Coral host
Under heat stress, symbionts are expected to parasitize the coral host by sequestering host resources and proliferating in host tissues without giving photosynthates to the host (Lesser et al., 2013;Baker et al., 2018;Morris et al., 2019). Higher competition between symbionts typically results in higher virulence towards the host (Bremermann and Pickering, 1983;Chao et al., 2000). In the codominant state the symbionts might be expected to compete more, and, as our data indicate, they also grow more, potentially withholding resources from the host. All this could result in host stress, but our data do not support this prediction. Functional profiles of corals with codominant symbionts did not significantly correlate with profiles from stressed corals (Fig. 4 A, B). Instead, under codominance the corals upregulated their translational machinery (Fig. 3C, Fig. 4A), which might be an indication of higher growth rate (Elser et al., 2003;López-Maury et al., 2008;Giordiano et al., 2015;Bosdriez et al., 2015). Down-regulation of neuronal components (Fig. 4A) is also notable, but cannot be easily interpreted. More importantly, in corals with codominant symbionts the response to elevated temperature treatment was reduced (Fig. 4D), indicating higher stress resilience.