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Symbiotic microorganisms in reef-building corals, including algae, bacteria, archaea, fungi, and viruses, play critical roles in the adaptation of coral hosts to adverse environmental conditions. However, their adaptation and functional relationships in nutrient-rich environments have yet to be fully explored. This study investigated Duncanopsammia peltata and the surrounding seawater and sediments from protected and non-protected areas in the summer and winter in Dongshan Bay. High-throughput sequencing was used to characterize community changes, co-occurrence patterns, and factors influencing symbiotic coral microorganisms (zooxanthellae, bacteria, and archaea) in different environments. The results showed that nutrient enrichment in the protected and non-protected areas was the greatest in December, followed by the non-protected area in August. In contrast, the August protected area had the lowest nutrient enrichment. Significant differences were found in the composition of the bacterial and archaeal communities in seawater and sediments from different regions. Among the coral symbiotic microorganisms, the main dominant species of zooxanthellae is the C1 subspecies (42.22–56.35%). The dominant phyla of bacteria were Proteobacteria, Cyanobacteria, Firmicutes, and Bacteroidota. Only in the August protected area did a large number (41.98%) of SAR324_cladeMarine_group_B exist. The August protected and non-protected areas and December protected and non-protected areas contained beneficial bacteria as biomarkers. They were Nisaea, Spiroplasma, Endozoicomonas, and Bacillus. No pathogenic bacteria appeared in the protected area in August. The dominant phylum in Archaea was Crenarchaeota. These symbiotic coral microorganisms' relative abundances and compositions vary with environmental changes. The enrichment of dissolved inorganic nitrogen in environmental media is a key factor affecting the composition of coral microbial communities. Co-occurrence analysis showed that nutrient enrichment under anthropogenic disturbances enhanced the interactions between coral symbiotic microorganisms. These findings improve our understanding of the adaptations of coral holobionts to various nutritional environments. [ABSTRACT FROM AUTHOR]

• Trophic flexibility of corals with different symbionts (PdC or PdD) was assessed. • PdC had a higher autotrophic capacity than PdD, despite lower symbiont density. • Trophic strategy of PdC was more flexible than that of PdD across seasons. • Corals relied more on autotrophy in summer, but heterotrophy in winter. • Symbiont genus can determine the nutritional strategy and adaptability of corals. Coral reefs are typically found in nutrient-limited waters, which may restrict the growth and expansion of corals. Nevertheless, corals are mixotrophs that may adjust to the variation in the availability of energy sources by switching their major nutritional mode between autotrophy (i.e., synthesizing their own food by symbionts) and heterotrophy (i.e., consuming external food sources). Such trophic plasticity may, however, be subject to symbiont genus and environmental conditions. Using a widespread coral species (Pocillopora damicornis), we examine how symbiont genus (Cladocopium vs. Durusdinium) affects the physiological traits (biomass, symbiont density, C:N ratio and chlorophyll content) of corals and their trophic strategy across seasons (summer and winter) based on stable isotope markers (δ13C and δ15N). We found that corals dominated by Cladocopium (PdC) had a lower symbiont density, but higher autotrophic capacity (higher cellular chlorophyll content and higher δ13C) than those by Durusdinium (PdD). Across seasons, the trophic strategy of PdC was flexible that the autotrophic capacity was reduced by 37.1 % from summer to winter, whereas the trophic strategy of PdD remained relatively unchanged. PdC had a greater heterotrophic capacity in winter, while PdD in summer. Our findings suggest that symbiont genus can determine the trophic flexibility of corals and drive trophic niche differentiation between conspecific corals. By adaptively changing trophic strategy, intraspecific competition of corals for energy sources can be reduced, possibly allowing them to persist in oligotrophic waters and adapt to the future oceanic climate. [ABSTRACT FROM AUTHOR]

Elevated nutrients decrease the healthy status of coral communities, which can be stressful on reef corals, always resulting in decreased live scleractinian coral cover (LSCC). Here, we tested the relationship between benthic communities (scleractinian coral, macroalgae, and soft coral) and corresponding environmental parameters to illustrate the current state and potential threats of coral communities in Dongshan, Southern China. These stations were influenced to varying degrees by nutrient enrichment due to severe anthropogenic pressures. Both correlation and regression analysis indicated that increasing concentration of nitrate, nitrite and dissolved inorganic phosphate (DIP) was adversely correlated with LSCC. Conversely, a positive relationship between LSCC and aragonite saturation state (Ω arag) in the field was also confirmed. Moreover, when the regression models incorporated historical data, potential nutrient tolerance thresholds of the scleractinian coral in Dongshan were identified across the evidences of sharply LSCC decreasing (~ 2.3 μM for nitrate, ~ 0.2 μM for nitrite, and ~ 0.15 μM for DIP). This work demonstrates the environmental parameters varied under frequent anthropogenic disturbances, which in our case shaped LSCC in a potential future reef "refuge" for tropical scleractinian corals. We also confirm the forms-dependent effects of nitrogen on LSCC in a eutrophic environment, suggesting that proper management of reef associated waters, and interpretation of nutrient tolerance thresholds in coral may take nitrogen forms into consideration. [Display omitted] • Anthropogenic activities increase the nutrient levels in coastal waters • Anthropogenic nutrients shaped the distribution of coral communities. • Elevated nutrients negatively impacted live scleractinian coral cover. • Nitrogen forms are critical for excess nitrogen management in coral protection. • The coral ecological thresholds provide an efficient management framework. [ABSTRACT FROM AUTHOR]

The high demand for live corals for decorative and reef-restoration purposes has caused increased interest in land-based coral farming. However coral cultivation is frequently threatened by disease outbreaks and corallivores. Small corallivores, such as the Acropora -eating flatworm (AEFW), can cause the loss of tissues and result in the death of the acroporid corals within a short period. Here, for the first time, this study reports a new Acropora -eating flatworm (new AEFW) that feeds on corals (Acropora). There were significant differences in the size, shape, and appearance between the new AEFW and the polyclad AEFW, Prosthiostomum acroporae , which is the only reported AEFW to date. The new AEFW underwent a life history of direct development. Transmission electron microscope (TEM) observations revealed that the dorsal epidermis of the new AEFW consisted of a single layer of epidermal cells with numerous microvilli and cilia with a typical "9 + 2" pattern in the axoneme structure. The vertical rootlet was approximately three times longer than the horizontal rootlet in the anchored-rootlet system. The basal membrane was well developed with a fibrous microfibrillar layer. The new AEFWs secreted rhabdites that did not have a lamellated structure. All of these morphological and physiological traits suggested that the new AEFWs were different from the polyclad AFEW from the order Polycladida. They were likely from the order Tricladida or Proseriata. Additionally, new AEFWs exhibited acroporid-specific feeding behavior when given seven scleractinian corals. The results indicated that the new AEFWs could be a potential threat to captive acroporid corals. • This study first reports a new type of Acropora -eating flatworm. • There are significant differences in the morphometrics, ultrastructure of the epidermis, life history, and appearance between the new AEFW and the previously reported polyclad AEFW, Prosthiostomum acroporae. • The results suggest that the new AEFW is more likely from the order Tricladida or Proseriata. • This finding expands the taxa of invertebrate corallivores and indicates a potential new threat to captive acroporids. [ABSTRACT FROM AUTHOR]