Causation direction
Two alternative hypotheses can be put forward to explain these results, differing in the direction of the causation. The first explanation is that symbionts switch into a highly competitive physiological mode when they are near 50:50 ratio in the host, increasing their productivity in the effort to outgrow each other. This “growth race” benefits the host because of higher symbiont productivity, improving the host’s growth and boosting its stress resilience. The second explanation starts with a host that is very healthy: it is less susceptible to stress and experiences a high growth rate. The host’s growth promotes symbiont growth to occupy the newly available space, while reduced competition inside the new space allows for symbiont codominance where one genus would otherwise outcompete the other. In support of this latter hypothesis, corals with codominant symbionts had lower symbiont densities than non-codominant corals (Fig. 5); this result would have been the opposite if the codominance was associated with the higher competition between symbionts, as postulated under the first explanation. The second explanation also appears more parsimonious because it does not assume an unknown mechanism of the two symbionts sensing each other’s abundances. In addition, the notable reduction in neuronal investment in codominant corals (Fig. 2C) suggests that codominance might in part be promoted by reduced host control over the symbionts’ proliferation, assuming host neurons are actually involved in such control, which remains to be investigated in the future.
Conclusions
We have documented a strong gene expression response to the presence of a mixed symbiont community in both the symbionts and the coral host. Overall, the presence of a mixed symbiont community is associated with higher physiological fitness of all symbiotic partners involved, manifested as higher growth rate and productivity in symbionts and higher cellular growth and stress resilience in the host. It appears more likely that symbiont growth and productivity are elevated as a consequence of higher host fitness, not the other way around, which is a testable hypothesis for future research. Irrespective of the causation direction, the presence of mixed symbiont communities could potentially be used as an instant indicator of coral well-being, which would be a useful tool for coral conservation and restoration.
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