A single coral polyp is shown. The coral animal synthesises green fluorescent host pigments, which fluoresce when exposed to blue light.
Corals provide the building blocks of the most productive and diverse marine ecosystem, the coral reef; arguably one of the most spectacular manifestations of life on Earth. Reef building corals are invertebrates belonging to the family Cnidaria, akin to jellyfish and anemones, but in contrast to those soft-bodied animals, corals secrete calcium carbonate and form an intricate skeleton matrix that supports the animal tissue. Although corals partly rely on heterotrophic energy via particle and prey capture, most (>95%) of their energy demand is covered by endosymbiotic microscopic algae of the genus Symbiodinum that carry out photosynthesis within the gastrodermis of the coral tissue.
Previous work has shown that corals are highly efficient at using light and that the optical properties of corals modulate the radiative exposure of their algae. Corals have highly optimised optical properties. The coral tissue can scatter and redistribute the incident radiation, leading to a more homogeneous photosynthesis over the coral surface. At the same time the coral skeleton can redistribute light through the coral matrix to otherwise shaded areas. Corals also synthesise green fluorescent pigments which can block incident radiation, effectively photoprotecting the coral. Such GFPs also have a presumable function in transforming the spectral quality of light, by e.g. absorbing damaging UV light and emitting light that can be used for photosynthesis.
Diffuse reflectance spectroscopy
The lateral attenuation of diffuse reflectance can be mapped over a photosynthetic tissue. Diffusion theory can then be used to extract the scattering and absorption coefficient. The image shows the mapping of lateral light attenuation over the heterogeneous coral surface.
The optical and photonic properties of corals are currently characterised in the Vignolini and Deheyn lab in collaboration with the microenvironmental ecology lab (Michael Kuhl) as well as with Prof Steve Jacques. A key focus is placed on studying the optical properties of coral tissue and the light scattering of green fluorescent proteins. The extraction of coral tissue optics is a difficult task due to the high optical density and the strong coupling of absorption and multiple scattering in coral tissue. However, suitable experimental and theoretical approaches have been developed in biomedical optics for non-invasive optical characterisation of complex tissues (Tuchin, 2007) driven by the need for early detection of tissue abnormalities and the design of efficient radiative treatment of cancer. A key approach involves the use of optical reflection spectroscopy in combination with radiative transfer modeling of the experimental data with Monte Carlo simulations calculating the photon probability distribution within biological tissues (Wang et al., 1995). Addtionally, more recent biomedical optics approaches to extract optical properties include optical coherence tomography. Direct measurements of the light distribution in tissue are possible with fiber optics microsensors.
The spatial distribution of light scattering can be mapped with a goniometer. A collimated beam of light is incident on a tissue slab and light is collected with a photodecter at different angles (see Vignolini lab)
The integrating sphere allows for mesaurements of total diffuse refelctance and transmittance of a sample. These values can be matched to theory to calcuate the scattering coefficient of a sample (see Vignolini lab)
Optical coherence tomography
OCT measures characteristic patterns of directly elastically backscattered (low coherent ballistic and near ballistic) photons from different reflective layers in a sample, e.g. at refractive index mismatches between tissue compartments with different microstructural properties. Signal contrast in OCT thus depends on the optical scattering properties of the investigated material, where light scattering creates good image contrast, while highly NIR absorbing media result in poor images. Recently, OCT imaging has been applied in the environmental sciences in order to understand the structure and function of biofilms, higher plants, aquatic vertebrates and corals.
Tuchin, V.V. 2007. Tissue optics: light scattering methods and instruments for medical diagnosis(Vol. 13). Bellingham: SPIE press.
Wang, L., Jacques, S.L. and Zheng, L., 1995. MCML—Monte Carlo modeling of light transport in multi-layered tissues. Computer methods and programs in biomedicine, 47(2), pp.131-146.
Wangpraseurt, D., Jacques, S.L., Petrie, T. and Kühl, M., 2016. Monte Carlo modeling of photon propagation reveals highly scattering coral tissue. Frontiers in plant science, 7.
Wangpraseurt, D., Wentzel, C., Jacques, S.L., Wagner, M. and Kühl, M., 2017. In vivo imaging of coral tissue and skeleton with optical coherence tomography. Journal of The Royal Society Interface, 14(128), p.20161003.