Diatoms as bio-sentinels to probe the dose-dependent impact of copper on aquatic environment: a multi-scale fractal analysis in Fourier Ptychographic Microscopy

The increasingly high concentration of heavy metals (HMs) in marine environments has become an urgent problem to be addressed. Among HMs, copper (Cu) is widely spread. At low concentrations, Cu plays an important role for the microalgal growth, activity, and reproductions. However, these functions are altered when the Cu concentration exceeds a certain level. Effects on the aquatic ecosystem can be extremely detrimental and they can reach also the human species through the food chain. Hence, sensors to detect with high sensitivity the Cu concentration within water samples are highly requested. Here we exploit Fourier Ptychographic Microscopy (FPM) for the multi-scale quantitative phase imaging of water samples containing the Skeletonema pseudocostatum diatom population exposed to several Cu doses. FPM has the unique feature to allow imaging a large field of view while preserving a high-resolution at the single-diatom level. Therefore, we show that the multi-scale analytical tool provided by fractal geometry is particularly suited to quantify the dose-dependent morphometric changes inside the water sample imaged by FPM. The proposed technique is potentially able to detect the several Cu doses by analyzing their effects on diatoms, here used as bio-sentinels.