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.

中文翻译：

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荚膜红杆菌SB1003荧光的时间依赖性增强及其对浓度温度和静磁场的关键依赖性

连续照射 395 nm 光会增加光合紫色非硫细菌，荚膜红杆菌 (SB1003) 的荧光发射强度。我们表明，来自这些光合细菌的细胞外色素复合物 (PC) 荧光发射的这种增加取决于色素的浓度和温度，也可以通过静磁场进行调节。在 300 K 或低于 300 K 或低于阈值样品浓度 (0.1 mg/ml) 时，时间依赖性增强发射消失。如果在荧光测量期间引入静态磁场 (395 mT)，则增强的发射会在此条件下 (T < 278 K) 重新出现。发射的时间依赖性用一阶速率常数表示，k = dF/(Fdt)。随着 PC 浓度低于阈值，k 的符号从正变为负，这意味着开始荧光衰减 (k < 0) 而不是放大 (k > 0)。在 PC 浓度高于阈值时，如果温度降低，k 变为负值。但是，令人惊讶的是，在低温下，静磁场会将 k 值恢复为正值。我们通过 PC 中存在的原卟啉环的聚集来解释上述微生物荧光发射的 k 开关和光动力学的逻辑性质。虽然同时存在荧光衰减和对静磁场的敏感性表明三重态在低温下占主导地位，但该过程被 SMF 诱导的自旋简并消除逆转。在更高的温度下，光激发性和缺乏磁响应表明单线态占主导地位。我们建议通过系统间交叉重构单重态-三重态分布可能是这种逻辑行为的基础。在微生物功能的背景下，荧光的时间依赖性增强也意味着将红色光子传递给未直接暴露于入射辐射的邻近微生物，从而充当间接光合作用调节剂。