In recent years, the porous silica structures (frustules) created by living diatoms have been studied for several nanoengineering applications based on biomimetic approaches. We focus on the gas-sensing properties of diatoms: investigation of different species shows that the photoluminescence emission of frustules is affected by even small modifications of the surrounding gas environment, exhibiting a detection limit of few tenths of ppm in the case of nitrogen dioxide. A new understanding of this phenomenon is discussed here in terms of "static-type" luminescence quenching through suppression of radiative states (most probably surface oxygen vacancies) induced by adsorption of gas molecules. The modeling allows the free energy of desorption to be measured by all-optical means: the value obtained suggests that a chemisorption process is involved, in agreement with the observed absorption/desorption kinetics. The findings encourage investigation of diatoms as low-cost biological transducers for detection of gas species.