Water Journal : Water Journal July 2011
water JULY 2011 111 respectively, photoreactivation and dark repair. The ability to reactivate varies significantly depending on the type of UV damage inflicted and by the level of biological organisation of the microorganism. The repair mechanism is not universal and there are no clearly defined characteristics determining which species can repair themselves and those that cannot. The part of cells most vulnerable to UV damage is the DNA and RNA. This is due partly to its unique function as the repository of the cell's genetic code, and also because of its highly complex structure and large size. It is hardly surprising, therefore, that all known molecular repair mechanisms have evolved to act upon the macromolecular nucleic acids, particularly DNA. In photoreactivation, repair is carried out by an enzyme called photolyase, which reverses the UV-induced damage, while in the case of dark repair it is carried out by a complex combination of more than a dozen enzymes. To begin reactivation (both light and dark), these enzymes must first be activated by an energy source -- in photoreactivation this energy is supplied by visible light (300-- 500nm), and in dark repair it is provided by nutrients within the cell. In both cases, reactivation is achieved by the enzymes repairing the damaged DNA, allowing replication to take place again. Common strains of E. coli contain about 20 photolyase enzymes, each of which can repair up to five thymine dimers per minute -- this means that, in a single cell, up to 100 such dimers can be repaired per minute. 1mJ/cm2 of UV produces approximately 3000--4000 dimers (Oguma, 2002) so, theoretically, damage induced by 1mJ/cm2 of UV can be repaired in just 30 minutes. Repair After Exposure to Low and Medium Pressure UV Low-pressure UV lamps have traditionally been used in water treatment plants because their UV output at 254nm closely matches the absorption peak of DNA bases at 265nm. A number of studies, however, have shown that microbial DNA is capable of photoreactivation after exposure to low pressure UV (e.g. Sommer et al., 2000). Because of these findings, and because of the increased use of medium pressure UV lamps in water and effluent treatment, recent research has begun to look at whether medium pressure UV can permanently inactivate the DNA of microorganisms. It has been suggested that, because the broader wavelengths emitted by medium pressure lamps not only damage DNA but also cause damage to other molecules, it is, therefore, much more difficult for cells to repair their DNA. The recent research compared the effects of low pressure and medium pressure UV on the ability of microorganisms to repair their DNA. In their tests they compared the ability of E. coli to recover in photoreactivating light after being exposed to different amounts of low and medium pressure UV. E. coli was used in the study as it is a useful 'biological indicator' of disinfection efficiency in water systems. The results of these studies showed a significant difference in photoreactivation following low- and medium-pressure radiation. While high levels of photorepair were observed after low-pressure irradiation, with maximum repair occurring after 2--3 hours, there was virtually no photorepair after medium pressure treatment. This was particularly the case at higher log reductions (log 3 and above) (Oguma et al., 2002, Zimmer et al., 2002 and Hu et al., 2005. Zimmer et al. proposed a number of reasons why medium pressure UV causes irreparable damage, while low pressure UV does not. One hypothesis is that there is a synergistic effect between the various wavelengths emitted by medium-pressure lamps that causes irreparable damage to the DNA. Another possible explanation is that the repair enzymes themselves are damaged. According to Zimmer et al., while absorption of UV by proteins is considered of little importance to cells, any damage to repair enzymes would be critical due to that fact that there are so few of them present in the cell. UV disinfection Berson inline UV system for drinking water disinfection.
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