Holographic tomography of the diatom Skeletonema pseudocostatum used as a bioindicator of heavy metal-polluted waters.

Heavy metal contamination in aquatic environments poses a significant threat to microbial communities, yet the subcellular responses of phytoplankton to metal stress remain poorly understood. In particular, the effects of heavy metal exposure on the structural and physiological properties of diatoms require further investigation. Here, we analyze the impact of cadmium (Cd) and copper (Cu) exposure on the subcellular structures of the diatom Skeletonema pseudocostatum using holographic tomography. This imaging technique enables detailed visualization and quantitative analysis of diatom subcomponents, including frustules, protoplasm, vacuoles, and chloroplasts, under varying metal concentrations. The study aims to understand the changes in the mean refractive index (RI) and concentration (e.g., the ratio among cell dry mass and its biovolume) as indicators of cellular response to metal stress and to infer if such diatom can be used as sentinel species of heavy metal pollution. Findings indicate that diatoms exhibit significant variations in RI and internal cell density when exposed to different metal concentrations. Lower RI values observed at higher metal concentrations, can be considered as a sign of stress due to cytoplasm extrusion and/or vacuolization. The results highlight the potential of using S. pseudocostatum as a bioindicator for monitoring water metal pollution. Moreover, the results show that holographic tomography as useful tool for non-invasive, high-resolution cellular imaging of phytoplankton in environmental studies.