The establishment and development of symbiosis in coral larvae

Coral reefs thrive because of the symbiotic partnership between corals and Symbiodinium. While this partnership is one of the keys to the success of coral reef ecosystems, surprisingly little is known about coral symbiosis, in particular, the establishment and development of symbiosis in host species which acquire symbionts anew in each generation. In this thesis, I use larvae of the genus Acropora to explore initial patterns of association between the host and Symbiodinium spp., and how environmental conditions affect the establishment and development of symbiosis. I test how competition among Symbiodinium types affects these processes, and whether these competitive effects are mediated by environmental conditions. Finally, I describe the poorly known symbiosis between coral and the recently described alga, Chromera velia.\ud \ud To explore patterns of initial association between host and symbiont, larvae of two species of Acropora were exposed to sediment collected from three different locations on the Great Barrier Reef (GBR). Acropora larvae were found to be promiscuous, taking up multiple types of Symbiodinium, while conspecific adult colonies were mostly dominated by a single Symbiodinium type. The Symbiodinium types acquired from the sediments reflected the symbiont assemblage within a wide range of hosts at each of the three sites, suggesting potential regional differences in the free-living Symbiodinium assemblage. Promiscuity allows larvae to associate with available symbionts that may be more suitable for the new environments following dispersal.\ud \ud To test the hypothesis that environmental conditions affect the establishment and development of symbiosis, Acropora larvae were exposed to monotypic cultures of Symbiodinium with different thermal tolerance under a range of temperature and light levels. Both temperature and light had a strong effect on the proportion of larvae infected and the density of symbionts within larvae, but temperature had the greater effect. The development of symbiosis at different temperatures was not always related directly to the thermal tolerance of the symbiont. Instead, each Symbiodinium and host combination had a different environmental optimum for development, indicating both host and symbiont affect the physiology of the holobiont.\ud \ud To more closely replicate the natural environment, Acropora larvae were also exposed to mixed cultures of Symbiodinium C1 and D (defined using internal transcribed spacer-1 nomenclature) to investigate whether competition among symbionts influenced infection dynamics. In addition, temperature was manipulated in these experiments. At ambient temperature Symbiodinium C1 was most abundant in the larvae, at +3.0°C competitive interactions between Symbiodinium C1 and D occurred, while at +5.5°C Symbiodinium D dominated the larvae over time. However, at +5.5°C these mixed symbioses collapsed, suggesting competition among symbionts within the host was costly to the symbionts involved and may be deleterious for the coral holobiont. As sea surface temperatures rise in response to global warming, the promiscuity of larvae may benefit corals by enabling them to establish symbiosis with Symbiodinium types with greater heat tolerance. However, because mixed populations of Symbiodinium occur in nature, the establishment of a stable symbiosis may be more difficult as symbiont compete for space and resources within the host.\ud \ud In addition to the symbiosis between coral and Symbiodinium, another potential coral symbiont, Chromera velia, was investigated. C. velia is the closest known relative of the apicomplexan parasites, and share a common ancestor with Symbiodinium. Multiple cultures of C. velia were isolated from the tissues of three coral genera on the east coast of Australia. Acropora larvae established symbiosis with C. velia, suggesting this alga may be endosymbiotic in corals. However, clumps of C. velia were also seen digesting larval tissue, suggesting this symbiont can also become parasitic under certain conditions.\ud \ud In conclusion, coral larvae associate with a wider range of Symbiodinium than adult colonies. Larval promiscuity may be adaptive by providing corals with a mechanism to acquire symbionts appropriate for new environments encountered by larvae following dispersal. With incremental rises in sea-surface temperatures the Symbiodinium types dominating the initial stages of symbiosis are likely to change, thereby changing the relative abundance of different host-symbiont combinations on coral reefs