<p><strong>The cnidarian-dinoflagellate symbiosis underpins coral reef success; however, this association is under threat due to the impact of global warming and increasing sea temperatures. Changes in weather patterns have also been projected to occur as the climate warms, leading to more frequent and intense extreme weather events, such as heavy rainfall and droughts, which often trigger sudden and/or prolonged salinity fluctuations. However, our understanding of osmoregulation in the cnidarian-dinoflagellate symbiosis is very limited, especially the homoeostatic mechanisms and physiological effects concerning the dinoflagellate. In addition, some of the most thermotolerant corals in the world dwell in high salinity seas, so it has been proposed that high salinity might confer tolerance against thermal stress. Therefore, the aim of this thesis was to characterise the effects of salinity in Exaiptasia diaphana (Aiptasia) – a model system for the cnidarian-dinoflagellate symbiosis – and cultured symbiotic dinoflagellates (family: Symbiodiniaceae), and to elucidate the relationship between hyper-salinity and thermotolerance. In Chapter 2, Aiptasia in association with its native symbiont, Breviolum minutum, was acclimated to control (35‰) or high salinity (42‰) before being subjected to a thermal stress (26 vs. 33.5 °C). All heat-treated Aiptasia experienced reduced symbiont densities, but those at the control salinity bleached significantly more and exhibited lower photosynthetic performance. At the cellular level, hyper-salinity induced the downregulation of the immune response and protein homeostasis in the host at both temperatures (26 and 33.5 °C), while the differential regulation of betaine metabolism proteins suggested that accumulation of betaines is an important osmoregulatory mechanism in Aiptasia. At elevated temperature, an increased abundance of stress-related proteins and enzymes involved in lipid and protein degradation, along with a downregulation of metabolite transporters, was detected in the host irrespective of salinity, consistent with the host receiving less nutritional benefit from the symbiont. The symbiont proteomic response notably differed from that of the host, with exposure to hyper-salinity causing extensive differences under thermal stress, but nearly none at the control temperature. A greater impairment of photosynthetic processes and a more evident shift in metabolic pathways of energy production were observed in thermally-compromised dinoflagellates at the control salinity compared to those exposed to high salinity. These results support the idea that when exposed to thermal stress, hyper-salinity confers thermotolerance in the cnidarian-dinoflagellate symbiosis, resulting in a more stable and beneficial symbiosis. The cytoprotectant/antioxidant properties of the accumulated osmolytes and the modulation of the host immune response are some of the putative mechanisms involved in this higher resistance to elevated temperature at the high salinity. In Chapter 3, a study exposing different Symbiodiniaceae (Symbiodinium microadriaticum, Breviolum minutum and Durusdinium trenchii) to increasing or decreasing salinities showed that the three species were exceptionally tolerant to gradual changes, persisting at values below 1‰ and above 60–70‰, although cell numbers declined under these conditions. This considerably exceeds the salinity range at which coral reefs are exposed on a regular basis (25–42‰). In a subsequent experiment, the physiological and proteomic response of these species was evaluated when exposed to the final salinities of 25‰ (low), 35‰ (control) and 42‰ (high). As expected, physiological responses did not show a negative impact under these conditions, though the proteome exhibited significant differences depending on the species and the treatment. Photosynthetic proteins were upregulated in S. minutum and D. trenchii at low and high salinity, while B. minutum exhibited a contradictory pattern at high salinity, with increased abundance of proteins that fix carbon and a decreased abundance of proteins that supply carbon for photosynthesis. Lipid metabolism was affected by salinity, and this also depended on species and salinity treatment. Increased abundance of antioxidants and chaperones was observed in all species, with S. microadriaticum showing a greater abundance at low salinity, D. trenchii at high salinity, and B. minutum at both. High salinity induced an increase in methionine cycle enzymes in all species, suggesting that this is a conserved response to hyper-osmotic conditions. The differential regulation of protein kinases and phosphatases among salinities was noted, emphasising the role of post-translational modifications for dealing with environmental change. Finally, a few ion pumps and channels were differentially abundant, especially those that decrease the ion permeability of the membrane at low salinity, preventing the leakage of essential ions.</strong></p><p>This thesis has provided detailed cellular and molecular insight into the mechanisms underlying the effects of stressors associated with climate change in the model system Aiptasia and in different Symbiodiniaceae species, which has the potential to serve as a foundation for future research on coral bleaching, and for the development of tools to mitigate the impacts of global warming.</p>
