Arthur C. Giese

The Photobiology of Blepharisma

Blepharisma (Figs. 1A and 1B) is a genus of ciliate protozoans in which most of the 47 listed species contain a pink pigment named blepharismin that renders them sensitive to glass-transmitted visible and nearultraviolet (UV) radiation. Initially, the pigment was called zoopurpurin by Arcichovskij (1905), who first determined its absorption spectrum. The name was changed to blepharismin to conform to the terminology of hypericin-like pigments, using the genus name as the basis of nomenclature: hypericin from Hypericum, stentorin from Stentor, fabrein from Fabrea, and blepharismin from Blepharisma (Giese, 1973).

Photosensitization of Organisms, with Special Reference to Natural Photosensitizers

Colorless cells, not appreciably absorbing visible light, are little affected by light passing through them because only light absorbed is capable of promoting a photochemical reaction. A photosensitizer is a molecule with a chromophore capable of absorbing light and transferring the energy to other molecules, themselves not capable of absorbing light.

Is Sunlight Good for You

The conventional wisdom that the sun cures human illness comes to us from ancient times and persists to this day. Peasants in some parts of southern Europe expose sick people and sick animals to the sun in hopes of a cure. Most people believe that sunlight is highly beneficial to health, and they expose themselves to it whenever they can—on holidays, at beaches, or under sun lamps.

How Ultraviolet Radiation Affects the Cell

When cells are exposed to sunlight, they absorb some ultraviolet radiation, chiefly in the UV-B range. The cell’s nucleic acids and proteins take up most of the radiation, which often produces photochemical changes. After exposure, the altered molecules may affect one or more of the cell’s functions. In complex multicellular animals like ourselves, UV-B radiation only penetrates, and is absorbed by, the cells near the surface; cells deep in tissues remain protected. UV-A radiation, however, goes into deeper tissue cells through the outer surface (epidermis) and some of it even penetrates the entire skin layer.

The Sun, Sun Myths, and Sun Worship

Most of us take the sun for granted. We know it to be a heavenly body subject to the same laws of physics that govern events on earth. For example, we know that the earth’s axis is not vertical to the earth’s orbit around the sun; that the resultant oblique angles at which the sun’s rays strike the earth cause seasonal variation in radiation (as Figure 1:1 shows); and that the quantity is greatest when the days are longest. We have confidence that the sun will rise each morning and set each evening. We know, as Figure 1:2 demonstrates, that the length of the day will vary regularly; that in the Northern Hemisphere it is maximal at the summer solstice (June 21) and minimal at the winter solstice (December 22); and that the length of day is equal to the length of night at the vernal equinox (March 21) and the autumnal equinox (September 22). The sun presents us with our major cues to time and season, and figures prominently in agricultural practices as well.

The Relevance of Photobiology

Photobiology, once considered an esoteric discipline, has penetrated many aspects of biology. Although biologists are probably cognizant of photobiological studies in their special fields of endeavor-for example, botanists know about photic reactions in photosynthesis--they may not be aware of the broad spectrum of biological problems to which photobiology contributes. As individuals burrow deeper in their own special fields, insularity tends to increase. Even photobiologists are often unaware of the importance of contributions of their colleagues in contiguous fields of photobiology. A survey of the contribution of photobiology to biology, which is the intent of this review, also focuses attention on research opportunities, especially at the molecular level, for further analysis of photobiological problems that may interest investigators seeking uncrowded but promising fields. Photobiology, as a branch of photochemistry, has its roots in the principles of photochemistry. Only light absorbed by molecules in cells is effective in promoting photobiological reactions. The absorbed light raises cellular molecules to excited states. While the energy of the excited state may be dissipated as fluorescence or heat, excitation often induces a chemical change that affects the cell.

Sunlight and Cancer

The most common human cancer, skin cancer, has long been known to be localized mainly on the head and neck. Since these areas are most directly exposed to the sun, it is no surprise that sunlight was suspected as the culprit. Physicians keep two kinds of records on human skin cancer: incidence and prevalence. Incidence data record the number of cases of cancer appearing in a given geographical area per year, and prevalence data indicate the total number of cancers at one time in a given place. Squamous cell carcinoma and basal cell carcinoma, shown in Figures 8:1B,C, are the two most commonly found types of skin cancer. These carcinomas may form nodules on the skin or grow downward into the dermal connective tissue, or invade the dermis laterally. Skin cancers appear to be on the increase in the light-skinned population.

How Sunlight Ages Skin

That sunlight, more than the passage of time, removes the visage of youth has been known by dermatologists for years, but the rest of us remain oblivious to this dictum. In addition to immediate, acute deterioration and rapid healing of the epidermis following sunburn, more permanent changes are induced in both epidermis and dermis by repeated, excessive, long-term exposure of the skin to the sun (Table 6:1). Often the deleterious effects are not recognized as a consequence of overexposure to sunlight in youth, but more popularly interpreted as a result of aging. Frequently, weather-beaten, wrinkly, furrowed, splotchy, excrescence-marred faces characteristic of extreme old age develop in early middle age or sooner as a consequence of too much sun.

Sunlight and Life

Sunlight is a mixture of light visible to us, and ultraviolet and infrared radiation, both invisible to us. Most of the sun’s radiation, about 60 percent, is in the infrared region. About 37 percent occurs in the visible region and only about 3 percent in the ultraviolet. In recent years, x-rays and radio waves have been detected from the sun, but the amount of energy represented is vanishingly small. The maximum intensity of sunlight is in the yellow-green part of the spectrum where our eyes see best, presumably an evolutionary adaptation to the sun’s radiation.

Photosensitization of Cells

Biologists know that a cell is sensitive only to the radiation it absorbs, but that when the cell takes on a photosensitizer, it can absorb radiation it normally cannot. We know that cells of light-colored human skin normally absorb little visible and UV-A radiation, but all we need do is provide these cells with appropriate photosensitizers and they suddenly absorb these kinds of radiation. This can result in damage to the epidermal prickle cells, much like sunburn. All epidermal cells, whether in people, animals, or plants, and all free-swimming cells, are subject to photosen-sitization. By analogy, consider the photographic process, in which a pure silver chloride emulsion on film is sensitive only to the short end of the spectrum, the blue, violet and ultraviolet rays. If we add a dye, such as pinacyanol, it will absorb over the entire range of visible wavelengths. The dye acts as a sensitizer by transmitting the light energy from all the wavelengths it absorbs to the silver chloride, making the film sensitive to the entire rainbow of colors from violet to red seen by the human eye.