Coral reefs are increasingly threatened by bleaching driven by global warming, which disrupts the coral-algal symbiosis and leads to the expulsion of algal symbionts (Symbiodiniaceae), ultimately increasing coral mortality. In response, early conservation strategies focused on manipulating the algal symbionts to enhance thermal tolerance of corals, as different algal taxa can confer varying levels of stress resistance to their hosts. While this approach has shown promise, its broader application is constrained by the strong host-specificity that characterizes the coral-algal symbiosis, limiting the range of compatible and effective symbiont pairings. These limitations have prompted the exploration of alternative strategies within the coral holobiont. In particular, bacteria have emerged as a potential target for intervention, as they can influence thermal tolerance of both algal symbiont and their coral hosts and may be more flexible in the associations. This suggests that microbiome manipulation strategies could represent a more versatile and scalable approach for supporting coral conservation. In this thesis, we developed a novel method to transplant bacteria associated with a putatively thermally tolerant algal strain (Durusdinium trenchii) into a thermally sensitive algal strain (Symbiodinium microadriaticum) using contact-independent co-cultures. We then assessed whether the transplantation could confer increased thermal tolerance in the recipient algal strain by measuring the algal physiology of the cultures over a week. Microbiome transplantation resulted in the transference of 2.45% of the total ASVs between co-cultured strains. These microbial transferences were accompanied by improved photosynthetic efficiency after transplantation; however, they did not confer increased thermal tolerance in S. microadriaticum during heat stress. Despite this outcome, our study establishes a proof-of-concept framework for microbiome transplantation between algal cultures highlighting its potential as a complementary strategy for coral conservation.
