Thermal pre-treatment of algal symbiont species differentially affects coral development

The foundation of coral reef ecosystems centered around the nutritional relationship between corals and intracellular algal symbionts. Although these symbioses are highly obligate for coral hosts, many partnerships are re-established anew with each coral generation. Furthermore, climate change destabilizes the symbiosis, and the cellular mechanisms underlying successful symbiont colonization of hosts and host development, and how they are affected by thermal stress are poorly understood. Here, we explored the effect of algal species and thermal treatments on symbiont and host cell proliferation by offering Acropora tenuis larvae one of four algal species pre-exposed to elevated or ambient temperature. In addition, we characterized the cell-surface glycome of each species-temperature combination to understand its role in symbiont recognition and proliferation. We found that thermal pre-treatment negatively affected algal photosynthetic efficiency and initial symbiont density in hosts, but did not affect symbiont colonization rate or cell proliferation. In contrast, host cell proliferation was affected in a species-specific manner. Thermal pre-treatment of B. minutum and D. trenchii negatively affected host cell proliferation compared to control symbionts, whereas thermal treatment of S. microadriaticum did not affect developmental outcomes. Further, uptake of thermally pre-treated D. trenchii decreased host cell proliferation below that of larvae not offered any symbionts, indicating that this relationship is costly to host development despite the high thermal tolerance of this species. Algal surface glycan composition varied across species but not by thermal pre-treatment, suggesting reductions in density of thermally pre-treated algae may be due to changes in physiology rather than altered surface chemistry. Further, variation in glycan abundance across species did not track differences in colonization rate or symbiont density, hinting towards a smaller role of glycans in host-symbiont specificity.