Microscale optics and coral photobiology

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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- Irradiance is among the most important factors determining the photobiology and ecophysiology of corals. However, almost nothing is known about the light field that Symbiodinium experiences within the coral host and the basic optical properties of coral tissues are unknown. This PhD thesis investigates the in vivo light field of Symbiodinium within the tissue, the optical properties of coral tissue and how coral optics modulate the light field for coral photosynthesis. The present work reveals that steep vertical light gradients and optical microniches exist in coral tissues. Yet, despite such stratified resource distribution, it is found that corals are highly efficient photosynthesisers, with local quantum efficiencies that approach theoretical limits. It is shown that coral tissues are light scattering media that cause an effective redistribution of the incident radiative energy. The light scattering mechanisms of coral tissues work like a photon trap and irradiance is ‘concentrated’ within the tissue, reaching scalar irradiance maxima over two times the incident photosynthetically active radiation. Additionally, corals have evolved mechanisms to transfer light laterally and such light transfer was identified as an important mechanism to redistribute solar energy and optimise photosynthesis across the heterogeneous coral surface. Further, unique optical properties were identified in corals, including tissue movement and the presence of highly reflective green fluorescent pigments. A nano-scale characterisation of Symbiodinium photophysiology revealed that Symbiodinium fixes carbon efficiently in low light niches, suggesting that the photosystem of Symbiodinium is adapted to the stratified resource distribution within the coral tissue. In a comparative analysis between coral tissues and plant canopies it is shown that the two systems share basic characteristics and it is thus proposed that coral tissues operate conceptually similar to plant canopies. In a coral tissue canopy, tissue optical properties and Symbiodinium photoadaptation in response to local light microgradients work in concert to optimise the energy acquisition of the coral tissue system. The evolution of flexible light scattering tissues is an important and hitherto ignored feature that enables corals to optimise the acquisition of solar energy and sustain a highly efficient photosynthetic system.
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