The Impact of Symbiont Diversity on Cellular Integration and Function in the Cnidarian-Dinoflagellate Symbiosis

Coral reef ecosystems are one the most important tropical ecosystems, providing a wide variety of ecological services. Reef-building corals form a symbiosis with phototrophic dinoflagellates of the family Symbiodiniaceae, however elevated temperatures disrupt this relationship in a process known as coral bleaching and worldwide bleaching events are increasing in frequency and intensity due to climate change. There is considerable interest in whether corals might adapt to warming conditions by changing their symbionts to a more thermally-tolerant type. The potential to respond is related to capacity for cellular integration between the partners, which underlies the specificity of the host-symbiont relationship. The aim of this thesis was to elucidate the cellular processes associated with hosting native vs. non-native symbionts in the model symbiotic cnidarian Exaiptasia pallida (commonly referred to as ‘Aiptasia’), to assess the potential for establishing a novel partnership. A multidisciplinary approach, including proteomics and quantitative immunocytochemistry, was used, with a particular focus on processes involved in inter-partner nutritional exchange.

In Chapter 2, a new method for characterising the proteome of Symbiodiniaceae is described. This method was used to compare the molecular and metabolic pathways underlying a successful symbiosis. Specifically, the proteome of Breviolum minutum, the native symbiont of Aiptasia, was compared when in culture (i.e., free-living) vs. in symbiosis, under a range of nutritional regimes (i.e., N-deplete or N-enriched in culture; starved or well-fed anemones in symbiosis). These various treatments induced distinct proteomes in B. minutum, especially related to immunosuppression to avoid host cell phagosome maturation, metabolic integration, and increased oxidative stress in the symbiotic state. Moreover, the different nutritional regimes impacted the B. minutum proteome, with evidence for increased efficiency nutrient uptake and assimilation under more nutrient-limited conditions.

Chapter 3 further characterised the proteome of B. minutum during the process of host colonization, and then compared this to the proteome of a non-native symbiont, the putatively opportunistic but thermally tolerant Durusdinium trenchii, when in symbiosis with Aiptasia. During host colonization over a fourteen-week period, the proteome of B. minutum showed changes related to photosynthesis and chloroplast maintenance as the symbiont density increased, until it eventually reached its peak. Conversely, the proteome of the non-native D. trenchii exhibited a lower abundance of photosynthetic proteins in the fully-established symbiosis, but an upregulation of parasite-like immunosuppression mechanisms, consistent with the view of this species of Symbiodiniaceae being opportunistic.

Lastly, in Chapter 4, host nutrient transporters were localized and quantified in Aiptasia, colonized with either the native B. minutum or the non-native D. trenchii and Symbiodinium microadriaticum. Specific immunofluorescent antibodies were designed for four different host transporters of interest: ammonium transporter 1 (AMT1); V-type proton ATPase (VHA); facilitated glucose transporter member 8 (GLUT8); and aquaporin-3 (AQP3, a sugar transporter). Tissue sections were then analysed by confocal microscopy. Hosts harbouring the non-native symbionts showed different transporter localization patterns to those harbouring the native B. minutum, suggesting a lesser degree of host-symbiont integration and disrupted nutritional flux with the non-native symbionts, with S. microadriaticum-colonized anemones being particularly distinct.

This thesis therefore adds further weight to the view that the cellular integration necessary to establish a functional symbiosis, with efficient inter-partner nutritional exchange, is not necessarily replicated with non-native symbiont species, which in some cases may persist even when symbiosis function is compromised. Ultimately, this will likely reduce the likelihood that corals might adapt to climate change by changing their symbiont population. On a more positive note, however, this thesis provides new insights into the fundamental cellular mechanisms that underlie a successful cnidarian-dinoflagellate symbiosis and contribute to observable patterns of host-symbiont specificity. Such information is essential if we are to develop the tools necessary for saving the world’s coral reefs, for example through gene-editing and the engineering of more thermally tolerant coral-dinoflagellate partnerships.