Transplantation and immunoincompatibility reactions among reef-building corals

Intracolony or syngeneic transplants of pieces of living coral, either occurring naturally on reefs or produced experimentally, were consistently compatible. In contrast, intercolony allografts performed extensively inAcropora formosa and inPorites andrewsii were incompatible, as were allografts occurring naturally in additional species observed in reef communities. Diverse interspecific combinations of coral xenografts generally displayed greater incompatibilities. Four levels of immuno-reactivity, with manifestations ranging from mild to severe, were distinguishable as follows: a) contact avoidance reactions, b) allogeneic contact incompatibility, c) chronic xenogeneic incompatibility, and d) acute interspecific aggression.Allogeneic incompatibility, most studied inA. formosa, was characterized by soft tissue contact avoidance wherever possible; absence of tissue death despite extensive enforced contact (18–20 days); and late interfacial cementation terminating allogeneic soft tissue contact. The properties ofchronic xenogeneic incompatibility as found in diverse species combinations, are slow onset (>7 days), bidirectional or unidirectional killing in contact zones, and short-range or localized effectiveness.Acute interspecific aggression initiated byFungia fungites as a dominant “aggressor ” is distinguished by early occurrence (2–7 days) and unidirectional contact killing. The presence or absence of these acute xenogeneic reactions sharply discriminated between species within each of the generaAcropora, Pocillopora, andPorites. No hierarchy of aggressive interactions was evident at the generic level.     

Conservation genetics and the resilience of reef-building corals

Coral reefs have suffered long-term decline due to a range of anthropogenic disturbances and are now also under threat from climate change. For appropriate management of these vulnerable and valuable ecosystems it is important to understand the factors and processes that determine their resilience and that of the organisms inhabiting them, as well as those that have led to existing patterns of coral reef biodiversity. The scleractinian (stony) corals deposit the structural framework that supports and promotes the maintenance of biological diversity and complexity of coral reefs, and as such, are major components of these ecosystems. The success of reef-building corals is related to their obligate symbiotic association with dinoflagellates of the genus Symbiodinium. These one-celled algal symbionts (zooxanthellae) live in the endodermal tissues of their coral host, provide most of the host's energy budget and promote rapid calcification. Furthermore, zooxanthellae are the main primary producers on coral reefs due to the oligotrophic nature of the surrounding waters. In this review paper, we summarize and critically evaluate studies that have employed genetics and/or molecular biology in examining questions relating to the evolution and ecology of reef-building corals and their algal endosymbionts, and that bear relevance to coral reef conservation. We discuss how these studies can focus future efforts, and examine how these approaches enhance our understanding of the resilience of reef-building corals.     

Surface flow for colonial integration in reef-building corals

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Abundant betaines in reef-building corals and ecological indicators of a photoprotective role

Betaines are well known as compatible solutes that exert protein- and membrane-stabilizing effects, including protective effects on photosynthesis in plants and free-living algae stressed by high irradiance or unusual temperatures. Betaines, however, have received minimal attention in reef-building corals. One goal of this research was to identify and quantify the betaines of reef-building corals with chemically definitive methods. Metabolite profiling was conducted on 10 species (6 genera) of Curaçao corals by liquid chromatography/time-of-flight mass spectrometry calibrated using six stable-isotope-labeled internal standards. Glycine betaine (GlyB), proline betaine (ProB), alanine betaine (AlaB), β-alanine betaine, hydroxyproline betaine (HProB), taurine betaine (TauB), trigonelline (Trig) and the chemically related sulfonium compound dimethylsulfoniopropionate were found in all species. Relative levels of betaines varied across species, with GlyB and ProB being most prominent. Betaines were collectively abundant; estimated total concentrations were 12 to 204 (mean = 75) mmol per liter of tissue. A second goal was to examine ecological patterns in betaine concentrations in field populations of Curaçao corals. Betaine concentrations exhibited intraspecific patterns that matched a priori predictions for molecules that defend photosynthesis against negative effects of high irradiance. In Madracis mirabilis—which occupies unshaded locations—GlyB, ProB, AlaB, HProB, and Trig were 37–94% more abundant in colonies at 5 m depth (high irradiance) than 20 m. In M. pharensis—which occupies exposed and shaded locations—GlyB, ProB, and AlaB were 30–44% more abundant in unshaded than shaded colonies at one depth. M. senaria exhibited 45–93% increases in concentrations of betaines—GlyB, AlaB, HProB, TauB, and Trig—between early and late in the day, although M. mirabilis and pharensis did not. The results indicate that multiple betaines occur commonly in reef-building coral species, and betaine concentrations are modulated in response to growth light conditions in ways consistent with betaines acting as agents of photoprotection of coral photosynthesis.     

The presence of multiple phenoloxidases in Caribbean reef-building corals

The melanin-synthesis pathways, phenoloxidase (PO) and laccases, are staple components of invertebrate immunity and have been shown to be vital in disease resistance. The importance of this pathway in immunity is a consequence of the release of oxygen radicals with cytotoxic effects and the production of insoluble melanin, which aids in the encapsulation of pathogens and parasites. Recently, melanization has been demonstrated as a critical immune response in several coral systems, although the biochemical components have not been thoroughly investigated. Coral diseases are posing a serious threat to coral reef survival, necessitating a full understanding of resistance mechanisms. In this study, we take a comparative approach to probe potential pathway components of melanin-synthesis in seven species from four different families of healthy Caribbean reef-building corals. Using different quinone substrates, we tested for the activity of the POs catecholase and cresolase, as well as laccase activity in each coral species. Since many invertebrate POs demonstrate some dependence on cations such as copper, calcium and magnesium, we treated the coral extracts with the chelators EDTA and EGTA to test the reliance of coral catecholase on these cations. The activity of the antioxidants peroxidase, superoxide dismutase and catalase was also tested in each coral and correlated to PO activity. All corals had demonstrable catecholase, cresolase and laccase activities, but only catecholase and cresolase activities varied significantly among species. Catecholase activity in each coral species was reduced by treatment with EDTA and EGTA, although some coral species were less affected than the others. Overall, these data show remarkable heterogeneity among the seven coral species of boulder-like reef building Caribbean coral. These differences may originate from the level of investment of each coral species into immunity and may explain disease ecology on the reef.     

Heat stress differentially impacts key calcification mechanisms in reef-building corals

Coral Reefs - Coral reefs are increasingly threatened by climate change, mass bleaching events and ocean acidification (OA). Coral calcification, a process that is critical to build and maintain...     

Major colour patterns of reef-building corals are due to a family of GFP-like proteins

Reef-building corals are renowned for their brilliant colours yet the biochemical basis for the pigmentation of corals is unknown. Here, we show that these colours are due to a family of GFP-like proteins that fluoresce under ultraviolet (UV) or visible light. Pigments from ten coral species were almost identical to pocilloporin (Dove et al. 1995) being dimers or trimers with approximately 28-kDa subunits. Degenerative primers made to common N-terminal sequences yielded a complete sequence from reef-building coral cDNA, which had 19.6% amino acid identity with green fluorescent protein (GFP). Molecular modelling revealed a `β-can' structure, like GFP, with 11 β-strands and a completely solvent-inaccessible fluorophore composed of the modified residues Gln-61, Tyr-62 and Gly-63. The molecular properties of pocilloporins indicate a range of functions from the conversion of high-intensity UV radiation into photosynthetically active radiation (PAR) that can be regulated by the dinoflagellate peridinin-chlorophyll-protein (PCP) complex, to the shielding of the Soret and Q_x bands of chlorophyll a and c from scattered high-intensity light. These properties of pocilloporin support its potential role in protecting the photosynthetic machinery of the symbiotic dinoflagellates of corals under high light conditions and in enhancing the availability of photosynthetic light under shade conditions. Accepted: 29 May 2000     

造礁石珊瑚共生虫黄藻离体培养方法的优化

【目的】开发一种高效地从造礁石珊瑚中分离、培养共生虫黄藻的技术方法,为珊瑚共生虫黄藻藻种资源储备和生理功能研究积累基础。【方法】首先采用微孔滤网过滤法和密度梯度离心法从造礁石珊瑚组织中直接分离或富集共生虫黄藻细胞,然后用改良的L1培养基在96孔板上对所得细胞进行离体培养,最后进行单细胞分离、培养和(或)平板划线培养获得单克隆虫黄藻细胞系。对所得虫黄藻单克隆藻株进行聚合酶链式反应-限制性内切酶片段长度多态性(polymerase chainreaction-restrictionfragmentlengthpolymorphism,PCR-RFLP)分析,结合内转录间隔区2(internal transcribed spacer2,ITS2)和大亚基(large subunit,LSU)测序进行物种鉴定及系统发育分析。【结果】采用上述方法从涠洲岛的霜鹿角珊瑚(Acropora pruinose)和西沙群岛的丛生盔形珊瑚(Galaxea fascicularis)及柔枝鹿角珊瑚(Acropora tenuis)中分离、培养得到3个虫黄藻株系,编号分别为AP21C1、GF21D1和AT21A...     

A connection between colony biomass and death in Caribbean reef-building corals

Increased sea-surface temperatures linked to warming climate threaten coral reef ecosystems globally. To better understand how corals and their endosymbiotic dinoflagellates (Symbiodinium spp.) respond to environmental change, tissue biomass and Symbiodinium density of seven coral species were measured on various reefs approximately every four months for up to thirteen years in the Upper Florida Keys, United States (1994-2007), eleven years in the Exuma Cays, Bahamas (1995-2006), and four years in Puerto Morelos, Mexico (2003-2007). For six out of seven coral species, tissue biomass correlated with Symbiodinium density. Within a particular coral species, tissue biomasses and Symbiodinium densities varied regionally according to the following trends: Mexico≥Florida Keys≥Bahamas. Average tissue biomasses and symbiont cell densities were generally higher in shallow habitats (1-4 m) compared to deeper-dwelling conspecifics (12-15 m). Most colonies that were sampled displayed seasonal fluctuations in biomass and endosymbiont density related to annual temperature variations. During the bleaching episodes of 1998 and 2005, five out of seven species that were exposed to unusually high temperatures exhibited significant decreases in symbiotic algae that, in certain cases, preceded further decreases in tissue biomass. Following bleaching, Montastraea spp. colonies with low relative biomass levels died, whereas colonies with higher biomass levels survived. Bleaching- or disease-associated mortality was also observed in Acropora cervicornis colonies; compared to A. palmata, all A. cervicornis colonies experienced low biomass values. Such patterns suggest that Montastraea spp. and possibly other coral species with relatively low biomass experience increased susceptibility to death following bleaching or other stressors than do conspecifics with higher tissue biomass levels.     

Climate change, genotypic diversity and gene flow in reef-building corals

In the ocean, large‐scale dispersal and replenishment by larvae is a key process underlying biological changes associated with global warming. On tropical reefs, coral bleaching, degradation of habitat and declining adult stocks are also likely to change contemporary patterns of dispersal and gene flow and may lead to range contractions or expansions. On the Great Barrier Reef, where adjacent reefs form a highly interconnected system, we use allozyme surveys of c. 3000 coral colonies to show that populations are genetically diverse, and rates of gene flow for a suite of five species range from modest to high among reefs up to 1200 km apart. In contrast, 700 km further south on Lord Howe Island, genetic diversity is markedly lower and populations are genetically isolated. The virtual absence of long‐distance dispersal of corals to geographically isolated, oceanic reefs renders them extremely vulnerable to global warming, even where local threats are minimal.     

Sensitivity of calcification to thermal stress varies among genera of massive reef-building corals

Reductions in calcification in reef-building corals occur when thermal conditions are suboptimal, but it is unclear how they vary between genera in response to the same thermal stress event. Using densitometry techniques, we investigate reductions in the calcification rate of massive Porites spp. from the Great Barrier Reef (GBR), and P. astreoides, Montastraea faveolata, and M. franksi from the Mesoamerican Barrier Reef (MBR), and correlate them to thermal stress associated with ocean warming. Results show that Porites spp. are more sensitive to increasing temperature than Montastraea, with calcification rates decreasing by 0.40 g cm(-2) year(-1) in Porites spp. and 0.12 g cm(-2) year(-1) in Montastraea spp. for each 1°C increase. Under similar warming trends, the predicted calcification rates at 2100 are close to zero in Porites spp. and reduced by 40% in Montastraea spp. However, these predictions do not account for ocean acidification. Although yearly mean aragonite saturation (Ω(ar)) at MBR sites has recently decreased, only P. astreoides at Chinchorro showed a reduction in calcification. In corals at the other sites calcification did not change, indicating there was no widespread effect of Ω(ar) changes on coral calcification rate in the MBR. Even in the absence of ocean acidification, differential reductions in calcification between Porites spp. and Montastraea spp. associated with warming might be expected to have significant ecological repercussions. For instance, Porites spp. invest increased calcification in extension, and under warming scenarios it may reduce their ability to compete for space. As a consequence, shifts in taxonomic composition would be expected in Indo-Pacific reefs with uncertain repercussions for biodiversity. By contrast, Montastraea spp. use their increased calcification resources to construct denser skeletons. Reductions in calcification would therefore make them more susceptible to both physical and biological breakdown, seriously affecting ecosystem function in Atlantic reefs.     

Short term viability of soft tissue detached from the skeleton of reef-building corals

We have induced soft tissue detachment from the skeleton of two colonial hard corals of the Pocilloporid family, both in vivo and in vitro. A parallel was made between polyp “bail-out”, i.e. field and laboratory-observed detachment of tissue fragments alone from the skeleton, and the dissociation method used for initiation of coral primary cell cultures. The in vitro approach provided insights into the active cellular re-arrangement mechanisms underlying coral tissue detachment. Functional polyps were not regenerated. Viability of tissue isolates detached from coral skeleton was probed for their use as a model for short-term biological assays. Cell viability dropped from 70% to 30% within the first week maintenance in vitro. Short-term isolate cultures limited to 3 days are a compromise allowing attachment of coral cells, yet preserving viability at about 70% of the total coral cell population.     

Ciliate and bacterial communities associated with White Syndrome and Brown Band Disease in reef-building corals

White Syndrome (WS) and Brown Band Disease (BrB) are important causes of reef coral mortality for which causal agents have not been definitively identified. Here we use culture-independent molecular techniques (DGGE and clone libraries) to characterize ciliate and bacterial communities in these diseases. Bacterial (16S rRNA gene) and ciliate (18S rRNA gene) communities were highly similar between the two diseases. Four bacterial and nine ciliate ribotypes were observed in both diseases, but absent in non-diseased specimens. Only one of the bacteria, Arcobacter sp. (JF831360) increased substantially in relative 16S rRNA gene abundance and was consistently represented in all diseased samples. Four of the eleven ciliate morphotypes detected contained coral algal symbionts, indicative of the ingestion of coral tissues. In both WS and BrB, there were two ciliate morphotypes consistently represented in all disease lesion samples. Morph1 (JN626268) was observed to burrow into and underneath the coral tissues at the lesion boundary. Morph2 (JN626269), previously identified in BrB, appears to play a secondary, less invasive role in pathogenesis, but has a higher population density in BrB, giving rise to the visible brown band. The strong similarity in bacterial and ciliate community composition of these diseases suggests that they are actually the same syndrome.     

Species richness and community structure of reef-building corals on the nearshore Great Barrier Reef

Species richness, cover and community structure of reef-building corals were assessed at 599 sites on 135 reefs along the Great Barrier Reef (GBR) between 1994 and 2001, with focus on the nearshore area. Communities were described hierarchically, with smaller regional communities forming part of higher level communities at increasing spatial scales. Site richness increased from the coast to the mid-continental shelf, declining on the outer shelf. Richness also increased with depth to 5 m, stabilizing thereafter. An anomaly was present in a 400 km section adjacent to the northern, ‘wet tropics’ coast, where site richness was 67 and 41% lower than the adjacent far northern and central GBR, respectively; this was probably due to the disturbance regime, with an apparent anthropogenic component. Site richness also declined in the Southern GBR, probably due to naturally marginal conditions. All indicator species had highest values in five small Far Northern and Central GBR communities. In the eight depauperate communities no indicator species had high values, indicating that these communities represent degraded, yet potentially transitional forms of the more diverse communities of the Far Northern and Central GBR. The study shows that on the GBR, disturbance results in the local removal of corals rather than a shift to suites of other coral species.     

Symbiodiniaceae-bacteria interactions: rethinking metabolite exchange in reef-building corals as multi-partner metabolic networks

The intimate relationship between scleractinian corals and their associated microorganisms is fundamental to healthy coral reef ecosystems. Coral-associated microbes (Symbiodiniaceae and other protists, bacteria, archaea, fungi and viruses) support coral health and resilience through metabolite transfer, inter-partner signalling, and genetic exchange. However, much of our understanding of the coral holobiont relationship has come from studies that have investigated either coral-Symbiodiniaceae or coral-bacteria interactions in isolation, while relatively little research has focused on other ecological and metabolic interactions potentially occurring within the coral multi-partner symbiotic network. Recent evidences of intimate coupling between phytoplankton and bacteria have demonstrated that obligate resource exchange between partners fundamentally drives their ecological success. Here, we posit that similar associations with bacterial consortia regulate Symbiodiniaceae productivity and are in turn central to the health of corals. Indeed, we propose that this bacteria-Symbiodiniaceae-coral relationship underpins the coral holobiont's nutrition, stress tolerance and potentially influences the future survival of coral reef ecosystems under changing environmental conditions. Resolving Symbiodiniaceae-bacteria associations is therefore a logical next step towards understanding the complex multi-partner interactions occurring in the coral holobiont.     

Repair machinery of symbiotic photosynthesis as the primary target of heat stress for reef-building corals

In a coral-algae symbiotic system, heat-dependent photoinhibition of photosystem II (PSII) leads to coral bleaching. When the reef-building coral Acropora digitifera was exposed to light, a moderate increase of temperature induced coral bleaching through photobleaching of algal pigments, but not through expulsion of symbiotic algae. Monitoring of PSII photoinhibition revealed that heat-dependent photoinhibition was ascribed to inhibition of the repair of photodamaged PSII, and heat susceptibility of the repair machinery varied among coral species. We conclude that the efficiency of the photosynthesis repair machinery determines the bleaching susceptibility of coral species under elevated seawater temperatures.     

贵州雅水早石炭世维宪晚期的造礁四射珊瑚

伴随晚泥盆世弗拉期–法门期之交的Kellwasser和泥盆纪–石炭纪之交的Hangenberg生物灭绝事件, 泥盆纪的主要造礁动物层孔海绵消失、珊瑚多样性急剧减少, 之后的早石炭世是造礁珊瑚的关键复苏期。贵州雅水剖面下石炭统维宪阶上司组发育了丰富的原位珊瑚层, 即层状珊瑚礁, 依据有孔虫化石确定珊瑚层的时代为维宪晚期。上司组珊瑚层主要由丛状复体四射珊瑚组成, 多样性低, 可识别出2属2种: Siphonodendron pauciradiale 和Stylostrotion pentalaxoidea。其中, S. pauciradiale广泛分布于西欧和北非地区维宪晚期的珊瑚礁中, 而S. pentalaxoidea分布局限, 目前仅见于华南板块。雅水剖面维宪晚期全球广布的造礁珊瑚S. pauciradiale的出现, 指示华南地区珊瑚礁的复苏演化过程与全球具有一致性。     

Environmental symbiont acquisition may not be the solution to warming seas for reef-building corals

Background

Coral reefs worldwide are in decline. Much of the mortality can be attributed to coral bleaching (loss of the coral''s intracellular photosynthetic algal symbiont) associated with global warming. How corals will respond to increasing oceanic temperatures has been an area of extensive study and debate. Recovery after a bleaching event is dependent on regaining symbionts, but the source of repopulating symbionts is poorly understood. Possibilities include recovery from the proliferation of endogenous symbionts or recovery by uptake of exogenous stress-tolerant symbionts.

Methodology/Principal Findings

To test one of these possibilities, the ability of corals to acquire exogenous symbionts, bleached colonies of Porites divaricata were exposed to symbiont types not normally found within this coral and symbiont acquisition was monitored. After three weeks exposure to exogenous symbionts, these novel symbionts were detected in some of the recovering corals, providing the first experimental evidence that scleractinian corals are capable of temporarily acquiring symbionts from the water column after bleaching. However, the acquisition was transient, indicating that the new symbioses were unstable. Only those symbiont types present before bleaching were stable upon recovery, demonstrating that recovery was from the resident in situ symbiont populations.

Conclusions/Significance

These findings suggest that some corals do not have the ability to adjust to climate warming by acquiring and maintaining exogenous, more stress-tolerant symbionts. This has serious ramifications for the success of coral reefs and surrounding ecosystems and suggests that unless actions are taken to reverse it, climate change will lead to decreases in biodiversity and a loss of coral reefs.     

The Vibrio core group induces yellow band disease in Caribbean and Indo-Pacific reef-building corals

Aims: To determine the relationship between yellow band disease (YBD)-associated pathogenic bacteria found in both Caribbean and Indo-Pacific reefs, and the virulence of these pathogens. YBD is one of the most significant coral diseases of the tropics. Materials and Results: The consortium of four Vibrio species was isolated from YBD tissue on Indo-Pacific corals: Vibrio rotiferianus, Vibrio harveyi, Vibrio alginolyticus and Vibrio proteolyticus. This consortium affects Symbiodinium (zooxanthellae) in hospite causing symbiotic algal cell dysfunction and disorganization of algal thylakoid membrane-bound compartment from corals in both field and laboratory. Infected corals have decreased zooxanthella cell division compared with the healthy corals. Vibrios isolated from diseased Diploastrea heliopora, Fungia spp. and Herpolitha spp. of reef-building corals display pale yellow lesions, which are similar to those found on Caribbean Montastraea spp. with YBD. Conclusions: The Vibrio consortium found in YBD-infected corals in the Caribbean are close genetic relatives to those in the Indo-Pacific. The consortium directly attacks Symbiodinium spp. (zooxanthellae) within gastrodermal tissues, causing degenerated and deformed organelles, and depleted photosynthetic pigments in vitro and in situ. Infected Fungia spp. have decreased cell division compared with the healthy zooxanthellae: 4·9%vs 1·9%, (P ≥ 0·0024), and in D. heliopora from 4·7% to 0·7% (P ≥ 0·002). Significance and Impact of the Study: Pathogen virulence has major impacts on the survival of these important reef-building corals around the tropics.