Degradation and corrosive activities of fungi in a diesel–mild steel–aqueous system Fa´ tima Menezes Bento 1 , Iwona Boguslava Beech 2 , Christine Claire Gaylarde 3, * , Gelsa Edith Englert 4 and Iduvirges Lourdes Muller 4 1 Department of Soil, Faculty of Agronomy, UFRGS, 7712 Bento Gonc ßalves Avenue, CEP 91540-001, POA, RS, Brazil 2 School of Pharmacy and Biomedical Sciences, Microbiology Research Laboratory, St Michael’s Building, White Swan Road, Portsmouth PO1 2DT,UK 3 Department of Soil, MIRCEN, UFRGS, 7712 Bento Gonc ¸ alves Avenue, CEP: 90001-970, POA, RS, Brazil 4 Department of Metallurgy, Biocorrosion and Biofilms Lab, UFRGS, 99 Osvaldo Aranha Avenue s.615D, CEP: 90035-190, POA, RS, Brazil *Author for correspondence: Tel.: +55-51-33-16-6026, Fax: +55-51-3316-6029, E-mail: cgaylarde@yahoo.com Received 22 December 2003; accepted 22 June 2004 Keywords: Aspergillus fumigatus, biocorrosion, biodegradation, diesel oil, fungi, mild steel, propionic acid, storage tanks Summary The fungi Aspergillus fumigatus, Hormoconis resinae and Candida silvicola were isolated from the fuel/water interfacial biomass in diesel storage tanks in Brazil. Their corrosive activities on mild steel ASTM A 283-93-C, used in storage tanks for urban diesel, were evaluated after various times of incubation at 30 °C in a modified Bushnell– Haas mineral medium (without chlorides) with diesel oil as sole source of carbon. Their ability to degrade diesel oil was evaluated after growth for 30 and 60 days. The fungus Aspergillus fumigatus and the consortium of all three organisms showed the highest production of biomass; A. fumigatus gave the greatest value for steel weight loss and produced the greatest reduction in pH of the aqueous phase. Solid phase microextraction (SPME) showed that the main acid present in the aqueous phase after 60 days incubation with A. fumigatus was propionic acid. Polarization curves indicated that microbial activity influenced the anodic process, probably by the production of corrosive metabolites, and that this was particularly important in the case of A. fumigatus. This fungus preferentially degraded aliphatic hydrocarbons of chain lengths C 11 AC 13 in the diesel, producing 47.7, 37.5 and 51% reductions in C 11 ,C 12 and C 13 , respectively. It produced more degradation than the consortium after 60 days incubation. It is likely that the presence of other species in the consortium inhibited the growth of A. fumigatus, thus resulting in a lower rate of diesel fuel degradation. Introduction Biocorrosion or microbiologically influenced corrosion (MIC) is corrosion that is influenced by the presence and activities of microorganisms or their metabolites. MIC is recognized by many industries as a significant factor in reducing the useful life of equipment (Westbrook et al. 1988; Rosales et al. 1994; Machado et al. 1998; Beech & Gaylarde 1999; Gaylarde et al. 1999; Videla 2001) and has been documented for metals exposed to crude and distillate fuels during storage (Videla et al. 1994; Stoec- ker 1995; Machado et al. 1998). Microbial contamina- tion of hydrocarbon fuels is the cause of serious problems in the quality of the product, as well as the corrosion of metallic structures in contact with the fuels. Microbial growth facilitates passivity breakdown and induces localized corrosion of fuel storage tanks through the action of organic acidic metabolites derived from hydrocarbon degradation. Some hydrocarbons, such as n-alkanes, are easily degraded by a large number of microorganisms which use them as sources of carbon and energy. Their corrosive effect depends on the pH and electrolyte composition of the medium, especially chlo- ride and phosphate levels. According to Walker & Cooney (1975), monoterminal oxidation appears to be the major pathway of n-alkane oxidation; this involves conversion to the homologous primary alcohol, aldehyde and monoic acid, and the fatty acid is oxidized via b-oxidation rather than being incorporated directly into cell components. However, in addition to the structure of the compound, the compo- sition of the hydrocarbon mixture also affects the degradability of individual components (Olson et al. 1999). Diesel oil is a complex mixture of hydrocarbons, which varies according to the production process. Many World Journal of Microbiology & Biotechnology 2005 21: 135–142 DOI: 10.1007/s11274-004-3042-2 Ó Springer 2005