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1.
    
As coral bleaching events become more frequent and intense, our ability to predict and mitigate future events depends upon our capacity to interpret patterns within previous episodes. Responses to thermal stress vary among coral species; however the diversity of coral assemblages, environmental conditions, assessment protocols, and severity criteria applied in the global effort to document bleaching patterns creates challenges for the development of a systemic metric of taxon‐specific response. Here, we describe and validate a novel framework to standardize bleaching response records and estimate their measurement uncertainties. Taxon‐specific bleaching and mortality records (2036) of 374 coral taxa (during 1982–2006) at 316 sites were standardized to average percent tissue area affected and a taxon‐specific bleaching response index (taxon‐BRI) was calculated by averaging taxon‐specific response over all sites where a taxon was present. Differential bleaching among corals was widely variable (mean taxon‐BRI = 25.06 ± 18.44%, ±SE). Coral response may differ because holobionts are biologically different (intrinsic factors), they were exposed to different environmental conditions (extrinsic factors), or inconsistencies in reporting (measurement uncertainty). We found that both extrinsic and intrinsic factors have comparable influence within a given site and event (60% and 40% of bleaching response variance of all records explained, respectively). However, when responses of individual taxa are averaged across sites to obtain taxon‐BRI, differential response was primarily driven by intrinsic differences among taxa (65% of taxon‐BRI variance explained), not conditions across sites (6% explained), nor measurement uncertainty (29% explained). Thus, taxon‐BRI is a robust metric of intrinsic susceptibility of coral taxa. Taxon‐BRI provides a broadly applicable framework for standardization and error estimation for disparate historical records and collection of novel data, allowing for unprecedented accuracy in parameterization of mechanistic and predictive models and conservation plans.  相似文献   

2.
    
Reef‐building corals are at risk of extinction from ocean warming. While some corals can enhance their thermal limits by associating with dinoflagellate photosymbionts of superior stress tolerance, the extent to which symbiont communities will reorganize under increased warming pressure remains unclear. Here we show that corals in the hottest reefs in the world in the Persian Gulf maintain associations with the same symbionts across 1.5 years despite extreme seasonal warming and acute heat stress (≥35°C). Persian Gulf corals predominantly associated with Cladocopium (clade C) and most also hosted Symbiodinium (clade A) and/or Durusdinium (clade D). This is in contrast to the neighbouring and milder Oman Sea, where corals associated with Durusdinium and only a minority hosted background levels of Cladocopium. During acute heat stress, the higher prevalence of Symbiodinium and Durusdinium in bleached versus nonbleached Persian Gulf corals indicates that genotypes of these background genera did not confer bleaching resistance. Within symbiont genera, the majority of ITS2 rDNA type profiles were unique to their respective coral species, confirming the existence of host‐specific symbiont lineages. Notably, further differentiation among Persian Gulf sites demonstrates that symbiont populations are either isolated or specialized over tens to hundreds of kilometres. Thermal tolerance across coral species was associated with the prevalence of a single ITS2 intragenomic sequence variant (C3gulf), definitive of the Cladocopium thermophilum group. The abundance of C3gulf was highest in bleaching‐resistant corals and at warmer sites, potentially indicating a specific symbiont genotype (or set of genotypes) that may play a role in thermal tolerance that warrants further investigation. Together, our findings indicate that co‐evolution of host–Symbiodiniaceae partnerships favours fidelity rather than flexibility in extreme environments and under future warming.  相似文献   

3.
    
For cnidarians that can undergo shifts in algal symbiont relative abundance, the underlying algal physiological changes that accompany these shifts are not well known. The sea anemone Anthopleura elegantissima associates with the dinoflagellate Symbiodinium muscatinei and the chlorophyte Elliptochloris marina, symbionts with very different tolerances to light and temperature. We compared the performance of these symbionts in anemones maintained in an 8–11.5 month outdoor common garden experiment with simulated intertidal conditions and three levels of shading (2, 43, and 85% ambient irradiance). Symbiont densities, mitotic indices, photophysiology and pigments were assessed at three time points during the summer, a period of high irradiance and solar heating during aerial exposure. Whereas S. muscatinei was either neutrally or positively affected by higher irradiance treatments, E. marina responded mostly negatively to high irradiance. E. marina in the 85% irradiance treatment exhibited significantly reduced Pmax and chlorophyll early in the summer, but it was not until nearly 3 months later that a shift in symbiont relative abundance toward S. muscatinei occurred, coincident with bleaching. Symbiont densities and proportions remained largely stable in all other treatments over time, and displacement of S. muscatinei by E. marina was not observed in the 2% irradiance treatment despite the potentially better performance of E. marina. While our results support the view that rapid changes in symbiont relative abundance are typically associated with symbiont physiological dysfunction and bleaching, they also show that significant temporal lags may occur between the onset of symbiont stress and shifts in symbiont relative abundances.  相似文献   

4.
    
Coral bleaching involves the loss of essential photosynthetic dinoflagellates (Symbiodinium sp.) from host gastrodermal cells in response to temperature or light stress. Although numerous potential cellular bleaching mechanisms have been proposed, there remains much uncertainty regarding which cellular events occur during early breakdown of the host–dinoflagellate symbiosis. In this study, transmission electron microscopy was used to conduct a detailed examination of symbiotic tissues of the tropical anemone Aiptasia pallida during early stages of host stress. Bleaching was induced by exposing specimens to a stress treatment of 32.5±0.5°C at 140±7 μ mol photons m?2 s?1 light intensity for 12 h, followed by 12 h at 24±1°C in darkness, repeated over a 48 h period. Ultrastructural examinations revealed numerous dense autophagic structures and associated cellular degradation in tentacle tissues after ~12 h of the stress treatment. Anemones treated with rapamycin, a known autophagy inducer, exhibited the same ultrastructural characteristics as heat‐stressed tissues, confirming that the structures observed during heat stress treatment were autophagic. In addition, symbionts appeared to be expelled from host cells via an apocrine‐like detachment mechanism from the apical ends of autophagic gastrodermal cells. This study provides the first ultrastructural evidence of host autophagic degradation during thermal stress in a cnidarian system and also supports earlier suggestions that autophagy is an active cellular mechanism during early stages of bleaching.  相似文献   

5.
    
  1. Seasonal droughts are predictable components of arid‐land stream hydrology, and many arid‐land aquatic taxa have adapted to their extreme environment. However, climate change is altering this predictable hydrology, producing longer and more severe droughts and creating novel disturbance regimes for resident organisms.
  2. The hydrologic transitions from flowing stream to fragmented pools to dry stream bed are frequently associated with steep decreases in taxonomic and functional diversity, referred to as thresholds of biodiversity loss. Less is known about how taxa respond between these thresholds, as fragmented pools gradually dry and abiotic conditions intensify. While an increasingly extreme environment may be expected to reduce taxonomic and trait richness, species adapted to predictable seasonal fragmentation may be resistant to declining water levels until all surface water is lost. We used aquatic mesocosms to test two competing hypotheses of the relationship between richness and pool drying for arid‐land stream invertebrates: (i) the drought vulnerability hypothesis (richness gradually decreases with drying) and (ii) the drought resistance hypothesis (richness remains constant until complete drying occurs).
  3. We inoculated replicate mesocosms with aquatic invertebrates from arid‐land streams in Arizona, U.S.A., and applied three drying treatments representing a continuum of drying stress commonly observed in local streams during the summer dry season (water depths: 10, 7 and 1 cm). Mesocosms were covered to restrict dispersal and colonisation processes and to isolate resistance (in situ survival of species) from resilience (community recovery following disturbance). After 45 days, we destructively sampled all invertebrates in the mesocosms and calculated various taxonomic and functional trait metrics.
  4. Taxonomic richness and composition did not differ between drying treatments, providing strong support for the drought resistance hypothesis. Severe drying was associated with lower invertebrate abundances and higher densities than the moderate and control treatments. This finding suggests that density‐dependent processes generated by decreased available habitat may be more important determinants of community composition during droughts than abiotic stress in this system.
  5. We observed a near‐complete overlap of trophic traits (body size and functional feeding group) and resistance traits (respiration mode and diapause) among the three treatments. This high functional redundancy may provide a buffer against changes to ecosystem functioning, even in cases of severe‐drying‐induced habitat contraction and fragmentation.
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6.
    
Spatially intimate symbioses, such as those between scleractinian corals and unicellular algae belonging to the genus Symbiodinium, can potentially adapt to changes in the environment by altering the taxonomic composition of their endosymbiont communities. We quantified the spatial relationship between the cumulative frequency of thermal stress anomalies (TSAs) and the taxonomic composition of Symbiodinium in the corals Montipora capitata, Porites lobata, and Porites compressa across the Hawaiian archipelago. Specifically, we investigated whether thermally tolerant clade D Symbiodinium was in greater abundance in corals from sites with high frequencies of TSAs. We recovered 2305 Symbiodinium ITS2 sequences from 242 coral colonies in lagoonal reef habitats at Pearl and Hermes Atoll, French Frigate Shoals, and Kaneohe Bay, Oahu in 2007. Sequences were grouped into 26 operational taxonomic units (OTUs) with 12 OTUs associated with Montipora and 21 with Porites. Both coral genera associated with Symbiodinium in clade C, and these co‐occurred with clade D in M. capitata and clade G in P. lobata. The latter represents the first report of clade G Symbiodinium in P. lobata. In M. capitata (but not Porites spp.), there was a significant correlation between the presence of Symbiodinium in clade D and a thermal history characterized by high cumulative frequency of TSAs. The endogenous community composition of Symbiodinium and an association with clade D symbionts after long‐term thermal disturbance appear strongly dependent on the taxa of the coral host.  相似文献   

7.
    
Environmental change, including global warming and chemical pollution, can compromise cnidarian‐(e.g., coral‐) dinoflagellate symbioses and cause coral bleaching. Understanding the mechanisms that regulate these symbioses will inform strategies for sustaining healthy coral–reef communities. A model system for corals is the symbiosis between the sea anemone Exaiptasia pallida (common name Aiptasia) and its dinoflagellate partners (family Symbiodiniaceae). To complement existing studies of the interactions between these organisms, we examined the impact of menthol, a reagent often used to render cnidarians aposymbiotic, on the dinoflagellate Breviolum minutum, both in culture and in hospite. In both environments, the growth and photosynthesis of this alga were compromised at either 100 or 300 µM menthol. We observed reduction in PSII and PSI functions, the abundances of reaction‐center proteins, and, at 300 µM menthol, of total cellular proteins. Interestingly, for free‐living algae exposed to 100 µM menthol, an initial decline in growth, photosynthetic activities, pigmentation, and protein abundances reversed after 5–15 d, eventually approaching control levels. This behavior was observed in cells maintained in continuous light, but not in cells experiencing a light–dark regimen, suggesting that B. minutum can detoxify menthol or acclimate and repair damaged photosynthetic complexes in a light‐ and/or energy‐dependent manner. Extended exposures of cultured algae to 300 µM menthol ultimately resulted in algal death. Most symbiotic anemones were also unable to survive this menthol concentration for 30 d. Additionally, cells impaired for photosynthesis by pre‐treatment with 300 µM menthol exhibited reduced efficiency in re‐populating the anemone host.  相似文献   

8.
1. This study analysed changes occurring in Mediterranean stream fish assemblages over a sequence of dry years followed by a generally wet period (1991–98). Variations in assemblage attributes were quantified at the basin and stream reach scales, and related to variables reflecting the occurrence of unusually dry or wet conditions. 2. Assemblage variability increased along with the resolution of analysis, with little changes in species richness, composition and rank abundances, but significant variation in individual species abundances. Fluctuations in relative abundances were significantly affected by variables reflecting the severity of summer droughts and the occurrence of rainy springs. These patterns were evident at the basin scale, while variability at individual stream reaches tended to be higher and less related to rainfall patterns. 3. At least three response guilds to rainfall variation could be identified: two of the four abundant and widespread species (chub and loach) declined following dry years, whereas the two other core species (nase and eel) declined after rainy spring; one scarce native species (stickleback) increased in dry years. 4. Except at the two most upstream reaches, the assemblages tended to recover quickly to previous configuration after the changes occurring during the sequence of dry years. 5. Temporal variability of local assemblages was concordant among reaches but did not follow any consistent spatial pattern. Instead, spatial patterns in assemblage attributes changed over time in response to environmental variability, with a tendency for a disruption of upstream–downstream gradients following dry years. 6. Results supported the view that present‐day droughts cause relatively small and transient changes to Mediterranean stream fish assemblages. However, longer and more severe droughts expected under altered future climates, may result in declines or local extinctions of the most sensitive species and their potential replacement by more resistant species. Changing drought regimes thus need to be duly considered in the development of conservation strategies for Mediterranean stream fish.  相似文献   

9.
    
Symbiodinium reside intracellularly in a complex symbiosome (host and symbiont‐derived) within cnidarian hosts in a specific host‐symbiont association. Symbiodinium is a diverse genus with variation greater than other dinoflagellate orders. In this paper, our investigation into specificity examines antigenic variation in the algal mucilage secretions at the host‐symbiont interface. Cultured Symbiodinium from a variety of clades were labeled with one of two antibodies to symbiont mucilage (PC3, developed using a clade B alga cultured from Aiptasia pallida; BF10, developed using a clade F alga cultured from Briareum sp.). The labeling was visualized with a fluorescent marker and examined with epifluorescence and confocal microscopy. PC3 antigen was found in cultured Symbiodinium from clades A and B, but not clades C, D, E and F. The correlation between labeling and clade may account for some of the specificity between host and symbiont in the field. Within clades A and B there was variation in the amount of label present. BF10 antigen was more specific and only found in cultures of the same cp23S‐rDNA strain the antibody was created against. These results indicate that the mucilage secretions do vary both qualitatively and quantitatively amongst Symbiodinium strains. Since the mucilage forms the host‐symbiont interface, variation in its molecular composition is likely to be the source of any signals involved in recognition and specificity.  相似文献   

10.
    
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11.
    
Global increases in coral disease prevalence have been linked to ocean warming through changes in coral‐associated bacterial communities, pathogen virulence and immune system function. However, the interactive effects of temperature and pathogens on the coral holobiont are poorly understood. Here, we assessed three compartments of the holobiont (host, Symbiodinium and bacterial community) of the coral Montipora aequituberculata challenged with the pathogen Vibrio coralliilyticus and the commensal bacterium Oceanospirillales sp. under ambient (27°C) and elevated (29.5 and 32°C) seawater temperatures. Few visual signs of bleaching and disease development were apparent in any of the treatments, but responses were detected in the holobiont compartments. V. coralliilyticus acted synergistically and negatively impacted the photochemical efficiency of Symbiodinium at 32°C, while Oceanospirillales had no significant effect on photosynthetic efficiency. The coral, however, exhibited a minor response to the bacterial challenges, with the response towards V. coralliilyticus being significantly more pronounced, and involving the prophenoloxidase‐activating system and multiple immune system‐related genes. Elevated seawater temperatures did not induce shifts in the coral‐associated bacterial community, but caused significant gene expression modulation in both Symbiodinium and the coral host. While Symbiodinium exhibited an antiviral response and upregulated stress response genes, M. aequituberculata showed regulation of genes involved in stress and innate immune response processes, including immune and cytokine receptor signalling, the complement system, immune cell activation and phagocytosis, as well as molecular chaperones. These observations show that M. aequituberculata is capable of maintaining a stable bacterial community under elevated seawater temperatures and thereby contributes to preventing disease development.  相似文献   

12.
    
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13.
    
The binary designation Symbiodinium thermophilum was invalid due to the absence of an illustration as required by Article 44.2 of the ICN. Herein, it is validated. This species is the most common symbiont in reef corals in the southern Persian/Arabian Gulf, the world's hottest body of water sustaining reef coral growth.  相似文献   

14.
    
The molecular structure of the carotenoid lactoside P457, (3S,5R,6R,3′S,5′R,6′S)‐13′‐cis‐5,6‐epoxy‐3′,5′‐dihydroxy‐3‐(β‐d ‐galactosyl‐(1→4)‐β‐d ‐glucosyl)oxy‐6′,7′‐didehydro‐5,6,7,8,5′,6′‐hexahydro‐β,β‐caroten‐20‐al, was confirmed by spectroscopic methods using Symbiodinium sp. strain NBRC 104787 cells isolated from a sea anemone. Among various algae, cyanobacteria, land plants, and marine invertebrates, the distribution of this unique diglycosyl carotenoid was restricted to free‐living peridinin‐containing dinoflagellates and marine invertebrates that harbor peridinin‐containing zooxanthellae. Neoxanthin appeared to be a common precursor for biosynthesis of peridinin and P457, although neoxanthin was not found in peridinin‐containing dinoflagellates. Fucoxanthin‐containing dinoflagellates did not possess peridinin or P457; green dinoflagellates, which contain chlorophyll a and b, did not contain peridinin, fucoxanthin, or P457; and no unicellular algae containing both peridinin and P457, other than peridinin‐containing dinoflagellates, have been observed. Therefore, the biosynthetic pathways for peridinin and P457 may have been coestablished during the evolution of dinoflagellates after the host heterotrophic eukaryotic microorganism formed a symbiotic association with red alga that does not contain peridinin or P457.  相似文献   

15.
    
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16.
    
The broad range in physiological variation displayed by Symbiodinium spp. has proven imperative during periods of environmental change and contribute to the survival of their coral host. Characterizing how host and Symbiodinium community assemblages differ across environmentally distinct habitats provides useful information to predict how corals will respond to major environmental change. Despite the extensive characterizations of Symbiodinium diversity found amongst reef cnidarians on the Great Barrier Reef (GBR) substantial biogeographic gaps exist, especially across inshore habitats. Here, we investigate Symbiodinium community patterns in invertebrates from inshore and mid‐shelf reefs on the southern GBR, Australia. Dominant Symbiodinium types were characterized using denaturing gradient gel electrophoresis fingerprinting and sequencing of the ITS2 region of the ribosomal DNA. Twenty one genetically distinct Symbiodinium types including four novel types were identified from 321 reef‐invertebrate samples comprising three sub‐generic clades (A, C, and D). A range of host genera harbored C22a, which is normally rare or absent from inshore or low latitude reefs in the GBR. Multivariate analysis showed that host identity and sea surface temperature best explained the variation in symbiont communities across sites. Patterns of changes in Symbiodinium community assemblage over small geographic distances (100s of kilometers or less) indicate the likelihood that shifts in Symbiodinium distributions and associated host populations, may occur in response to future climate change impacting the GBR.  相似文献   

17.
    
The dinoflagellate photosymbiont Symbiodinium plays a fundamental role in defining the physiological tolerances of coral holobionts, but little is known about the dynamics of these endosymbiotic populations on coral reefs. Sparse data indicate that Symbiodinium populations show limited spatial connectivity; however, no studies have investigated temporal dynamics for in hospite Symbiodinium populations following significant mortality and recruitment events in coral populations. We investigated the combined influences of spatial isolation and disturbance on the population dynamics of the generalist Symbiodinium type C2 (ITS1 rDNA) hosted by the scleractinian coral Acropora millepora in the central Great Barrier Reef. Using eight microsatellite markers, we genotyped Symbiodinium in a total of 401 coral colonies, which were sampled from seven sites across a 12‐year period including during flood plume–induced coral bleaching. Genetic differentiation of Symbiodinium was greatest within sites, explaining 70–86% of the total genetic variation. An additional 9–27% of variation was explained by significant differentiation of populations among sites separated by 0.4–13 km, which is consistent with low levels of dispersal via water movement and historical disturbance regimes. Sampling year accounted for 6–7% of total genetic variation and was related to significant coral mortality following severe bleaching in 1998 and a cyclone in 2006. Only 3% of the total genetic variation was related to coral bleaching status, reflecting generally small (8%) reductions in allelic diversity within bleached corals. This reduction probably reflected a loss of genotypes in hospite during bleaching, although no site‐wide changes in genetic diversity were observed. Combined, our results indicate the importance of disturbance regimes acting together with limited oceanographic transport to determine the genetic composition of Symbiodinium types within reefs.  相似文献   

18.
    
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19.
    
The decline of reef‐building corals in conjunction with shifts to short‐lived opportunistic species has prompted concerns that Caribbean reef framework‐building capacity has substantially diminished. Restoring herbivore populations may be a potential driver of coral recovery; however, the impact of herbivores on coral calcification has been little studied. We performed an exclusion experiment to evaluate the impact of herbivory on Orbicella faveolata coral growth over 14 months. The experiment consisted of three treatments: full exclusion cages; half cage procedural controls; and uncaged control plates, each with small O. faveolata colonies. We found that herbivorous fish exclusion had a substantial impact on both macroalgal cover and coral growth. Fleshy macroalgae reached 50% cover within some exclusion cages, but were almost absent from uncaged control plates. Critically, O. faveolata calcification rates were suppressed by almost half within exclusion cages, with monthly coral growth negatively related to overgrowth by fleshy macroalgae. These findings highlight the importance of herbivorous fishes for coral growth and the detrimental impact of macroalgal proliferation in the Caribbean. Policy makers and local managers should consider measures to protect herbivorous fishes and reduce macroalgal proliferation to enable coral communities to continue to grow and function.  相似文献   

20.
    
Identifying which factors lead to coral bleaching resistance is a priority given the global decline of coral reefs with ocean warming. During the second year of back‐to‐back bleaching events in the Florida Keys in 2014 and 2015, we characterized key environmental and biological factors associated with bleaching resilience in the threatened reef‐building coral Orbicella faveolata. Ten reefs (five inshore, five offshore, 179 corals total) were sampled during bleaching (September 2015) and recovery (May 2016). Corals were genotyped with 2bRAD and profiled for algal symbiont abundance and type. O. faveolata at the inshore sites, despite higher temperatures, demonstrated significantly higher bleaching resistance and better recovery compared to offshore. The thermotolerant Durusdinium trenchii (formerly Symbiondinium trenchii) was the dominant endosymbiont type region‐wide during initial (78.0% of corals sampled) and final (77.2%) sampling; >90% of the nonbleached corals were dominated by D. trenchii. 2bRAD host genotyping found no genetic structure among reefs, but inshore sites showed a high level of clonality. While none of the measured environmental parameters were correlated with bleaching, 71% of variation in bleaching resistance and 73% of variation in the proportion of D. trenchii was attributable to differences between genets, highlighting the leading role of genetics in shaping natural bleaching patterns. Notably, D. trenchii was rarely dominant in O. faveolata from the Florida Keys in previous studies, even during bleaching. The region‐wide high abundance of D. trenchii was likely driven by repeated bleaching associated with the two warmest years on record for the Florida Keys (2014 and 2015). On inshore reefs in the Upper Florida Keys, O. faveolata was most abundant, had the highest bleaching resistance, and contained the most corals dominated by D. trenchii, illustrating a causal link between heat tolerance and ecosystem resilience with global change.  相似文献   

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