The following are extracts from a paper called "History of Ultraviolet Photobiology", and it really lives up to the title. The paper is a very thorough background to the study of ultraviolet light, and its effects. It was written by Philip E. Hockberger. I thought it was sheer brilliant, and very, very well researched. It was the kind of stuff that can make one feel a bit like a diver swimming around the coral reef. Loads of interesting things going on, lots to take on-board. Below are a few juicy bits which caught my eye....
Starting in the late 17th century, a new mythology arose in Europe that was based upon scientific principles and provided the basis for a more reliable understanding of the relationship between humans and sunlight. By the start of the 19th century, the application of these principles led to the realization that sunlight is not a single stimulus but, rather, a collection of stimuli of different wavelengths (e.g., infrared, visible, ultraviolet). This realization inspired additional studies aimed at determining whether different wavelengths might be responsible for the different effects of sunlight
In 1801, Ritter made the hallmark observation. He noticed that invisible rays just beyond the violet end of the spectrum were even more effective at darkening silver chloride-soaked paper. He called them "deoxidizing rays" to emphasize their chemical reactivity and to distinguish them from the "heat rays" at the other end of the visible spectrum.
In 1832, Picton (44) was the first to document the detrimental effects of sunlight on patients with smallpox. He reported that soldiers confined to dungeons during a smallpox epidemic contracted the disease but recovered without suppuration or scarring.
In 1866, Schutze (cited in 31) demonstrated that vertebrate eyes possess two kinds of photoreceptors: rods for dim vision and cones for color vision. In 1877, Boll (131, 132) and Kühne (133, 134) independently published their classical studies on visual purple (rhodopsin), the photoreceptor pigment of rods, and established that it was involved in the detection of light. Sixty years later, Hosoya (135) showed that rhodopsin absorbs UV as well as visible rays.
In 1875, Von Platen (112) found that illumination of the frog retina stimulated oxygen uptake, CO2 production, and increased metabolism. The same year, Pott (113) showed that an individual mouse produced more CO2 under green or yellow light than under violet, red or sunlight. It also produced less CO2 at night.
In 1883, Graber (141) showed that blinded salamaders and naturally blind ringworms avoided UV and violet-blue light, and he suggested that the response was mediated through the skin.
In 1885, Moleschott (117) reported that light-induced CO2 production in frogs was mediated locally through the skin as well as through the visual system. By 1887, Fubini & Spallitta (118) showed that all colors were effective at increasing CO2 production, though not to the same degree.
Several studies showed that light stimulated the motility of contractile tissues. Between 1844-59, Arnold (146), Reinhardt (147) and Brown-Sequard (148) observed that artificial light induced contraction of the iris muscle in the extracted eyes of eels and frogs. Brown-Sequard further demonstrated that it was due to a direct effect of light on the pupillary sphincter muscle. In 1892, Steinach (149) extended these results to fish and amphibians by showing contraction of the papillary muscle in response to light in isolated eyes even after carefully removing the optic and oculomotor nerves.
In 1888, Gaillard (260) found that sunlight was damaging to many kinds of bacteria and spores but not to molds or yeast. He agreed that the rate of destruction was dependent upon the intensity of sunlight, the composition of the medium, and the presence of oxygen.
Between 1893-95, Ward (260) performed a remarkable series of experiments demonstrating superb technical skill and ingenuity. Using improved versions of Buchner's assay and Geisler's apparatus, he showed that violet-blue and near UV (UVA) rays were the most damaging part of sunlight on bacteria. He also noted that pigmented fungi were resistant, consistent with the notion that pigments serve as protective filters. Finsen (50) showed that sunlight concentrated by a lens and passed through the ear of a white rabbit was capable of bactericidal action. In 1896, Westbrook (265, 266) showed that the bactericidal effect of sunlight was greatest at the surface of cultures, whereas bacterial growth was facilitated deeper in the medium due to elevated temperature and decreased oxygen availability.
In 1893, Richardson (267) showed that sunlight had a sterilizing effect on human urine, and that irradiation of urine in the presence of oxygen resulted in the generation of hydrogen peroxide. D'Arcy & Hardy (268) showed that UVA and violet-blue rays from a high intensity electric arc lamp stimulated production of an oxidizing substance in water, possibly ozone. This, they suggested, might explain the bactericidal action reported by Ward. In 1927, Bedford (269) showed that UV light stimulated hydrogen peroxide production in culture medium. This led him to suggest that the destructive action of UV light on bacteria is caused by the interaction of light with photosensitizers in the medium resulting in hydrogen peroxide production leading to irreparable damage to the bacteria.
In 1898, Anderson (70) reported that two patients exhibiting seasonal sunburn (hydroa aestivale) possessed an unusual porphyrin-like pigment in their urine. Ehrman (71) suggested that this pigment was hematoporphyrin, although Günther (72) noted that not all patients with porphyrinuria were light-sensitive. In 1913, Meyer-Betz (73) confirmed the photosensitizing properties of hematoporphyrin by administering it to himself.
In 1916, Jüngling (94) showed that melanin production was enhanced by light rays longer than 330 nm, whereas sunburn was induced by rays below 330 nm.
In 1921, Fabry & Buisson measured the spectral composition of sunlight and the absorption characteristics of ozone. They surmised that ozone in the upper atmosphere is responsible for filtering most of the solar UV radiation. In 1919, Dorno demonstrated that the intensity of UV radiation penetrating the atmosphere varies throughout the day (greatest when directly overhead) and with the seasons of the year (greatest in summer).
They [biologists] used the term "UVC" to refer to the solar region that was absorbed by the ozone layer in the Earth's upper atmosphere, i.e., below 290 nm, and therefore had no biological impact. The term "UVA" was used for the region 320-400 nm that penetrated window glass and had physiological effects on organisms. The term "UVB" was applied to the region between the UVC and UVA, i.e., 290-320 nm, and this region was believed to be responsible for the deleterious effects of sunlight on living organisms.
In 1919, Adler (154) showed that UV, but not visible, rays stimulated smooth muscle contraction in the frog, rabbit, and guinea pig. In 1954, Giese & Furshpan (155) showed that low intensity UV rays increased the frequency of discharge of the stretch receptor of a crayfish muscle, whereas high intensity UV rays decreased it. In 1957, Pierce & Giese (156) found that high intensity UV rays reduced the amplitude of action potentials in the axons of frogs and crabs, but irradiation with blue light immediately afterwards reversed the effect (photoreactivation)
Very recently, Berson, Yau and colleagues (185, 186) have demonstrated that rat retinal ganglion cells are photosensitive, due to the photosensitive pigment melanopsin that absorbs throughout the UV and visible spectrum, and that these cells are responsible for setting the circadian clock.
Wykoff (289, 290) reported that the energy required to kill bacteria with X-rays was 100 times less than that required with even the most potent UV rays (i.e., 265 nm). He calculated that only one in four million absorbed UV photons is capable of causing cell death.
Raab (245) found that Paramecia stained with the fluorescent dye acridine red were killed when exposed to visible light. He also showed that animals treated with eosin and exposed to visible light suffered from edema and necrosis in the irradiated area. While investigating the cause of the toxicity, he found that neither the light nor the dye was toxic when given alone. Furthermore, the dye was non-toxic if exposed to light separately and then applied. He concluded that it was the combination of dye and light that was responsible for the effect.
Between 1900-1910, von Tappeiner (Raub's mentor), Jodlbauer, and their colleagues went on to show that this toxic effect (which they called "photodynamic sensitization") could be produced using any fluorescent dye and any wavelength (UV or visible) that excited the dye. This led von Tappeiner (246) to propose that it was the emitted light that was responsible for the toxicity.
In 1932, Blum (3) reviewed the results of 121 papers related to this topic, and he concluded that it was not the light but rather some chemical toxin produced by the interaction of light with the dyes. This effect, he pointed out, was clearly distinct from the direct effect of UV rays on cells. Photodynamic actions required a dye or some other chemical to interact with the light, and the response was dependent upon the presence of oxygen. The latter was demonstrated by Straub (247) who hypothesized that the photodynamic effect was due to direct oxidation of cellular constituents. Blum (3) surmised that cellular damage was an indirect effect caused by photooxidation of the dye resulting in the generation of a toxic by-product, probably a peroxide. He also ventured that the photosensitivity of range animals feeding on either buckwheat or St. John's wort was due to the same kind of photochemical reaction.
In 1964, Setlow & Carrier (304) and Pettijohn & Hanawalt (305) independently found that DNA is spontaneously repaired in bacteria following UV exposure.
Webb (15) reviewed the literature showing that UVA rays cause lethal and mutagenic effects in microorganisms even in the absence of exogenous photosensitizers. Unlike UVB effects, UVA effects are oxygen-dependent. In 1980, D'Aoust and colleagues (298) showed that flavins are endogenous photosensitizers which underly the damaging effect of visible light in bacteria. Hartman (299) reported that irradiation of E. coli with UV rays (300-400 nm) induced hydrogen peroxide production, a process that probably involves flavins (300).
Most studies of UVA and violet-blue light responses have implicated carotenoids and flavins as molecular photoreceptors. In 1935-37, Castle (332) and Bünning (333) proposed that carotenes were involved in phototropism in the fruiting bodies of Phycomyces and Pilobolus (fungi) and in the coleoptiles of the plant Avena. In 1950, Galston (334) proposed the alternative "flavin hypothesis" in which riboflavin acts as a photosensitizing agent in the photooxidation and stimulation of the growth hormone (auxin) indole acetic acid. Forty years later, Galland (335) reported that flavins are still regarded as the most common photoreceptors in blue light responses, although carotenoids and pterins have been implicated in some cases.
Philip E. Hockberger
Northwestern University, Feinberg School of Medicine
Department of Physiology, M211
303 E. Chicago Ave., Chicago, IL 60611-3008, USA