Optical Insight: Spectrophotometry as a Tool to Quantify Cell Density of Green Microalgae in Suspension, with Emphasis on Growing Conditions and Toxicological Evaluations

Microalgae are often used in different industrial sectors and can be used as indicators of aquatic environmental health. An essential step for cultivating microalgae is assessing the cell density, which is traditionally performed through cell counting by optical microscopy (OM). However, this method has limitations, mainly in terms of runtime and low reproducibility. Assessing the optical density (OD) by spectrophotometry was investigated as an affordable alternative. The OD of the green microalgae Desmodesmus subspicatus and Tetraselmis sp. was detected at 680 nm. The correlation between the cell density and OD was suitable for both microalgae and the accuracy was comparable to traditional OM counting. In toxicological tests, the OD technique allowed the monitoring of microalgal growth in the presence of the chemicals K 2 Cr 2 O 7 , KCl and sodium dodecyl sulfate. This study sheds some light on this well-known technique, which is an important contribution since few articles have explored its potentialities and fragilities. Graphical Abstract