The fluorescence emission of individual photosystem I complexes from Synechocystis PCC 6803 in protonated and deuterated buffer shows zero-phonon lines as well as broad intensity distributions. The number and the line width of the zero phonon lines depend strongly on the solvent (H2O/D2O). The spectral diffusion rate of the whole fluorescence emission from photosystem I is significantly reduced upon deuteration of the solvent. This leads to a substantial increase of well-resolved zero-phonon lines. Since the chlorophyll a chromophores lack exchangeable protons, these observed changes in the spectral diffusion have to be assigned to exchangeable protons at the amino acids and structural water molecules in the chromophore binding pocket.