Studying Microplastic Incorporation into Corals Using CARS

Microplastic pollution is a serious threat to marine organisms, including reef-building corals. Corals are known to incorporate microplastics, which could potentially provide an archive of information to trace present and past microplastic pollution. In this study, a label-free chemical analytical method for rapid visualization of microplastics incorporated in coral tissue and skeletons is proposed by using a coherent anti-Stokes Raman scattering (CARS) microscope combined with two-photon excited fluorescence (TPEF) detection. Acropora polystoma was maintained in an environment with a high concentration of polyethylene (PE) beads ranging from 0.7 to 5 μm in size. PE beads were successfully visualized, and the number of incorporated beads per skeleton area was quantitatively analyzed using the proposed technique. Notably, plastic beads were only found in the skeleton of coral colonies showing signs of localized tissue loss. Further CARS analysis of tissue found that a number of microplastics were trapped in the areas where tissue loss was observed due to bleaching or physical tissue removal, whereas minimal amounts were observed within healthy tissue. These results suggest that unhealthy conditions which lead to tissue loss and bleaching may accelerate microplastic incorporation into coral tissue and further into skeletons. Our study shows the ability of our CARS-based method to help understand how microplastics are incorporated into corals and will lead to improved tracking of their accumulation in coral reefs.