Horizontal transmission of heat-evolved microalgal symbionts in adult corals

Traditional coral reef restoration methods often fail to consider rising sea-surface temperatures driven by climate change. The introduction of experimentally heat-evolved algal symbionts into corals offers a promising solution by enhancing coral holobiont thermotolerance in a relatively short timeframe. However, the scalability of this approach remains a key challenge. Coral expulsion of viable symbiont cells may provide a passive pathway for upscaling this intervention by facilitating the widespread transmission of heat-evolved symbionts and their physiological benefits across coral reefs. Here, we investigated the expulsion and horizontal transmission dynamics of heat-evolved Cladocopium proliferum (strain SS8) in the scleractinian coral Galaxea fascicularis. First, we assessed the 24-hour symbiont expulsion dynamics of three colonies of G. fascicularis hosting SS8 in addition to homologous symbionts. SS8 was detected in the expelled symbiont community of all colonies, with diel peaks in mitotic index and photochemical efficiency observed at night and the majority of expelled cells appearing morphologically intact. Second, we tested whether expelled SS8 could be acquired by chemically bleached adult G. fascicularis fragments in a custom-designed multi-lane raceway experiment. After 55 days of exposure to an SS8-expelling G. fascicularis donor, SS8 was detected at background levels (≤0.06%) in 11.1% of recipient fragments (5/45). These findings provide the first empirical evidence that viable, heat-evolved symbionts can be expelled and acquired by bleached adult corals, highlighting a potential natural pathway for the scaling up of this intervention to enhance coral thermal resilience.