The most immediate organismal response to environmental change is acclimatization through plastic changes in physiology and behavior. In terms of symbiotic interactions, the effects of climate change can result in increased reliance on partners, or the complete failure of the interaction leading to symbiont replacements or extinction of one or both partners. The specific goal of Theme I seeks to develop an integrated understanding—from genotype to phenotype—of the immediate impacts of climate change on each partner of the symbiosis. We aim to compare the mechanisms that govern these responses and outcomes across the closed-open-complex symbiosis spectrum.