Abstract
The symbiosis of coral-Symbiodiniaceae is the quintessential basis of the coral reef ecosystem, and its breakdown results in coral bleaching, one of the most severe ecological catastrophes in the ocean. Critical to the establishment of the symbiosis is the host’s specific recognition of the symbionts through the binding of the coral host’s pattern recognition receptors (PRRs) to the symbiont cell surface’s glycoconjugates. However, the molecular basis for this recognition process is poorly understood. The present study investigated the binding affinities of the coral galectin PdGLT-1 to different symbiodiniacean species under different temperatures. At 25°C, the PdGLT-1 recombinant protein (rPdGLT-1) exhibited different binding affinities to different symbiodiniacean species from five genera, with a significantly higher binding affinity (p < 0.05) to Fugacium kawagutii (2.6-fold) and Cladocopium goreaui (1.9-fold) than Symbiodinium microadriaticum. The binding topology of rPdGLT-1 differed among the five symbiodiniacean species; for S. microadriaticum, Breviolum minutum, and Durusdinium trenchii, the binding was on some specific sites on the cell surface, whereas for C. goreaui and F. kawagutii, the binding signals were detected over the whole cell surface. Interestingly, PdGLT-1 binding induced agglutination of F. kawagutii cells but not of C. goreaui, explaining why C. goreaui was the most dominant symbiodiniacean symbionts in corals. Moreover, the affinity of rPdGLT-1 to Symbiodiniaceae was affected by temperature, and the highest binding affinities were observed at 30, 20, 30, 35, and 30°C for S. microadriaticum, B. minutum, C. goreaui, D. trenchii, and F. kawagutii, respectively. The optimal binding temperatures were consistent with the current understanding that D. trenchii was the most thermal resistant among these species. These results suggest that the binding affinity of the PRR PdGLT-1 may determine the specificity of host-symbiont pairing and explain why Cladocopium is the dominant symbionts of coral P. damicornis at normal temperature, and corals with Durusdinium symbionts may survive better at high temperature.
Highlights
Due to the unrelentless global warming, coral reefs are facing the danger of massive bleaching predicted for the end of this century (Hoegh-Guldberg et al, 2007; Magel et al, 2019; Weis, 2019)
A binding band was observed for each of the five symbiodiniacean species examined at 25◦C, and the band intensity increased with increasing algal concentration from 10 to 105 cells mL−1, especially for B. minutum and D. trenchii (Figure 1)
The establishment of the coral-Symbiodiniaceae symbiosis is essential to the prosperity of the coral reef ecosystem (Wood-Charlson et al, 2006)
Summary
Due to the unrelentless global warming, coral reefs are facing the danger of massive bleaching predicted for the end of this century (Hoegh-Guldberg et al, 2007; Magel et al, 2019; Weis, 2019). The cell surface of Symbiodiniaceae is populated with glycoconjugates, such as mannose-mannose and galactose-β(1-4)-N-acetylglucosamine (Markell et al, 1992; Tortorelli et al, 2021) These glycoconjugates, which form the microbe-associated molecular patterns (MAMPs), can be recognized by the pattern recognition receptors (PRRs) such as lectins on the surface of coral cells (Wood-Charlson et al, 2006; Weis et al, 2008). Such glycan-lectin interaction is critical for initiating the coral-Symbiodiniaceae symbiosis (Tortorelli et al, 2021). The molecular mechanisms and factors affecting the establishment and maintenance of the symbiosis between corals and Symbiodiniaceae are still poorly understood
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