The Responses of Cyanobacteria to UV-B Irradiation

Introduction

Cyanobacteria are a deep branching phylum of prokaryotes that are of incredible importance to the biosphere. These ancient organisms perform photosynthesis through the use of photosystems (PS) I and II, a design unique among photosynthesizing bacteria and archaea. Pigments such as phycocyanin and chlorophylls a and b are embedded in the cyanobacterial thylakoid membrane. In particular, phycocyanin is associated with cyanobacteria, as it is responsible for the blue-green appearance for which these organisms are named. Pigments absorb wavelengths of light from the visible spectrum of 400 to 700 nm (Allen, 2001). When energy from light is captured, H2O or H2S is split. When water is the substrate, this reaction releases oxygen gas, protons (H+) and donates an excited electron to PS II. As this initial electron relaxes back to its ground state, it passes its energy to electrons in adjacent atoms until the reaction center of PS II is reached. From this point, electrons reduce various proteins in PS II, allowing H+ to be pumped across the thylakoid membrane to produce a chemical gradient. The potential energy stored in this gradient is then harnessed to make a usable energy source for the cell in the form of ATP. Even more energy is stored when a second electron from PS I is excited. The protein ferredoxin can then produce the energy carrier NADPH.

It is important to note the production of oxygen gas (O2) that occurs during photosynthesis in cyanobacteria. As it turns out, cyanobacteria are the only known bacteria that produce O2 (Slonczewski). The evolution of this type of metabolism probably caused the increase of oxygen in Earth’s early atmosphere. Thus, without the presence of cyanobacteria or a similarly photosynthesizing predecessor, aerobic life as we know it could not have evolved.

Today, cyanobacteria continue to act as important contributors to the air we breathe and provide a number of other important services to ecosystems. The diverse forms seen amongst species of cyanobacteria allow this phylum to fill key roles in a range of ecosystems including the most extreme aquatic and terrestrial environments on Earth. Cyanobacteria have been found to act as a primary source of usable nitrogen to living organisms. Most species of cyanobacteria are capable of fixing nitrogen, converting atmospheric nitrogen into nitrate or ammonia. In habitats like those found in arctic regions where nitrogen is the most common limiting element, the presence of cyanobacteria accounts for up to 80% of the total annual nitrogen input (Solheim et al., 2006). The growth of organisms in these regions would be severely retarded without the nitrogen contributions of cyanobacteria. Similarly, cyanobacteria compose a large portion of microbial biomass and productivity in many ecosystems on which a multitude of higher organisms depend (Vincent, 2000).

Given these essential functions of cyanobacteria in ecosystems, it is important to understand how these organisms are adapted to their environment and how global change might impact them. The level of ozone-depleting gases in our atmosphere has increased dramatically in Earth’s recent history due to anthropogenic emissions. With a reduction of the ozone layer, organisms on Earth are being exposed to an increased level of ultraviolet (UV) radiation. UV light has shorter wavelengths (290-400 nm) than visible light, meaning that it has higher energy (Allen, 2001). Exposure to this high-energy irradiation can lead to the damage of proteins and DNA, as well as changes in growth, cellular differentiation, photo orientation, and the motility of cells (Ehling-Schulz & Scherer, 1999). UV-A (320-400 nm) radiation is not impacted by the presence of the ozone layer, and thus exposure to this form of UV light will not change due to ozone depletion (Ehling-Schulz & Scherer, 1999). However, UV-B (290-320 nm) photons are typically absorbed by the protective ozone layer, so a decrease in ozone will lead to an increased amount of UV-B hitting Earth’s surface (Ehling-Schulz & Scherer, 1999). This will be of particular importance to cyanobacteria, as these organisms depend on near-constant exposure to sunlight for survival. Accordingly, cyanobacteria have been found to poses mechanisms to avoid UV-B rays, defend against them, and repair any damage that they might cause to cellular components (Ehling-Schulz & Scherer, 1999).

Cellular Damage Caused by UV-B Irridation

UV-B irradiation has been found to have a large number of damaging effects on cyanobacteria and many other organisms. While a small amount of potentially toxic forms of oxygen are normally produced within cyanobacteria, uncommonly high levels of UV light can lead to the creation of an increased number of reactive oxygen species (ROS) from excited photosynthetic pigments (He & Hader, 2002). These ROS can cause oxidative damage to polyunsaturated fatty acids in the cellular membrane and breaks in DNA strands (He & Hader, 2002). Proteins and DNA absorb UV-B irradiation, which makes them common targets for damage (Ehling-Schulz & Scherer, 1999). It has been found that UV-B wavelengths often harm the D1 reaction center protein of PS II, Rubisco, phycobili proteins (like phycocyanin) and nitrogenase (Ehling-Schulz & Scherer, 1999). When DNA absorbs UV-B photons, double-stranded breaks, single-stranded breaks, DNA-protein cross links, cyclobutane dimmers, and pyrimidine-(6,4)-pyrimidone photoproducts have all been shown to occur (Peak & Peak, 1986; Mitchel & Niara, 1989 as cited in Ehling-Schulz & Scherer, 1999). The energy that must be used to protect against and repair this damage impacts the cells’ energy budget. Organisms affected by UV-B exposure have thus been shown to grow more slowly and replicate less frequently than those animals with less of a UV-B burden (Garcia-Pichel et al., 1993; Ehling-Schulz et al., 1997).

Motility in Response to UV-B Irradiation

Cyanobacteria can often evade the damaging effects of UV-B irradiation by moving. Some species form mat communities in soils by migrating down to the level at which each species can absorb the maximum amount of light and receive the minimum amount of threat from UV irradiation (Quesada & Vincent, 1997). Because even closely related species of cyanobacteria show a large degree of variability in their sensitivity to UV light (Quesada & Vincent, 1997), mat communities tend to show distinct layers of cyanobacteria. Layers closer to the surface will consist of those species that have a higher tolerance for UV-B irradiation than the species found in lower layers. In fact, it has been found that this distinct layering is lost when cyanobacteria are grown in the absence of UV-B radiation (Sheridan, 2001). Likewise, aquatic cyanobacteria can sink to lower levels in the water column to avoid intense radiation (Reynolds et al., 1987 as cited in Ehling-Schulz & Scherer, 1999). Moving up and down occurs daily in some species to avoid the higher light intensity of the afternoon (Garcia-Pichel et al., 1994), whereas other species may only respond to long-term changes. Migration, however, comes at a price. Moving to areas with less light penetration can lead to a decrease in productivity of these photosynthetic organisms (Bebout & Garcia-Pichel, 1995).

The motility of cyanobacteria is based on the little-understood method of gliding and the simple concept of floating. Gliding involves no obvious change in cell shape or external organelle, and may be related to internal rotation or contraction of some unidentified cell components (Burchard, 1980). This movement can occur over semi-solid or solid surfaces with a film of water on top (Burchard, 1980). It has been shown that cyanobacteria exposed to UV-B light will move to less intensely irradiated areas (Quesada & Vincent, 1997; Bebout & Garcia-Pichel, 1995), indicating that these organisms are photoactive (Burchard, 1980). Despite this similarity in response by different species of cyanobacteria, it has also been shown that the rate at which species react and glide away is variable (Quesada & Vincent, 1997). When in the water column, gliding becomes less useful, and cyanobacteria rely on gas-filled vesicles to maintain buoyancy. Cyanobacteria are capable of changing their buoyancy by altering the volume of their gas vesicles as a response to environmental conditions including light intensity (Brookes and Ganf, 2001). Altering gliding patterns and buoyancy based on the strength of UV-B radiation is thus an important way in which cyanobacteria can avoid UV damage.

References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500. Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2010, Kenyon College.