Light from a firefly at temperatures considerably higher and lower than normal

www.nature.com/scientificreports OPEN Light from a firefly at temperatures considerably higher and lower than normal Mana Mohan Rabha1, Upamanyu Sharma2 & Anurup Gohain Barua2* Bioluminescence emissions from a few species of fireflies have been studied at different temperatures. Variations in the flash-duration have been observed and interesting conclusions drawn in those studies. Here we investigate steady-state and pulsed emissions from male specimens of the Indian species Sclerotia substriata at temperatures considerably higher and lower than the ones at which they normally flash. When the temperature is raised to 34 °C, the peak wavelength gets red-shifted and the emitted pulses become the narrowest which broaden considerably thereafter for small increases in temperature; this probably indicates denaturation of the enzyme luciferase catalyzing the light-producing reaction. When the temperature is decreased to the region of 10.5–9 °C, the peak gets blue-shifted and the flash-duration increased abnormally with large fluctuation; this possibly implies cold denaturation of the luciferase. We conclude that the first or hot effect is very likely to be the reason of the species being dark-active on hot days, and the second or cold one is the probable reason for its disappearance at the onset of the winter. Our study makes the inference that these two happenings determine the temperature-tolerance, which plays a major role in the selection of the habitat for the firefly. The light of the firefly is the outcome of a very efficient reaction, called chemiluminescent reaction. It is wellknown that oxygen is the biochemical trigger which excites the substrate luciferin, and produces the photoemitter molecule oxyluciferin in presence of ATP and M g2+, the reaction being catalyzed by the enzyme luciferase. In the normal flashing state of a live firefly, visible light is produced as the excited state oxyluciferin decays to the ground state via a pathway followed by molecules indicating p hosphorescence1. It has been shown that the pulses produced by the firefly are manifestations of an oscillating chemical r eaction2. Very recently, assuming the firefly lighting cycle to be a nonlinear oscillator with a robust periodic cycle, a low dimensional nonlinear mathematical model based on the basic lighting mechanism of a firefly has been proposed3. A few studies have been carried out on the effect of temperature on in vivo emissions of fireflies. It has been observed that flash periods of four Luciola species of fireflies of Melanesia decrease with an increase in temperature4. A significant negative correlation between the ambient temperature and inter flash interval has been observed in specimens of Luciola cruciata at five different sites in central J apan5. For the Indian species Luciola praeusta at 20–40 °C, the pulse duration has been found to decrease approximately linearly with temperature for male specimens6 and exponentially with temperature for female specimens7. These imply that the speed of the light-producing reaction increases approximately linearly for males and exponentially for females of this species in this range of temperature. At the temperature of approximately 42 °C, the duration of a flash from this species becomes the minimum and thereafter increases considerably with a slight increase in the temperature, implying that denaturation of the enzyme occurs at this temperature optimum. The peak wavelength also shows a red shift of about 5 nm at this temperature. On the other hand, at temperatures lower than approximately 21 °C, pulse-durations show large increase in a non-linear manner with lowering of the temperature8—almost like the ones observed with fireflies positioned under a strong static magnetic fi eld9. Below 17 °C for females and 15 °C for males of this species, and from below the normal flashing temperature of about 28 °C for females and below about 22 °C for males of another Indian species Asymmetricata circumdata, peaks in flashes get split into three, manifesting the three luminescent forms of the emitter oxyluciferin, with lifetimes of the order of milliseconds1. Sclerotia substriata is the fourth species of firefly found in India after the summer species L. praeusta and A. circumdata, and the winter one Diaphanes sp. This species has been identified by Dr. Lezley Ballantyne of Charles Sturt University, Australia. A male specimen of this species is shown in Fig. 1. It is found near the banks of two large ponds, separated by a lane, in the campus of Gauhati University in very small numbers from April to 1 Department of Physics, Pandit Deendayal Upadhyaya Adarsha Mahavidyalaya, Behali 784184, India. 2Department of Physics, Gauhati University, Guwahati 781014, India. *email: Scientific Reports | (2021) 11:12498 | https://doi.org/10.1038/s41598-021-91839-3 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1.  A specimen of male adult firefly S. substriata. (a) Ventral side. (b) Dorsal side. Average lengths and weights of the collected specimens are 9 mm and 10 mg, respectively. In the campus of Gauhati University, this species is mostly found flashing from trees on the banks of two large ponds lying side by side. September, and in even smaller numbers in March and October. The geographic location of the place is 26.1543° N and 91.6632° E. A survey of literature indicates that no study on the characteristics of light from S. substriata has been carried out till now. In this article, we present emission spectra and pulses of this species at temperatures substantially higher and lower than its normal flashing ones. Results and discussion Steady‑state measurements at different temperatures. Steady-state emission spectra recorded at high temperatures of 34, 35, 37, 39, 41, 43 and 45 ºC along with the one at the normal laboratory temperature of 28 ºC are shown in the wavelength scale in Fig. 2a, and in the energy scale (eV) in Fig. 2b. Emission spectra at low temperatures of 11.5, 10.5, 9.5, 8.5, 7.5 and 6.5 ºC with the normal one at 28 ºC are presented in the wavelength scale in Fig. 2c, and in the energy scale in Fig. 2d. A typical spectrum in the normal range of temperature for this species is asymmetric in nature. The wavelength peak appears at 558 nm, and the full width at half maximum (FWHM) is measured as 61 nm, spreading from 532.5 to 593.5 nm, at the laboratory temperature of 28 ºC (Supplementary Table 1). It has been hypothesized that different species of fireflies emit light at slightly different wavelength peaks because of slight differences in their enzyme s tructures10. Changes in temperature from 20 up to 34 ºC do not change the values of the emission peak (558 nm) and FWHM (61 nm). From 34 ºC onwards, the peak begins to shift towards red; the lower and upper positions of the FWHM begin red-shifting as well, and the spread becomes broad. At 45 °C, the maximum employed temperature that only a few of the specimens could (...truncated)