Heat generation and light scattering of green fluorescent protein-like pigments in coral tissue

Green fluorescent protein (GFP)-like pigments have been proposed to have beneficial effects on coral photobiology. Here, we investigated the relationships between green fluorescence, coral heating and tissue optics for the massive coral Dipsastraea sp. (previously Favia sp.). We used microsensors to measure tissue scalar irradiance and temperature along with hyperspectral imaging and combined imaging of variable chlorophyll fluorescence and green fluorescence. Green fluorescence correlated positively with coral heating and scalar irradiance enhancement at the tissue surface. Coral tissue heating saturated for maximal levels of green fluorescence. The action spectrum of coral surface heating revealed that heating was highest under red (peaking at 680 nm) irradiance. Scalar irradiance enhancement in coral tissue was highest when illuminated with blue light, but up to 62% (for the case of highest green fluorescence) of this photon enhancement was due to green fluorescence emission. We suggest that GFP-like pigments scatter the incident radiation, which enhances light absorption and heating of the coral. However, heating saturates, because intense light scattering reduces the vertical penetration depth through the tissue eventually leading to reduced light absorption at high fluorescent pigment density. We conclude that fluorescent pigments can have a central role in modulating coral light absorption and heating

Monte Carlo modeling of photon propagation reveals highly scattering coral tissue

Corals are very efficient at using solar radiation, with photosynthetic quantum efficiencies approaching theoretical limits. Here, we investigated potential mechanisms underlying such outstanding photosynthetic performance through extracting inherent optical properties of the living coral tissue and skeleton in a massive faviid coral. Using Monte Carlo simulations developed for medical tissue optics it is shown that for the investigated faviid coral, the coral tissue was a strongly light scattering matrix with a reduced scattering coefficient of µs’ =10 cm-1 (at 636 nm). In contrast, the scattering coefficient of the coral skeleton was µs’ =3.4 cm-1, which facilitated the efficient propagation of light to otherwise shaded coral tissue layers, thus supporting photosynthesis in lower tissues. Our study provides a quantification of coral tissue optical properties in a massive faviid coral and suggests a novel light harvesting strategy, where tissue and skeletal optics act in concert to optimize the illumination of the photosynthesizing algal symbionts embedded within the living coral tissue

Bio-optical properties and radiative energy budgets in fed and unfed scleractinian corals (Pocillopora sp.) during thermal bleaching

© 2019 The authors. Corals live in symbiosis with algal dinoflagellates, which can achieve outstanding photo - synthetic energy efficiencies in hospite approaching theo retical limits. However, how such photosynthetic efficiency varies with environmental stress remains poorly known. Using fiber-optic and electrochemical microsensors in combination with variable chlorophyll fluorescence imaging, we investigated the combined effects of thermal stress and active feeding on the radiative energy budget and photosynthetic efficiency of the symbiotic coral Pocillopora sp. At ambient temperature (25°C), the percentage of ab sorbed light energy used for photosynthesis under low irradiance was higher for fed (∼5-6%) compared to unfed corals (4%). Corals from both feeding treatments responded equally to stress from high light ex posure (2400 μmol photons m-2 s-1), exhibiting a de crease in photosynthetic efficiency, down to 0.5-0.6%. Fed corals showed increased resilience to thermal-induced bleaching (loss of symbionts) compared to unfed corals. In addition, while unfed corals decreased their photosynthetic efficiency almost immediately when exposed to thermal stress, fed corals maintained a constant and high photosynthetic efficiency for 5 more days after onset of thermal stress. We conclude that active feeding is beneficial to corals by prolonging coral health and resilience during thermal stress as a result of an overall healthier symbiont population

Light respiratory processes and gross photosynthesis in two scleractinian corals

© 2014 Schrameyer et al. The light dependency of respiratory activity of two scleractinian corals was examined using O2 microsensors and CO2 exchange measurements. Light respiration increased strongly but asymptotically with elevated irradiance in both species. Light respiration in Pocillopora damicornis was higher than in Pavona decussata under low irradiance, indicating species-specific differences in light-dependent metabolic processes. Overall, the coral P. decussata exhibited higher CO2 uptake rates than P. damicornis over the experimental irradiance range. P. decussata also harboured twice as many algal symbionts and higher total protein biomass compared to P. damicornis, possibly resulting in self-shading of the symbionts and/or changes in host tissue specific light distribution. Differences in light respiration and CO2 availability could be due to host-specific characteristics that modulate the symbiont microenvironment, its photosynthesis, and hence the overall performance of the coral holobiont

Spectral effects on Symbiodinium photobiology studied with a programmable light engine

©2014 Wangpraseurt et al. The spectral light field of Symbiodinium within the tissue of the coral animal host can deviate strongly from the ambient light field on a coral reef and that of artificial light sources used in lab studies on coral photobiology. Here, we used a novel approach involving light microsensor measurements and a programmable light engine to reconstruct the spectral light field that Symbiodinium is exposed to inside the coral host and the light field of a conventional halogen lamp in a comparative study of Symbiodinium photobiology. We found that extracellular gross photosynthetic O 2 evolution was unchanged under different spectral illumination, while the more red-weighted halogen lamp spectrum decreased PSII electron transport rates and there was a trend towards increased light-enhanced dark respiration rates under excess irradiance. The approach provided here allows for reconstructing and comparing intra-tissue coral light fields and other complex spectral compositions of incident irradiance. This novel combination of sensor technologies provides a framework to studying the influence of macro- and microscale optics on Symbiodinium photobiology with unprecedented spectral resolution

Elevated CO2 Leads to Enhanced Photosynthesis but Decreased Growth in Early Life Stages of Reef Building Coralline Algae

Crustose coralline algae (CCA) are key organisms in coral reef ecosystems, where they contribute to reef building and substrate stabilization. While ocean acidification due to increasing CO2 can affect the biology, physiology and ecology of fully developed CCA, the impacts of elevated CO2 on the early life stages of CCA are much less explored. We assessed the photosynthetic activity and growth of 10-day-old recruits of the reef-building crustose coralline alga Porolithon cf. onkodes exposed to ambient and enhanced CO2 seawater concentration causing a downward shift in pH of ∼0.3 units. Growth of the CCA was estimated using measurements of crust thickness and marginal expansion, while photosynthetic activity was studied with O2 microsensors. We found that elevated seawater CO2 enhanced gross photosynthesis and respiration, but significantly reduced vertical and marginal growth of the early life stages of P. cf. onkodes. Elevated CO2 stimulated photosynthesis, particularly at high irradiance, likely due to increased availability of CO2, but this increase did not translate into increased algal growth as expected, suggesting a decoupling of these two processes under ocean acidification scenarios. This study confirms the sensitivity of early stages of CCA to elevated CO2 and identifies complexities in the physiological processes underlying the decreased growth and abundance in these important coral reef builders upon ocean acidification

The in situ light microenvironment of corals

We used a novel diver-operated microsensor system to collect in situ spectrally resolved light fields on corals with a micrometer spatial resolution. The light microenvironment differed between polyp and coenosarc tissues with scalar irradiance (400-700 nm) over polyp tissue, attenuating between 5.1- and 7.8-fold from top to base of small hemispherical coral colonies, whereas attenuation was at most 1.5-fold for coenosarc tissue. Fluctuations in ambient solar irradiance induced changes in light and oxygen microenvironments, which were more pronounced and faster in coenosarc compared with polyp tissue. Backscattered light from the surrounding benthos contributed > 20% of total scalar irradiance at the coral tissue surface and enhanced symbiont photosynthesis and the local O2 concentration, indicating an important role of benthos optics for coral ecophysiology. Light fields on corals are species and tissue specific and exhibit pronounced variation on scales from micrometers to decimeters. Consequently, the distribution, genetic diversity, and physiology of coral symbionts must be coupled with the measurements of their actual light microenvironment to achieve a more comprehensive understanding of coral ecophysiology. © 2014, by the Association for the Sciences of Limnology and Oceanography, Inc

Lateral light transfer ensures efficient resource distribution in symbiont-bearing corals

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 ∼2cm. Such light transport stimulates O2 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. © 2014. Published by The Company of Biologists Ltd

Effective light absorption and absolute electron transport rates in the coral Pocillopora damicornis

Pulse Amplitude Modulation (PAM) fluorometry has been widely used to estimate the relative photosynthetic efficiency of corals. However, both the optical properties of intact corals as well as past technical constrains to PAM fluorometers have prevented calculations of the electron turnover rate of PSII. We used a new Multi-colour PAM (MC-PAM) in parallel with light microsensors to determine for the first time the wavelength-specific effective absorption cross-section of PSII photochemistry, σII(λ), and thus PAM-based absolute electron transport rates of the coral photosymbiont Symbiodinium both in culture and in hospite in the coral Pocillopora damicornis. In both cases, σII of Symbiodinium was highest in the blue spectral region and showed a progressive decrease towards red wavelengths. Absolute values for σII at 440nm were up to 1.5-times higher in culture than in hospite. Scalar irradiance within the living coral tissue was reduced by 20% in the blue when compared to the incident downwelling irradiance. Absolute electron transport rates of P.damicornis at 440nm revealed a maximum PSII turnover rate of ca. 250 electrons PSII-1 s-1, consistent with one PSII turnover for every 4 photons absorbed by PSII; this likely reflects the limiting steps in electron transfer between PSII and PSI. Our results show that optical properties of the coral host strongly affect light use efficiency of Symbiodinium. Therefore, relative electron transport rates do not reflect the productivity rates (or indeed how the photosynthesis-light response is parameterised). Here we provide a non-invasive approach to estimate absolute electron transport rates in corals. © 2014 Elsevier Masson SAS