A Citrobacter sp. originally isolated from metal-polluted soil accumulates heavy metals via metalphosphate deposition utilizing inorganic phosphate liberated via PhoN phosphatase activity. Further strain development was limited by the non-transformability of this environmental isolate. Recombinant Escherichia coli DH5 alpha bearing cloned phoN or the related phoC acquired metal-accumulating ability, which was compared with that of the Citrobacter sp. with respect to removal of uranyl ion (UO2(2+)) from dilute aqueous flows and its deposition in the form of polycrystalline hydrogen uranyl phosphate (HUO2PO4). Subsequently, HUO2PO4-laden cells removed Ni2+ from dilute aqueous flows via intercalation of Ni2+ into the HUO2PO4 lattice. Despite comparable acid phosphatase activity in all three strains, the E. coli DH5 alpha (phoN) construct was superior to Citrobacter N14 in both uranyl and nickel accumulation, while the E. coli DH5 alpha (phoC) construct was greatly inferior in both respects. Expression of phosphatase activity alone is not the only factor that permits efficient and prolonged metal phosphate accumulation, and the data highlight possible differences in the PhoN and PhoC phosphatases, which are otherwise considered to be related in many respects.
Ozone concentrations of a few parts per hundred million (pphm) sometimes occur in urban areas, unaccompanied by significant concentrations of other pollutants. The Santa Ana winds provide an important example of such ozone episodes, for they invariably contain 3 to 6 pphm of ozone with no diurnal variation. The origin of these winds in a large-scale weather system, along with the absence of other pollutants and the lack of a diurnal cycle, indicates that this ozone is of natural origin. Occasionally the sea breeze develops a sharp front as it moves inland from the Pacific Ocean. In such episodes, the air which the marine sea breeze replaces is free of pollution except for several parts per hundred million of ozone. This is also regarded as not man-made because of the absence of other pollutants. Elevated ground-level concentrations of ozone have also been occasionally reported from other areas in association with storms. Downward transport from the ozonosphere seems the most probable source for these ozone episodes.
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