Abstract. One known attribute of the photosynthetic apparatus is photon capture and generation of metabolic energy. The thermodynamic implications of fluorescence, invariably associated with the photosynthetic components is however poorly understood. In this paper we report a density dependent amplification of such fluorescence which can be interpreted as a thermodynamic strategy of controlled energy release by the cell. We show in support of this hypothesis that prolonged photoexitation of cell free extract of Rhodobacter capsulatus SB1003 at 395 nm, induces fluorescence emission amplifying with time as long as the fluorophore density is above a critical level. The fact that the amplification disappears at low temperature and at dilute condition, is in accordance with the thermodynamic interpretation that energy is released as per requirement. Live cell imaging is also validation of the phenomenon even at the cellular level. Single cells of Rhodobacter capsulatus SB1003 shows time dependent loss of fluorescence, the process being reversed for cellular clusters. To explain the mechanism of this bistable fluorescence (F) amplification, variation of the scale free kinetic constant k=1/F (dF/dt) is studied at varying temperatures in presence and absence of static magnetic field. The sign of k shifts from positive to negative if T is lowered or if the system is diluted. But at low T, k again switches from negative to positive value,if static magnetic field is applied. The chain of events can be explained by a simple model assuming excretion of a porphyrin by the microbe and possible photon dependent aggregation behavior of such porphyrin complex ,differential temperature and magnetic field sensitivity of the monomeric or aggregated forms of porphyrin being reported earlier.All rights reserved. No reuse allowed without permission.was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
Bistability has been observed for photosynthetic purple non-sulfur bacteria Rhodobacter capsulatus SB1003. The microbes respond to UV excitation (at 365nm) in a switchable manner. A time dependent increase in fluorescence (590-685nm) following roughly a first order kinetics is observed at normal temperature (298K). When either concentration is lowered the state switches to a steady state first order bleaching. Similar bistability is also observed with respect to temperature variations , as amplification occuring at say 298K disappears below 280K. One can synthetically control bistability and force a re-emergence of fluorescence amplification by exposing the system to a static magnetic field ( 0.25T). Differential excitability of singlet and triplet states and selective sensitivity of triplets to static magnetic field may explain this result. The triplets dominate at low temperature preventing amplification of emission, and the reversal may be caused by the Zeeman splitting of the triplets , wherein the excitable singlets re-emerge. Evolutionary and ecological implication of bistable photosynthetic system is discussed in the text.
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