The effects of long-term exposure to low pH on the skeletal microstructure of the sea urchin Heliocidaris erythrogramma

Anthropogenic CO2 - driven ocean acidification (OA) is causing a decrease in seawater pH and the saturation state of calcium carbonate minerals, compromising the ability of calcifying species to produce and maintain their skeletons. Sea urchins are ecologically important calcifying species and we investigated the impacts of long-term (9 month) exposure to near-future OA (Ambient pH(NBS) 8.01; OA - pH(NBS) 7.6) on the skeleton microstructure of Heliocidaris erythrogramma using scanning electron microscopy (SEM), micro-computed tomography (mu CT) and nanoindentation. SEM revealed that the youngest plates (apical plates) which had likely grown in experimental conditions had larger pores in the OA group (pore surface area similar to 72% larger) compared with those of urchins maintained in ambient pH. High-resolution, mu CT 3-D reconstructions of the apical plates revealed that the experimental OA treatment urchins had a similar to 14% greater porosity and similar to 17% less biomineral, suggesting an inability to finely regulate skeletogenesis. The mid-test ambital plates established prior to this study did not show any OA associated change in porosity. Nanoindentation of the apical plates indicated that OA reduced skeletal hardness and elasticity. Stereom pore size is a key trait of the sea urchin endoskeleton and increased porosity in H. erythrogramma is likely to impact its biological functions as well as its biomechanical capacity to defend against predation and physical disturbances.