A new acid-tolerant nitrogen-fixing bacterium associated with sugarcane

Abstract

During surveys of bacteria possibly responsible for N2 fixation in sugarcane, root and stem samples were collected in four sugarcane-growing regions in Brazil. A new microaerobic N2-fixing bacterium was isolated from most samples of washed roots and stems from all regions. Isolation procedures were based on semisolid diluted sugarcane juice medium followed by replication to N-free 10% sugar medium acidified with acetic acid to pH 4.5. The new bacterium is an aerobic rod, motile by 1 to 3 lateral flagella, fixes N2 in semisolid media under air but not in liquid media except when a starter dose of N is added. It has no nitrate, reductase and N2 fixation proceeds in the presence of 10mM NO3−. Best growth occurs with high sucrose concentrations (10%). Growth occurs up to 30% sucrose but not at 35%. Acid is formed reaching a final pH of below 3.0. Growth and N2 fixation proceed at this acidity. Ethanol is used for growth and is “overoxidised” (oxidized to CO2 and H2O). Acetic and lactic acids are also oxidized to CO2 and H2O. Acids produced from glucose are consumed with precipitation of CaCO3. Dark brown colonies are formed on potato agar with 10% sugar and dark orange colonies on N poor agar (20 mg yeast extract per 1) containing bromothymol blue. In view of the distinct characteristics which do not allow identification within either Frateuria, Gluconobacter, Acetobacter or any known N2-fixing bacterium a new genus and species are proposed and namedSaccharobacter nitrocaptans.