Interpreting the sign of coral bleaching as friend vs. foe

Coral bleaching is a major concern to researchers, conservationists and the general public worldwide. To date, much of the high profile attention for bleaching has coincided with major environmental impacts and for many the term coral bleaching is synonymously associated with coral mortality (so‐called ‘lethal’ bleaching episodes). While this synonymous association has undoubtedly been key in raising public support, it carries unfair representation: nonlethal bleaching is, and always has been, a phenomenon that effectively occurs regularly in nature as corals acclimatize to regular periodic changes in growth environment (days, seasons etc). In addition, corals can exhibit sublethal bleaching during extreme environmental conditions whereby mortality does not occur and corals can potentially subsequently recover once ambient environmental conditions return. Perhaps not surprisingly it is the frequency and extent of these non and sublethal processes that yield key evidence as to how coral species and reef systems will likely withstand environmental and thus climatic change. Observations of non and sublethal bleaching (and subsequent recovery) are arguably not as readily reported as those of lethal bleaching since (1) the convenient tools used to quantify bleaching yield major ambiguity (and hence high potential for misidentification) as to the severity of bleaching; and (2) lethal bleaching events inevitably receive higher profile (media) attention and so are more readily reported. Under‐representation of non and sublethal bleaching signs may over‐classify the severity of bleaching, under‐estimate the potential resilience of reefs against environmental change, and thus ultimately limit (if not depreciate) the validity and effectiveness of reef management policies and practices. While bleaching induced coral mortality must remain our key concern it must be better placed within the context of bleaching signs that do not result in a long‐term loss of reef viability.     

Studies on mediators of manganese peroxidase for bleaching of wood pulps

In order to enhance the bleaching effect of manganese peroxidase (MnP), unsaturated fatty acids, thiol-containing compounds and various other organic compounds were applied in pulp bleaching experiments with MnP. Thiol-containing compounds did not improve the pulp bleaching effect by MnP. Some unsaturated fatty acids, linoleic acid and linolenic acid provided a better pulp bleaching effect than Tween 80. The correlation between the number of C=C bonds in a fatty acid and its pulp bleaching effect was also investigated. The MnP pulp bleaching capability was shown to depend on the carboxylic acid used. A combination of Tween 80 and a carboxylic acid resulted in higher pulp brightness than that obtained with Tween 80 alone. A laccase mediator, 3-hydroxy-1,2,3-benzotriazin-4(3H)-one, could also enhance the MnP pulp bleaching effect.     

Validation of degree heating weeks as a coral bleaching index in the northwestern Pacific

Mass bleaching is the most significant threat to coral reefs. The United States National Oceanic and Atmospheric Administration monitors world sea surface temperature (SST) and releases warnings for bleaching based on degree heating weeks (DHW), which is the accumulation of temperature anomalies exceeding the monthly maximum mean SST for a given region. DHW values >4.0 °C-weeks are thought to induce bleaching, and those >8.0 °C-weeks are thought to result in widespread bleaching and some mortality. This study validates the effectiveness of DHW as a mass bleaching index by on-site historical observation at eight sites in the northwestern Pacific Ocean. The mass bleaching events occurred during different years at different sites. The recorded years of the bleaching events matched well with DHW values >8 °C-weeks, and the logistically projected probability of bleaching against DHW showed a positive relationship. DHW provides a reasonable threshold for bleaching.     

Hydrogen Peroxide and Sodium Perborate Bleaching of Pulp from Olive Tree Residues

This paper will consider the influence of the operating conditions used in the hydrogen peroxide bleaching (concentration 1–5 % and process time 30–210 min) and sodium perborate bleaching (sodium perborate concentration 1–5 %, hydrogen peroxide 0–2% and process time 60–180 min) of olive wood trimmings pulp on the yield, kappa index and viscosity of the resulting pulp, and on strength related properties of paper sheets (stretch index and burst index) in order to determine the best bleaching conditions of this pulp. Medium to low hydrogen peroxide concentrations (1–3 %) and a high operation time (210 min) were desired in the bleaching of pulp. A high sodium perborate concentration and hydrogen peroxide concentration (5 % and 2 % respectively) and medium to low operation time (60–120 min) were desired for the sodium perborate bleaching. A comparison of both bleaching agents, under similar or under optimum operating conditions, revealed that sodium perborate bleaching results in lower brightness, a higher kappa index and also higher viscosity than hydrogen peroxide bleaching. Moreover, both provided similar stretch index and burst index values for sodium perborate bleaching with respect to hydrogen peroxide bleaching.     

The cumulative impact of annual coral bleaching can turn some coral species winners into losers

Mass coral bleaching events caused by elevated seawater temperatures result in extensive coral loss throughout the tropics, and are projected to increase in frequency and severity. If bleaching becomes an annual event later in this century, more than 90% of coral reefs worldwide may be at risk of long‐term degradation. While corals can recover from single isolated bleaching and can acclimate to recurring bleaching events that are separated by multiple years, it is currently unknown if and how they will survive and possibly acclimatize to annual coral bleaching. Here, we demonstrate for the first time that annual coral bleaching can dramatically alter thermal tolerance in Caribbean corals. We found that high coral energy reserves and changes in the dominant algal endosymbiont type (Symbiodinium spp.) facilitated rapid acclimation in Porites divaricata, whereas low energy reserves and a lack of algal phenotypic plasticity significantly increased susceptibility in Porites astreoides to bleaching the following year. Phenotypic plasticity in the dominant endosymbiont type of Orbicella faveolata did not prevent repeat bleaching, but may have facilitated rapid recovery. Thus, coral holobiont response to an isolated single bleaching event is not an accurate predictor of its response to bleaching the following year. Rather, the cumulative impact of annual coral bleaching can turn some coral species ‘winners’ into ‘losers’, and can also facilitate acclimation and turn some coral species ‘losers’ into ‘winners’. Overall, these findings indicate that cumulative impact of annual coral bleaching could result in some species becoming increasingly susceptible to bleaching and face a long‐term decline, while phenotypically plastic coral species will acclimatize and persist. Thus, annual coral bleaching and recovery could contribute to the selective loss of coral diversity as well as the overall decline of coral reefs in the Caribbean.     

Incorporating adaptive responses into future projections of coral bleaching

Climate warming threatens to increase mass coral bleaching events, and several studies have projected the demise of tropical coral reefs this century. However, recent evidence indicates corals may be able to respond to thermal stress though adaptive processes (e.g., genetic adaptation, acclimatization, and symbiont shuffling). How these mechanisms might influence warming‐induced bleaching remains largely unknown. This study compared how different adaptive processes could affect coral bleaching projections. We used the latest bias‐corrected global sea surface temperature (SST) output from the NOAA/GFDL Earth System Model 2 (ESM2M) for the preindustrial period through 2100 to project coral bleaching trajectories. Initial results showed that, in the absence of adaptive processes, application of a preindustrial climatology to the NOAA Coral Reef Watch bleaching prediction method overpredicts the present‐day bleaching frequency. This suggests that corals may have already responded adaptively to some warming over the industrial period. We then modified the prediction method so that the bleaching threshold either permanently increased in response to thermal history (e.g., simulating directional genetic selection) or temporarily increased for 2–10 years in response to a bleaching event (e.g., simulating symbiont shuffling). A bleaching threshold that changes relative to the preceding 60 years of thermal history reduced the frequency of mass bleaching events by 20–80% compared with the ‘no adaptive response’ prediction model by 2100, depending on the emissions scenario. When both types of adaptive responses were applied, up to 14% more reef cells avoided high‐frequency bleaching by 2100. However, temporary increases in bleaching thresholds alone only delayed the occurrence of high‐frequency bleaching by ca. 10 years in all but the lowest emissions scenario. Future research should test the rate and limit of different adaptive responses for coral species across latitudes and ocean basins to determine if and how much corals can respond to increasing thermal stress.     

Axial gradients of rhodopsin in light-exposed retinal rods of the toad

Exposure of an intact vertebrate eye to light bleaches the rhodopsin in the photoreceptor outer segments in spatially nonuniform patterns. Some axial bleaching patterns produced in toad rods were determined using microspectrophotometric techniques. More rhodopsin was bleached at the base of the outer segment than at the distal tip. The shape of the bleaching gradient varied with the extent of bleach and with the spectral content of the illuminant. Monochromatic light at the lambda max of the rhodopsin gave rise to the steepest bleaching gradients and induced the greatest changes in the form of the gradient with increasing extent of bleach. These results were consistent with a mathematical model for pigment bleaching in an unstirred sample. The model did not fit bleaching patterns resulting from special lighting conditions that promoted the photoregeneration of rhodopsin from the intermediates of bleaching. Prolonged light adaptation of toads could also produce axial rhodopsin gradients that were not fit by the bleaching model. Under certain conditions the axial gradient of rhodopsin in a rod outer segment reversed with time in the light: the rhodopsin content became highest at the base. This result could be explained by an interaction between the pattern of bleaching and the intracellular topography of regeneration.     

Dietary shift in corallivorous Drupella snails following a major bleaching event at Koh Tao, Gulf of Thailand

The island Koh Tao in the western Gulf of Thailand suffered severe coral bleaching in 2010. Its mushroom coral fauna of 20 species was surveyed during the bleaching in 2010 and after the bleaching in 2011. Multi-species assemblages of free-living mushroom corals occurred around the island, two of which were invaded by corallivorous Drupella snails after the bleaching. Previously these gastropods were known to mainly consume branching corals and hardly any mushroom corals. The snails were found preying on four fungiid species, three of which were susceptible to bleaching. The dietary shift became apparent after populations of preferred prey species (Acroporidae and Pocilloporidae) had died during the bleaching event. It seems that bleaching mortality reduced the availability of preferred prey, causing the corallivores to switch to less preferred species that occur in dense aggregations.     

Taxonomic patterns of bleaching within a South African coral assemblage

In 1998, the Indian Ocean coral reefs suffered a severe and extensive mass bleaching event. The thermal tolerances of corals were exceeded and their photosynthetic symbionts (zooxanthellae) lost. Mortalities of up to 90% were recorded on the reefs of Seychelles, Maldives, Kenya and Tanzania. South African coral reefs were among the few that largely escaped the 1998 mass bleaching event, but may be threatened in the future if global warming increases. This study assessed the extent of coral bleaching and partial recovery at Sodwana Bay, South Africa during 2000 and 2001. Bleaching levels in this study varied over the course of a year, which suggested that seasonally varying parameters such as sea temperature were the most likely cause of bleaching. Bleaching levels were highest at the shallowest site. However, these bleaching levels were very low in comparison with those of reefs elsewhere in the Indian Ocean. The greater volume of water over the relatively deeper reefs of Sodwana Bay may have protected the reefs from severe bleaching. Field measurements on the three reefs indicated that, although the reefs at Sodwana Bay are still healthy, bleaching increased from <1% in 1998 to 5–10% in 2002. Bleaching occurred in 26 coral genera. The Alcyonacea were highly susceptible to bleaching, especially Sarcophyton sp. Among the hard corals, Montipora spp. were the species most susceptible to bleaching. The sensitivity of these genera to early and slight increases in temperature suggests that they can forewarn of a possible greater bleaching event. In contrast, the coral genera Turbinaria and Stylophora were most resistant to bleaching.     

Large-scale bleaching of corals on the Great Barrier Reef

 The Great Barrier Reef (GBR) experienced its most intensive and extensive coral bleaching event on record in early 1998. Mild bleaching commenced in late January and intensified by late February/early March 1998. Broad-scale aerial surveys conducted of 654 reefs (∼23% of reefs on the GBR) in March and April 1998, showed that 87% of inshore reefs were bleached at least to some extent (>1% of coral cover) compared to 28% of offshore (mid- and outer-shelf) reefs. Of inshore reefs 67% had high levels of bleaching (>10% of coral) and 25% of inshore reefs had extreme levels of bleaching (>60% of coral). Fewer offshore reefs (14%) showed high levels of bleaching while none showed extreme levels of bleaching. Ground-truth surveys of 23 reefs, which experienced bleaching in intensities ranging from none to extreme, showed that the aerial survey data are likely to be underestimates of the true extent and intensity of bleaching on the GBR. The primary cause of this bleaching event is likely to be elevated sea temperature and solar radiation, exacerbated by lowered salinity on inshore and some offshore reefs in the central GBR. Accepted: 30 July 1998     

Susceptibility of central Red Sea corals during a major bleaching event

A major coral bleaching event occurred in the central Red Sea near Thuwal, Saudi Arabia, in the summer of 2010, when the region experienced up to 10–11 degree heating weeks. We documented the susceptibility of various coral taxa to bleaching at eight reefs during the peak of this thermal stress. Oculinids and agaricids were most susceptible to bleaching, with up to 100 and 80 % of colonies of these families, respectively, bleaching at some reefs. In contrast, some families, such as mussids, pocilloporids, and pectinids showed low levels of bleaching (<20 % on average). We resurveyed the reefs 7 months later to estimate subsequent mortality. Mortality was highly variable among taxa, with some taxa showing evidence of full recovery and some (e.g., acroporids) apparently suffering nearly complete mortality. The unequal mortality among families resulted in significant change in community composition following the bleaching. Significant factors in the likelihood of coral bleaching during this event were depth of the reef and distance of the reef from shore. Shallow reefs and inshore reefs had a higher prevalence of bleaching. This bleaching event shows that Red Sea reefs are subject to the same increasing pressures that reefs face worldwide. This study provides a quantitative, genus-level assessment of the vulnerability of various coral groups from within the Red Sea to bleaching and estimates subsequent mortality. As such, it can provide valuable insights into the future for reef communities in the Red Sea.     

Effects of coral bleaching on the feeding response of two species of coral-feeding fish

Coral bleaching is an increasingly prominent threat to coral reef ecosystems, not only to corals, but also to the many organisms that rely on coral for food and shelter. Coral-feeding fishes are negatively affected by coral loss caused by extensive bleaching, but it is unknown how feeding behaviour of most corallivorous fishes changes in response to coral bleaching. In this study, coral bleaching was experimentally induced in situ to examine the feeding response of two obligate corallivorous fish, Labrichthys unilineatus (Labridae) and Chaetodon baronessa (Chaetodontidae). Feeding rates were monitored before, during, and immediately after experimental bleaching of prey corals. L. unilineatus significantly increased its feeding on impacted corals during bleaching, but showed a steady decline in feeding once corals were fully bleached. Feeding response of L. unilineatus appears to parallel the expected stress-induced mucous production by bleaching colonies. In contrast, C. baronessa preferentially fed from healthy colonies over bleached colonies, although bleached colonies were consumed for five days following manipulation. Feeding by corallivorous fishes can play an important role in determining coral condition and mortality of corals following stress induced bleaching.     

Apoptosis and autophagy as mechanisms of dinoflagellate symbiont release during cnidarian bleaching: every which way you lose

Cnidarian bleaching results from the breakdown in the symbiosis between the host cnidarian and its dinoflagellate symbiont. Coral bleaching in recent years has increasingly caused degradation and mortality of coral reefs on a global scale. Although much is understood about the environmental causes of bleaching, the underlying cellular mechanisms of symbiont release that drive the process are just beginning to be described. In this study, we investigated the roles of two cellular pathways, host cell apoptosis and autophagy, in the bleaching process of the symbiotic anemone Aiptasia pallida. Host cell apoptosis was experimentally manipulated using gene knockdown of an anemone caspase by RNA interference, chemical inhibition of caspase using ZVAD-fmk and an apoptosis-inducer wortmannin. Autophagy was manipulated by chemical inhibition using wortmannin or induction using rapamycin. The applications of multiple single treatments resulted in some increased bleaching in anemones under control conditions but no significant drop in bleaching in individuals subjected to a hyperthermic stress. These results indicated that no single pathway is responsible for symbiont release during bleaching. However, when multiple inhibitors were applied simultaneously to block both apoptosis and autophagy, there was a significant reduction in bleaching in heat-stressed anemones. Our results allow us to formulate a model for cellular processes involved in the control of cnidarian bleaching where apoptosis and autophagy act together in a see-saw mechanism such that if one is inhibited the other is induced. Similar interconnectivity between apoptosis and autophagy has previously been shown in vertebrates including involvement in an innate immune response to pathogens and parasites. This suggests that the bleaching response could be a modified immune response that recognizes and removes dysfunctional symbionts.     

Predictability of coral bleaching from synoptic satellite and in situ temperature observations

Satellite and compiled in situ observations of sea surface temperatures have greatly increased the ability to detect anomalous and persistent warm water and are being widely used to predict climate change, coral bleaching and mortality. A field-based synoptic view of coral bleaching spanning eight countries and ∼35° of latitude in the western Indian Ocean tested the accuracy of synoptic temperature data derived from satellites and shipboard data to detect and predict bleaching during 2005. The ability to predict the degree of bleaching based on degree heating weeks data was moderate, but increased when past temperature anomalies and coral community susceptibility were included. It is estimated that slightly more than half of the bleaching response is due to anomalous warm water and nearly half due to taxa and community level acclimation or adaptation, where these two factors have opposing effects. Cumulative temperature anomalies do identify general areas with bleaching but both large over and underestimates of bleaching intensity were observed. Consequently, field observations are needed to confirm the synoptic satellite predictions for particular reefs, particularly where acclimation and reorganization of the coral community have occurred due to past bleaching events.     

卫星遥感珊瑚礁白化概述

珊瑚礁白化是由于珊瑚失去体内共生的虫黄藻或者共生的虫黄藻失去体内色素而导致五彩缤纷的珊瑚礁变白的现象,严重的白化可以带来珊瑚礁的死亡.国内外研究表明海水温度升高和珊瑚礁白化关系最为紧密.卫星遥感能够提供大范围、同步与连续的海洋数据,如海水表层温度和海色数据,从而能够及时监测和预测珊瑚礁的白化.基于AVHRR (Advanced Very High Resolution Radiometer),NOAA(National Oceanic and Atmospheric Administration,US)开发了全球监测珊瑚礁白化的方法,热点(HotSpot)和周热度(DHW)两种主要指数.目前,我国珊瑚礁白化现象的监测和研究明显滞后于国际动态,迫切需要发展和利用卫星遥感的方法监测南海珊瑚礁白化状况.     

Annual coral bleaching and the long-term recovery capacity of coral

Mass bleaching events are predicted to occur annually later this century. Nevertheless, it remains unknown whether corals will be able to recover between annual bleaching events. Using a combined tank and field experiment, we simulated annual bleaching by exposing three Caribbean coral species (Porites divaricata, Porites astreoides and Orbicella faveolata) to elevated temperatures for 2.5 weeks in 2 consecutive years. The impact of annual bleaching stress on chlorophyll a, energy reserves, calcification, and tissue C and N isotopes was assessed immediately after the second bleaching and after both short- and long-term recovery on the reef (1.5 and 11 months, respectively). While P. divaricata and O. faveolata were able to recover from repeat bleaching within 1 year, P. astreoides experienced cumulative damage that prevented full recovery within this time frame, suggesting that repeat bleaching had diminished its recovery capacity. Specifically, P. astreoides was not able to recover protein and carbohydrate concentrations. As energy reserves promote bleaching resistance, failure to recover from annual bleaching within 1 year will likely result in the future demise of heat-sensitive coral species.     

Laccase-mediated system pretreatment to enhance the effect of hydrogen peroxide bleaching of cotton fabric

This study evaluates the bleaching efficiency of the hydrogen peroxide bleaching process combined with laccase-mediated system pretreatment (LMS-HPBP) in the treatment of scoured cotton fabric. By changing the factors of laccase-mediated system pretreatment and the hydrogen peroxide bleaching process and examining the subsequent whiteness value and retained tensile strength of the samples, we find three LMS-HPBP processes that are more environment friendly than the conventional hydrogen peroxide bleaching process (CHPBP): (i) bleaching with lower dosage of hydrogen peroxide; (ii) bleaching at reduced temperature; (iii) bleaching for shortened duration. Whiteness, retained tensile strength and K/S values of cotton fabric samples treated by i-iii processes were similar to or higher than those by CHPBP. X-ray diffraction (XRD) analysis also demonstrated that the three processes rendered fabric of both lower crystallinity and bigger crystallite size than those by CHPBP. In addition, the "green" short-flow process was developed to treat cotton fabric and the results obtained shows this method is feasible as a new energy-saving process.     

Bleaching of corals on the Great Barrier Reef: differential susceptibilities among taxa

Large-scale coral bleaching episodes are potentially major disturbances to coral reef systems, yet a definitive picture of variation in assemblage response and species susceptibilities is still being compiled. Here, we provide a detailed analysis of the bleaching response of 4160 coral colonies, representing 45 genera and 15 families, from two depths at four sites on reefs fringing inshore islands on the Great Barrier Reef. Six weeks after the onset of large-scale bleaching in 1998, between 11 and 83% of colonies along replicate transects were affected by bleaching, and mortality was 1 to 16%. There were significant differences in bleaching response between sites, depths and taxa. Cyphastrea, Turbinaria and Galaxea were relatively unaffected by bleaching, while most acroporids and pocilloporids were highly susceptible. The hydrocorals (Millepora spp.) were the most susceptible taxa, with 85% mortality. Spatial variation in assemblage response was linked to the taxonomic composition of reef sites and their bleaching history. We suggest, therefore, that much of the spatial variation in bleaching response was due to assemblage composition and thermal acclimation. Accepted: 14 January 2000     

Radiation induced-like effects of four home bleaching agents used for tooth whitening: effects on bacterial cultures with different capabilities of repairing deoxyribonucleic acid (DNA) damage.

"Home bleaching" methods are commonly used in dentistry to correct tooth discoloration. This technique employs carbamide peroxide, in several concentrations, where the active component is hydrogen peroxide. In patients undertaking this treatment, this exposure can cause biological effects mainly due to the activity of hydrogen peroxide. Hydrogen peroxide is associated with effects induced by chemical (natural and synthetic substances) and physical agents (ionizing radiations). We have evaluated the cytotoxic effects of four commercial dental bleaching agents: Insta-Brite, Karisma, Opalescence and Whiteness. We have studied the effects of these agents on the survival of different E. coli strains with various capabilities to repair damages on the deoxyribonucleic acid (DNA): AB1157 (wild type), AB2463 (recA) and BW9091 (xthA). To determine the effect of the bleaching agents on the survival of E. coli AB1157, AB2463 and BW9091, cultures in exponential growth-phase were incubated with the bleaching agent or with 0.9% NaCl, as a control. After plating, the survival fractions were determined. The bleaching agents tested decreased the survival fractions of all strains studied and the E. coli BW9091 was the most sensitive and, moreover, these bleaching agents are capable of inducing damage to the DNA molecule. In conclusion, our results indicate that dental bleaching agents can generate biological effects like the ionizing radiations, and we suggest that dental professionals involved in bleaching to correct tooth discoloration, should control the clinical environment strictly, thus preventing contact between the oral mucosa/gingival tissues and the bleaching agents.     

Optimization of hydrogen peroxide in totally chlorine free bleaching of cellulose pulp from olive tree residues

The influence of the operating conditions used in the bleaching of olive wood trimmings pulp (viz. hydrogen peroxide concentration and time) on the yield, kappa index and viscosity of the resulting pulp and on strength-related properties of paper sheets was studied to determine the optimal bleaching conditions of this pulp. Hydrogen peroxide bleached pulps at different sequences (oxygen, ozone, chlorine dioxide and alkaline extractions) were compared. Hydrogen peroxide bleaching proved to be suitable for this pulp. Considerable improvements in viscosity were obtained with respect to other bleaching sequences such as oxygen, ozone and chlorine dioxide. Hydrogen peroxide bleaching decreased the kappa index 51.3% less than ozone bleaching, 25.0% less than chlorine dioxide (D) and 6.3% less combined chlorine dioxide-alkaline extraction (DE). To obtain kappa indices 50.9% and 37.9% lower than the index achieved by hydrogen peroxide, oxygen (LaO(p)) and ozone (LaO(LaZ)R) sequences respectively were needed. Lower-medium levels of hydrogen peroxide concentrations (1-3%) and high reaction times (210 min) proved to be suitable for bleaching of pulp olive trimming residues. This approach could be used on this residue to produce adequately bleached pulp.