Radiogenic metabolism

Ionizing radiation, in appropriate doses, stimulates many physiological parameters in protista, plants, invertebrates, and vertebrates (l). Radiogenic metabolism is that metabolism which is promoted by radiation. It seems improbable that evolving microorganisms would ignore a significant, ubiquitous, and relatively constant energy source at a time when sunlight was intermittently obliterated by volcanic dust clouds. Possibly, radiogenic metabolism was involved in the prephotosynthetic transduction of radiant energy into chemical energy. Metabolic adaptation to the utilization of free radicals from the radiolysis of water could be the evolutionary precursor to the use of active oxygen radicals in photosynthesis and respiration. Vestiges of radiogenic metabolism may be one factor in the intestinal microflora competition (2). The biochemical rationale of this work is that oxidant depleted fermentations slow or stop with the accumulation of protons and electrons (or H·) Ionizing radiation provides a constant supply of oxidants (free radicals) as illustrated in the radiolysis of water: HOH→hvH.+HO.. Each free radical is a potential acceptor of H· from metabolic energy production. The model we used to test the hypothesis was the anaerobic growth in the dark of a light and oxidant adapted culture of Rhodopseudomonas capsulata, strain B 100, using the method of Madigan et al. (3). Little growth was obtained in the dark unless oxidant, e.g., dimethyl sulfoxide (DMSO), was added. The experiments were made with a 24 hr light and oxidant adapted culture of R. capsulata inoculated into full, screw capped tubes and incubated in the dark at 32 C for 2½ days. Positive control cultures contained 60 mM DMSO; negative controls had no oxidant added and no irradiation. The experimental cultures were continuously irradiated with 60Co rays of different intensities. The respective culture O.D.s for uninoculated, no oxidant, 90 R/hr γ rays, and DMSO were: 0.0,0.46, 1.05, and 2.00 at 2½ days; and 0.0, 0.30, 1.00, and 4.0 at 5 days. Washed cells were used in the 2nd experiment. These results indicate that appropriate doses of continuous ionizing radiation allowed a degree of growth of this photosynthetic organism in the dark. Implications of this experiment for biochemical evolution, prephotosynthetic prototypes and vestigial systems for the transduction of radiant into chemical energy, the complementation of sunlight and ionizing radiation for a photosynthetic organism, and the int