Quantitative tracking of the cellular and skeletal response of the Caribbean coral Orbicella annularis to gradients in marine sewage pollution

Orbicella annularis is an abundant framework-building Scleractinian coral that serves as an ecological cornerstone throughout the Caribbean Sea. The O. annularis holobiont (all interacting biotic and abiotic components making up the coral) is negatively impacted by increased exposure to anthropogenic pollution, which results in altering coral physiology and increasing the risk of disease infection. Changes in O. annularis tissue cellular composition and skeletal structure have been tracked across a 75 km-long gradient from clean to polluted seawater on the fringing reef of the leeward coast of the island nation of Curaçao. A unidirectional ocean current flows to the northwest past the capital city of Willemstad, a large point source of human sewage and ship bilge. Apparently healthy coral colonies were evaluated and sampled by extracting 2.5 cm-diameter biopsies at five sites from Water Plant to Playa Kalki within the back-reef carbonate sedimentary depositional facies. As a result, environmental variables other than pollution, such as water depth, currents, water temperature, oxygenation and other factors remained constant between sites. Two-photon confocal laser scanning microscopy (TP-LCM) was used for three-dimensional (3D) quantification of the density of zooxanthellae and chromatophore cells within the coral tissues. X-ray computed tomography (BioCT) was used to determine the density of the outermost layer of the coral skeletal being precipitated at the time of sampling in May 2014. Results indicate that zooxanthellae cell tissue densities decreased as pollution concentration increased. Conversely, chromatophore cell tissue densities exhibited no significant covariant changes along the pollution gradient and varied significantly within corals at each site. This implies zooxanthellae cell tissue density is strongly influenced by environmental stress due to pollution, while changes in chromatophore cell tissue density is controlled by other unknown factors. O. annularis skeletal density showed no significant changes across the geographic pollution gradient, as well as significant within site variations. This suggests skeletal density is not as strongly impacted by pollution as by other unknown biological, physical and chemical factors. There was a significant positive relationship between chromatophore and zooxanthellae tissue density as well as a negative relationship between chromatophore density and skeletal density. These results have been used to create a new model for healthy coral physiology, integrating the relationship between zooxanthellae, chromatophore and skeletal density of individual O. annularis polyps to better understand their collective role in coral metabolism