An Extended Dose–Response Model for Microbial Responses to Ionizing Radiation

Life on Earth has always existed in the flux of ionizing radiation. However, fungi seem to interact with the ionizing radiation differently from other inhabitants of the Earth. Recent data show that melanized fungal species like those from Chernobyl's reactor respond to ionizing radiation with enhanced growth. Fungi colonize space stations and adapt morphologically to extreme conditions. Radiation exposure causes upregulation of many key genes, and an inducible microhomology-mediated recombination pathway could be a potential mechanism of adaptive evolution in eukaryotes. The discovery of melanized organisms in high radiation environments, the space stations, Antarctic mountains, and in the reactor cooling water combined with phenomenon of 'radiotropism' raises the tantalizing possibility that melanins have functions analogous to other energy harvesting pigments such as chlorophylls.

Few data are available on how quickly free-living microorganisms evolve. We analyzed biofilms collected from a well-defined acid mine drainage system over 9 years to investigate the processes and determine rates of bacterial evolution directly in the environment. Population metagenomic analyses of the dominant primary producer yielded the nucleotide substitution rate, which we used to show that proliferation of a series of recombinant bacterial strains occurred over the past few decades. The ecological success of hybrid bacterial types highlights the role of evolutionary processes in rapid adaptation within natural microbial communities.

Natural levels of radioactivity on the Earth vary by more than a thousand-fold; this spatial heterogeneity may suffice to create heterogeneous effects on physiology, mutation and selection. We review the literature on the relationship between variation in natural levels of radioactivity and evolution. First, we consider the effects of natural levels of radiation on mutations, DNA repair and genetics. A total of 46 studies with 373 effect size estimates revealed a small, but highly significant mean effect that was independent of adjustment for publication bias. Second, we found different mean effect sizes when studies were based on broad categories like physiology, immunology and disease frequency; mean weighted effect sizes were larger for studies of plants than animals, and larger in studies conducted in areas with higher levels of radiation. Third, these negative effects of radiation on mutations, immunology and life history are inconsistent with a general role of hormetic positive effects of radiation on living organisms. Fourth, we reviewed studies of radiation resistance among taxa. These studies suggest that current levels of natural radioactivity may affect mutational input and thereby the genetic constitution and composition of natural populations. Susceptibility to radiation varied among taxa, and several studies provided evidence of differences in susceptibility among populations or strains. Crucially, however, these studies are few and scattered, suggesting that a concerted effort to address this lack of research should be made. © 2012 The Authors. Biological Reviews © 2012 Cambridge Philosophical Society.