  Recent experimental observations of time-dependent beatings in the
two-dimensional echo-spectra of light-harvesting complexes at ambient
temperatures have opened up the question whether coherence and wave-like
behaviour plays a significant role in photosynthesis. We perform a numerical
study of the absorption and echo-spectra of the Fenna-Matthews-Olson (FMO)
complex in chlorobium tepidum and analyse the requirements in the theoretical
model needed to reproduce beatings in the calculated spectra. The energy
transfer in the FMO pigment-protein complex is theoretically described by an
exciton Hamiltonian coupled to a phonon bath which account for the pigments
electronic and vibrational excitations respectively. We use the hierarchical
equations of motions method to treat the strong couplings in a non-perturbative
way. We show that the oscillations in the two-dimensional echo-spectra persist
in the presence of thermal noise and static disorder.
