Endosymbiosis is such an intimate interaction that impacts on either partner may disrupt the other, which makes endosymbiotic species quite sensitive to environmental changes, such as elevated temperature, decreasing pH, etc. In some circumstances, the changes are so rapid that the whole ecosystem is under threaten as the endosymbiosis is disrupted, for example, the coral reefs experience bleaching when the sea temperature exceeds certain threshold. The hologenome theory proposed by E. Rosenberg and I. Zilber Rosernberg suggests that endosymbionts can adapt to such changes and confer their resistance to hosts, as they usually have shorter generation time and larger population size compared with hosts, and thus the holobiont, which is the host plus the endosymbiont, is the unit under selection. My dissertation research uses green hydra Hydra viridissima as the model system, in which green algae provide carbonate to hydra while the hydra provide CO2 and proteins in returns. The hydra can be bleached and survive without algae, and can re-associate with algae later, allowing manipulations on both the host and the endosymbiont. To date I have compared survival rate of symbiotic hydra and aposymbiotic hydra (i.e bleached hydra that algae have been deprived) under heat stress, and tried to select for UV-B tolerant algae. The result shows high variation exist across both symbiotic and aposymbiotic hydra populations, and there is correlation between the thermal tolerance of these two groups. In addition, the aposymbiotic hydra is more tolerant than their symbiotic ancestral strains. This provides a direct evidence that holobiont stress tolerance is correlated to host stress tolerance, which is usually overlooked in the past. For the artificial selection experiment, the selected algae didn’t show improved tolerance under UV-B. However, different mutated algal strains exhibit various UV-B tolerance, indicating there is possibility that a stress tolerant mutation can be induced. The next step would be testing UV-B resistance of hydra receiving these selected algae to see if hologbiont stress tolerant is related to algal stress tolerance. I will also compare acclimation/deacclimation rate of symbiotic/aposymbiotic hydra, as well as adaptation rate of algae within/without host in the future.