Menthol-DCMU bleaching and re-inoculation experiments with different diatom strains provide evidence of symbiont switching in benthic foraminifera

Marine heat waves are increasing in frequency, highlighting the urgent need to understand the thermal resilience and symbiont-switching potential of non-coral calcifiers such as large benthic foraminifera (LBF), important calcium carbonate producers and sediment generators in coral reefs. This study investigates the potential for symbiont-switching to enhance thermal resilience in the diatom-bearing foraminifer Amphistegina lobifera by conducting a controlled bleaching and re-inoculation experiment. A 4-week menthol-DCMU bleaching protocol was used to generate aposymbiotic hosts, followed by a 10-day re-inoculation experiment with three diatom strains: Nitzschia spp., Fragilaria spp. (both native to A. lobifera), and Minutocellus spp. (native to the thermally resilient Pararotalia calcariformata). Re-inoculation success was assessed by symbiont coverage (% surface area occupied by the symbiont via confocal laser scanning microscopy) and photosynthetic efficiency (Fv:Fm using pulse-amplitude modulated fluorometry). All specimens exhibited increased photosynthetic efficiency. Symbiont coverage increased by 67–102% for Fragilaria spp. and 22–108% for Nitzschia spp., with half of the individuals fully re-browning. Minutocellus spp. reached >85% coverage in two individuals and ~7% in others. These results demonstrate that A. lobifera can establish new symbioses with non-native symbionts, suggesting symbiont-switching as a viable strategy for thermal adaptation under ocean warming.