Azospirillum humicireducens sp. nov., a nitrogen-fixing bacterium isolated from a microbial fuel cell

Abstract

A Gram-negative, facultative anaerobic, motile, spiral, straight-to-slightly curved rod-shaped and nitrogen-fixing strain, designated SgZ-5T, was isolated from a microbial fuel cell (MFC) and was characterized by means of a polyphasic approach. Growth occurred with 0–1 % (w/v) NaCl (optimum 1 %) and at pH 5.5–8.5 (optimum pH 7.2) and at 25–37 °C (optimum 30 °C) in nutrient broth (NB). The strain had the ability to grow under anaerobic conditions via the oxidation of various organic compounds coupled to the reduction of anthraquione-2,6-disulfonate (AQDS). Chemotaxonomic characteristics (main ubiquinone Q-10, major fatty acid C18 : 1ω7c/C18 : 1ω6c and DNA G+C content 67.7 mol%) were similar to those of members of the genus Azospirillum . According to the results of phylogenetic analyses, strain SgZ-5T belonged to the genus Azospirillum within the family Rhodospirillaceae of the class Alphaproteobacteria , and was related most closely to the type strains of Azospirillum lipoferum , Azospirillum thiophilum and Azospirillum oryzae (98.0, 97.6 and 97.1 % 16S rRNA gene sequence similarity, respectively). DNA–DNA pairing studies showed that the unidentified organism displayed reassociation values of 36.7±3.7, 24.1±2.2 and 22.3±2.4 % to the type strains of A. lipoferum , A. thiophilum and A. oryzae , respectively. Similarities between nifH gene sequences of strain SgZ-5T and members of the genus Azospirillum ranged from 94.0 to 97.0 %. A combination of phenotypic, chemotaxonomic, phylogenetic and genotypic data clearly indicated that strain SgZ-5T represents a novel species, for which the name Azospirillum humicireducens sp. nov. is proposed. The type strain is SgZ-5T ( = CCTCC AB 2012021T = KACC 16605T).