Consequences of the Prolonged Ionizing Radiation Treatment on Bacteria in an Extremely Radioactively Polluted Zone of the Chernobyl Nuclear Power Plant
Victoria ROMANOVSKAYA*
Biology of Gas Oxidizing Microorganisms, Institute of Microbiology and Virology NAS of Ukraine, 154 Zabolotny str, Kiev 143, Ukraine
The accident at the Chernobyl Nuclear Power Plant (ChNPP) in 1986 has created a natural model for a study of the consequences of prolonged ionizing radiation (1-128 μCi per kg of soil) on microorganisms as part of the biota in an extremely radioactively polluted region (10 km ChNPP zone). In soils of the 10 km ChNPP zone subjected to a long-duration radiation treatment the species diversity indices of heterotrophic bacteria were very low compared with those in unpolluted soils. Bacillus cereus was the dominant heterotrophic bacterium. Also pink pigmented facultatively methylotrophic bacteria (Methylobacteriuth extorquens or Methylobacterium mesophilicum) were found in all soil samples. The frequency of occurrence and species structure of Methylobacterium strains in soil and plant samples within the 10 km ChNPP zone did not differ significantly from those in similar samples collected outside the zone. Isolated strains of the genera Bacillus and Methylobacterium displayed a high resistance to hydrogen peroxide (1-3%), which causes cell damage in a rather similar way as ionizing radiation. Other species of heterotrophic bacteria were not present in all soil samples of the 10 km ChNPP zone; those isolated were characterized by a low resistance to hydrogen peroxide. In soils of the 10 km ChNPP zone the numbers of cellulose fermenting, nitrifying and sulfate reducing bacteria were much lower than in similar unpolluted soils. These results thus show that the number and diversity of soil bacteria under anthropogenic radiation strongly decreases. The level of genetic variability of Methylobacterium strains was higher in strains isolated within the 10 km ChNPP zone. Methylobacterium strains were highly resistant to UV and ionizing irradiation. UV had only a low mutagenic effect on Methylobacterium strains. Dozes of nuclear radiation under 0.6 kGy did not affect the viability of the investigated Methylobacterium strains. Sublethal dozes were as a high as 5-10 kGy for some strains. Apparently, Methylobacterium strains are able to efficiently repair radiation damage, that enables them to survive under high radiation levels. Thus, the selective pressure of long-duration treatment of soil bacteria with nuclear radiation has resulted in dramatic change in their qualitative and quantitative population structures. Constant inhabitants of all investigated radioactively polluted soils were Bacillus cereus, and also Methylobacterium extorquens or M. mesophilicum; these species thus must possess highly active mechanisms for detoxification of the active oxygen species generated and/or efficient system for repair of DNA damage.
