Southern Taiwan’s Kenting National Park is a popular retreating place for many domestic and international tourists, with increasing tourist numbers potentially over-burdening the coastal ecosystems. To better understand human impacts, a long-term ecological research program was initiated in 2001 to track water quality at 14 coral reef-abutting sites throughout the park since then. Extracting the data from this 20-year survey, we found that increasing in the nutrient levels during the summer rainy season, together with the drops in salinity led by freshwater inputs (land- & rainfall-derived), was the main impact to coral reef ecosystem of Kenting. Cluster analysis further confirmed the nutrient influx was mainly attributed to the local discharge outlets with dense of villages and hotels at upstream. Therefore, more efforts are needed to input to control tourist number, treat waste water discharge and strengthen land protection facilities.

Global warming threatens reef-building corals with large-scale bleaching events; therefore, it is important to discover potential adaptive capabilities for increasing their temperature resistance before it is too late. This study presents two coral species (Platygyra verweyi and Isopora palifera) surviving on a reef having regular hot water influxes via a nearby nuclear power plant that exhibited completely different bleaching susceptibilities to thermal stress, even though both species shared several so-called “winner” characteristics (e.g., containing Durusdinium trenchii, thick tissue, etc.). During acute heating treatment, algal density did not decline in P. verweyi corals within three days of being directly transferred from 25 to 31 °C; however, the same treatment caused I. palifera to lose < 70% of its algal symbionts within 24 h. The most distinctive feature between the two coral species was an overwhelmingly higher constitutive superoxide dismutase (ca. 10-fold) and catalase (ca. 3-fold) in P. verweyi over I. palifera. Moreover, P. verweyi also contained significantly higher saturated and lower mono-unsaturated fatty acids, especially a long-chain saturated fatty acid (C22:0), than I. palifera, and was consistently associated with the symbiotic bacteria Endozoicomonas, which was not found in I. palifera. However, antibiotic treatment and inoculation tests did not support Endozoicomonas having a direct contribution to thermal resistance. This study highlights that, besides its association with a thermally tolerable algal symbiont, a high level of constitutive antioxidant enzymes in the coral host is crucial for coral survivorship in the more fluctuating and higher temperature environments.

Background: Large-scale coral bleaching events have become increasingly frequent in recent years. This process occurs when corals are exposed to high temperatures and intense light stress, leading to an overproduction of reactive oxygen species (ROS) by their endosymbiotic dinoflagellates. The ROS buildup prompts corals to expel these symbiotic microalgae, resulting in the corals’ discoloration. Reducing ROS production and enhancing detoxification processes in these microalgae are crucial to prevent the collapse of coral reef ecosystems. However, research into the cell physiology and genetics of coral symbiotic dinoflagellates has been hindered by challenges associated with cloning these microalgae. Results: A procedure for cloning coral symbiotic dinoflagellates was developed in this study. Several species of coral symbionts were successfully cloned, with two of them further characterized. Experiments with the two species isolated from Turbinaria sp. showed that damage from light intensity at 340 μmol photons/m²/s was more severe than from high temperature at 36 °C. Additionally, preincubation in high salinity conditions activated their endogenous tolerance to bleaching stress. Pretreatment at 50 ppt salinity reduced the percentage of cells stained for ROS by 59% and 64% in the two species under bleaching stress compared to those incubated at 30 ppt. Furthermore, their Fv’/Fm’ during the recovery period showed a significant improvement compared to the controls. Conclusions: These findings suggest that intense light plays a more important role than high temperatures in coral bleaching by enhancing ROS generation in the symbiotic dinoflagellates. The findings also suggest the genomes of coral symbiotic dinoflagellates have undergone evolutionary processes to develop mechanisms, regulated by gene expression, to mitigate damages caused by high temperature and high light stress. Understanding this gene expression regulation could contribute to strengthening corals’ resilience against the impact of global climate change.

Coral bleaching is caused by the exocytosis of endosymbiotic dinoflagellates due to overproduction of reactive oxygen species (ROS) in the symbionts when corals are under high temperature and high light stress. Strategies to reduce ROS production and enhance ROS scavenging efficiency in coral symbionts are crucial for preventing the collapse of coral reef ecosystems. However, cell physiology studies of coral symbionts have been hampered by the difficulty of cloning the symbionts. A simple procedure was developed for cloning coral symbionts in this work. Experiments performed with two species of symbionts isolated from Turbinaria sp. revealed that the damages caused by high light at 340 umol photon/m2/s1 was more severe than those caused by high temperature at 36 °C, and high salinity activated endogenous tolerance against the bleaching stress. Pretreatment at 50 ppt salinity reduced the percentage of cells stained for ROS by 59% and 64% in the two species exposed to the bleaching stress compared with those incubated at 30 ppt, and their Fv’/Fm’ recoveries were also improved significantly. These findings suggest that the genomes of coral symbionts have undergone evolutionary processes to develop mechanisms for ameliorating damages caused by high salinity, temperature, and light stress.

Phototactic behavior, e.g., locomotion toward light direction, in symbiotic sea anemones is thought to be driven by algal photosynthesis, yet the mechanism by which photosynthetically derived signals influence host muscle activity remains unclear. This is especially challenging to study due to the semi-sessile nature of anemones, which limits their physical responsiveness to light. Here, we report a novel light-induced column-bending response in the model symbiotic anemone Exaiptasia pulchella , and assess the role of algal photosynthesis in mediating this behavior. Following light exposure, anemones exhibited a phototropic response with a latent period of 5.3 ± 2.7 s and a maximum bending angle of 40° ± 17°. This response was featured a refractory period of approximately 120 s. Application of DCMU (a photosystem II inhibitor) and acetazolamide (a carbonic anhydrase inhibitor) resulted in a dose-dependent suppression of the bending response, implicating photosynthetic activity in behavioral initiation. Inhibition by 2-hydroxyethanesulfonic acid (a taurine transporter antagonist) and MgCl₂ further suggests that taurine may function as a signaling molecule released from endodermal cells, triggering neural and muscular activation. While the complete signaling pathway remains unresolved, these results offer new insight into host-symbiont integration and photosynthetically induced behavior in cnidarians.