Oxidation of elemental sulfur, tetrathionate and ferrous iron by the psychrotolerant Acidithiobacillus strain SS3 Daniel Kupka a , Maria Liljeqvist b , Pauliina Nurmi c,1 , Jaakko A.Puhakka c , Olli H. Tuovinen c,d , Mark Dopson b, * a Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, SK-043 53 Kosˇice, Slovakia b Department of Molecular Biology, Umea˚ University, SE-901 87 Umea˚ , Sweden c Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finland d Department of Microbiology, Ohio State University, 484 West 12th Avenue, Columbus, OH 43210, USA Received 20 May 2009; accepted 25 August 2009 Available online 24 September 2009 Abstract Mesophilic iron and sulfur-oxidizing acidophiles are readily found in acid mine drainage sites and bioleaching operations is known about their activities at suboptimal temperatures and in cold environments. The purpose of this work was to char of elemental sulfur (S 0 ), tetrathionate (S 4 O 6 2 ) and ferrous iron (Fe 2þ ) by the psychrotolerant Acidithiobacillus strain SS3. The rates of el sulfur and tetrathionate oxidation had temperature optima of 20 and 25C, respectively, determined using a temperature gradient incub involved narrow (1.1 C) incremental increases from 5 to 30 C. Activation energies calculated from the Arrhenius plots were 61 an 89 kJ mol 1 for tetrathionate and 110 kJ mol 1 for S 0 oxidation. The oxidation of elemental sulfur produced sulfuric acid at 5 C and decreased the pH to approximately 1. The low pH inhibited further oxidation of the substrate. In media with both S 0 and Fe 2þ , oxidation of elemental sulfur did not commence until all available ferrous iron was oxidized. These data on sequential oxidation of the two substrates are in keepin upregulation and downregulation of several proteins previously noted in the literature. Ferric iron was reduced to Fe 2þ in parallel with elemental sulfur oxidation, indicating the presence of a sulfur:ferric iron reductase system in this bacterium. Ó 2009 Elsevier Masson SAS. All rights reserved. Keywords: Acidithiobacillus strain SS3; Iron oxidation; Psychrotolerant; Redox coupling; Sulfur oxidation; Temperature dependency 1. Introduction Bioleaching processes utilize acidophilic Fe 2þ and ele- mentalsulfur(S 0 )-oxidizing microorganisms to aid in the solubilization ofmetalsfrom sulfideminerals [27,31].In bioleaching, the sulfur entity in sulfide minerals is oxidized chemically by Fe 3þ or directly by bacteria with oxygen as the electron acceptor. Typicalintermediates from these reactions are secondary sulfide minerals (e.g. CuS from CuFeS 2 ), thio- sulfate (S 2 O 3 2 ), polythionates (S n O 6 2 ), polysulfides (S n 2 ) and elemental sulfur (S 0 ) [34]. These sulfur compounds are furthe oxidized to sulfate in acid-producing reactions. S 0 is biologi- cally oxidized to sulfate in the presence of oxygen accord to the netequation 1 (the major sulfate species in acidic solutions is bisulfate (pK a2 1.92)). 2S 0 þ 3O 2 þ 2H 2 O / 2HSO 4 þ 2H þ ð1Þ The use of Fe 3þ as an external electron acceptor for Acidithiobacillus ferrooxidans mediatedoxidationofele- mental sulfur (Eq. (2)) under oxic and anoxic conditions has been reported [6,28,35]. * Corresponding author. E-mail addresses: dankup@saske.sk (D. Kupka), maria.liljeqvist@ molbiol.umu.se (M. Liljeqvist), pauliina.nurmi@inbox.com (P. Nurmi), jaakko.puhakka@tut.fi (J.A. Puhakka), tuovinen.1@osu.edu (O.H. Tuovinen), mark.dopson@molbiol.umu.se (M. Dopson). 1 Presentaddress: MTT Agrifood Research Finland, Biotechnology and Food Research, FI-31600 Jokioinen, Finland. 0923-2508/$ - see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.resmic.2009.08.022 Research in Microbiology 160 (2009) 767e774 www.elsevier.com/locate/resmic