Response Diversity of Coral Holobionts and Recruitment Under Thermal Stress
Date Issued
2016
Date
2016
Author(s)
Hsu, Chia-Min
Abstract
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.
Subjects
coral holobionts
thermal stress
coral bleaching
Symbiodinium shuffling
recruitment
DNA barcode
SDGs
Type
thesis