Assessment of coral bleaching using symbiotic zooxanthellae density and satellite remote sensing data in the Nansha Islands,South China Sea

Coral bleaching,characterized by a significant loss of symbiotic zooxanthellae,is the primary cause of mass coral mortality and reef degradation throughout the world.The characteristics,processes,and resistance of corals to bleaching varies significantly and is dependent on environmental conditions.We documented a mass coral bleaching event in June 2007 at the Meiji and Zhubi Reefs,Nansha Islands (NS),South China Sea using ecological surveys and measurement of coral zooxanthellae density and sea surface temperatures (SST).More than 35 species of corals (between 0-20 m in depth) were bleached.These bleached corals accounted for 15.6% of total corals in the investigated quadrats.The branching corals Pocillopora and Acropora were the most vulnerable species whereas the massive corals Porites and Favia were more tolerant of the high SSTs.Surprisingly,we found no evidence of bleaching in Agariciidae corals suggesting that this family is resistant to thermal stresses.The bleached corals had lost 72%-90% of their symbiotic zooxanthellae.Furthermore,corals that had no visual signs of bleaching had also lost 31%-53% of their zooxanthellae suggesting that most corals were experiencing the early stage of bleaching.The monthly mean SST during June 2007 was 30.8°C,the highest since 1998.Based on measurements of SST and the Hotspots and DHW data (NOAA),we conclude that it the extremely high SSTs triggered this coral bleaching event.Our results suggest that the previously accepted temperature thresholds used to predict coral bleaching based on satellite data are likely to underestimate the extent and intensity of coral bleaching,at least in the NS.     

Fluorescent pigments in corals are photoprotective

All reef-forming corals depend on the photosynthesis performed by their algal symbiont, and such corals are therefore restricted to the photic zone. The intensity of light in this zone declines over several orders of magnitude--from high and damaging levels at the surface to extreme shade conditions at the lower limit. The ability of corals to tolerate this range implies effective mechanisms for light acclimation and adaptation. Here we show that the fluorescent pigments (FPs) of corals provide a photobiological system for regulating the light environment of coral host tissue. Previous studies have suggested that under low light, FPs may enhance light availability. We now report that in excessive sunlight FPs are photoprotective; they achieve this by dissipating excess energy at wavelengths of low photosynthetic activity, as well as by reflecting of visible and infrared light by FP-containing chromatophores. We also show that FPs enhance the resistance to mass bleaching of corals during periods of heat stress, which has implications for the effect of environmental stress on the diversity of reef-building corals, such as enhanced survival of a broad range of corals allowing maintenance of habitat diversity.     

涠洲岛西局部造礁石珊瑚盖度和大小频率分布

本研究通过分析珊瑚大小频率分布,探讨涠洲岛西局部造礁石珊瑚群落结构特点和种群动态,以了解珊瑚的生活史和所经历的变化。所分析的珊瑚大小数据来自2021年2月24-26日开展的涠洲岛西珊瑚礁的造礁石珊瑚样带调查,利用对数转换将珊瑚大小变换为珊瑚迭代,然后作直方图对珊瑚大小频率分布加以分析。分析结果表明,片状的牡丹珊瑚属Pavona和块状的滨珊瑚属Porites、角蜂巢珊瑚属Favites、盘星珊瑚属Dipsastraea等在涠洲岛西局部珊瑚群落中占优势(珊瑚面积和珊瑚个数)。对数转换的大小频率分布呈正态分布,偏斜不明显,变异系数小于0.5,表明这些珊瑚目前处于正常的自然补充、生长和死亡过程。涠洲岛西局部造礁石珊瑚盖度降低,主要是因为个体大的枝状珊瑚锐减。     

Communication arising. Is coral bleaching really adaptive?

From an experiment in which corals are transplanted between two depths on a Panamanian coral reef, Baker infers that bleaching may sometimes help reef corals to survive environmental change. Although Baker's results hint at further mechanisms by which reef-building corals may acclimatize to changing light conditions, we do not consider that the evidence supports his inference.     

中国珊瑚化学成分与生物活性研究新进展

综述了近10年来中国珊瑚(软珊瑚、柳珊瑚和石珊瑚)的化学成分和药理研究新进展;从珊瑚中分离出了许多结构新颖并具有强烈生理活性的化合物;化合物的类型主要涉及到甾醇类、萜类、含氮化合物类、长链脂肪酸酯和长链醇类.     

贵州布寨泥盆纪生物礁的古生物学和古生态学特征

贵州布寨泥盆纪生物礁发育在独山组的鸡泡段和鸡窝寨段,大部分是骨架礁,主要由层孔虫和床板珊瑚构成。部分是障积礁,主要由枝状床板珊瑚和藻类构成。在礁组合中可识别出4个生物群落和一个生物组合,各生物群落和生物组合发育于各自互不相同的环境。根据生物群落和生物组合的生态分析及与邻区礁组合的生物群落对比,认为布寨礁发育在环境条件较为稳定的浅海,海水的温度、含盐度、清洁度、深度和水动力条件都比较适合造礁生物生长。图2,参9。     

Predicted recurrences of mass coral mortality in the Indian Ocean

In 1998, more than 90% of shallow corals were killed on most Indian Ocean reefs. High sea surface temperature (SST) was a primary cause, acting directly or by interacting with other factors. Mean SSTs have been forecast to rise above the 1998 values in a few decades; however, forecast SSTs rarely flow seamlessly from historical data, or may show erroneous seasonal oscillations, precluding an accurate prediction of when lethal SSTs will recur. Differential acclimation by corals in different places complicates this further. Here I scale forecast SSTs at 33 Indian Ocean sites where most shallow corals died in 1998 (ref. 1) to identify geographical patterns in the timing of probable repeat occurrences. Reefs located 10-15 degrees south will be affected every 5 years by 2010-2025. North and south from this, dates recede in a pattern not directly related to present SSTs; paradoxically, some of the warmest sites may be affected last. Temperatures lethal to corals vary in this region by 6 degrees C, and acclimation of a modest 2 degrees C by corals could prolong their survival by nearly 100 years.     

涠洲岛珊瑚健康及其影响因子分析

根据2007年秋至2008年春间,涠洲岛6条主剖面15条断面的珊瑚和底质覆盖率、珊瑚补充量、死亡指数的一次两阶段的调查数据和同期4个季度月的水环境调查数据,结合以往的研究成果,初步分析了涠洲岛珊瑚健康状况及其影响因子。结果表明,调查断面珊瑚死亡指数平均为40.90%,珊瑚死亡发生于本次调查的两年前;珊瑚有恢复的迹象,但珊瑚补充量小,珊瑚补充量与活珊瑚+死珊瑚覆盖率显著相关(r=0.520,P<0.05);除汞、铅、锌和油类外,涠洲岛珊瑚礁海区水体的理化、生物因子符合一类海水水质标准,大体上呈微营养水平,春季的水体营养指数明显高于其他季节。涠洲岛珊瑚死亡率高和恢复慢,表明珊瑚处于退化中的亚健康状态。     

Interspecies and spatial diversity in the symbiotic zooxanthellae density in corals from northern South China Sea and its relationship to coral reef bleaching

Coral reef bleaching is usually characterized by expulsion of symbiotic zooxanthellae, loss of zooxanthellae pigmentation, or both. We collected 128 samples comprising 39 species of 21 genera of reef-building corals from Luhuitou and Xiaodonghai in Sanya of Hainan Island and Daya Bay of Guangdong Province, respectively, and analyzed the symbiotic zooxanthellae population density. The results show that： （1） the symbiotic zooxanthella density varies from 0.67×10^6 to 8.48×10^6 cell/cm^2, displaying significant interspecies variability, with branch corals usually having relatively less zooxanthellae （ranging from 0.67×10^6 to 2.47×10^6 cell/cm^2） than massive species （from 1.0×10^6 to 8.48×10^6 cell/cm^2）; （2） corals inhabiting within 4 m water depth have higher levels of symbiotic zooxanthellae than those living at the bottom （-7 m depth） of the reef area; （3） there is no discernable difference in the zooxanthellae density between corals from relatively high latitude Daya Bay （-22°N） and those from relatively low latitude Sanya （-18°N） at comparable sea surface temperatures （SST）; （4） in partially-bleached corals, the density of zooxanthellae shows the following order： healthy-looking part〉 semi-bleached part 〉 bleached part. Based on the above results, we suggest that （1） the zooxanthellae density difference between branching and massive coral species is the main cause that branching corals are more vulnerable to bleaching than massive corals. For example, symbiotic zooxanthellae levels are low in branching Acropora and Pocillopora corals and thus these corals are more susceptible to bleaching and mortality; （2） symbiotic zooxanthellae density can also be affected by environmental conditions, such as sediment loads, diving-related turbidity, and aquaculture-related nitrate and phosphate input, and their increase may reduce symbiotic zooxanthellae density in corals.     

Emerged fossil corals on the coast of northwestern Hainan Island, China: Implications for mid-Holocene sea level change and tectonic uplift

Fossil corals are widespread on the coast of northwestern Hainan Island, China. Most of these corals are exposed during low tide levels, indicating that a higher sea level may have existed during their life time. The radiocarbon data introduced by this paper in combination with those from other sources show that the corals were alive mainly during the midto late-Holocene. Mid-Holocene sea levels 1-3 m higher than the present level were confirmed for adjacent coasts; however, the emerged corals on the investigated coast are not necessarily indicators of a higher eustatic sea level. They do predict relative sea levels of 1.5-3.4 m when using 1 m below the tidal datum as the upper limit of coral growth. However, using the Mean Lower Low Water as the coral’s upper growth limit, the relative sea level on the non-volcanic coast was almost as high as the present level, whereas that on the volcanic coast was 0.4-2.0 m higher than present. Therefore, the exposure of these dead coral heads and their discrepancy in elevation may be the result of tectonic uplift caused by volcanic activities. From these results, the highest sea level of upward coral growth must be defined first when using fossil corals to reconstruct past sea levels. Meanwhile, vertical tectonics have to be taken into account as a significant factor when conducting high-resolution sea level reconstruction, although the South China coast is located in a "tectonically stable" region.     

涠洲岛珊瑚礁特点、演变及保护与修复对策的思考

涠洲岛珊瑚礁最早形成于约7 000年前的末次海侵,是中国大陆沿海成礁珊瑚分布最北缘的典型边缘珊瑚礁（Marginal Coral Reefs）,与区域内其他珊瑚礁缺少生态关联,其特征不同于热带珊瑚礁,即块状珊瑚占优势。20多年来,涠洲岛珊瑚礁持续退化,枝状珊瑚大量死亡,石珊瑚盖度急剧降低,反映珊瑚礁良好状况的指标生物如鹦嘴鱼、蝴蝶鱼等的数量大幅减少。近10年来,珊瑚虽未出现异常死亡现象,但珊瑚自然修复不明显,表明珊瑚补充不足。功能性植食性鱼类的缺乏,显示珊瑚礁恢复力不足。涠洲岛珊瑚礁国家级海洋公园的建立,体现了保护涠洲岛珊瑚礁的政治意愿。只有恢复鹦嘴鱼等功能性植食性鱼类的数量,才能恢复和增强珊瑚礁固有恢复力,进而促进涠洲岛珊瑚礁的自然恢复。人为修复可采取移植珊瑚断枝的方式,并以实现生态、经济和社会增益为目标,构建能应对人为干预且能持续演化的珊瑚礁。     

Coral bleaching: thermal adaptation in reef coral symbionts

Many corals bleach as a result of increased seawater temperature, which causes them to lose their vital symbiotic algae (Symbiodinium spp.) - unless these symbioses are able to adapt to global warming, bleaching threatens coral reefs worldwide. Here I show that some corals have adapted to higher temperatures, at least in part, by hosting specifically adapted Symbiodinium. If other coral species can host these or similar Symbiodinium taxa, they might adapt to warmer habitats relatively easily.     

Conventional taxonomy obscures deep divergence between Pacific and Atlantic corals

Only 17% of 111 reef-building coral genera and none of the 18 coral families with reef-builders are considered endemic to the Atlantic, whereas the corresponding percentages for the Indo-west Pacific are 76% and 39%. These figures depend on the assumption that genera and families spanning the two provinces belong to the same lineages (that is, they are monophyletic). Here we show that this assumption is incorrect on the basis of analyses of mitochondrial and nuclear genes. Pervasive morphological convergence at the family level has obscured the evolutionary distinctiveness of Atlantic corals. Some Atlantic genera conventionally assigned to different families are more closely related to each other than they are to their respective Pacific 'congeners'. Nine of the 27 genera of reef-building Atlantic corals belong to this previously unrecognized lineage, which probably diverged over 34 million years ago. Although Pacific reefs have larger numbers of more narrowly distributed species, and therefore rank higher in biodiversity hotspot analyses, the deep evolutionary distinctiveness of many Atlantic corals should also be considered when setting conservation priorities.     

High-precision U-series dating of Holocene corals from South China Sea by thermal ionization mass spectrometry (TIMS)

Using the isotopic diluting and thermal ionization mass spectrometric (TIMS) technique, the U-series dating procedure was established on the basis of measurement of national standard GBW04412. With this highly precise technique the Holocene corals from the South China Sea were dated. Comparison between this method and α-spectrometry was made. The results show that a thriving period of corals in the South China Sea appeared at 6 800-5 800 a BP, during which the paleosea level was the higher than at present in the Holocene.     

Permanent El Niño during the Pliocene warm period not supported by coral evidence

The El Ni?o/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3-5 million years ago) may have existed in a permanent El Ni?o state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean. This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change, ENSO variability--and related global climate teleconnections-could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial. Here we show that permanent El Ni?o conditions did not exist during the PWP. Our spectral analysis of the δ(18)O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, δ(18)O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ(18)O events in the fossil corals closely resemble the decreases in δ(18)O seen in the live coral during El Ni?o events. Prior research advocating a permanent El Ni?o state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.     

Coral reefs: corals' adaptive response to climate change

The long-term response of coral reefs to climate change depends on the ability of reef-building coral symbioses to adapt or acclimatize to warmer temperatures, but there has been no direct evidence that such a response can occur. Here we show that corals containing unusual algal symbionts that are thermally tolerant and commonly associated with high-temperature environments are much more abundant on reefs that have been severely affected by recent climate change. This adaptive shift in symbiont communities indicates that these devastated reefs could be more resistant to future thermal stress, resulting in significantly longer extinction times for surviving corals than had been previously assumed.     

Using the Acropora digitifera genome to understand coral responses to environmental change

Despite the enormous ecological and economic importance of coral reefs, the keystone organisms in their establishment, the scleractinian corals, increasingly face a range of anthropogenic challenges including ocean acidification and seawater temperature rise. To understand better the molecular mechanisms underlying coral biology, here we decoded the approximately 420-megabase genome of Acropora digitifera using next-generation sequencing technology. This genome contains approximately 23,700 gene models. Molecular phylogenetics indicate that the coral and the sea anemone Nematostella vectensis diverged approximately 500 million years ago, considerably earlier than the time over which modern corals are represented in the fossil record (～240 million years ago). Despite the long evolutionary history of the endosymbiosis, no evidence was found for horizontal transfer of genes from symbiont to host. However, unlike several other corals, Acropora seems to lack an enzyme essential for cysteine biosynthesis, implying dependency of this coral on its symbionts for this amino acid. Corals inhabit environments where they are frequently exposed to high levels of solar radiation, and analysis of the Acropora genome data indicates that the coral host can independently carry out de novo synthesis of mycosporine-like amino acids, which are potent ultraviolet-protective compounds. In addition, the coral innate immunity repertoire is notably more complex than that of the sea anemone, indicating that some of these genes may have roles in symbiosis or coloniality. A number of genes with putative roles in calcification were identified, and several of these are restricted to corals. The coral genome provides a platform for understanding the molecular basis of symbiosis and responses to environmental changes.     

砗磲人工繁育、资源恢复与南海岛礁生态牧场建设

南海珊瑚岛礁是海洋生态系统的重要组成部分,是区域特色海洋和渔业生物资源及其多样性的保障。砗磲是珊瑚岛礁的构成物种,具有优良的造礁、固礁护礁功能,并为其它岛礁生物提供栖息地、繁育和庇护场所；近二十多年来,在人类活动和全球环境变化影响下,砗磲及珊瑚等岛礁生物资源受到比较严重的破坏,其岛礁生态牧场天然参与者和构建者的功能受损明显。我们突破砗磲幼虫虫黄藻植入难点和变态率极低的瓶颈问题,率先在海南成功研发了砗磲规模化人工繁育和中培技术,并进行了放流增殖试验,取得了良好的效果。但目前和今后砗磲及珊瑚等生物资源情况依然严峻,期望今后开展更系统、科学、更大规模的基于砗磲、珊瑚及其它重要礁栖生物的放流增殖与生态牧场构建,全面恢复南海珊瑚岛礁的生态环境和生物资源,推动南海的可持续发展。     

Heterotrophic plasticity and resilience in bleached corals

Mass coral bleaching events caused by elevated seawater temperatures have resulted in extensive coral mortality throughout the tropics over the past few decades. With continued global warming, bleaching events are predicted to increase in frequency and severity, causing up to 60% coral mortality globally within the next few decades. Although some corals are able to recover and to survive bleaching, the mechanisms underlying such resilience are poorly understood. Here we show that the coral host has a significant role in recovery and resilience. Bleached and recovering Montipora capitata (branching) corals met more than 100% of their daily metabolic energy requirements by markedly increasing their feeding rates and CHAR (per cent contribution of heterotrophically acquired carbon to daily animal respiration), whereas Porites compressa (branching) and Porites lobata (mounding) corals did not. These findings suggest that coral species with high-CHAR capability during bleaching and recovery, irrespective of morphology, will be more resilient to bleaching events over the long term, could become the dominant coral species on reefs, and may help to safeguard affected reefs from potential local and global extinction.     

Coral mucus functions as an energy carrier and particle trap in the reef ecosystem

Zooxanthellae, endosymbiotic algae of reef-building corals, substantially contribute to the high gross primary production of coral reefs, but corals exude up to half of the carbon assimilated by their zooxanthellae as mucus. Here we show that released coral mucus efficiently traps organic matter from the water column and rapidly carries energy and nutrients to the reef lagoon sediment, which acts as a biocatalytic mineralizing filter. In the Great Barrier Reef, the dominant genus of hard corals, Acropora, exudes up to 4.8 litres of mucus per square metre of reef area per day. Between 56% and 80% of this mucus dissolves in the reef water, which is filtered through the lagoon sands. Here, coral mucus is degraded at a turnover rate of at least 7% per hour. Detached undissolved mucus traps suspended particles, increasing its initial organic carbon and nitrogen content by three orders of magnitude within 2 h. Tidal currents concentrate these mucus aggregates into the lagoon, where they rapidly settle. Coral mucus provides light energy harvested by the zooxanthellae and trapped particles to the heterotrophic reef community, thereby establishing a recycling loop that supports benthic life, while reducing loss of energy and nutrients from the reef ecosystem.