Abstract
Thorium is one of the most widespread radioactive elements in natural ecosystems, along with uranium, it is the most important source of nuclear energy. However, the effects of thorium on living organisms have not been thoroughly studied. Marine luminescent bacteria and their enzymes are optimal bioassays for studying low-dose thorium exposures. Luminescent bioassays provide a quantitative measure of toxicity and are characterized by high rates, sensitivity, and simplicity. It is known that the metabolic activity of bacteria is associated with the production of reactive oxygen species (ROS). We studied the effects of thorium-232 (10−11–10−3 M) on Photobacterium phosphoreum and bacterial enzymatic reactions; kinetics of bacterial bioluminescence and ROS content were investigated in both systems. Bioluminescence activation was revealed under low-dose exposures (<0.1 Gy) and discussed in terms of “radiation hormesis”. The activation was accompanied by an intensification of the oxidation of a low-molecular reducer, NADH, during the enzymatic processes. Negative correlations were found between the intensity of bioluminescence and the content of ROS in bacteria and enzyme systems; an active role of ROS in the low-dose activation by thorium was discussed. The results contribute to radioecological potential of bioluminescence techniques adapted to study low-intensity radioactive exposures.
Highlights
Luminous marine bacterial bioassays are appropriate systems for radiotoxicity monitoring in complex multicomponent media
Activation of bacterial bioluminescence was previously observed in diluted solutions of another alpha-emitting radionuclide, americium-241, with higher energy of radioactive decay [14,18,20,21]; the effect was attributed to radiation hormesis
Opposite correlations with reactive oxygen species (ROS) were observed in the aforementioned paper: 300% increase of ROS content was associated with bacterial bioluminescence activation by tritium, while our current study demonstrates a moderate ROS decrease at low-concentration exposure to thorium (Figures 1 and 2 curves 2)
Summary
Luminous marine bacterial bioassays are appropriate systems for radiotoxicity monitoring in complex multicomponent media. This bioassay has been utilized extensively [1,2,3,4,5]. By utilizing bioluminescence intensity as a physiological testing parameter The advantages of this bioassay include high sensitivity, simplicity, high-throughput capacity (1–3 min), and availability of devices for toxicity registration. These advantages provide the possibility of numerous sample analyses and proper statistical processing, with these being important for investigating low-dose effects that are characterized as stochastic [6,7]. Bacterial bioluminescent enzyme systems were suggested as a bioassay for the first time in 1990 [8] and the advantages of this approach were later demonstrated [2,3,4,9,10]
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