Single-molecule spectroscopy at low temperature was used to study the spectral properties, heterogeneities, and spectral dynamics of the chlorophyll a (Chl a) molecules responsible for the fluorescence emission of photosystem I monomers (PS I-M) from the cyanobacterium Thermosynechococcus elongatus. The fluorescence spectra of single PS I-M are dominated by several red-shifted chlorophyll a molecules named C708 and C719. The emission spectra show broad spectral distributions and several zero-phonon lines (ZPLs). Compared with the spectra of the single PS I trimers, some contributions are missing due to the lower number of C719 Chl's in monomers. Polarization-dependent measurements show an almost perpendicular orientation between the emitters corresponding to C708 and C719. These contributions can be assigned to chlorophyll dimers B18B19, B31B32, and B32B33.