Adaptive strategies of carbon transformation amongst coral symbionts (Symbiodiniaceae)

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Algal endosymbionts (family Symbiodiniaceae) fuel the metabolism of reef-forming corals through uptake and utilisation of inorganic carbon (Ci) from photosynthesis. Changes in photosynthetic performance both within, and between endosymbiont taxa influence the extent of organic carbon ultimately translocated to the host coral. However, how such changes are regulated by plasticity in light harvesting, versus Ci assimilation processes remains unknown. In this thesis, I therefore built on novel approaches to assess functional diversity of fitness traits across Symbiodiniaceae to identify the extent with which Ci-uptake and incorporation differed amongst taxa and the extent with which differences could be reconciled against evolutionary adaptation across the family to sustain reef functioning in response to climate change. This thesis focused on direct assessment of Ci-uptake, and how it is linked to light harvesting and utilisation by Symbiodiniaceae both 𝘦𝘹 𝘩𝘰𝘴𝘱𝘪𝘵𝘦 (in culture) and 𝘪𝘯 𝘩𝘰𝘴𝘱𝘪𝘵𝘦 (in symbiosis with their host). I first cultured a broad range of Symbiodiniaceae taxa to assess how Ci was invested into cellular uptake, excretion, and growth; and how these metrics changed when three isolates of different thermal tolerances were subjected to sub-optimal conditions of growth. I further examined how these different thermo-tolerant Symbiodiniaceae coped with a stress-inducing increase of temperature. In parallel with photophysiology and Ci-uptake rate measurements, transcriptomics were carried out to resolve the underlying molecular network driving physiological response to heat stress. Finally, I extended this laboratory-based approach to examine Ci-uptake performance of natural coral communities across complex environmental gradients (mangrove vs. reef corals) on the Great Barrier Reef to resolve the adaptations of symbionts linked to their survival to extreme environments. My results revealed that environmental regulation outweighed evolutionary adaptation of Symbiodiniaceae in their capacity for Ci-uptake, suggesting that their ecological success predominantly relies on plasticity of upstream photosynthetic processes (efficiency of light-harvesting and non-photochemical energy quenching) rather than those downstream (Ci-uptake, assimilation, and excretion). Despite exhibiting similar trends in functional gene expression, each studied Symbiodiniaceae isolate exhibited different photophysiology and Ci-uptake rates in response to thermal stress for both (previously well studied) light reactions and dark reactions of photosynthesis. When in symbiosis, flexibility in the major Symbiodiniaceae taxa between reef and mangrove corals was associated with a reduced Ci incorporation in mangrove corals compared to reef corals. Together, these results will serve as a stepping stone to future research on the long term, aiming to improve worldwide reef health in response to global climate change.
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