Ultraviolet light makes up the portion of the electromagnetic spectrum with wavelengths just shorter than those visible to the human eye, ranging from approximately 100 nm to 400 nm. Within this portion of the spectrum, UV light can be divided into three subcategories: UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (100-280 nm) (1).
Each type of UV light has different effects on living organisms and are present in different levels at Earth’s surface. UV-A is the most abundant, comprising about 6.3% of the total light that passes through the ozone layer and reaches the surface, while UV-B makes up approximately 1.5% and UV-C cannot pass through the ozone layer at all. (4)
Because UV-A is the most prevalent, it has the fewest harmful effects on Earth’s life forms. In fact, it plays an important role in Vitamin D formation in human skin. UV-A is the type of radiation that is responsible for sunburns and cataracts. (7)
UV-C is unable to get through the ozone layer and is therefore not found naturally on Earth’s surface. It is extremely damaging to DNA and is used commercially as a tool to sterilize scientific instruments.
Our project focuses on the middle part of the ultraviolet spectrum, UV-B. While only a small portion of the spectrum is composed of UV-B light, it can still pose a significant threat to many organisms. When UV-B strikes a cell, it is absorbed by the cell’s DNA. this absorption excites the atoms in the nucleic acid thymine and promotes for formation of cyclobutane pyrimidine dimers (CPDs), causing a disruption in the double-helix structure of the genetic material. (6)
While irradiation by UV-B light leads to harmful effects in a wide variety of species, including the causation of skin cancer in humans, we will be focusing primarily on its effects in photosynthetic organisms.
The Effects of UV Radiation on Photosynthetic Organisms
UV-B radiation affects photosynthetic organisms in a variety of ways. While the bulk of our team’s research concerns the damage done to DNA by ultraviolet radiation, UV-B also has adverse effects on other cellular components, including proteins, lipids, membranes, and pigments. (4)
Plants are not entirely defenseless against the dangers of UV-B radiation. Because they are photosynthetic organisms and must live their entire lives in the sun in order to survive, plants have developed multi-faceted systems to deal with the stress of normal radiation levels. For example, many plants produce compounds called flavenoid photoprotectants that absorb UV radiation before it can harm the organism (2). Plants can also produce antioxidants, which neutralize harmful reactive oxygen species produced by UV light (9).
While these and other mechanisms of protection have historically been enough to guard plants against significant damage due to UV-B irradiation, ozone depletion due to the production of greenhouse gasses is leading to increasing levels of UV-B at the earth’s surface. While some plants are largely unaffected by this change, many, including vital crops like corn and rice, are sensitive to these rising levels of radiation (5, 7).
A study by _______ et al examining the effects of UV-B on corn found that increased levels of UV-B are correlated with decreased leaf area, decreased levels of protein, sugar, and starch, and decreased rates of photosynthesis. The study concluded that overall corn yield decreased with increases in UV-B radiation (5). A second study by Rousseaux et al examined DNA damage in plants in South America under the passage of an ozone hole. The results of these two students are consistent with those found by similar studies, demonstrating that increased levels of UV-B radiation could pose a credible threat to the world’s food supply (10).