Early experimental comparison of natural and assisted symbiont acquisition in Stylophora pistillata

Abstract Acquisition of Symbiodiniaceae following a bleaching event is essential for coral recovery, yet most experimental approaches rely on cultured symbionts, which require specialized infrastructure and may exhibit laboratory-induced physiological alterations. Natural acquisition (NA), in which bleached corals acquire symbionts directly from healthy donors, represents a potentially relevant but understudied alternative. Here, we experimentally compared short-term recolonization dynamics under assisted acquisition (AA; laboratory-cultured symbiont cells) and natural acquisition (NA; passive transfer of released symbiont cells between coral nubbins) in menthol-bleached Stylophora pistillata over a three-week period. Partial symbiont recolonization was observed across all treatments, restoring approximately 10–15% of symbiont densities relative to unbleached controls. However, recolonization also occurred in the negative control, indicating that recovery was at least partly driven by residual symbionts remaining after bleaching. Accordingly, neither AA nor NA resulted in a statistically significant increase in symbiont density relative to the negative control. These findings suggest that, under the conditions tested, external symbiont supply did not clearly enhance early recolonization beyond background recovery. Instead, early recovery dynamics may be strongly influenced by residual symbiont proliferation. While NA remains a logistically simple and ecologically relevant approach, further optimization including increased replication, extended exposure time, and improved experimental conditions will be necessary to evaluate its potential in coral reinoculation studies.