Cnidarians and their symbiotic dinoflagellates form a productive mutualism that shapes marine environments. In this symbiosis, dinoflagellate species from the family Symbiodiniacea reside within cnidarian host gastrodermal cells and provide the host with photosynthetically fixed carbon in exchange for host metabolites. This nutritional exchange allows both partners to thrive in nutrient-limited tropical environments. One important consequence of this relationship is the formation of coral reef ecosystems, which serve as important marine habitats for biodiversity. As sea surface temperatures continue to warm as a result of anthropogenic climate change, these cnidarian-Symbiodiniaceae symbioses face physiological challenges that can result in cellular stress and changes in host-symbiont biomass ratios. The success of endosymbionts relies on (1) effective recognition and uptake by host cells, (2) population growth and distribution through cell proliferation of host and symbiont cells, and (3) resilience in the face of environmental stressors. This dissertation therefore examines these aspects of host-symbiont cellular regulation during the establishment, maintenance, and breakdown of symbiosis in the sea anemone Exaiptasia pallida (commonly referred to as Aiptasia).
In cnidarians, symbiont uptake is mediated through innate immune pathways of recognition. Glycan-lectin interactions are an important subset of these pathways, in which symbiont surface glycans are recognized by cnidarian host proteins known as lectins during the onset of symbiosis. In Chapter 2, the surface glycans of symbionts were experimentally manipulated and characterized to determine the effect of altered N-glycan composition on uptake by Aiptasia. The biosynthesis pathway of N-glycans was characterized and inhibited in the Symbiodinacea species Breviolum minutum. Inhibition of the N-glycan biosynthesis pathway resulted in a significant increase in the proportion of high-mannose glycans but not in the abundance of N-glycans. Hosts experienced a decrease in the uptake of experimentally treated Breviolum minutum. This work reveals that glycan complexity plays a functional role during the establishment of symbiosis.
In Chapter 3, the examination of host-symbiont regulation continued during the proliferative colonization phase. The cell proliferation of Aiptasia was investigated in the symbiotic and aposymbiotic state, and the cell cycles of two Breviolum symbionts were analyzed from algal cultures and host isolates. Localized host cell proliferation was found to correlate with regions containing proliferating symbionts. Overall, hosts undergoing colonization had increased levels of cell proliferation compared to aposymbiotic hosts. The location of cell proliferation also significantly shifted from the epidermis in aposymbiotic hosts towards the gastrodermis in colonizing symbiotic hosts. In contrast to the relationship between proliferating host cells and their colonizing symbionts, the cell cycles of symbionts in fully symbiotic hosts appeared to be restricted. The cell cycles of Breviolum species in hospite exhibited increased S-phase populations but decreased G2M-phase populations, which resembled their respective cell cycles in nutrient-limited cultures. B. psygmophilum appeared to have increased S-phase populations and wider G1-phase population peaks than B. minutum. These cell cycle differences between species suggest a role for cell cycle regulation in mechanisms governing nutrition and host-symbiont specificity.
In Chapter 4, a noninvasive method was developed to monitor the patterns of symbiont proliferation during recolonization and thermal stress. Successful recolonization by symbiont populations consisted of local growth from symbiont clusters as well as the consistent establishment of new symbiont clusters during the first two weeks. Clusters with increased densities of symbionts declined immediately after thermal stress, whereas singlet symbiont populations persisted for a longer period. The importance of host-symbiont specificity was observed when comparing the rapid, consistent recolonization by homologous symbiont B. minutum to the slower, inconsistent recolonization by heterologous symbionts Symbiodinium microadriaticum and Durusdinium trenchii. However, after recolonization was established, B. minutum colonization was more susceptible to bleaching from the effect of thermal stress. Symbionts S. microadriaticum and D. trenchii persisted longer in Aiptasia under thermal stress. These differences in the establishment and resilience of symbiont recolonization emphasize the need for understanding the underlying mechanisms that govern successful cnidarian-dinoflagellate associations.
In summary, the work presented in this dissertation details the cellular regulation of cnidarian-Symbiodiniaceae symbioses. Differences between symbiont species and the composition of their cell surfaces have an effect on the success and nature of their symbioses with their cnidarian hosts. The results of this dissertation underscore the importance of shared cellular mechanisms that control many aspects of these symbioses, including the establishment and homeostasis of the association.