live coral habitats adding to the growing body of literature highlighting that live corals (especially erect branching corals) are critically important for sustaining high abundance and diversity of fishes on coral reefs.     

Spatial and seasonal reef calcification in corals and calcareous crusts in the central Red Sea

The existence of coral reef ecosystems critically relies on the reef carbonate framework produced by scleractinian corals and calcareous crusts (i.e., crustose coralline algae). While the Red Sea harbors one of the longest connected reef systems in the world, detailed calcification data are only available from the northernmost part. To fill this knowledge gap, we measured in situ calcification rates of primary and secondary reef builders in the central Red Sea. We collected data on the major habitat-forming coral genera Porites, Acropora, and Pocillopora and also on calcareous crusts (CC) in a spatio-seasonal framework. The scope of the study comprised sheltered and exposed sites of three reefs along a cross-shelf gradient and over four seasons of the year. Calcification of all coral genera was consistent across the shelf and highest in spring. In addition, Pocillopora showed increased calcification at exposed reef sites. In contrast, CC calcification increased from nearshore, sheltered to offshore, exposed reef sites, but also varied over seasons. Comparing our data to other reef locations, calcification in the Red Sea was in the range of data collected from reefs in the Caribbean and Indo-Pacific; however, Acropora calcification estimates were at the lower end of worldwide rates. Our study shows that the increasing coral cover from nearshore to offshore environments aligned with CC calcification but not coral calcification, highlighting the potentially important role of CC in structuring reef cover and habitats. While coral calcification maxima have been typically observed during summer in many reef locations worldwide, calcification maxima during spring in the central Red Sea indicate that summer temperatures exceed the optima of reef calcifiers in this region. This study provides a foundation for comparative efforts and sets a baseline to quantify impact of future environmental change in the central Red Sea.

     

Scleractinian coral cell proliferation is reduced in primary culture of suspended multicellular aggregates compared to polyps

Cell cultures from reef-building scleractinian corals are being developed to study the response of these ecologically important organisms to environmental stress and diseases. Despite the importance of cell division to support propagation, cell proliferation in polyps and in vitro is under-investigated. In this study, suspended multicellular aggregates (tissue balls) were obtained after collagenase dissociation of Pocillopora damicornis coral, with varying yields between enzyme types and brands. Ultrastructure and cell type distribution were characterized in the tissue balls (TBs) compared to the polyp. Morphological evidence of cellular metabolic activity in their ciliated cortex and autophagy in their central mass suggests involvement of active tissue reorganization processes. DNA synthesis was evaluated in the forming multicellular aggregates and in the four cell layers of the polyp, using BrdU labeling of nuclei over a 24 h period. The distribution of BrdU-labeled coral cells was spatially heterogeneous and their proportion was very low in tissue balls (0.2 ± 0.1 %), indicating that suspended multicellular aggregate formation does not involve significant cell division. In polyps, DNA synthesis was significantly lower in the calicoderm (<1 %) compared to both oral and aboral gastroderm (about 10 %) and to the pseudostratified oral epithelium (15–25 % at tip of tentacle). DNA synthesis in the endosymbiotic dinoflagellates dropped in the forming tissue balls (2.7 ± 1.2 %) compared to the polyp (14 ± 3.4 %) where it was not different from the host gastroderm (10.3 ± 1.2 %). A transient (24 h) increase was observed in the cell-specific density of dinoflagellates in individually dissociated coral cell cultures. These results suggest disruption of coral cell proliferation processes upon establishment in primary culture.     

The calcifying interface in a stony coral primary polyp: An interplay between seawater and an extracellular calcifying space

Stony coral exoskeletons build the foundation for the most biologically diverse marine ecosystems on Earth, coral reefs, which face major threats due to many anthropogenic–related stressors. Therefore, understanding coral biomineralization mechanisms is crucial for coral reef management in the coming decades and for using coral skeletons in geochemical studies. This study combines in–vivo imaging with cryo-electron microscopy and cryo–elemental mapping to gain novel insights into the biological microenvironment and the ion pathways that facilitate biomineralization in primary polyps of the stony coral Stylophora pistillata. We document increased tissue permeability in the primary polyp and a highly dispersed cell packing in the tissue directly responsible for producing the coral skeleton. This tissue arrangement may facilitate the intimate involvement of seawater at the mineralization site, also documented here. We further observe an extensive filopodial network containing carbon-rich vesicles extruding from some of the calicoblastic cells. Single-cell RNA-Sequencing data interrogation supports these morphological observations by showing higher expression of genes involved in filopodia and vesicle structure and function in the calicoblastic cells. These observations provide a new conceptual framework for resolving the ion pathway from the external seawater to the tissue-mineral interface in stony coral biomineralization processes.     

The Symbiosis between Lophelia pertusa and Eunice norvegica Stimulates Coral Calcification and Worm Assimilation

We investigated the interactions between the cold-water coral Lophelia pertusa and its associated polychaete Eunice norvegica by quantifying carbon (C) and nitrogen (N) budgets of tissue assimilation, food partitioning, calcification and respiration using ¹³C and ¹⁵N enriched algae and zooplankton as food sources. During incubations both species were kept either together or in separate chambers to study the net outcome of their interaction on the above mentioned processes. The stable isotope approach also allowed us to follow metabolically derived tracer C further into the coral skeleton and therefore estimate the effect of the interaction on coral calcification. Results showed that food assimilation by the coral was not significantly elevated in presence of E. norvegica but food assimilation by the polychaete was up to 2 to 4 times higher in the presence of the coral. The corals kept assimilation constant by increasing the consumption of smaller algae particles less favored by the polychaete while the assimilation of Artemia was unaffected by the interaction. Total respiration of tracer C did not differ among incubations, although E. norvegica enhanced coral calcification up to 4 times. These results together with the reported high abundance of E. norvegica in cold-water coral reefs, indicate that the interactions between L. pertusa and E. norvegica can be of high importance for ecosystem functioning.     

On some features of embryonic development and metamorphosis of Aurelia aurita (Cnidaria, Scyphozoa)

Aurelia aurita is a cosmopolite species of scyphomedusae. Its anatomy and life cycle are well investigated. This work provides a detailed study on development and structure of A. aurita planula before and during its metamorphosis. Intravital observations and histology study during the settlement and metamorphosis of the planulae demonstrated that the inner manubrium lining of primary polyp (gastroderm) develops from the ectoderm of the planula posterior end. The spatial and temporal dynamics of serotonergic cells from the early embryonic stages until the formation of the primary polyp were studied for the first time. In addition, the distribution of tyrosinated tubulin and neuropeptide RF-amide at different stages of A. aurita development was traced.     

Spatial distribution of calcification and photosynthesis in the scleractinian coral Galaxea fascicularis

The spatial heterogeneity of photosynthesis and calcification of single polyps of the coral Galaxea fascicularis was investigated. Photosynthesis was investigated with oxygen microsensors. The highest rates of gross photosynthesis (Pg) were found on the tissue covering the septa, the tentacles, and the tissues surrounding the mouth opening of the polyp. Lower rates were found on the tissues of the wall and the coenosarc. Calcification was investigated by radioactive tracers. The incorporation pattern of ⁴⁵Ca and ¹⁴C in the corallites was imaged with use of a Micro-Imager. The -images obtained showed that the incorporation of the radioactive tracers coincided with the Pg distribution pattern with the highest incorporation rates found in the corallite septa. Thus, the high growth rate of the septa is supported by the high rates of Pg by the symbiont in the adjacent tissues. The total incorporation rates were higher in light than in dark, however, the distribution pattern of the radioisotope incorporation was not affected by illumination. This further emphasizes the close relation between calcification and photosynthesis.     

Light microenvironment and single-cell gradients of carbon fixation in tissues of symbiont-bearing corals

Recent coral optics studies have revealed the presence of steep light gradients and optical microniches in tissues of symbiont-bearing corals. Yet, it is unknown whether such resource stratification allows for physiological differences of Symbiodinium within coral tissues. Using a combination of stable isotope labelling and nanoscale secondary ion mass spectrometry, we investigated in hospite carbon fixation of individual Symbiodinium as a function of the local O₂ and light microenvironment within the coral host determined with microsensors. We found that net carbon fixation rates of individual Symbiodinium cells differed on average about sixfold between upper and lower tissue layers of single coral polyps, whereas the light and O₂ microenvironments differed ~15- and 2.5-fold, respectively, indicating differences in light utilisation efficiency along the light microgradient within the coral tissue. Our study suggests that the structure of coral tissues might be conceptually similar to photosynthetic biofilms, where steep physico-chemical gradients define form and function of the local microbial community.The quantity and quality of solar radiation are arguably the most important environmental resources that affect the structure and function of photosynthetic communities in both terrestrial and aquatic environments. Sunlight is of key importance for symbiont-bearing corals, driving the symbiotic interaction between the coral animal and its photosynthetic microalgae of the genus Symbiodinium (Roth, 2014). Light attenuation through the water mass and over the reef matrix has a fundamental role in structuring morphology, function and distribution of corals and their symbiotic algae with depth (Falkowski et al., 1990). Recent studies on the optical properties of corals have shown that light is also a highly stratified resource at the level of individual coral polyps and tissue layers (Wangpraseurt et al., 2014). Steep light gradients exist within the polyp tissues of some corals and light can attenuate by more than an order of magnitude within tissues, that is, comparable to the attenuation that can occur in open oceanic waters between the surface and >25 m of water depth (Kirk, 1994; Wangpraseurt et al., 2012). In this study, we investigated whether such light gradients within coral tissues are correlated with a stratification of Symbiodinium physiology in hospite.We used fibre-optic and electrochemical microsensors together with stable isotopic labelling and nanoscale secondary ion mass spectrometry (NanoSIMS) to estimate single-cell carbon fixation rates across light gradients within coral tissues. We collected several fragments of Favites sp. from the Heron Island reef flat (152°69'' E, 20°299'' S), Great Barrier Reef, Australia. Fragments were cultured under a downwelling photon irradiance (400–700 nm) of ~100 μmol photons per m² per s (12/12 h cycle), in aerated seawater (25 °C, salinity 33). Photosynthesis-irradiance curves for the investigated corals were determined with an imaging pulse amplitude modulated fluorometer (I-PAM, Walz GmbH, Effeltrich, Germany; Ralph et al., 2005). Values for saturating irradiance, E_max, and irradiance at onset of saturation, E_k, were ~350 μmol photons per m² per s and ~160 μmol photons per m² per s, respectively (data not shown). These values are typical for healthy corals kept under moderate irradiance (Ralph et al., 2005). To ensure incubations at irradiance levels where photosynthesis and irradiance correlated linearly, that is, on the linearly increasing part of the P vs I curve, all experiments were performed at ~80 μmol photons per m² per s (12/12 h cycle). Microsensor measurements of scalar irradiance (tip size ~60 μm; Lassen et al., 1992) and O₂ concentration (OX-50, tip size 50 μm, Unisense A/S, Aarhus, Denmark) were performed within the polyp and coenosarc tissues of corals as described previously (Figures 1a and b; Wangpraseurt et al., 2012). After microsensor measurements, corals were incubated with ¹³C-bicarbonate (Supplementary Text S1). NanoSIMS imaging was then applied on coral tissue sections, as described by Pernice et al. (2014) to quantify the assimilation of dissolved inorganic carbon into individual Symbiodinium cells across polyp (oral and aboral) and coenosarc tissues of corals. Briefly, corals were incubated in small aquaria with 2 mm NaH¹³CO₃ in artificial sea water (recipe adapted from Harrison et al., 1980). After 24 h of isotopic incubation, coral fragments were sampled, chemically fixed and processed for NanoSIMS analyses (see Kopp et al., 2013; Pernice et al., 2012, 2014; and Supplementary Text S1,Supplementary Figure S1).Open in a separate windowFigure 1Internal microenvironment and single-cell ¹³C assimilation by Symbiodinium cells within Favites sp. (a) Representative measurement locations indicating connecting tissue (c, coenosarc; white circle) and polyp tissue (p; red circle). Scale bar is 0.5 cm. (b) Schematic diagram of the vertical arrangement of the polyp tissue structure (not drawn to scale). The coral tissue consists of oral and aboral gastrodermal tissues that contain photosymbiont cells (~10 μm in diameter). The two tissue layers are separated by a flexible gastrodermal cavity and the entire mean polyp tissue thickness was 1150 μm (±385 s.d., n=8) as determined by microsensor profiles. The NanoSIMS images (c–e) show the ¹³C/¹²C isotopic ratio for Symbiodinium cells in coenosarc tissue (c), the upper oral polyp tissue (d) and in the lowest layer of aboral polyp tissue (e). Scale bars are 10 μm. The colour scale of the NanoSIMS images is in hue saturation intensity ranging from 220 in blue (which corresponds to natural ¹³C/¹²C isotopic ratio of 0.0110) to 1000 in red (which corresponds to ¹³C/¹²C isotopic ratio of 0.05, ~4.5 times above the natural ¹³C/¹²C isotopic ratio). Quantification of ¹³C enrichment of individual Symbiodinium cells was obtained by selecting regions of interest that were defined in Open_MIMS (http://nrims.harvard.edu/software/openmims) by drawing the contours of the Symbiodinium cells directly on the NanoSIMS images. (f) Mean enrichment measured in Symbiodinium cells by NanoSIMS, in coenosarc tissue (in white, n=33), in upper oral polyp tissue (in grey, n=25), in the lowest layer of polyp tissue (in turquoise, n=17) and in the control treatment (n=20). Bars in the histograms indicate the s.e.m. enrichment quantified for the different whole Symbiodinium cells for each tissue category. Microsensor measurements of (g) scalar irradiance and (h) O₂ performed along depth gradients within the polyp tissue (mean±s.d., n=4). Measurements were averaged for the first 100 μm from the tissue surface (oral) and the last 100 μm from the skeleton (aboral). The oral and aboral depth was defined through gentle touching of the microsensor tip at the surface of the coral tissue and skeleton, respectively.Our combined approach of using NanoSIMS and microsensors within the tissue of corals provides, to the best of our knowledge, the first evidence for physiological differences of individual Symbiodinium cells in hospite in relation to the local microenvironmental conditions across different coral tissue layers, that is, oral vs aboral parts of polyp and coenosarc. Quantitative analysis based on tissue sections from different coral tissue layers showed that mean incorporation of ¹³C-bicarbonate by individual Symbiodinium cells was up to 6.5-fold higher in the upper oral polyp and coenosarc tissues compared with the lowermost layer of polyp tissues (δ¹³C: 1609±147‰, n=25 for Symbiodinium cells in upper oral polyp tissue; 1696±205‰, n=33 for Symbiodinium cells in coenosarc tissue and 246±82‰, n=17 for Symbiodinium cells in the lowest aboral layer of polyp tissue). Although the sample sizes in this study are small and the ¹³C signal is heterogeneous within individual Symbiodinium cells (because of carbon fixation hotspots in specific compartments; Supplementary Figure S2; Kopp et al., 2015), the magnitude of the difference in mean ¹³C incorporation between the aboral part of the polyp and the two other parts of coral tissue was clear and statistically significant (one-way analysis of variance (ANOVA) F_2,75=15.91; P<0.0001; 6.5-fold increase in polyp oral vs aboral polyp tissue, Fischer''s least significant difference (LSD) P<0.0001; 6.9-fold increase in coenosarc vs aboral polyp tissue Fischer''s LSD P<0.0001; and no significant difference between oral polyp vs coenosarc tissue, Fischer''s LSD P=0.718; Figure 1c–f; Supplementary Table S1). The internal microenvironment within the corresponding polyp tissues was highly stratified with respect to light and O₂ (Figures 1g and h). Scalar irradiance decreased about 15-fold from the surface to the bottom of the polyp tissues. Gradients of O₂ were less steep but still significant, with an approximate reduction in O₂ concentration by about 2.5 times (Figure 1; Supplementary Table S2; ANOVA F_1,6= 16.4; P=0.006).These results suggest that coral tissues are vertically stratified systems that affect the physiological activity of their symbionts along a fine-scale microenvironmental gradient. The presence and role of microscale heterogeneity has hitherto largely been ignored in the field of coral symbiosis research, while much is known for other photosynthetic tissues. For instance, for terrestrial plant leaves and for aquatic photosynthetic biofilms, it is known that the photosynthetic unit can adapt to microenvironmental light gradients, where chloroplasts/phototrophs harboured in low-light niches show increased photosynthetic quantum efficiencies at low light levels (Terashima and Hikosaka, 1995; Al-Najjar et al., 2012). Although the steady-state O₂ concentration values reported here are a function of the different metabolic processes of the coral holobiont (that is, Symbiodinium photosynthesis and the combined respiration by the coral host, Symbiodinium and microbes), the NanoSIMS approach allowed us to separate ¹³C fixation of Symbiodinium from the host metabolic activity. Our study provides the first experimental evidence from carbon fixation measurements that Symbiodinium cells can adapt to optical microniches in coral tissues. The 15-fold reduction in irradiance with depth in the coral tissue led only to an ~6.5-fold reduction in net carbon fixation suggesting enhanced light-harvesting efficiency or a reduced P/R ratio for Symbiodinium harboured in aboral tissues. Although such enhanced efficiency under low light often reflects the adaptation of the photosynthetic apparatus (for example, an increase in light-harvesting complexes (Walters, 2005) and reduced cell respiration (Givnish, 1988), it might additionally be the result of physiologically distinct populations or clades of Symbiodinium. Several studies have revealed remarkable genetic and physiological diversities among different Symbiodinium clades (Loram et al., 2007; Stat et al., 2008; Baker et al., 2013; Pernice et al., 2014). Although Favites sp. corals from Southern Great Barrier Reef are generally reported in association with one specific Symbiodinium type (clade C3; Tonk et al., 2013), Symbiodinium diversity within the microenvironment of these common corals could have been overlooked and such physiological diversity could further provide selective advantage to different genotypes in microenvironments within coral tissue. Coral tissues might thus exhibit similar characteristics to photosynthetic biofilms where steep physico-chemical microgradients give rise to different pheno- and ecotypes of phototrophs along those gradients (Musat et al., 2008; Ward et al., 1998).These first experiments were performed under sub-saturating irradiance of ~80 μmol photons per m² per s. Earlier studies showed that the local scalar irradiance in upper vs deeper tissue layers relates to the incident photon irradiance in a linear fashion such that at stressful incident irradiance levels of, for example, 2000 μmol photons per m² per s, light levels in the lowermost polyp tissue layers are ~200 μmol photons per m² per s (Wangpraseurt et al., 2012), still representing optimal conditions for photosynthesis. We thus consider it likely that excess irradiance triggering photoinhibition in oral tissues is unlikely to cause photoinhibition of Symbiodinium in aboral polyp tissues. The internal light field is species specific and in some thin-tissued, branching corals such as Pocillopora damicornis, intra-tissue light attenuation is not very pronounced (Wangpraseurt et al., 2012; Szabó et al., 2014). The ability to harbour Symbiodinium cells in low-light niches might be an important resilience factor for thick-tissued corals, such as massive faviids, during and after coral bleaching. Our study gives first insights to the functional diversity of Symbiodinium along microscale gradients in coral tissue and underscores the importance of considering such heterogeneity in studies linking symbiont diversity and coral physiology responses to environmental stress factors.     

Calcification rate and the effect of temperature in a zooxanthellate and an azooxanthellate scleractinian reef coral

Calcification rates, normalized to skeletal mass, in the zooxanthellate Galaxea fascicularis and the azooxanthellate Dendrophyllia sp. were similar over the whole temperature range of 18–29 °C. Calcification was measured by Ca⁴⁵ incorporation in corals that were naturally acclimated to the prevailing seawater temperature. In both species maximum calcification rate occurred at about 25 °C and calcification rates can be fitted to a Gaussian distribution with respect to temperature. The similarity in temperature dependence of the zooxanthellate and azooxanthellate coral suggests that temperature affects some fundamental process of calcification that is independent of light effects. It is shown that two different populations of Galaxea fascicularis have distinctly different ratios of tissue protein to skeletal mass per polyp. This indicates that tissue protein may not be suitable for normalizing calcification rates in individual coral polyps, both within and between species. Intra- and interspecific comparisons of calcification rates may be better made on the basis of skeletal mass when polyps are similar in size and shape.Communicated by Topic Editor C. Barnes     

Genetic Susceptibility,Colony Size,and Water Temperature Drive White-Pox Disease on the Coral Acropora palmata

Outbreaks of coral diseases are one of the greatest threats to reef corals in the Caribbean, yet the mechanisms that lead to coral diseases are still largely unknown. Here we examined the spatial-temporal dynamics of white-pox disease on Acropora palmata coral colonies of known genotypes. We took a Bayesian approach, using Integrated Nested Laplace Approximation algorithms, to examine which covariates influenced the presence of white-pox disease over seven years. We showed that colony size, genetic susceptibility of the coral host, and high-water temperatures were the primary tested variables that were positively associated with the presence of white-pox disease on A. palmata colonies. Our study also showed that neither distance from previously diseased individuals, nor colony location, influenced the dynamics of white-pox disease. These results suggest that white-pox disease was most likely a consequence of anomalously high water temperatures that selectively compromised the oldest colonies and the most susceptible coral genotypes.     

Multimodal Optical Microscopy Methods Reveal Polyp Tissue Morphology and Structure in Caribbean Reef Building Corals

An integrated suite of imaging techniques has been applied to determine the three-dimensional (3D) morphology and cellular structure of polyp tissues comprising the Caribbean reef building corals Montastraeaannularis and M. faveolata. These approaches include fluorescence microscopy (FM), serial block face imaging (SBFI), and two-photon confocal laser scanning microscopy (TPLSM). SBFI provides deep tissue imaging after physical sectioning; it details the tissue surface texture and 3D visualization to tissue depths of more than 2 mm. Complementary FM and TPLSM yield ultra-high resolution images of tissue cellular structure. Results have: (1) identified previously unreported lobate tissue morphologies on the outer wall of individual coral polyps and (2) created the first surface maps of the 3D distribution and tissue density of chromatophores and algae-like dinoflagellate zooxanthellae endosymbionts. Spectral absorption peaks of 500 nm and 675 nm, respectively, suggest that M. annularis and M. faveolata contain similar types of chlorophyll and chromatophores. However, M. annularis and M. faveolata exhibit significant differences in the tissue density and 3D distribution of these key cellular components. This study focusing on imaging methods indicates that SBFI is extremely useful for analysis of large mm-scale samples of decalcified coral tissues. Complimentary FM and TPLSM reveal subtle submillimeter scale changes in cellular distribution and density in nondecalcified coral tissue samples. The TPLSM technique affords: (1) minimally invasive sample preparation, (2) superior optical sectioning ability, and (3) minimal light absorption and scattering, while still permitting deep tissue imaging.     

The vermetid gastropod Dendropoma maximum reduces coral growth and survival

Coral reefs are one of the most diverse systems on the planet; yet, only a small fraction of coral reef species have attracted scientific study. Here, we document strong deleterious effects of an often overlooked species—the vermetid gastropod, Dendropoma maximum—on growth and survival of reef-building corals. Our surveys of vermetids on Moorea (French Polynesia) revealed a negative correlation between the density of vermetids and the per cent cover of live coral. Furthermore, the incidence of flattened coral growth forms was associated with the presence of vermetids. We transplanted and followed the fates of focal colonies of four species of corals on natural reefs where we also manipulated presence/absence of vermetids. Vermetids reduced skeletal growth of focal corals by up to 81 per cent and survival by up to 52 per cent. Susceptibility to vermetids varied among coral species, suggesting that vermetids could shift coral community composition. Our work highlights the potential importance of a poorly studied gastropod to coral dynamics.     

The Possible Role of Cyanobacterial Filaments in Coral Black Band Disease Pathology

Black band disease (BBD), characterized by a black mat or line that migrates across a coral colony leaving behind it a bare skeleton, is a persistent disease affecting massive corals worldwide. Previous microscopic and molecular examination of this disease in faviid corals from the Gulf of Eilat revealed a number of possible pathogens with the most prominent being a cyanobacterium identified as Pseudoscillatoria coralii. We examined diseased coral colonies using histopathological and molecular methods in order to further assess the possible role of this cyanobacterium, its mode of entry, and pathological effects on the coral host tissues. Affected areas of colonies with BBD were sampled for examination using both light and transmission electron microscopies. Results showed that this dominant cyanobacterium was found on the coral surface, at the coral–skeletal interface, and invading the polyp tissues and gastrovascular cavity. Although tissues surrounding the invasive cyanobacterial filaments did not show gross morphological alterations, microscopic examination revealed that the coral cells surrounding the lesion were dissociated, necrotic, and highly vacuolated. No amoebocytes were evident in the mesoglea of affected tissues suggesting a possible repression of the coral immune response. Morphological and molecular similarity of the previously isolated BBD-associated cyanobacterium P. coralii to the current samples strengthens the premise that this species is involved in the disease in this coral. These results indicate that the cyanobacteria may play a pivotal role in this disease and that the mode of entry may be via ingestion, penetrating the coral via the gastrodermis, as well as through the skeletal–tissue interface.