For some bacteria and algae, it has been proposed that inorganic polyphosphates and transport of metalphosphate complexes could participate in heavy metal tolerance. To test for this possibility in Acidithiobacillus ferrooxidans, a microorganism with a high level of resistance to heavy metals, the polyphosphate levels were determined when the bacterium was grown in or shifted to the presence of a high copper concentration (100 mM). Under these conditions, cells showed a rapid decrease in polyphosphate levels with a concomitant increase in exopolyphosphatase activity and a stimulation of phosphate efflux. Copper in the range of 1 to 2 M greatly stimulated exopolyphosphatase activity in cell extracts from A. ferrooxidans. The same was seen to a lesser extent with cadmium and zinc. Bioinformatic analysis of the available A. ferrooxidans ATCC 23270 genomic sequence did not show a putative pit gene for phosphate efflux but rather an open reading frame similar in primary and secondary structure to that of the Saccharomyces cerevisiae phosphate transporter that is functional at acidic pH (Pho84). Our results support a model for metal detoxification in which heavy metals stimulate polyphosphate hydrolysis and the metal-phosphate complexes formed are transported out of the cell as part of a possibly functional heavy metal tolerance mechanism in A. ferrooxidans. Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans)is a chemolithoautotrophic bacterium that obtains its energy from the oxidation of ferrous iron, elemental sulfur, or partially oxidized sulfur compounds (19,24). This ability makes it of great industrial importance due to its application in biomining to recover metals such as copper, gold, and uranium (19,23). These microorganisms are normally subjected to stress in their environment, such as temperature and pH changes and the presence of toxic heavy metals and nutrient starvation, which affect their physiological state (30).Unlike most heterotrophic bacteria, A. ferrooxidans is capable of resisting high concentrations of heavy metals such as copper, zinc, arsenic, and uranium (9). The genetic basis for mercury and arsenic resistance has been studied in detail in this acidophile (6,26). Copper is an essential trace element for all cells. However, it can cause serious cell damage through radical formation (10). Information regarding copper resistance in A. ferrooxidans is scarce. Although copper-tolerant strains have been obtained by growth and adaptation to increasingly higher concentrations of this metal (5,8,18), only a few genes were recently identified by RNA arbitrarily primed PCR as being induced or repressed in A. ferrooxidans subjected to copper (21). Nevertheless, the role of these genes in the mechanism of copper resistance is still unclear, and their expression may be related to indirect metabolic responses to stress (21).Many heavy metal resistance systems involve either active efflux or detoxification of metal ions by different transformations (27). For copper, these include intracell...

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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 in Sulfolobus acidocaldarius, Sulfolobus metallicus and Sulfolobus solfataricus. All three micro-organisms synthesized polyP during growth, but only S. metallicus greatly accumulated polyP granules. The differences in the capacity to accumulate polyP between these archaea may reflect adaptive responses to their natural environment. Thus, S. metallicus could 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. solfataricus could not grow in or tolerate more than 1-5 mM copper sulfate, most likely due to its low levels of polyP. Shifting S. metallicus cells 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 mM greatly stimulated PPX activity in cell-free extracts from S. metallicus. The results strongly suggest that a metal tolerance mechanism mediated through polyP is functional in members of the genus Sulfolobus. 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.