Innate immune responses of a scleractinian coral to vibriosis

Scleractinian corals are the most basal eumetazoan taxon and provide the biological and physical framework for coral reefs, which are among the most diverse of all ecosystems. Over the past three decades and coincident with climate change, these phototrophic symbiotic organisms have been subject to increasingly frequent and severe diseases, which are now geographically widespread and a major threat to these ecosystems. Although coral immunity has been the subject of increasing study, the available information remains fragmentary, especially with respect to coral antimicrobial responses. In this study, we characterized damicornin from Pocillopora damicornis, the first scleractinian antimicrobial peptide (AMP) to be reported. We found that its precursor has a segmented organization comprising a signal peptide, an acidic proregion, and the C-terminal AMP. The 40-residue AMP is cationic, C-terminally amidated, and characterized by the presence of six cysteine molecules joined by three intramolecular disulfide bridges. Its cysteine array is common to another AMP and toxins from cnidarians; this suggests a common ancestor, as has been proposed for AMPs and toxins from arthropods. Damicornin was active in vitro against Gram-positive bacteria and the fungus Fusarium oxysporum. Damicornin expression was studied using a combination of immunohistochemistry, reverse phase HPLC, and quantitative RT-PCR. Our data show that damicornin is constitutively transcribed in ectodermal granular cells, where it is stored, and further released in response to nonpathogenic immune challenge. Damicornin gene expression was repressed by the coral pathogen Vibrio coralliilyticus. This is the first evidence of AMP gene repression in a host-Vibrio interaction.     

Photochemical response of the scleractinian coral Stylophora pistillata to some sunscreen ingredients

Coral Reefs - Ultraviolet (UV) filters and preservatives, which are common constituents of sunscreens and other cosmetics, are reported as a threat for coastal coral reef ecosystems; however, few...     

Environmental variation in a fossil scleractinian coral

Sample populations of Solenastrea fairbanksi , collected from different environments in the Pliocene Imperial Formation, can be distinguished by measurements of linear dimensions of the coenosteum and thickness of septal structures. In these characters, magnitudes of intercolony variance components are lower than interpopulation or intracolony components, suggesting that the variation is largely caused by environmental factors. Three patterns of mean variation exist between populations: (1) Highest means occur in shallow, offshore (high energy) environments. (2) Highest means occur in shallow environments with low turbidity (high light intensity). (3) Highest means occur in clear, deeper offshore environments (favorable nutrient supply). The distance between adjacent corallites and the inverse of coenosteal porosity follow the first pattern, measures of thickness of vertical corallite structures follow the second, whereas estimates of annual growth rate follow the third. Corallite diameters and columella thicknesses do not vary between populations. Comparisons with modern species show that many characters in S. fairbanksi respond to environmental factors in a manner different from similar characters in other scleractinian species. In fact, some inflexible characters in S. fairbunksi are highly flexible in other species.     

Mono- and multispecies biofilms from a crustose coralline alga induce settlement in the scleractinian coral Leptastrea purpurea

Coral Reefs - Microorganisms have been reported to induce settlement in various marine invertebrate larvae but their specificity of inductive capacities for the settlement of coral larvae remains...     

Uranium in scleractinian coral skeletons

Accurate determinations have been made of the distribution of uranium in fresh and diagenetically altered coral skeletons occurring both naturally and grown under a variety of experimental conditions. Whereas live coral skeletons are homogeneous in uranium distribution, dead skeletons show heterogeneities relating to lithothamnioid algal encrustations and endolithic sponges. In the analyses of over 100 live coral skeletons, no zonal uranium distributions, described by previous workers, were found. In skeletons, free from organic material, uranium was found to exchange readily with the coral skeleton and/or to be precipitated along trabecular axes and skeletal margins. Bioeroded specimens contained higher uranium concentrations than freshly formed aragonite; they were similar to fossil coral skeletons used by previous researchers for uranium scrics dating.     

Changes in coral microbial communities in response to a natural pH gradient

Surface seawater pH is currently 0.1 units lower than pre-industrial values and is projected to decrease by up to 0.4 units by the end of the century. This acidification has the potential to cause significant perturbations to the physiology of ocean organisms, particularly those such as corals that build their skeletons/shells from calcium carbonate. Reduced ocean pH could also have an impact on the coral microbial community, and thus may affect coral physiology and health. Most of the studies to date have examined the impact of ocean acidification on corals and/or associated microbiota under controlled laboratory conditions. Here we report the first study that examines the changes in coral microbial communities in response to a natural pH gradient (mean pH_T 7.3–8.1) caused by volcanic CO₂ vents off Ischia, Gulf of Naples, Italy. Two Mediterranean coral species, Balanophyllia europaea and Cladocora caespitosa, were examined. The microbial community diversity and the physiological parameters of the endosymbiotic dinoflagellates (Symbiodinium spp.) were monitored. We found that pH did not have a significant impact on the composition of associated microbial communities in both coral species. In contrast to some earlier studies, we found that corals present at the lower pH sites exhibited only minor physiological changes and no microbial pathogens were detected. Together, these results provide new insights into the impact of ocean acidification on the coral holobiont.     

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.     

Whole-metagenome amplification of a microbial community associated with scleractinian coral by multiple displacement amplification using phi29 polymerase

Limitations in obtaining sufficient specimens and difficulties in extracting high quality DNA from environmental samples have impeded understanding of the structure of microbial communities. In this study, multiple displacement amplification (MDA) using phi29 polymerase was applied to overcome these hindrances. Optimization of the reaction conditions for amplification of the bacterial genome and evaluation of the MDA product were performed using cyanobacterium Synechocystis sp. strain PCC6803. An 8-h MDA reaction yielded a sufficient quantity of DNA from an initial amount of 0.4 ng, which is equivalent to approximately 10(5) cells. Uniform amplification of genes randomly selected from the cyanobacterial genome was confirmed by real-time polymerase chain reaction. The metagenome from bacteria associated with scleractinian corals was used for whole-genome amplification using phi29 polymerase to analyse the microbial diversity. Unidentified bacteria with less than 93% identity to the closest 16S rDNA sequences deposited in DNA Data Bank of Japan were predominantly detected from the coral-associated bacterial community before and after the MDA procedures. Sequencing analysis indicated that alpha-Proteobacteria was the dominant group in Pocillopora damicornis. This study demonstrates that MDA techniques are efficient for genome wide investigation to understand the actual microbial diversity in limited bacterial samples.     

A poecilostome copepod parasitic in a scleractinian coral in the moluccas

Wedanus inconstans, n. gen., n. sp., a lichomolgid copepod living in the polyps of the reef coral Goniopora tenuidens (Quelch), is described from Halmahera in the Moluccas. The genus is apparently related to Xenomolgus Humes & Stock, 1972.     

Crustose coralline algae and associated microbial biofilms deter seaweed settlement on coral reefs

Crustose coralline algae (CCA), a group of calcifying red algae found commonly in benthic marine ecosystems worldwide, perform essential ecological functions on coral reefs, including creating benthic substrate, stabilizing the reef structure and inducing coral settlement. An important feature of CCA is the ability to keep their surfaces free of epiphytic algae, thereby reducing algal overgrowth and allowing them access to light. However, the mechanisms by which CCA prevent settlement of opportunistic seaweeds (fleshy macroalgae) are not fully understood, nor is whether these mechanisms vary among CCA species. In our study based on the Great Barrier Reef, we demonstrate that three common CCA species (Titanoderma pustulatum, Porolithon onkodes and Neogoniolithon sp.) have a remarkable ability to deter settlement of seaweed spores. We provide experimental evidence that the CCA use allelopathy and microbial inhibition against the settlement of spores of the brown seaweed Padina boergesenii. Methanol extracts of allelopathic compounds from T. pustulatum, Po. onkodes and Neogoniolithon sp. significantly reduced the settlement of Pa. boergesenii spores by 4.3 times, 3.0 and 3.8 times, respectively. Further, we found that microbial biofilms, while having a lower inhibitory effect than allelopathic compounds, also reduced seaweed settlement of Pa. boergesenii. Our study demonstrates that allelopathy and microbial inhibition, in addition to epithallial tissue sloughing, are mechanisms employed by CCA to prevent the settlement of epiphytic algae. Understanding the mechanisms by which CCA avoid seaweed overgrowth contributes to our understanding of the dynamics of seaweed proliferations on reefs and to the ecological knowledge of this important group of reef-building organisms.     

Chemical diversity in the scleractinian coral Astroides calycularis

The detailed chemical inspection of the Mediterranean coral Astroides calycularis led to the isolation and structure characterization of two families of alkaloids. Derivatives of orthidine were for the first time isolated from this species. The second family of alkaloids includes the aplysinopsins among which a new derivative is described. The structure was identified on the basis of extensive NMR data interpretation. These results are of chemotaxonomic relevance in order to link this species to the Atlantic Tubastrea aurea.     

Bioaccumulation of zinc in the scleractinian coral Stylophora pistillata

The uptake kinetics of zinc (Zn), an essential nutrient for both photosynthesis and calcification, in the tissue of S. pistillata showed that the transport of Zn is composed of a linear component (diffusion) at high concentrations and an active carrier-mediated component at low concentrations. The carrier affinity (K _m=28 pmol l⁻¹) was very low, indicating a good adaptation of the corals to low levels of Zn in seawater. Zn accumulation in the skeleton was linear; its level was dependent on the length of the incubation as well as on the external concentration of dissolved Zn. There was also a light-stimulation of Zn uptake, suggesting that zooxanthellae, through photosynthesis, are involved in this process. An enrichment of the incubation medium with 10 nM Zn significantly increased the photosynthetic efficiency of S. pistillata. This result suggests that corals living in oligotrophic waters might be limited in essential metals, such as zinc.     

Response of a scleractinian coral, Stylophora pistillata, to iron and nitrate enrichment

The purpose of this study was to determine whether the addition of iron alone or in combination with nitrate affects growth and photosynthesis of the scleractinian coral, Stylophora pistillata, and its symbiotic dinoflagellates. For this purpose, we used three series of two tanks for a 3-week enrichment with iron (Fe), nitrate (N) and nitrate+iron (NFe). Two other tanks were kept as a control (C). Stock solutions of FeCl(3) and NaNO(3) were diluted to final concentrations of 6 nM Fe and 2 &mgr;M N and continuously pumped from batch tanks into the experimental tanks with a peristaltic pump. Results obtained showed that iron addition induced a significant increase in the areal density of zooxanthellae (ANOVA, p=0.0013; change from 6.3+/-0.7x10(5) in the control to 8.5+/-0.6x10(5) with iron). Maximal gross photosynthetic rates normalized per surface area also significantly increased following iron enrichment (ANOVA, p=0.02; change from 1.23+/-0.08 for the control colonies to 1.81+/-0.24 &mgr;mol O(2) cm(-2) h(-1) for the iron-enriched colonies). There was, however, no significant difference in the photosynthesis normalized on a per cell basis. Nitrate enrichment alone (2 &mgr;M) did not significantly change the zooxanthellae density or the rates of photosynthesis. Nutrient addition (both iron and nitrogen) increased the cell-specific density of the algae (CSD) compared to the control (G-test, p=0.3x10(-9)), with an increase in the number of doublets and triplets. CSD was equal to 1.70+/-0.04 in the Fe-enriched colonies, 1.54+/-0.12 in the N- and NFe-enriched colonies and 1.37+/-0.02 in the control. Growth rates measured after 3 weeks in colonies enriched with Fe, N and NFe were 23%, 34% and 40% lower than those obtained in control colonies (ANOVA, p=0.011).     

Low temperature FESEM of the calcifying interface of a scleractinian coral

The ultrastructural nature of the calcifying interface in the scleractinian coral Galaxea fascicularis has been investigated using high-resolution, low temperature field emission scanning electron microscopy (FESEM). This technique permitted structural analyses of soft tissue and skeleton in G. fascicularis in a frozen-hydrated state, without the need for chemical fixation or decalcification. Structural comparisons are made between frozen-hydrated polyps and polyps that have undergone conventional fixation and decalcification. Vesicles expelled by the calicoblastic ectodermal cells into sub-skeletal spaces and previously suggested to play a role in calcification were commonly observed in fixed samples but were distinctly absent in frozen-hydrated preparations. We propose that these vesicles are fixation artefacts. Two distinct types of vesicles (380 and 70 nm in diameter, respectively), were predominant throughout the calicoblastic ectodermal cells of frozen-hydrated preparations, but these were never seen to be entering, or to be contained within, sub-skeletal spaces, nor did they contain any crystalline material. In frozen-hydrated preparations, membranous sheets were seen to surround and isolate portions of aboral mesogloea and to form junctional complexes with calicoblastic cells. The calicoblastic ectoderm was closely associated with the underlying skeleton, with sub-skeletal spaces significantly smaller (P<0.0001) in frozen-hydrated polyps compared to fixed polyps. A network of organic filaments (26 nm in diameter) extended from the apical membranes of calicoblastic cells into these small sub-skeletal cavities. A thin sheath was also frequently observed adjacent to the apical membrane of calicoblastic cells.     

Sperm dispersal distances estimated by parentage analysis in a brooding scleractinian coral

Within populations of brooding sessile corals, sperm dispersal constitutes the mechanism by which gametes interact and mating occurs, and forms the first link in the network of processes that determine specieswide connectivity patterns. However, almost nothing is known about sperm dispersal for any internally fertilizing coral. In this study, we conducted a parentage analysis on coral larvae collected from an area of mapped colonies, to measure the distance sperm disperses for the first time in a reef‐building coral and estimated the mating system characteristics of a recently identified putative cryptic species within the Seriatopora hystrix complex (ShA; Warner et al. 2015). We defined consensus criteria among several replicated methods (colony 2.0, cervus 3.0, mltr v3.2) to maximize accuracy in paternity assignments. Thirteen progeny arrays indicated that this putative species produces exclusively sexually derived, primarily outcrossed larvae (mean t_m = 0.999) in multiple paternity broods (mean r_p = 0.119). Self‐fertilization was directly detected at low frequency for all broods combined (2.8%), but comprised 23% of matings in one brood. Although over 82% of mating occurred between colonies within 10 m of each other (mean sperm dispersal = 5.5 m ± 4.37 SD), we found no evidence of inbreeding in the established population. Restricted dispersal of sperm compared to slightly greater larval dispersal appears to limit inbreeding among close relatives in this cryptic species. Our findings establish a good basis for further work on sperm dispersal in brooding corals and provide the first information about the mating system of a newly identified and abundant cryptic species.     

Overview of microbial biofilms

As the success of this two-issue special section of the Journal of Industrial Microbiology attests, the study of microbial biofilms is truly burgeoning as the uniqueness and the importance of this mode of growth is increasingly recognized. Because of its universality the biofilm concept impacts virtually all of the subdivisions of Microbiology (including Medical, Dental, Agricultural, Industrial and Environmental) and these two issues incorporate contributions from authors in all of these disciplines. Some time ago we reasoned that bacteria cannot possibly be aware (sic) of their precise location, in terms of this spectrum of anthrocentric subspecialties, and that their behavior must be dictated by a standard set of phenotypic responses to environmental conditions in what must seem to them (sic) to be a continuum of very similar aquatic ecosystems. In this overview I will, therefore, stress the common features of microbial biofilms that we should bear in mind as we use this simple universal concept to seek to understand bacterial behavior in literally hundreds of aquatic ecosystems traditionally studied by dozens of subspecies of microbiologists reared in sharply different scientific and academic conventions.     

Isolation of microsatellites from two species of scleractinian coral

Microsatellites were isolated from two broadcast‐spawning species of scleractinian coral (Platygyra daedalea and Goniastrea favulus) from Australia's Great Barrier Reef. We found 27 microsatellites across both species, although only five loci were polymorphic in each species. Microsatellite loci displayed a wide range of diversity levels with four to 11 alleles per locus (H_O = 0.26–0.91) in P. daedalea and two to seven alleles per locus (H_O = 0.16–0.96) in G. favulus. Most loci showed departures from Hardy–Weinberg equilibrium which may reflect nonrandom mating but may also be related to difficulties associated with coral DNA.     

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.