Fluorescence line-narrowing measurements at low temperature were performed on the Fenna-Matthews-Olson complex of Prosthecochloris aestuarii. Superimposed on the phonon wing, several vibronic bands could be observed. By use of these data, the temperature dependence of the lowest-energy absorption band was modeled based on the linear harmonic Franck-Condon approximation. The overall Huang-Rhys factor was estimated to be 0.45. The maximum of the phonon distribution was located at 20 cm(-1). Thirty vibrational modes could be observed, and their Franck-Condon factors were estimated. The strongest modes were located at 36, 701 and similar to 195 cm(-1). For the full width at half-maximum of the inhomogeneous broadening, a value of 80 cm(-1) was determined. We did not find any evidence for the presence of different excitonic states in the lowest-energy absorption band.