Variable thermal stress tolerance of the reef-associated symbiont-bearing foraminifera Amphistegina linked to differences in symbiont type

Adaptation, acclimatization and symbiont diversity are known to regulate thermal tolerance in corals, but the role of these mechanisms remains poorly constrained in other photosymbioses, such as large benthic foraminifera (LBFs), which are known to bleach at temperatures that are likely to be exceeded in the near future. LBFs inhabit a broad range of shallow-water settings. Within species, differences in thermal tolerance have been found among populations from different habitats, but it is not clear whether such differences occur among LBFs inhabiting similar habitats, but differing in other aspects, such as symbiont type. To this end, we compared responses to thermal stress in specimens from a population of Amphistegina lessonii, an abundant Indo-Pacific species, to specimens of Amphistegina gibbosa, its Atlantic counterpart, from a similar environment but two different water depths (5 and 18 m). Test groups of each species were exposed in a common experiment to three thermal stress scenarios over a four-week period. Growth, respiration, mortality and motility were measured to characterize the holobiont response. Coloration, photosynthesis and chlorophyll a content were measured to determine the response of the endosymbiotic diatoms. The photosymbionts were characterized by genetic fingerprinting. Our results show that, although groups of A. gibbosa were collected from different habitats, their responses were similar, indicating only marginally higher tolerance to thermal peaks in specimens from the shallower site. In contrast, species-specific differences were stronger, with A. lessonii showing higher tolerance to episodic stress and less pronounced impacts of chronic stress on motility, growth and photosymbiont performance. These interspecies variations are consistent with the presence of different and more diverse symbiont assemblages in A. lessonii compared with A. gibbosa. This study demonstrates the importance of considering symbiont diversity in the assessment of intra- and interspecific variations in stress responses in LBFs.