Partner Switching and Metabolic Carbon Flux under Thermal Stress in the Cnidarian-Dinoflagellate Symbiosis

Reef corals depend heavily on their symbiotic relationship with dinoflagellates of the family Symbiodiniaceae, which are their primary source of metabolic energy and hence allow them to survive in oligotrophic tropical seas. The symbiotic relationship between these two partners is exceptionally sensitive to environmental change, however, and global warming is known to induce dysbiosis (i.e., breakdown of the symbiosis) in a process referred to as ‘coral bleaching’. The adaptive bleaching hypothesis posits that the host may acquire a new dominant Symbiodiniaceae species after a bleaching event, either from a shift in the relative abundance of the resident symbionts (‘shuffling’) or the uptake of new symbionts from the environment (‘switching’), better equipping the holobiont (i.e., the whole symbiosis) for the new environmental regime. However, different symbiont types may have different nutritional implications for the coral, potentially limiting the potential for partner shuffling or switching. Energy-rich carbon compounds are primarily provided by the symbiont to their coral host as glucose, glycerol, and lipids. Yet, it is poorly understood how climate change impacts this carbon translocation and how symbiont identity influences the response. This thesis, therefore, addressed this topic, using the sea anemone Exaiptasia diaphana (‘Aiptasia’), a globally adopted model system for the study of the cnidarian-dinoflagellate symbiosis.

First, I aimed to quantify and compare pools of carbon-based metabolites under thermal stress in the host and symbiont, focusing on glucose, glycerol, and total carbohydrates (Chapter 2). It was hypothesized that thermal stress (33 °C) would cause a decline in the pools of these various metabolites, likely due to decreased photosynthetic function of the symbiont and carbon translocation to the host, combined with elevated catabolism in the host under thermal stress. Metabolites were measured using a range of commercially available metabolite-specific assay kits. As predicted, total carbohydrates in the symbionts decreased in abundance at high temperature, however, host and symbiont glucose and glycerol pools either remained constant or even increased under thermal stress relative to controls. This latter observation is consistent with gluconeogenesis (i.e., the synthesis of glucose from the likes of glycerol) in response to increased metabolic demands at high temperatures. The increased pools of glycerol, on the other hand, are consistent with the use of this metabolite as an osmolyte or moderator of cellular stress. While the test-kit approach used here was associated with a considerable amount of inter-sample variability, it nevertheless confirmed and added to previous observations gained using much more expensive, technically complex metabolomics methods.

I then compared the synthesis and translocation of photosynthates in Aiptasia under low, control, and high temperature (15, 25, 33 oC), when colonized by either Breviolum minutum, which is the only known symbiont of Aiptasia through the Indo-Pacific region (the source of the Aiptasia used here), Durusdinium trenchii or Breviolum psygmophilum (Chapter 3). D. trenchii is a thermally tolerant but opportunistic species not typically associated with Aiptasia, while Breviolum psygmophilum is often associated with temperate and sub-tropical species, including Aiptasia in the western Atlantic Ocean. I hypothesized that B. minutum would translocate more photosynthate to its host at the control temperature but that D. trenchii and B. psygmophilum would out-perform B. minutum at high and low temperatures, respectively. To test this, a radiotracer (NaH14CO3) was used to measure photosynthetic fixation and translocation. Contrary to expectations, anemones hosting D. trenchii bleached completely at both low and high temperatures. However, the population density of B. psygmophilum increased at low temperature relative to B. minutum and controls, and anemones containing B. psygmophilum did not bleach as extensively as those containing B. minutum at high temperature, confirming the reputation of the former as a ‘thermal generalist.’ With respect to carbon metabolism, B. psygmophilum performed similarly or slightly out-performed B. minutum across all temperatures, though both species provided the most nutritional benefit at the low temperature. However, at the high temperature, symbiont density had a major influence on total carbon flux to the host, lessening the impact of bleaching. Specifically, a low symbiont population density facilitated proportionally higher rates of symbiont-cell specific photosynthesis and translocation, presumably due to reduced competition for CO2, such that heat-stressed anemones continued to receive similar amounts of photosynthate to controls.

In summary, this thesis demonstrates that both thermal stress and symbiont type impact carbon flux and metabolism in the cnidarian-dinoflagellate symbiosis. Most interestingly, my findings highlight the potential importance for more research on the cellular processes that underlie the physiology of thermal generalists such as B. psygmophilum and the potential for such generalists to aid the adaptation of reef corals to climate change, either through natural processes or the development of tools for coral reef conservation.