*4.7. Thiols Determination*

Three independent *S. marinoi* cultures were carried out under control (Sin150) and high sinusoidal light (Sin600) conditions. Cultures were collected at 6 h from dawn in both light conditions (i.e., at the maximal peak of light intensity) by centrifugation at 2600 rcf speed, 4 ◦C for 15 min (Eppendorf 5810 R, Eppendorf AG, Hamburg, Germany). Pellets were lyophilized using an Edwards lyophilizer and the dried pellets were weighed and kept at room temperature until analysis. Samples were finally analyzed by HPLC/LC–MS for ovothiol and glutathione determination, according to the protocol fully described in Milito et al., 2020 [23]. Briefly, thiols were extracted using a solution of acetonitrile:perchloric acid (1:1), reduced by adding DTT, and finally derivatized with 4-bromomethyl-7-methoxycoumarin (BMC). Samples were analyzed by reversed phase HPLC and thiol-BMC conjugates were detected with a diode-array detector at 330 nm. Isolated ovothiol-BMC adducts were characterized by High-resolution electrospray ionization mass spectrometry (HR-ESI-MS). Ovothiol B identification was confirmed by coelution with an authentic standard [23].
