**5.2 Terrestrial organisms**

UVB radiation is known to both negatively and positively affect species. While growth of leaves and stems are inhibited by UVB along with reduced daytime seedling emergence and biomass in early stages of growth, some species of plants exposed to the radiation are less likely to be attacked by leaf beetles (Ballere' et al., 1996). Other plants such as soybeans also receive reduced herbivorous damage when exposed to UVB (Zavala et al., 2000). Thrips, insects that feeds on plant leaves, actually avoid exposure to UVB solar radiation, suggesting that insects can behaviorally respond to that particular wavelength presence (Mazza et al., 1999). UVB can also have an indirect effect upon some species of insect larvae, as those who eat UVB radiated plant material are found to have decreased growth rates and suffer more mortalities versus larvae that feed on non-radiated plant matter (McCloud & Berenbaum, 1994).

Humans are sensitive to solar radiation and may experience sunburn, heat strokes, eye diseases, and skin cancer when overexposed. Low doses of UV radiation are required for vitamin D synthesis and can be used to treat some illnesses. By regulating dosages of UVB radiation to humans with allergic chronic dermatitis, nickel sensitivity, and psoriasis, physicians are able to successfully treat and suppress hypersensitivity. Photoimmunology is a relatively new field and human experiments are often rare due to ethical guidelines and research protocols. Immune system responses by humans to prolonged exposure to UV radiation are usually detrimental. UV damage often includes changes in intracellular signaling, T-cell numbers in exposed skin and inhibition of natural killer cell activity (Duthie et al., 1999). Even humans with varying levels of pigmentation in their skin, such as those of diverse racial descent, are affected differently when exposed to UVB (Coelho et al., 2009; Duthie et al., 1999). Langerhans cells, antigen-presenting cells within the epidermis, die due to membrane disruption and organelle damage in Celtic descendants while cells in the darker skin of Aboriginal or Asian Australians are depleted by apoptosis (Duthie et al., 1999). The melanin produced in the skin when humans are exposed to UV radiation is actually able to absorb UV, shielding nuclear DNA (Coelho et al., 2009). Long term exposure can lead to degeneration of skin cells, underlying fibrous tissue and/or blood vessels leading to premature skin aging and skin cancer. Other possible effects include ocular inflammation and cataracts that can cause blindness. Most tissue damage is due to high levels of UVB wavelengths while more indirect damage via reactive oxygen intermediates affecting DNA, proteins, and lipids is caused by UVA radiation (Figure 8) (Rafanelli et al., 2010).

Fig. 8. Relationship of exposure to UV-R and burden of disease. Source: WHO, Ultraviolet radiation and the INTERSUN Programme in Rafanelli et al., 2010

Vitamin D is necessary for healthy growth and function of most terrestrial vertebrates. Exposure to direct sunlight allows UVB photons to enter the skin and begin a chain reaction resulting in the formation of vitamin D. This nutrient is essential for efficient intestinal absorption of calcium, especially in humans (Holick, 2008). However, an organism's diet is often a supplemental source of vitamin D in addition to providing the building blocks of vitamin D synthesis. In bats, diet strongly affects the levels of circulating vitamin D metabolite. Certain cave dwelling species, receiving little to no direct sunlight, actually have the highest recorded concentrations in vertebrate taxa. Unlike the plant visiting bats, which have levels not even sufficient for humans, sanguivorous and piscivorous species have access to dietary vitamin D, increasing their serum concentrations of the vitamin D metabolite (Southworth et al., 2009). Ectotherms, who often bask in sunlight, are able to use this behavior to regulate their own vitamin D levels when dietary intake is insufficient (Karsten et al., 2009). Deficiency of vitamin D in humans can lead to increased risks of bone fractures from osteoporosis, common cancers, cardiovascular disease, autoimmune and infectious diseases (Holick, 2008).
