Symbiotic zooxanthellae diversity and stability during coral bleaching in a marginal subtropical reef: A meta-haplotype approach

The symbiosis between corals and photosynthetic zooxanthellae is the foundation for the success of reef corals. Adaptive changes in zooxanthellae communities have been reported in tropical coral species during and after bleaching events, but little is known about such shifts in subtropical species. Hong Kong, located in the northern edge of the South China Sea, is a subtropical marginal environment for coral growth due to its high seasonal and short-term variability in water temperatures. Previous studies have documented the pairings of coral host and zooxanthellae in Hong Kong’s hard corals. However, most of these studies have used a low throughput approach with limited sampling effort, which could not detect rare components of symbiont communities that may become dominant in response to warming. Here we propose a comprehensive baseline study to determine the zooxanthellae community structure in subtropical Hong Kong corals. We will conduct field surveys at three locations along a sheltered to exposed gradient, and trace the spatial and temporal changes in the zooxanthellae composition. We will conduct high throughput sequencing of the internal transcribed spacer region 2 (ITS2) marker to provide a comprehensive baseline of zooxanthellae composition in four representative coral species. For a resilient species (Platygyra carnosa) and a sensitive species (Montipora peltiformis) identified in our previous field surveys conducted during natural bleaching events, we will conduct comparative laboratory thermal exposure experiments to understand the roles of symbiont communities in heat-induced bleaching and recovery, followed by functional analysis of metagenomes to identify the shifts in stress and metabolic genes underlying the differential bleaching susceptibility of the two species. Overall, this study will fill in a knowledge gap in our understanding of the roles of zooxanthellae in the subtropical corals, especially the resilience of coral–symbiont assemblages to thermal stress. The results will help us predict the responses of subtropical coral reefs to global climate change and anthropogenic disturbances.