Nutrient dynamics in coral symbiosis depend on both the relative and absolute abundance of Symbiodiniaceae species

Coral-associated Symbiodiniaceae exhibit host specificity but lack phylosymbiosis, with Cladocopium and Durusdinium showing different cophylogenetic patterns

Altering the composition of the Symbiodiniaceae community to adapt to anomalous sea water warming represents a potential survival mechanism for scleractinian corals. However, the processes of Symbiodiniaceae assembly and long-standing evolution of coral-Symbiodiniaceae interactions remain unclear. Here, we utilized ITS2 (internal transcribed spacer 2) amplicon sequencing and the SymPortal framework to investigate the diversity and specificity of Symbiodiniaceae across 39 scleractinian coral species. Furthermore, we tested phylosymbiosis and cophylogeny between coral hosts and their Symbiodiniaceae. In our study, environmental samples exhibited the highest Symbiodiniaceae diversity. Cladocopium and Durusdinium dominated the Symbiodiniaceae communities, with significant β-diversity differences among coral species. Additionally, host specificity was widespread in Symbiodiniaceae, especially Durusdinium spp., yet lacked a phylosymbiotic pattern. Moreover, Cladocopium spp. showed cophylogenetic congruence with their hosts, while there was no evidence for Durusdinium spp. Furthermore, host switching was the predominant evolutionary event, implying its contribution to Cladocopium diversification. These findings suggest that Symbiodiniaceae assembly does not recapitulate host phylogeny, and host specificity alone does not drive phylosymbiosis or cophylogeny. As environmental reservoirs, free-living Symbiodiniaceae may influence symbiotic communities. Additionally, Durusdinium-coral associations lack cophylogenetic signals, indicating more flexible partnerships than Cladocopium. Overall, our results enhance understanding of Symbiodiniaceae assembly and coral-Symbiodiniaceae evolutionary interactions.

Coral-Symbiodiniaceae symbiotic associations under antibiotic stress: Accumulation patterns and potential physiological effects in a natural reef

Antibiotics threaten scleractinian corals, but their accumulation patterns and physiological effects on corals in natural reefs remain unclear. This study investigated antibiotic occurrence in seawater and two coral species, Galaxea fascicularis and Pocillopora damicornis, and explored the physiological effects of bioaccumulated antibiotics in a fringing reef of the South China Sea. Nineteen antibiotic components were detected in seawater, with total antibiotic concentrations (ΣABs) ranging from 17.69 to 44.22 ng L⁻¹ . Eleven antibiotic components were accumulated in the coral hosts, and five components were observed in their algal symbionts. Higher ΣABs were significantly associated with increased total antioxidant capacity in the coral hosts of P. damicornis, while G. fascicularis exhibited a significant increase in algal symbiont density. Furthermore, ofloxacin was linked to increased algal symbiont density of G. fascicularis, while several antibiotic components, including tilmicosin, sulfapyridine, ofloxacin, and lincomycin hydrochloride, were observed to reduce antioxidant levels in the algal symbionts of G. fascicularis. No significant correlations between antibiotic components and physiological activities were detected in P. damicornis. These results highlight species-specific bioaccumulation patterns and physiological responses to antibiotics, suggesting that prolonged contaminations could destabilize coral-Symbiodiniaceae symbiosis. The findings improve understanding of the ecological risks of antibiotic pollution in reefs.

The dispersant Corexit 9500 and (dispersed) oil are lethal to coral endosymbionts

Endosymbionts (Symbiodiniaceae) play a vital role in the health of corals. Seawater pollution can harm these endosymbionts and dispersants used during oil spill cleanup can be extremely toxic to these organisms. Here, we examined the impact of oil and a specific dispersant, Corexit-9500, on two representative endosymbionts - Symbiodinium and Cladocopium - from the Southwestern endemic coral Mussismilia braziliensis. The survival and photosynthetic potential of the endosymbionts decreased dramatically after exposure to the dispersant and oil by ~25 % after 2 h and ~50 % after 7 days. Low concentrations of dispersant (0.005 ml/l) and dispersed oil (Polycyclic Aromatic Hydrocarbons, 1132 μg/l; Total Petroleum Hydrocarbons, 595 μg/l) proved highly toxic to both Symbiodinium and Cladocopium. These levels triggered a reduction in growth rate, cell size, and cell wall thickness. After a few hours of exposure, cellular organelles were damaged or destroyed. These acute toxic effects underline the fragile nature of coral endosymbionts.

Unlocking the Complex Cell Biology of Coral-Dinoflagellate Symbiosis: A Model Systems Approach

Symbiotic interactions occur in all domains of life, providing organisms with resources to adapt to new habitats. A prime example is the endosymbiosis between corals and photosynthetic dinoflagellates. Eukaryotic dinoflagellate symbionts reside inside coral cells and transfer essential nutrients to their hosts, driving the productivity of the most biodiverse marine ecosystem. Recent advances in molecular and genomic characterization have revealed symbiosis-specific genes and mechanisms shared among symbiotic cnidarians. In this review, we focus on the cellular and molecular processes that underpin the interaction between symbiont and host. We discuss symbiont acquisition via phagocytosis, modulation of host innate immunity, symbiont integration into host cell metabolism, and nutrient exchange as a fundamental aspect of stable symbiotic associations. We emphasize the importance of using model systems to dissect the cellular complexity of endosymbiosis, which ultimately serves as the basis for understanding its ecology and capacity to adapt in the face of climate change.

Coupled carbon and nitrogen cycling regulates the cnidarian-algal symbiosis

Efficient nutrient recycling underpins the ecological success of cnidarian-algal symbioses in oligotrophic waters. In these symbioses, nitrogen limitation restricts the growth of algal endosymbionts in hospite and stimulates their release of photosynthates to the cnidarian host. However, the mechanisms controlling nitrogen availability and their role in symbiosis regulation remain poorly understood. Here, we studied the metabolic regulation of symbiotic nitrogen cycling in the sea anemone Aiptasia by experimentally altering labile carbon availability in a series of experiments. Combining 13C and 15N stable isotope labeling experiments with physiological analyses and NanoSIMS imaging, we show that the competition for environmental ammonium between the host and its algal symbionts is regulated by labile carbon availability. Light regimes optimal for algal photosynthesis increase carbon availability in the holobiont and stimulate nitrogen assimilation in the host metabolism. Consequently, algal symbiont densities are lowest under optimal environmental conditions and increase toward the lower and upper light tolerance limits of the symbiosis. This metabolic regulation promotes efficient carbon recycling in a stable symbiosis across a wide range of environmental conditions. Yet, the dependence on resource competition may favor parasitic interactions, explaining the instability of the cnidarian-algal symbiosis as environmental conditions in the Anthropocene shift towards its tolerance limits.

Host nutrient sensing is mediated by mTOR signaling in cnidarian-dinoflagellate symbiosis

To survive in the nutrient-poor waters of the tropics, reef-building corals rely on intracellular, photosynthetic dinoflagellate symbionts. Photosynthates produced by the symbiont are translocated to the host, and this enables corals to form the structural foundation of the most biodiverse of all marine ecosystems. Although the regulation of nutrient exchange between partners is critical for ecosystem stability and health, the mechanisms governing how nutrients are sensed, transferred, and integrated into host cell processes are largely unknown. Ubiquitous among eukaryotes, the mechanistic target of the rapamycin (mTOR) signaling pathway integrates intracellular and extracellular stimuli to influence cell growth and cell-cycle progression and to balance metabolic processes. A functional role of mTOR in the integration of host and symbiont was demonstrated in various nutritional symbioses, and a similar role of mTOR was proposed for coral-algal symbioses. Using the endosymbiosis model Aiptasia, we examined the role of mTOR signaling in both larvae and adult polyps across various stages of symbiosis. We found that symbiosis enhances cell proliferation, and using an Aiptasia-specific antibody, we localized mTOR to symbiosome membranes. We found that mTOR signaling is activated by symbiosis, while inhibition of mTOR signaling disrupts intracellular niche establishment and symbiosis altogether. Additionally, we observed that dysbiosis was a conserved response to mTOR inhibition in the larvae of a reef-building coral species. Our data confim that mTOR signaling plays a pivotal role in integrating symbiont-derived nutrients into host metabolism and symbiosis stability, ultimately allowing symbiotic cnidarians to thrive in challenging environments.