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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