The influence of nutrient availability on the thermotolerance of symbiotic dinoflagellates of family Symbiodiniaceae

Coral reefs are the most biodiverse ocean ecosystems on the planet, providing essential habitat for over 25% of the world’s marine organisms. Their structural complexity and stability contribute to essential coastline defence against erosion, as well as provide billions of dollars per year in economic value in the form of tourism and artisanal fishing. Fundamental to this unique and indispensable habitat is the symbiotic relationship between cnidarian corals and algal dinoflagellates of family Symbiodiniaceae. Reef-building corals gain a crucial majority of their daily energy needs from their endosymbiotic dinoflagellates, which facilitates coral growth, reproduction and formation of the reef structure on which countless other organisms thrive. However, this symbiosis has come under threat from warming oceans as a consequence of anthropogenic climate change. Under thermal stress the cnidarian-dinoflagellate symbiosis breaks down, resulting in expulsion of the dinoflagellate (‘coral bleaching’), followed by the eventual death of the coral animal. Tolerance of elevated temperatures is known to vary among coral and Symbiodiniaceae species, and may be influenced by the interaction of nutrient availability and photosynthetic function of the endosymbiont. The aim of this thesis was to investigate photophysiological mechanisms in thermotolerant and thermally sensitive Symbiodiniaceae to determine how they are affected by thermal and nutritive stress, both in and out of symbiosis. Cultured Symbiodiniaceae phylotypes A4 (thermotolerant) and B2 (thermally sensitive) were subjected to high and low nutrient and temperature treatments. The photosynthetic health (quantum yield, FV/FM), rate of chloroplastic protein synthesis (D1 protein) and photophysiological response to light (NPQ) of each phylotype was monitored to determine if thermotolerance was related to nutrient utilization under heat stress. Phylotype A4 showed considerably increased D1 synthesis regardless of nutrient treatment when compared to phylotype B2, but only minor differences in FV/FM. Also, correlation between D1 concentration and FV/FM was observed in A4, but not B2 during recovery from heat stress. Responses to short term light exposure contrasted significantly between the two phylotypes under all conditions, indicating marked differences in the photophysiological apparatus. To examine nutrient use and thermotolerance in symbiosis, the model symbiotic anemone Aiptasia pallida (commonly Aiptasia) was inoculated with either Symbiodiniaceae species Breviolum minutum, phylotype A4 or phylotype B2. The holobionts (host and symbiont) were fed or starved for a period of six weeks and then subjected to heat stress. D1protein concentration and FV/FM was similar in all fed holobionts, regardless of symbiont type and heat treatment. Following heat stress, all starved holobionts showed extremely low concentrations of D1 protein, but comparable FV/FM, while in low temperature starved treatments, only Aiptasia hosting B. minutum showed any recovery of D1 protein. The study shows that efficiency of nutrient utilization in photosynthetic pathways is not necessarily an indicator of thermotolerance, nor does it dictate the ability of the symbiont to confer physiological benefits to the host under conditions of heat or nutrient stress. Rather, host-symbiont pairings most likely reflect responses to external pressures dictated by the local environment. The implications of the physiological disparities between the Symbiodiniaceae types tested are discussed in the context of environmental adaptations and host-symbiont nutrient dynamics. The complexity of symbiotic interactions highlighted by this study reinforces the imperative necessity of further investigations into cnidarian-dinoflagellate symbioses, particularly in regard to thermotolerance and photophysiology. Only through understanding the physiological effects of rising ocean temperatures on this essential partnership can we begin the work of protecting the coral reef habitat for future generations.