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FLUORESCENCE QUENCHING INDUCED BY OXIDIZING CONDITIONS IN THE GREEN NON-SULFUR BACTERIUM CHLOROFLEXUS AURANTIACUS: A PROPOSED ECOPHYSIOLOGICAL SIGNIFICANCE IN HOT SPRING MICROBIAL MATS Alberto Vianelli, Deborah Armiento, Guido Domingo, Leonardo Leonforte, Paolo D.Gerola Università degli Studi dell’Insubria, Dipartimento di Biologia Strutturale e Funzionale, via J.H.Dunant 3, 21100 Varese, Italy Excess excitation energy dissipation via non-photochemical quenching and its possible role in photoprotection is currently under active investigation in photosynthetic eukaryotes, albeit relatively little attention has been paid to its presence in photosynthetic prokaryotes. A major goal of the work in our laboratory is to analyze redox-dependent fluorescence quenching in chlorosomes-containing photosynthetic bacteria (green bacteria). In particular we focus on Chloroflexus aurantiacus, a facultative phototroph isolated from hot spring microbial mats in Japan and Yellowstone National Park. In our previos work, we have shown that under oxidizing conditions, obtained by controlled addition of potassium ferri/ferrocyanide,. bacteriochlorophyll (bchl) a and c fluorescence yield in isolated chlorosomes is controlled by two types of redox-sensitive mechanisms. Aerobic redox titration indicated an apparent Em of 410 mV for a five-fold decrease, suggesting a novel mechanism, probably based on oxidizing bacteriochlorophyll (Bchl+) itself acting as a quencher, is operating in this bacterium. This mechanism is absent in green sulfur bacteria (Chlorobium sp.), contaning chlorosomes as well, but otherwise adapted to a highly reducing environment (sulfide-rich hot springs). In the present work, we have undertaken further studies in order to better characterize the fluorescence quenching in chlorosmes, and to relate the in vitro observations in chlorosomes to in vivo conditions: whole cells, isolated membranes, antenna complex LH1, reaction centers were studied by means of steady state absorption and fluorescence emission spectra. Finally, taking into account of redox equilibria in oxygenated waters, we propose a physiological role for this oxidant-induced fluorescence quenching, in view of the extreme environmental parameters (particularly oxygen concentration) variations observed in hot spring microbial mats. It is possibly worthy of note that an increasing amount of data from otehr labs point to a possible involvement of (B)chl + based quenching in highly oxidizing conditions in cyanobacteria and higher plants too.