Thermal responses and Symbiodiniaceae shuffling in a Red Sea foraminifer holobiont.

Symbiont-bearing benthic foraminifera, like corals, rely on endosymbionts for growth and nutrition and experience bleaching under heat stress. Sorites orbiculus, found in tropical and subtropical waters, hosts a diverse range of endosymbionts belonging to Symbiodiniaceae. We examined the thermal tolerance of the Red Sea population, which is the source of an established, invasive population of S. orbiculus in the Eastern Mediterranean. The Red Sea population is expected to exhibit enhanced thermotolerance, potentially due to its greater variety of endosymbionts. We conducted temperature-manipulation experiments, measuring calcification rates and net photosynthesis, evaluating the thermotolerance of S. orbiculus and its endosymbionts. We also analyzed the diversity of Symbiodiniaceae by sequencing the internal transcribed spacer 2 (ITS2). Our results show that exposure to 35°C initially induced stress in the endosymbionts during the first week; however, net photosynthesis gradually recovered in subsequent weeks. In contrast, host calcification rates remained low at 35°C, though not completely inhibited. Amplicon sequencing revealed that by the end of the experiment, a single ITS2 type, belonging to Symbiodiniaceae Clade F2 and initially present in field-collected specimens, became dominant in cultured individuals. Because this type became dominant across all temperature treatments, the observed shift likely reflects a response to laboratory conditions rather than direct thermal selection. Consequently, although the recovery at 35°C is consistent with acclimatory physiological responses, it does not provide direct evidence for symbiont-specific acclimation. Our findings suggest that holobiont physiological recovery and symbiont shuffling under laboratory conditions may contribute to the resilience of S. orbiculus in warming oceans.