||Proteomics, photosymbiosis, foraminifera, global change, ocean warming, physiology, coral reefs, adaptation, acclimatization
||Photosymbiosis is of central importance for the wellbeing and proliferation of many tropical marine organisms, but the balance within this relationship can react delicately to changes in environmental conditions. Many of such reef-building calcifiers, e.g. corals and large benthic foraminifera (LBF), construct ecologically important habitats along the oceans tropical coasts and contribute considerably to global carbonate sediment production. Ocean warming is among the most damaging factors to coral reef ecosystems, often leading to the disruption of the photosymbiotic associations. Seen as bleaching, it may ultimately lead to mass mortality of reef-calcifiers. Understanding the characteristics that influence adaptive mechanisms of these photosymbiotic holobionts is hence crucial to project their future fate. To disentangle the drivers of holobiont resilience, characteristics influencing the stress responses of both, host and photosynthesizing symbiont, need to be considered, including differences in species-specific adaptations and in local environmental conditions that may result in different acclimatization. This thesis aims to extend the understanding of adaptive mechanisms in photosymbiotic calcifiers in modern times of ocean warming by focusing on various levels of organismal responses of the common reef-associated diatom-bearing foraminifera Amphistegina, from populations to the proteomes of host and symbiont. To test for inter-species and intra-species variations in thermal stress responses and symbiont assemblages, the widespread Indo-Pacific species A. lessonii, and its Atlantic counterpart A. gibbosa, were exposed to different ocean warming scenarios. Three thermal-stress treatments were simulated over one month in an experiment: a single thermal peak, followed by lower control temperature; episodic stress, simulated by four thermal peaks that alternated with periods at control temperature; and chronic stress. In addition to determining various parameters indicating holobiont and photosymbiont performance, the photosymbionts were characterized by genetic fingerprinting. Although test populations of A. gibbosa were collected from habitats with different temperature ranges, their responses were similar, with only marginally higher tolerance to thermal peaks in specimens from a shallower-water site as compared to a deeper-water site in the direct vicinity. In contrast, differences between species bring evidence for higher tolerance of A. lessonii as compared to A. gibbosa, as episodic stress had no and chronic stress less pronounced impact, especially with regard to photosymbionts. These inter-species variations were consistent with the presence of different and more diverse symbiont assemblages in A. lessonii, which demonstrates the importance of considering symbiont diversity in the assessment of stress response and adaptive capacity of LBF. Monitoring performance of the deeper-dwelling group of A. gibbosa over the experimental period revealed that after three to twelve days, chronic stress led to bleaching, however, without inducing mortality, which may be a result of the steep increase in total antioxidant capacity in this treatment. Single and episodic stress induced both the same minor responses. As this population experiences fluctuating temperatures in its natural habitat, it is likely adapted to thermal peaks. This highlights the potential of such variable marine environments to support resilient physiological mechanisms among photosymbiotic organisms. Nonetheless, reproduction seemed to be suppressed by episodic and chronic stress. Such possible trade-offs may have far-reaching implications for LBF communities. To reveal underlying molecular mechanisms, changes in the proteome were analyzed. A quantitative bottom-up proteomics approach was employed to link the cellular mechanisms to the observed stress responses in A. gibbosa. This offered the opportunity to separate the effect of the LBF host and its photosymbiont. High congruency to physiological parameters validated the presented novel workflow and showed major changes in the abundance of manifold proteins, induced by the different thermal-stress treatments. The proteome regulations going along with bleaching included the impairment of symbiont carbon concentrating mechanisms, and led to cell death and degradation. In the host, efficient repair mechanisms and enhanced protein synthesis maintained homeostasis. Metabolic pathways were adjusted to the symbiont loss, which demonstrates the importance of shifting feeding modes as resilience mechanism. This thesis contributes to disentangling the underlying drivers of photosymbiotic reef organisms, including the flexibility in symbiotic associations, interactions between host and symbionts, and the role of environmental factors shaping the range of their ecological constraints.