Calcium carbonate skeletons of scleractinian corals amplify light availability to their algal symbionts by diffuse scattering, optimizing photosynthetic energy acquisition. However, the mechanism of scattering and its role in coral evolution and dissolution of algal symbioses during “bleaching” events are largely unknown. Here we show that differences in skeletal fractal architecture at nano/micro-lengthscales within 96 coral taxa result in an 8-fold variation in light-scattering and considerably alter the algal light environment. We identified a continuum of properties that fall between two extremes: (1) corals with low skeletal fractality that are efficient at transporting and redistributing light throughout the colony with low scatter but are at higher risk of bleaching and (2) corals with high skeletal fractality that are inefficient at transporting and redistributing light with high scatter and are at lower risk of bleaching. While levels of excess light derived from the coral skeleton is similar in both groups, the low-scatter corals have a higher rate of light-amplification increase when symbiont concentration is reduced during bleaching, thus creating a positive feedback-loop between symbiont concentration and light-amplification that exposes the remaining symbionts to increasingly higher light intensities. By placing our findings in an evolutionary framework, in conjunction with a novel empirical index of coral bleaching susceptibility, we find significant correlations between bleaching susceptibility and light-scattering despite rich homoplasy in both characters; suggesting that the cost of enhancing light-amplification to the algae is revealed in decreased resilience of the partnership to stress.

BackgroundAt the forefront of ecosystems adversely affected by climate change, coral reefs are sensitive to anomalously high temperatures which disassociate (bleaching) photosynthetic symbionts (Symbiodinium) from coral hosts and cause increasingly frequent and severe mass mortality events. Susceptibility to bleaching and mortality is variable among corals, and is determined by unknown proportions of environmental history and the synergy of Symbiodinium- and coral-specific properties. Symbiodinium live within host tissues overlaying the coral skeleton, which increases light availability through multiple light-scattering, forming one of the most efficient biological collectors of solar radiation. Light-transport in the upper ~200 μm layer of corals skeletons (measured as ‘microscopic’ reduced-scattering coefficient, ), has been identified as a determinant of excess light increase during bleaching and is therefore a potential determinant of the differential rate and severity of bleaching response among coral species.ResultsHere we experimentally demonstrate (in ten coral species) that, under thermal stress alone or combined thermal and light stress, low- corals bleach at higher rate and severity than high- corals and the Symbiodinium associated with low- corals experience twice the decrease in photochemical efficiency. We further modelled the light absorbed by Symbiodinium due to skeletal-scattering and show that the estimated skeleton-dependent light absorbed by Symbiodinium (per unit of photosynthetic pigment) and the temporal rate of increase in absorbed light during bleaching are several fold higher in low- corals.ConclusionsWhile symbionts associated with low- corals receive less total light from the skeleton, they experience a higher rate of light increase once bleaching is initiated and absorbing bodies are lost; further precipitating the bleaching response. Because microscopic skeletal light-scattering is a robust predictor of light-dependent bleaching among the corals assessed here, this work establishes as one of the key determinants of differential bleaching response.Electronic supplementary materialThe online version of this article (doi:10.1186/s12898-016-0061-4) contains supplementary material, which is available to authorized users.

Purpose: Endoscopic examination has proven effective in both detecting and preventing colorectal cancer; however, only about a quarter of eligible patients undergo screening. Even if the compliance rate increased, limited endoscopic capacity and cost would be prohibitive. There is a need for an accurate method to target colonoscopy to those most at risk of harboring colonic neoplasia. Exploiting field carcinogenesis seems to be a promising avenue. Our group recently reported that an early increase in blood supply (EIBS) is a reliable marker of field carcinogenesis in experimental models. We now investigate whether in situ detection of EIBS in the rectum can predict neoplasia elsewhere in the colon. Experimental Design:We developed a novel polarization-gated spectroscopy fiber-optic probe that allows depth-selective interrogation of microvascular blood content. Using the probe, we examined the blood content in vivo from the rectal mucosa of 216 patients undergoing screening colonoscopy. Results: Microvascular blood content was increased by f50% in the endoscopically normal rectal mucosa of patients harboring advanced adenomas when compared with neoplasia-free patients irrespective of lesion location. Demographic factors and nonneoplastic lesions did not confound this observation. Logistic regression using mucosal oxyhemoglobin concentration and patient age resulted in a sensitivity of 83%, a specificity of 82%, and an area under the receiver operating characteristic curve of 0.88 for the detection of advanced adenomas. Conclusions: Increased microvascular blood supply in the normal rectal mucosa is associated with the presence of clinically significant neoplasia elsewhere in the colon, supporting the development of rectal EIBS as a colon cancer risk-stratification tool.

Low-coherence enhanced backscattering (LEBS) spectroscopy is a light scattering technique which uses partial spatial coherence broadband illumination to interrogate the optical properties at sub-diffusion length scales. In this work, we present a post-processing technique which isolates the hemoglobin concentration at different depths within a sample using a single spectroscopic LEBS measurement with a fixed spatial coherence of illumination. We verify the method with scattering (spectralon reflectance standard and polystyrene microspheres) and absorbing (hemoglobin) phantoms. We then demonstrate the relevance of this method for quantifying hemoglobin content as a function of depth within biological tissue using the azoxymethane treated animal model of colorectal cancer.

Noninvasive optical techniques for tissue characterization, in particular, light scattering properties and blood supply quantification of mucosa, is useful in a wide variety of applications. However, fiber-optic probes that require contact with the tissue surface can present a challenging problem in the variability of in vivo measurements due the nature of interactions, for example affects due to variations in pressure applied to the probe tip. We present an in vivo evaluation of pressure, angle, and temporal effects on tissue properties for polarization-gated spectroscopy at superficial depths (within 100 to 200 microns of tissue surface) for oral mucosa.

Various staining techniques are commonly used in biomedical research to investigate cellular morphology. By inducing absorption of light, staining dyes change the intracellular refractive index due to the Kramers-Kronig relationship. We present a method for creating 2-D maps of real and imaginary refractive indices of stained biological cells using their thickness and absorptance. We validate our technique on dyed polystyrene microspheres and quantify the alteration in refractive index of stained biological cells. We reveal that specific staining of individual organelles can increase their scattering cross-section by orders of magnitudes implying a major impact in the field of biophotonics.

Flexible sigmoidoscopy is a robust, clinically-validated and widely available colorectal cancer (CRC) screening technique that is currently sanctioned by both major guideline organizations. Given that endoscopic visualization is generally limited to the distal third of the colon and women tend to have a proclivity for proximal lesions, flexible sigmoidoscopy’s performance is markedly inferior in women than men. Our group has demonstrated that by using a novel light scattering approach we were able to detect an early increase in blood supply (EIBS) in the distal colonic mucosa which served as a marker of field carcinogenesis and hence proximal neoplasia. Therefore, we sought to ascertain whether rectal EIBS would improve flexible sigmoidoscopy, especially in women. A polarization-gated spectroscopy fiber-optic probe was utilized to assess EIBS in the endoscopically-normal rectum (n=366). When compared to gender-matched neoplasia-free controls, females with advanced proximal neoplasia (n=10) had a robust (60%, p=0.002) increase in rectal mucosal oxyhemoglobin (OHb) content (p = 0.001) whereas the effect size in males was less marked (33%, p=0.052). In women, addition of rectal OHb tripled the sensitivity for advanced neoplasia over flexible sigmoidoscopy alone. Indeed, the performance characteristics appeared to be excellent (sensitivity=100%, specificity=76.8%, positive predictive value =32.6% and negative predictive value of 100%). A variety of non-neoplastic factors were assessed and did not confound the relationship between rectal EIBS and advanced neoplasia. Therefore, using rectal EIBS in combination with flexible sigmoidoscopy mitigated the gender-gap and may allow flexible sigmoidoscopy to be considered a viable CRC screening test in women.

Enhanced backscattering (EBS), also known as weak localization of light, is derived using the Huygens–Fresnel principle and backscattering is generally shown to be the sum of an incoherent baseline and a phase conjugated portion of the incident wave that forms EBS. The phase conjugated portion is truncated by an effective aperture described by the probability function P(s) of coherent path-pair separations. P(s) is determined by the scattering properties of the medium and so characterization of EBS can be used for metrology of scattering materials. A three dimensional intensity peak is predicted in free space at a point conjugate to the source and is experimentally observed.