We investigated the effects of natural light at night (LAN) in the field and artificial LAN in the laboratory on the circadian rhythm of pupal eclosion in a tropical wild type strain of Drosophila jambulina captured at Galle, Sri Lanka (6.1(o)N, 80.2(o)E). The influence of natural LAN, varying in intensity from 0.004 lux (starlight intensity) to 0.45 lux (moonlight intensity), on the entrainment pattern of the circadian rhythm of eclosion at 25(o) +/- 0.5(o)C was examined by subjecting the mixed-aged pupae to natural cycles of light and darkness at the breeding site of this strain in the field. The eclosion peak was approximately 2 h prior to sunrise, and the 24 h rhythmicity was the most robust. Effects of artificial LAN at 25(o) +/- 0.5(o)C were determined in the laboratory by subjecting pupae to LD 12:12 cycles in which the light intensity of the photophase was 500 lux in all LD cycles, while that of the scotophase was either 0 lux (complete darkness, DD), 0.5, 5, or 50 lux. In the 0 lux LAN condition (i.e., the control experiment), the eclosion peak was approximately 2 h after lights-on, and the 24 h eclosion rhythm was not as strong as in the 0.5 lux LAN condition. The entrainment pattern in 0.5 lux LAN was strikingly similar to that in the field, as the 0.5 lux LAN condition is comparable to the full moonlight intensity in the tropics. LAN at 0.5 lux dramatically altered both parameters of entrainment, as the eclosion peak was advanced by approximately 4 h and the 24 h eclosion rhythm was better than that of the control experiment. LAN at 5 lux, however, resulted in a weak eclosion rhythm that peaked in the subjective forenoon. Interestingly, the 50 lux LAN condition rendered the eclosion events unambiguously arrhythmic. After-effects of LAN on the period (tau) of the free-running rhythm and the nature of eclosion rhythm were also determined in DD by a single LD 12:12 to DD transfer. After-effects of the LAN intensity were observed on both the tau and nature of the eclosion rhythm in all four experiments. Pupae raised in 0.5 lux LAN exhibited the shortest tau (20.6 +/- 0.2 h, N = 11 for this and subsequent values) and the most robust rhythm, while pupae raised in 50 lux LAN had the longest tau (29.5 +/- 0.2 h) and weakest rhythm in DD. Thus, these results demonstrate the intensity of LAN, varying from 0 to 50 lux, profoundly influences the parameters of entrainment as well as free-running rhythmicity of D. jambulina. Moreover, the observed arrhythmicity in LD 12:12 cycles caused by the 50 lux LAN condition appeared to be the masking effect of relatively bright light at night, as the LD 12:12 to DD transfer restored the rhythmicity, although it was rather weak.

Synergic contribution of light and temperature is known to cause a paradoxical masking effect (inhibition of activity by bright light and high temperature) on various rhythms of animals. The present study reports the paradoxical masking effects of 1000-lux photophase at 25°C on the locomotor activity rhythm of Drosophila malerkotliana. Flies were subjected to light (L)-dark (D) 12:12 cycles wherein the photophase was varied from 10 to 1000 lux, whereas the scotophase was set to 0 lux in these and subsequent LD cycles. At 10, 100, and 500 lux, the flies were diurnal; however, at 1000 lux they were nocturnal. Transfer from LD 12:12 cycles to continuous darkness (DD) initiated free-running rhythmicity in all flies. Free-running rhythms of the flies switched from the 10-lux to the 500-lux groups started from the last activity-onset phase of the rhythm following 3-5 transient cycles, suggesting involvement of the circadian pacemaker. In contrast, the free-running rhythm of the flies of the 1000-lux group began abruptly from the last lights-on phase of the LD cycle, indicating noninvolvement of the pacemaker. Furthermore, all flies showed nocturnal activity in the two types of LD 12:12 cycles when the photophase was 1000 lux. The first type of LD cycles had three succeeding photophases of 100, 1000, and again 100 lux, whereas the second type of LD cycles had only one photophase of 1000 lux, but the LD 12:12 cycles were reversed to DL 12:12 cycles. Apparently, the combined effects of light and temperature caused such paradoxical masking effects. This hypothesis was tested by repeating the above experiments at 20°C. Flies in all experiments exhibited a diurnal activity pattern, even when the photophase was 1000 lux. Thus, the present study demonstrates that the paradoxical masking effect in D. malerkotliana was caused by the additive influence of light intensity and temperature. This strategy appears to have physiological significance, i.e., to shun and thus protect against the bright photophase at high temperature in the field.

The effects of nocturnal irradiance tenfold dimmer than starlight intensity on the locomotor activity rhythm of Drosophila jambulina were investigated in two types of light-dark (12 h:12 h) cycles, in which light intensity of the photophase was 10 lux while that of the scotophase was either 0 lux for control flies or 0.0006 lux for experimental flies. Activity onset in the experimental flies was approximately 5.4 h prior to lights-on, so it occurred around midnight. However, activity onset of the control flies coincided almost with the lights-on. Nevertheless, activity offset was the same in both groups, occurring at lights-off. Duration of the active phase (alpha) and activity passes/fly/cycle (APC) in the experimental flies was far greater than in controls. After-effects of the nocturnal illumination of the light-dark cycles when the flies were transferred to constant darkness were evident as the period of the free-running rhythm was shortened, alpha was lengthened, and APC was enhanced in the experimental compared to control flies. Thus, very low photic sensitivity of these flies appears to be a physiological adaptation to dim-light ambiance in its natural breeding site in the field.

The role of melatonin (MLT) in micro-and macroalgae is the subject of continuous discussion. In eukaryotic unicellular organisms, the physiological role of MLT seems to differ from those of higher plants and mammals. Experimental data on the role of MLT in algae suggest that it acts mainly as an antioxidant. In the present study, the physiological role of MLT in phototrophic organisms was studied by measuring its effects on periodically controlled processes (cell division and HSP70B expression) as well as mechanisms involved in oxidative stress protection (carotenoids, FeSOD and MnSOD expression). The presence of MLT in Chlamydomonas reinhardtii was assessed and quantified using analytical thin-layer chromatography followed by fluorescence measurements. These data reveal that photoperiod had a significant influence on the processes and mechanisms studied in C. reinhardtii. Moreover, MLT had the greatest impact on C. reinhardtii under conditions promoting photo-oxidative stress. Addition of MLT to the growth medium down-regulated carotenoid content and the expression of FeSOD and MnSOD. These results support the idea that MLT has a physiological role as a direct and indirect antioxidant agent.

The efficacy of bright photophase (BP) in accelerating the re-entrainment of Drosophila biarmipes rhythm following 8 h phase advance and delay of light-dark (LD) cycle was examined by subjecting the flies to 24 h LD cycles with dim photophase (DP) at 30 lx and BP at 300 lx. Re-entrainment was analysed by using the activity onset, activity offset and the duration of activity. Following LD advance or delay, the BP flies re-entrained faster than the DP flies which was attributed to the enhanced zeitgeber strength of BP. Nevertheless, the re-entrainment was a protracted process even in the BP flies since the activity offsets underwent more transients than the activity onsets. Thus, this study demonstrates that the BP accelerates the re-entrainment in D. biarmipes. It, however, also reveals that the re-entrainment is a prolonged process when the activity onset and offset are regarded as the rhythm markers.