珊瑚共⽣體和⼊添在熱逆境下的反應多樣性

Response Diversity of Coral Holobionts and Recruitment Under Thermal Stress

許嘉閔(Chia-Min Hsu)

指導教授 ： 陳昭倫

國立臺灣大學/理學院/海洋研究所/博士(2016年)

https://doi.org/10.6342/NTU201603387

摘要

全球暖化持續威脅著世界的珊瑚礁,而珊瑚與共生藻形成的共生體,面對熱逆
境時有人提出其可能藉由(1)馴化:共生藻洗牌或是(2)適應:特定基因型宿主作為 存活的機制。先前研究發現墾丁潭子灣的籬枝同孔珊瑚與耐熱型 D1a 和敏感型 C3 兩型共生藻共生,且共生藻組成比例會隨著水表溫度呈現季節性洗牌變化,而廣 泛分佈在墾丁國家公園海域及第三核能發電廠熱水排放口(核三出水口)。結合 籬枝同孔珊瑚的共生特性與出水口較高的海溫環境,是為研究珊瑚共生體和珊瑚 群集面對熱逆境反應的最佳模式生物和實驗場址。本研究藉由分析其共生藻時空 變化和珊瑚宿主遺傳組成,來評估珊瑚共生體對海水高溫的反應。此外,除了珊 瑚成體階段,珊瑚幼苗入添代表著族群恢復的驅動力,但常因為不易觀察且難以 鑑種而忽略。因此本研究嘗試螢光技術來觀察珊瑚入添與估計存活率,並利用分 子生物條碼來分析入添珊瑚種類和共生藻組成,藉以連結珊瑚兩個生活史階段, 並預測未來暖化所可能造成的影響。
本研究調查 2006~2009 年間包括潭子灣與出水口等,共七個樣點的珊瑚共生藻 群聚變化,結果發現潭子灣的共生藻除了季節變動外,原先優勢的耐熱型 D1a 洗 牌成熱敏感型 C3,而樣點間呈現與海溫相關的空間差異:高溫出水口附近的珊瑚, 體內共生藻都是耐熱型 D1a;遠離出水口的礁區則共生藻多為敏感型 C3。這些結 果代表著籬枝同孔珊瑚可藉由調整體內不同生理的共生藻來調適高溫海水造成的 逆境,支持共生體藉由共生藻洗牌對逆境達成馴化。
進一步探討籬枝同孔珊瑚宿主其基因單型的多樣性與空間分布差異。利用核插 入子 PaxC 分析籬枝同孔珊瑚宿主的基因單型。於 155 個珊瑚樣本中發現 10 個基 因單型,其中 Hap1 和 Hap2 最為優勢並和 C3 或 D1a 共生,顯示不同礁區的 Hap1 和 Hap2 宿主藉由共生藻洗牌來反應海水溫度變化。然而僅出現於出水口的 D1a 專一性 Hap4,則有可能為墾丁地區最適應高溫的基因單型。未來進一步檢驗出水 口 Hap1、Hap2 和 Hap4 於高溫逆境下的反應,則可驗證之。
過去研究因為缺乏有效的型態特徵可供鑑定珊瑚入添種類,大多僅鑑定至科的 位階,為了暸解珊瑚成體族群和入添組成間的關係,本研究利用螢光偵測附著板上的入添珊瑚並利用:細胞色素氧化酶 I (COI)、核插入子 (PaxC)、粒線體開放讀 序框架 (ORF) 等進行生物條碼分析。成功利用螢光偵測收集到 150 塊附著板上共 248 個珊瑚苗,經 COI 標誌鑑定出六個屬包含軸孔 Acropora、同孔 Isopora、表孔 Montipora、鹿角 Pocillopora、微孔 Porites、雀屏 Pavona。而 PaxC 與 ORF 更可分 別將同孔屬與鹿角屬珊瑚鑑別至種。這樣的組合分析大幅改進了對珊瑚入添階段 的種類鑑定能力,以暸解「存活的」珊瑚入添組成和數量,將提供珊瑚礁入添研 究更多的資訊。
延續活珊瑚入添組成分析的成功,再整合傳統漂白附著板的分析方式,以了解 樣點的總附著率、死亡率和有效入添量。有效入添量計算成長超過存活臨界值的 固著幼苗數量,代表該樣點實際的有效入添量。以出水口當做未來暖化的模擬環 境,另外兩地點眺石和萬里桐為控制組,進一步瞭解暖化對珊瑚成功入添的影響。 結果顯示附著率於時間上的顯著差異,可能與白化事件有關,使出水口樣點第一 批 (白化隔年四月到八月) 附著率為零。此外,死亡率的結果顯示在出水口均高於 其他兩個樣點,而有效入添率於萬里桐 19.6% 和眺石 19.7%,遠大於出水口的 2.43%。依照出水口海域所呈現的低附著率以及高死亡率,未來熱逆境將可能大幅 限制造礁珊瑚成功的入添,珊瑚白化後的恢復力受限。
總結,籬枝同孔珊瑚顯示珊瑚共生體面對如 1998 年白化和熱排水等逆境的調 適韌性。雖然耐熱型共生藻和珊瑚宿主能夠在逆境下存活,但其成長和生殖卻可 能因耐熱性增加而遭受取捨。珊瑚族群的延續必須有新的有性生殖個體持續加 入,才能增加遺傳多樣性維持面對逆境反應的高度韌性。全球暖化將不僅衝擊珊 瑚成體族群的基因多樣性,生活史初期階段更容易受到傷害。這樣的結果預告著 未來全球暖化若無法控制升溫低於 20C,對全球珊瑚礁的續存將是一大考驗。藉由 分子技術分析珊瑚入添組成、存活率以及有效入添率,讓附著板研究產出更多珊 瑚礁恢復過程的資訊,將來更可推測珊瑚幼苗散佈範圍、分析上下游族群以及族 群連通性,有效的提供生態與分子資料,作為將來面對環境變遷的管理政策制定 基礎。

關鍵字

珊瑚共生體 ； 熱逆境 ； 珊瑚白化 ； 共生藻洗牌 ； 時空變化 ； 入添 ； 生物條碼

並列摘要

Global warming continues threatening the persistence of coral reefs worldwide. Acclimatization through Symbiodinium shuffling or adaptation with specific genotypes of hosts have been proposed as potential mechanisms for coral holobionts to survive through thermal stress. In previous study, Isopora palifera in Tantzei Bay of Kenting National Park (KNP) was demonstrated to shuffle thermal-tolerant Symbiodinium ITS2 D1a and thermal-sensitive Symbiodinium C3 in response to seasonal variation of sea surface temperatures. I. palifera was chosen as model species because its flexible symbiosis and wide distribution at shallow reefs in KNP, including the thermal effluent of nuclear power plant (NPP-OL), which is a suitable site to study how coral assemblage’s response to thermal stress. In this thesis, I aim to understand the capability of coral holobionts’ response to the high temperature stress. Temporal and spatial variations of Symbiodinium shuffling and host genetic analysis were used for evaluation holobionts’ responses facing warming. Additionally, coral recruitment as driving force for recovery of coral populations is generally ignored due to the difficulty in species identification. Nevertheless, fluorescence censusing and DNA barcoding were applied here to explore the diversity of coral recruitment and recruitment success at NPP-OL and other sites.
First, I re-examined the temporal dynamics of Symbiodinium community of I. palifera in TZB from 2006 to 2009. In addition, spatial variation of Symbiodinium communities in I. palifera was also examined from 6 other sites of Nanwan, KNP in 2009, including NPP-OL. Results showed that I. palifera colonies at TZB continued to show seasonal shuffling. However, symbiont community shifted from thermal-tolerant Symbiodinium D1a dominant after 1998 mass bleaching event during 2000 and 2001 to thermal-sensitive Symbiodinium C3 dominant during 2006 to 2009. Meanwhile, significant differences in spatial variation of the symbiont community in Nanwan were detected with I. palifera colonies at the NPP-OL dominated by Symbiodinium D1a. These results suggests that I. palifera can acclimatize to SST anomalies by shuffling different physiologically Symbiodinium in response to environmental change.
Second, I further explore on whether the spatial variations of Symbiodinium associated with specific host genotypes. The haplotypic variability of host genetics was examined by PaxC gene. Ten haplotypes (Hap1-Hap10) were identified from 155 samples, and Hap1 and Hap2 were the most common haplotypes found every site. Corals from both Hap 1 and 2 have association with D1a only at warmer sites, while corals from the same haplotype have association with clade C3 only, mixture of C3+ D1a, D1a only at the other sites. While Hap4 corals were exclusively found at the effluent area of NPP-OL to Maobitou (MBT), mainly associating with D1a. Overall this study demonstrates the possibility of I. palifera to acclimatize to temperature anomalies by shuffling to thermal-tolerant Symbiodinium and back shuffling to thermal-sensitive one when the stress disappeared, but also the adaptive response of the coral host to the long-term thermal stress by the selection of a specific haplotype preferentially associated with thermal-tolerant Symbiodinium.
Third, the knowledge gap of relationship between adult populations and recruit assemblages is mainly due to the lack of definitive morphological characters or any toolkit being available for higher taxonomic resolution. In order to know how the early life of coral reproduction, recruit barcoding analysis was developed here for better understand of coral populations resilience. We applied fluorescent detection of the coral recruits in combinations of different DNA-barcoding markers: cytochrome oxidase I gene (COI), open reading frame (ORF), and nuclear Pax-C intron (PaxC) methods to be useful tool for increasing the resolution of the coral spat identification in ecological studies. Using the COI DNA barcoding technique, 90.3% (224/248) of the coral spats were successfully identified into six coral genera including, Acropora, Isopora, Montipora, Pocillopora, Porites, and Pavona. PaxC further separated I. cuneata and I. palifera of Isopora from Acropora, and ORF could successfully identify the species of Pocillopora (except P. meandrina and P. eydouxi). This combination of existing approaches greatly improved the taxonomic resolution of “live” recruit identification to species levels.
Fourth, following up the success in “live” recruit identification; further examination on the remaining coral skeletons of recruits suffered post-settlement mortality was analyzed to reveal the mortality rate and effective recruitment. Effective recruitment was calculated from the number of live recruits growing over threshold size and having high possibility to join adult populations. Recruitment success plays an important role in sustaining coral populations and reef resilience after disturbances. How the thermal stress caused by rising seawater temperature impact on the recruitment success of reef-building corals remains unexplored. I deployed 120 artificial settlement plates on three sites in 2011 April along three reefs of the KNP, including NPP-OL, where yearly mean seawater temperature was 0.6-1.5 0C higher than the other two sites, Wanlitung (WLT) and Tiaoshi (TS). No spat was found on the plates in OL during April-August 2011 after bleaching event in 2010. No temporal variation of settlement rate was found in WLT and TS, but significantly different in OL. Post-settlement mortality ranged from 26.6-83.3 in WLT, 33.6-70.8 in TS, and 75-90.8% in OL. Overall, effective recruitment was estimated as 19.6% in WLT, 19.7% in TS, and 2.43% in OL. Low settlement and high post-settlement morality implied that thermal stress might suppress the recruitment success of reef-building corals in the era of climate change.
In conclusion, acclimatization through Symbiodinium shuffling or adaptation with specific genotypes of coral I. palifera have been demonstrated here as potential mechanisms for coral holobionts in response to thermal stress such as 1998 bleaching and thermal effluent in NPP-OL since 1984. The persistence of coral populations relies on successful recruitment produced sexually, which increases population genetic diversity and resilience when facing environmental stresses. Global warming not only impacts on the adult coral populations but affects on the early life phases. Bleaching event reduces fertile colony density, coral fecundity, settlement rate, and recruitment survival at NPP-OL. The lowest effective recruitment at NPP-OL suggests that thermal effluent strongly affect the early life history of coral reproduction. The low resilience potential at NPP-OL compared to the cooler sites without the warm water discharge represent the contrast reef fates in the near future. Molecular analysis on the coral recruits improves our understanding in the early ecological process of coral reef resilience. The dispersal of coral larvae, source or sink population, and genetic flow will become possible to be investigated in the future for better management policy.

並列關鍵字

coral holobionts ； thermal stress ； coral bleaching ； Symbiodinium shuffling ； recruitment ； DNA barcode

參考文獻

Abrego D, KE Ulstrup, BL Willis, MJH van Oppen. 2008. Species-specific interactions between algal endosymbionts and coral hosts define their bleaching response to heat and light stress. Proc R Soc Lond B Biol Sci 275: 2273-2282.

Ainsworth TD, SF Heron, JC Ortiz, PJ Mumby, A Grech, D Ogawa et al. 2016. Climate change disables coral bleaching protection on the Great Barrier Reef. Science 352: 338-342.

Ainsworth TD, RV Thurber, RD Gates. 2010. The future of coral reefs: A microbial perspective. Trends Ecol Evol 25: 233-240.

Amar K-O, NE Chadwick, B Rinkevich. 2008. Coral kin aggregations exhibit mixed allogeneic reactions and enhanced fitness during early ontogeny. BMC Evol Biol 8: 126-126.

Anthony KRN, DI Kline, G Diaz-Pulido, S Dove, O Hoegh-Guldberg. 2008. Ocean acidification causes bleaching and productivity loss in coral reef builders. Proc Natl Acad Sci USA 105: 17442-17446.