Chemical bleaching severity determines uptake of heat-evolved Symbiodiniaceae in corals

Coral reefs are increasingly threatened by marine heatwaves, driving efforts to enhance coral thermal resilience. One promising approach involves engineering coral symbioses by introducing heat-evolved (HE) algal symbionts into corals. To use this intervention with adult corals, the native symbiont community needs to be depleted, typically via chemical bleaching, prior to inoculation with HE symbionts. However, host recovery and community assembly dynamics in mildly bleached corals remain poorly understood. In this study, a dose-dependent chemical bleaching (CB) experiment was conducted using menthol and diuron (DCMU) to understand how different levels of CB impact host health and acquisition of HE symbionts ( Cladocopium and Durusdinium ). We monitored coral mortality, change in area, pigmentation, and photochemical efficiency through bleaching and twelve weeks of recovery and found that impacts on host health were proportional to bleaching level, although there were strong genotypic effects. Shuffling to the previously low abundance native Durusdinium was also proportional to CB treatment, and only fragments from the severe treatment experienced significant uptake of HE symbionts. Furthermore, significant uptake was not detected until 12 weeks post-deployment, suggesting delayed niche establishment and proliferation of these symbionts. Findings from this study provide insight into symbiont community assembly processes, informing the optimisation and scaling of symbiont engineering and manipulation interventions that enhance coral resilience.