Spatial and temporal dynamics of photosynthesis regulation of genetically defined coral/algal symbiosis associations

University of Technology, Sydney. Dept. of Environmental Sciences.Photosynthetic capacity of scleractinian corals relies predominantly on the productivity of single-celled endosymbiotic dinoflagellates of the genus Symbiodinium, known as zooxanthellae, residing intracellularly within coral endoderm tissue. The regulation of photosynthesis of zooxanthellae is in turn dependent on light and temperature. This thesis explores the genetic basis for variation in photosynthesis capacity of zooxanthellae by examining the photo-physiology of genetically characterised Symbiodinium communities at a range of spatial and temporal scales. In situ and manipulative experiments were conducted to improve our understanding of metabolic responses of zooxanthellae under climate change scenarios. Fine scale measurements of irradiance and photosynthesis allowed the assessment of photo-physiological changes across individual colonies of Pocillopora damicornis and Acropora valida. Pocillopora damicornis generally contain genetically homogeneous populations of Symbiodinium, whilst genetically diverse Symbiodinium communities exist within Acropora valida. Measurements of light absorption in P. damicornis were conducted using a scalar irradiance microprobe and it was found that light absorption was greatest in shade-adapted polyp tissue and smallest in sun-adapted coenosarc tissue. Genetic heterogeneities, found at the scale of individual polyps in A. valida, correlated with O2 concentration at the surface of the colony which was greater in polyps that harboured the two clades (A + C) than in polyps that only harboured clade C. In both corals, measurements using an O2 microelectrode and a fibre-optic microprobe yielded dissimilar results when used at moderate to high irradiances. Seasonal changes in photosynthetic capacity suggested that P. damicornis is more sensitive to combined effects of relatively higher temperature and irradiance in summer than A. valida suggesting that the symbiont community of A. valida may not be physiologically compromised possibly due to phylogenetic changes of Symbiodinium. Furthermore, thermal tolerances of conspecific corals were examined at narrow and wide spatial scales across the length of the Great Barrier Reef. Pocillopora damicornis, which harboured Symbiodinium type Cl, thus bleached in correlation with latitude, whereas Turbinaria reniformis bleached in correlation with the presence and absence of the known thermo-tolerant Symbiodinium clade D. The results, integrating over spatial and temporal scales suggest that the acclimatisation capacity of corals to light and temperature is determined by i) history of light and temperature exposure and in cases where corals associate with multiple Symbiodinium types ii) the distribution of Symbiodinium

Extracting the Temperature of Hot Carriers in Time- and Angle-Resolved Photoemission

The interaction of light with a material's electronic system creates an out-of-equilibrium (non-thermal) distribution of optically excited electrons. Non-equilibrium dynamics relaxes this distribution on an ultrafast timescale to a hot Fermi-Dirac distribution with a well-defined temperature. The advent of time- and angle-resolved photoemission spectroscopy (TR-ARPES) experiments has made it possible to track the decay of the temperature of the excited hot electrons in selected states in the Brillouin zone, and to reveal their cooling in unprecedented detail in a variety of emerging materials. It is, however, not a straightforward task to determine the temperature with high accuracy. This is mainly attributable to an a priori unknown position of the Fermi level and the fact that the shape of the Fermi edge can be severely perturbed when the state in question is crossing the Fermi energy. Here, we introduce a method that circumvents these difficulties and accurately extracts both the temperature and the position of the Fermi level for a hot carrier distribution by tracking the occupation statistics of the carriers measured in a TR-ARPES experiment.Comment: 17 pages, 5 figure

Photosynthetic impact of hypoxia on in hospite zooxanthellae in the scleractinian coral Pocillopora damicornis

Shallow water coral reefs may experience hypoxia under conditions of calm weather doldrums. Anaerobic responses of endosymbionts (i.e. zooxanthellae) within Pocillopora damicornis coral colonies were tested using both slow and fast chlorophyll a fluorescence induction kinetics. Zooxanthellae were examined in hospite when exposed to control conditions (26°C, 200 μmol photons m-2 s-1, 100% air-saturation, 4 cm s-1 flow) and to 2 treatments of reduced air content (40 and 0%), achieved by controlling the N2:O2 ratio in water circulating at 2 cm s -1. Furthermore, the impact of water flow on photosynthesis was examined at 0% air saturation by turning off the flow entirely (0 cm s -1), thereby mimicking the environmental conditions of calm weather doldrums. Corals exposed to depleted air content (0 % with and without flow) showed a significant decrease (p < 0.001) in effective quantum yield (φPSII) in comparison with controls. Maximum quantum yield was significantly reduced when gas exchange was inhibited (0% without flow), whereas non-photochemical quenching (NPQ) was not affected. Fast polyphasic fluorescence transients of chlorophyll a fluorescence showed a significant increase in minimum dark-adapted fluorescence, F0, when corals were exposed to anaerobic conditions. Furthermore, an increase in the J peak (2 ms) corresponding to the reduction of the primary electron acceptor, QA, was observed in 0% air-saturation with flow. We found that the most sensitive parameters for detecting physiological change associated with hypoxia were φPSII using slow (pulse-amplitude modulation) fluorescence kinetics, as well as an increase in the O peak, φPo(electron transport efficiency before QA), and an elevation of the J peak on a double-normalised transient using fast (Plant Efficiency Analyser) induction kinetics. © Inter-Research 2005