The temperature inside the leach dump often rises spontaneously as a result of microbial activities. Thus, thermophilic iron-oxidizing chemolithotrophs such as thermophilic "Acidithiobacillus" species and "Leptospirillum" and at even higher temperatures the thermoacidophilic archaeon "Sulfolobus (Metallosphaera sedula)" may become important in the leaching process above 40 °C. Similarly to copper, "Acidithiobacillus ferrooxidans" can oxidize U4+ to U6+ with O2 as electron acceptor. However, it is likely that the uranium leaching process depends more on the chemical oxidation of uranium by Fe3+, with "At. ferrooxidans" contributing mainly through the reoxidation of Fe2+ to Fe3+ as described above.