Microbial mats can accumulate heavy metals and radio nuclides through precipitation and complexation on and within the cell surface containing hydroxyl and carboxyl groups. Processes involving bacterial oxidation-reduction will alter the mobility of the hevy metal pollutants in water. The microbial immobilization of radio nuclides and other metals was compared for the high temperature and strongly acidic conditions of Tamagawa Hot Springs in Akita Prefecture and the low temperature and neutral pH conditions of Misasa Hot Springs in Tottori Prefecture, Japan. Both hot springs produced microbial mats indicating the Ra and Rn radioactivity was higher than that of the surrounding water. Here, bacterial biomineralization in the radioactive hot springs was described by nanometer−resolution observations using electron microscopic techniques. The analytical results showed the bacterial immobilization of radio nuclides and other metals. The reddish−brown microbial mats at Tamagawa Hot Springs had radioactivity of 5700 cps and were composed of Ca²⁺,Al³⁺,Fe²⁺+,HSO₄⁻ and SO₄²⁻, which had formed sulfur-bearing compounds of barite (BaSO₄), elemental sulfur and As−S elements. Bacterial cells were entirely encrusted with spherical grains having diameters of 100−200 nm, suggesting that heavy metals were transported from strongly acidic hot spring waters (pH of 1.2) to sediments. On the other hand, the green microbial mats at Misasa Hot Springs were characterized by a neutral pH of 7.2 and a temperature of 42°C. The radioactive microbial mats (11.9 μSv/h) were mainly composed of cyanobacteria of Oscillatoria spp. and Phormidium spp., which are associated with minerals of ferrihydrite, Mn oxides and calcite. The ²²⁶Ra content of the microbial mats was 6.9 × 10⁴ Bq/kg, which was higher than that of the hot spring water (1.4 ×10⁻¹ Bq/kg). In addition, in reddish-brown microbial mats, coccus and bacillus type bacteria associated with extracellular polymers around the cell. The results suggest that the microorganisms in the microbial mats grow and decay with a metabolic reaction, and then accumulate heavy metals. It is possible that the capability of radioactive immobilization can be used to counteract the disastrous effects of radionuclide-polluted water and sediments.
田崎 et al. (Thu,) studied this question.