Reactive oxygen species play a key role in cell signalling and oxidative stress mechanisms, therefore, sensing their production by living organisms is of fundamental interest. Here we describe a novel biosensing method for extracellular detection of endogenous hydrogen peroxide (H2O2). The method is based on the enhancement of the optical absorption spectrum of the hemoprotein cytochrome c when loaded into a highly scattering random medium. Such a configuration enables, in contrast to existing techniques, non-invasive and dynamic detection of the oxidation of cyt c in the presence of H2O2 with unprecedented sensitivity. Dynamic information on the modification of the cell oxidative status of Chlamydomonas reinhardtii, an aquatic green algae, was obtained under oxidative stress conditions induced by the presence of trace concentrations of Cd(II). Furthermore, the dynamics of H2O2 production was investigated under different lighting conditions confirming the impact of Cd(II) on the photosynthetic activity of those phytoplanktonic cells.