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Light quantity and quality are among the most important factors determining the physiology and stress response of zooxanthellate corals. Yet, almost nothing is known about the light field that Symbiodinium experiences within their coral host, and the basic optical properties of coral tissue are unknown. We used scalar irradiance microprobes to characterize vertical and lateral light gradients within and across tissues of several coral species. Our results revealed the presence of steep light gradients with photosynthetically available radiation decreasing by about one order of magnitude from the tissue surface to the coral skeleton. Surface scalar irradiance was consistently higher over polyp tissue than over coenosarc tissue in faviid corals. Coral bleaching increased surface scalar irradiance by ~150% (between 500 and 700 nm) relative to a healthy coral. Photosynthesis peaked around 300 μm within the tissue, which corresponded to a zone exhibiting strongest depletion of scalar irradiance. Deeper coral tissue layers, e.g., ~1000 μm into aboral polyp tissues, harbor optical microniches, where only ~10% of the incident irradiance remains. We conclude that the optical microenvironment of corals exhibits strong lateral and vertical gradients of scalar irradiance, which are affected by both tissue and skeleton optical properties. Our results imply that zooxanthellae populations inhabit a strongly heterogeneous light environment and highlight the presence of different optical microniches in corals; an important finding for understanding the photobiology, stress response, as well as the phenotypic and genotypic plasticity of coral symbionts.

SummaryDinoflagellates of the genus Symbiodinium express broad diversity in both genetic identity (phylogeny) and photosynthetic function to presumably optimize ecological success across extreme light environments; however, whether differences in the primary photobiological characteristics that govern photosynthetic optimization are ultimately a function of phylogeny is entirely unresolved.We applied a novel fast repetition rate fluorometry approach to screen genetically distinct Symbiodinium types (n = 18) spanning five clades (A-D, F) for potential phylogenetic trends in factors modulating light absorption (effective cross-section, reaction center content) and utilization (photochemical vs dynamic nonphotochemical quenching;The variability of PSII light absorption was independent of phylogenetic designation, but closely correlated with cell size across types, whereas PSII light utilization intriguingly followed one of three characteristic patterns: (1) Our functional trait-based approach shows, for the first time, how Symbiodinium photosynthetic function is governed by the interplay between phylogenetically dependent and independent traits, and is potentially a means to reconcile complex biogeographic patterns of Symbiodinium phylogenetic diversity in nature.

Coral surface temperature was investigated with multiple temperature sensors mounted on hemispherical and branching corals under (a) artificial lighting and controlled flow; (b) natural sunlight and controlled flow; and (c) in situ conditions in a shallow lagoon, under naturally fluctuating irradiance, water flow, and temperature. Under high irradiance and low flow conditions, hemispherical corals were 0.6uC warmer than the surrounding water. Hemispherical corals reached higher temperatures than branching corals, by a measure of 0.2uC to 0.4uC. Microsensor temperature measurements showed the presence of a thermal boundary layer (TBL). The TBL thickness was flow dependent, and under low flow conditions, a TBL up to 3 mm thick limited heat transfer to the ambient water. Combined microsensor measurements of temperature and oxygen showed that the TBL was approximately four times thicker than the diffusive boundary layer, as predicted from heat and mass transfer theory. A simple conceptual model describes coral surface temperature as a function of heat fluxes between coral tissue, skeleton, and surroundings. The slope of the predicted linear relationship between coral temperature and solar irradiance is fixed by the efficiencies of light absorption and the heat losses to the skeleton and the water. Although spectral absorptivity may play a significant role in coral warming, shape-related differences in thermal properties can cause hemispherical corals to reach higher temperatures than branching corals. Shape-related differences in thermal histories may thus help explain differences in susceptibility to coral bleaching between branching and hemispherical coral species.The increasing occurrence of coral bleaching over the last two decades has focused attention on temperature fluctuations on corals reefs (Brown 1997;Berkelmans and Willis 1999;Hoegh-Guldberg 1999). Under mass coral bleaching conditions, small excursions in the ambient water temperature on a coral reef (of just a few degrees Celsius above the normal average temperature maximum) induce the expulsion of the endosymbiotic dinoflagellates (zooxanthellae) and/or the loss of pigments from a wide variety of corals (Glynn 1996;Hoegh-Guldberg 1999). If such high temperature anomalies last for a week or more mass mortality of corals can occur (Glynn 1996; HoeghGuldberg 1999;Coles and Brown 2003).In most of the literature on coral bleaching, temperature of the ambient water is always assumed to be the same as the coral temperature. However, as a result of the shallow nature of many coral reef lagoons, radiant energy reaching the coral surface can increase its temperature relative to the surrounding water. Few studies have considered the 1 Corresponding author (Peter.Ralph@uts.edu.au). AcknowledgmentsWe thank N. Ralph for constructing the flow-through chamber and R. Bilger and J. Kent for valuable discussion of heat transfer theory. We thank M. Ball for discussions of heat transfer in corals at an early stage of this investigation and two anonymous reviewers ...

The cyanobacterium known as Acaryochloris marina is a unique phototroph that uses chlorophyll d as its principal light-harvesting pigment instead of chlorophyll a, the form commonly found in plants, algae and other cyanobacteria; this means that it depends on far-red light for photosynthesis. Here we demonstrate photosynthetic activity in Acaryochloris-like phototrophs that live underneath minute coral-reef invertebrates (didemnid ascidians) in a shaded niche enriched in near-infrared light. This discovery clarifies how these cyanobacteria are able to thrive as free-living organisms in their natural habitat.

Coral tissue optics has received very little attention in the past, although the interaction between tissue and light is central to our basic understanding of coral physiology. Here we used fibre-optic and electrochemical microsensors along with variable chlorophyll fluorescence imaging to directly measure lateral light propagation within living coral tissues. Our results show that corals can transfer light laterally within their tissues to a distance of ~2 cm. Such light transport stimulates O 2 evolution and photosystem II operating efficiency in areas >0.5-1 cm away from direct illumination. Light is scattered strongly in both coral tissue and skeleton, leading to photon trapping and lateral redistribution within the tissue. Lateral light transfer in coral tissue is a new mechanism by which light is redistributed over the coral colony and we argue that tissue optical properties are one of the key factors in explaining the high photosynthetic efficiency of corals.