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.