Strong thermally induced quenching of fluorescence emission intensity of the Fenna-Matthews-Olson trimeric bacteriochlorophyll a - protein antenna complex from the green photosynthetic bacterium Chlorobium tepidum is observed starting already from 2 K, very different from that for other antenna complexes as well as for bacteriochlorophyll a in common solutions where the emission yield decreases only slightly past 100 K. Close spacing of the trimer exciton states within the lowest-energy manifold of absorbing states around 826 nm along with significant increase of the electron-phonon coupling strength toward higher energies is proposed responsible for this effect. (c) 2006 Elsevier B.V. All rights reserved.