During experimental light-dark cycles, O9 in the tissue of the colonial scleractinian corals Favia sp. and Acropora sp reached >250 % of air saturation after a few minutes in light. Immediately after darkenmg, 0; was depleted rapidly, and within 5 mm the 0; concentration at the tissue surface reached <2 % of air saturation. The pH of the tissue changed within 10 min from about 8.5 in the light to 7.3 m the dark. Oxygen and pH profiles revealed a diffusive boundary layer of flow-dependent thickness, which limited coral respiration in the dark. The light field at the tissue surface (measured as scalar irradiance, Eo) differed strongly with respect to light intensity and spectral composition from the incident collimated light (measured as downwelling irradiance, Ed) Scalar irradiance reached up to 180 % of Ed at the coral tissue surface for wavelengths subject to less absorption by the coral tissue (600 to 650 run and >680 nm). The scalar irradiance spectra exhibited bands of chlorophyll a (chl a ) (675 run), chl c (630 to 640 nm) and pendinin (540 nm) absorption and a broad absorption band due to chlorophyl l~ and carotenoids between 400 and 550 nm. The shape of both action spectra and photosynthesis vs irradiance (Pvs I) curves depended on the choice of the light intensity parameter. Calculations of miha1 slopes and onset of light saturation, Ik, showed that P vs Eo curves exhibit a lower initial slope and a higher 4 than corresponding Pvs Ed curves. Coral respiration in light was calculated as the difference between the measured gross and net photosynthesis, and was found to be >6 times higher at a saturating irradiance of 350 pEm m 2 s 1 than the dark respiration measured under identical hydrodynamic conditions (flow rate of 5 to 6 cm ssl).

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Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for O2, pH and light

The clear waters surrounding tropical coral reefs typically are oligotrophic, yet these reefs are highly productive and support dense populations of marine organisms. This paradox is resolved in part because many coral reef invertebrates accommodate unicellular autotrophs (‘‘zooxanthellae,’’ Symbiodinium spp.; Prochlorales, Prochloron sp.; cyanobacteria) within their tissues. These photoautotrophic symbioses entail an exchange of nutrients between the endosymbionts and the animal hosts. Organic compounds produced by the microalgal partners are released to the hosts for their nutrition while inorganic metabolic wastes are recycled to fertilize the algae (Muscatine 1990). The requirement for photosynthetically active radiation (PAR, 400–700 nm) also exposes such algalinvertebrate symbioses and other primary producers to high levels of environmental ultraviolet radiation (UVR 5 295–400 nm) in tropical waters (Smith and Baker 1979, Fleischmann 1989). These waters are often low in UV-absorbing particulate and dissolved organic matter, and sea level fluences of solar UV are high because of the short atmospheric path length and thinness of the stratospheric ozone layer above the tropics (Baker et al. 1980). Photoautotrophic symbiosis thus presents an evolutionary challenge by precluding the morphological development of an optically opaque barrier (such as hair, scales, and feathers in higher vertebrates) and potentially allows damaging UV radiation to reach vulnerable biomolecules in both partners. The problem is exacerbated because the tissues are hyperoxic (.250% air saturation: Kuhl et al.

Review—ultraviolet radiation-absorbing mycosporine-like amino acids in coral reef organisms: a biochemical and environmental perspective

Radiographic studies of reef coral exoskeletons: Rates and patterns of coral growth

In the last two decades, genetic and genomic studies have revealed the astonishing diversity and ubiquity of microorganisms. Emergence and expansion of the human microbiome project has reshaped our thinking about how microbes control host health – not only as pathogens, but also as symbionts. In coral reef environments, scientists have begun to examine the role that microorganisms play in coral life history. Herein we review the current literature on coral-microbe interactions within the context of their role in evolution, development, and ecology. We ask the following questions, first posed by McFall-Ngai et al., 2013 in their review of animal evolution, with specific attention to how coral-microbial interactions may be affected under future environmental conditions: 1) How do corals and their microbiome affect each other’s genomes? 2) How does coral development depend on microbial partners? 3) How is homeostasis maintained between corals and their microbial symbionts? 4) How can ecological approaches deepen our understanding of the multiple levels of coral-microbial interactions? Elucidating the role that microorganisms play in the structure and function of the holobiont is essential for understanding how corals maintain homeostasis and acclimate to changing environmental conditions.

http://medinalab.org/new/wp-content/uploads/2015/01/2015-microbes-in-the-coral-holobiont.pdf

Microbes in the coral holobiont: partners through evolution, development, and ecological interactions.

Recent reports of worldwide coral bleaching events leading to devastating coral mortality have caused alarm among scientists and resource managers. Differential survival of coral species through bleaching events has been widely documented. We suggest that among the possible factors contributing to survival of coral species during such events are endolithic algae harboured in their skeleton, providing an alternative source of energy. We studied the dynamics of photosynthetic pigment concentrations and biomass of endoliths in the skeleton of the encrusting coral Oculina patagonica throughout a bleaching event. During repeated summer bleaching events these endolithic algae receive increased photosynthetically active radiation, increase markedly in biomass, and produce increasing amounts of photoassimilates, which are translocated to the coral. Chlorophyll concentrations and biomass of endoliths were 4.6 ± 1.57 and 1570 ± 427 μg cm−2 respectively, in skeletons of relatively healthy colonies (0–40%bleaching) but up to 14.8± 2.5 and 4036 ± 764 g cm−2 endolith chlorophyll and biomass respectively, in skeletons of bleached colonies (greater than 40% bleaching). The translocation dynamics of 14C–labelled photoassimilates from the endoliths to bleached coral tissue showed significantly higher 14C activity of the endoliths harboured within the skeletons of bleached corals than that of the endoliths in non–bleached corals. This alternative source of energy may be vital for the survivorship of O. patagonica , allowing gradual recruitment of zooxanthellae and subsequent recovery during the following winter.

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Endolithic algae: an alternative source of photoassimilates during coral bleaching.

Two species of algae were living symbiotically with a hard brain coral, Fa@ia, harvested in the environs of the Flinders Island Group on the Great Barrier Reef in Australia. One of the algae has the microscopic appearance and pigment corn position of dinoflagellates (Halldal, 1968, Jeffrey and Haxo, 1968) and it contrib utes the dark brown color to the coral tissues which form a surface layer over the colony. This alga resembles Symbiodiniuni microadriaticum Freudenthal. Green algae lived inside the spherical coral forming another colored layer (Jeffrey, 1968) , and there was an intermediate pale green (nearly white) layer between these brown and green layers. The green algae seemed to be of mixed genera and species, most of them probably belonging to Ostreobiuni Reineckei Bornet within the order Siphonales. The present paper describes the in vivo absorption spectra of these algal layers as well as the spectral distribution. EXPERIMENTAL METHODS The samples of Favia pallida DaIla were harvested in the environs of the Flinders Island Group, and were kept in running sea-water aquaria on the open deck of the research vessel. The spectral data presented in this paper were observed for the sample which was 12 cm in diameter and 8.5 cm in thickness

Francis T. Haxo

Papers

Light transmission and spectral distribution through epi- and endozoic algal layers in the brain coral, favia