Interaction of Chlorella vulgaris and Schizophyllum commune with U(VI)

For a risk assessment for released radionuclides a comprehensive elucidation of the migration behavior of uranium is necessary. Therefore not only the uranium interactions with the geosphere but also with the biosphere have to be investigated.
The aim of the study was to characterize the sorption of U(VI) by living Chlorella vulgaris and Schizophyllum commune and the molecular structure of formed complexes.
Both organisms bind significant amounts of U(VI) dependent on applied uranium concentration and pH in the range from 4 to 7. At uranium concentration of 0.2 mM Schizophyllum shows binding capacities of around 130 mg U/g dry weight independent of pH value. A higher initial uranium concentration leads to higher binding capacities of 230-280 mg U/g dry weight at pH 5 and 6 for still living Schizophyllum biomass. In contrast to that, Chlorella binds at a uranium concentration of 0.5 mM 75 mg U/g dry weight (pH 4.4). Furthermore, Chlorella cells die at concentration higher than 0.1 mM within 48 h. Therefore further experiments were carried out at a concentration of 5 µM. Under this condition living algae firstly bind almost all uranium within 5 min of incubation, but then again mobilize up to 80% of the bound uranium during ongoing incubation. The release of metabolism related substances is suggested to cause this mobilization of uranium. As potential leachates for algal-bound uranium oxalate, citrate and ATP were tested and found to be able to mobilize 55-87% of the algal-bound uranium within 24 h.
Differences in complexation of uranium by active and inactive algae cells as well as fungal biomass were investigated with a combination of different spectroscopic techniques. Obtained results demonstrated an involvement of carboxylic and organic/inorganic phosphate groups in the uranium complexation with varying contributions dependent on microbial biomass, cell status, uranium concentration and pH.