Time-dependent enhancement of fluorescence from Rhodobacter capsulatus SB1003 and its critical dependence on concentration temperature and static magnetic field

Continuous exposure of 395 nm light increases the fluorescence emission intensity of photosynthetic purple non-sulphur bacteria, Rhodobacter capsulatus (SB1003). We show that such an increase in fluorescence emission of extracellular pigment complexes (PC) from these photosynthetic bacteria depends on the concentration of the pigment and temperature and can also be modulated by the static magnetic field. The time-dependent enhanced emission disappears either at or below 300 K or below a threshold sample concentration (0.1 mg/ml). The enhanced emission reappears at this condition (T < 278 K) if a static magnetic field (395 mT) is introduced during fluorescence measurement. The time dependence of emission is expressed in terms of a first-order rate constant, k = dF/(Fdt). The sign of k shifts from positive to negative as PC concentration is lowered than a threshold value, implying onset of fluorescence decay (k < 0) rather than amplification (k > 0). At PC concentration higher than a threshold, k becomes negative if the temperature is lowered. But, surprisingly, at low temperature, a static magnetic field reverts the k value to positive. We explain the logical nature of k-switching and photo-dynamics of the aforesaid microbial fluorescence emission by aggregation of protoporphyrin rings present in the PC. While the simultaneous presence of decay in fluorescence and susceptibility to static magnetic field suggests the dominance of triplet states at low temperatures, the process is reversed by SMF-induced removal of spin degeneracy. At higher temperatures, the optical excitability and lack of magnetic response suggest the dominance of singlet states. We propose that the restructuring of the singlet-triplet distribution by intersystem crossing may be the basis of this logical behaviour. In context with microbial function, time-dependent enhancement of fluorescence also implies relay of red photons to the neighbouring microbes not directly exposed to the incident radiation, thus serving as an indirect photosynthetic regulator.