The relationship between corals and their symbiotic dinoflagellates: Environment and Host control

The ecological success and also the susceptibility of corals to bleaching have been attributed to their obligate relationship with Symbiodinium and its functional diversity. Deeper understanding of this symbiosis is essential to enable the existence of reef ecosystems. This thesis aimed to determine how the environment and the host exert control over the proteome of Symbiodinium and how this is related to the maintenance of symbiosis.Firstly, it was explored how the proteome related to the photoacclimation strategies of Symbiodinium, and its regulation under different light environments. An initial assessment of the proteome regulation using shotgun proteomics over Symbiodinium type C1 cultured at two experimental photon fluxes displayed differences in proteins associated with light harvesting, electron transport, carbon fixation and protein modification. Subsequent two-dimensional gel electrophoresis (2-DE) analysis between three Symbiodinium types (A1, A13 and C1) cultured at the two previous photon fluxes revealed unique proteome differences across these types and between both experimental light treatments. Database searches of the isoelectric point (pI) and the molecular weight (MW) tagged these proteins also associated with the same processes matched by previous shotgun analysis over C1 type. These results are in agreement with previous hypothesis on different photoacclimation strategies displayed among different types of Symbiodinium. These strategies comprised differential proteomic modifications in the photosynthetic unit (PSU) of the experimental types of Symbiodinium to balance ATP production and achieve homeostasis at different photon flux environments.The host control effect and regulation of the proteome and physiology of Symbiodinium ex- hospite was studied using two host release factors (HRFs) a free aminoacid mix (FAA) and the host tissue (HT) homogenate from the coral Pocillopora damicornis. To comprehend the relation between the proteome and physiology various parameters related with the photosynthate production and translocation of Symbiodinium were measured including: glucose released, excitation pressure over photosystem two (PSII), total protein, total glucose and chlorophyll a concentration in the presence and absence of the HRFs. The measured parameters showed variability among types. In general, the parameters directly related to photochemistry: Chl a and excitation pressure over PSII were controlled by photon flux and inversely correlated with each other. Protein and glucose also correlated with each other and were controlled by host factors although HT and FAA had an antagonistic effect of stimulation and inhibition of these parameters respectively. The principal component analysis (PCA) evidenced the existence of a general mechanism of glucose release which correlated with protein expression. The HRFs control over the proteome of Symbiodinium explored by SDS-PAGE analysis displayed different bands with differential expression among types and treatments. Some of these proteins may be related with the stimulation and inhibition of the mechanism of translocation of glucose that were found. Identification of these proteins would probably clarify the physiology behind the translocation of glucose from the Symbiodinium cell to its coral host.The proteome response and physiology of Symbiodinium was assessed by the interaction of different light environments with different temperatures. Growth patterns and photochemical measurements confirmed unique acclimation processes to light and temperature conditions across the three Symbiodinium types. These acclimation strategies appeared regulated by the differential expression of proteins involved in photosynthesis, protein modification, energy metabolism and cell maintenance. This differential expression of the proteome related to these processes seemed associated with the activation of alternative electron transport pathways to balance ATP production, regulate energy metabolism and fuel the cell at the different experimental light and temperature conditions. The results obtained provide a broader view on how the differential regulation of the proteome mediates the physiological plasticity across types of Symbiodinium to photoacclimate to different light enviroments, to acclimate to high temperature and to respond to stressful conditions determining the functional diversity existing in this genus of coral symbionts.