The technique of accumulated photon echoes was used to study optical dephasing of the lowest Q(Y) transitions of the bacteriochlorophyll a complex of the green sulfur bacteria Prosthecochloris aestuarii. Variations in the photon echo decay as a function of wavelength are interpreted in terms of (phonon-assisted) downward relaxation within the Q(Y) manifold as a mechanism of energy transfer at the lowest temperatures. A temperature dependence study revealed contributions to the total dephasing both from transitions within so-called two level systems (TLS) of the host as well as activation of transitions to higher electronic levels within the Q(Y) manifold at temperatures above 5 K. Conclusions are drawn about the nature of energy transfer at low temperatures.