Copper tolerance of the thermoacidophilic archaeon Sulfolobus metallicus: possible role of polyphosphate metabolism

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Abstract
It has been postulated that inorganic polyphosphate (polyP) and transport of metal–phosphate complexes could participate in heavy metal tolerance in some bacteria. To study if such a system exists in archaea, the presence of polyP was determined by the electron energy loss spectroscopy (EELS) procedure and quantified by using specific enzymic methods inSulfolobus acidocaldarius,Sulfolobus metallicusandSulfolobus solfataricus. All three micro-organisms synthesized polyP during growth, but onlyS. metallicusgreatly accumulated polyP granules. The differences in the capacity to accumulate polyP between these archaea may reflect adaptive responses to their natural environment. Thus,S. metallicuscould grow in and tolerate up to 200 mM copper sulfate, with a concomitant decrease in its polyP levels with increasing copper concentrations. On the other hand,S. solfataricuscould not grow in or tolerate more than 1–5 mM copper sulfate, most likely due to its low levels of polyP. ShiftingS. metallicuscells to copper sulfate concentrations up to 100 mM led to a rapid increase in their exopolyphosphatase (PPX) activity which was concomitant in time with a decrease in their polyP levels and a stimulation of phosphate efflux. Furthermore, copper in the range of 10 μM greatly stimulated PPX activity in cell-free extracts fromS. metallicus. The results strongly suggest that a metal tolerance mechanism mediated through polyP is functional in members of the genusSulfolobus. This ability to accumulate and hydrolyse polyP may play an important role not only in the survival of these micro-organisms in sulfidic mineral environments containing high toxic metals concentrations, but also in their applications in biomining.

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