Morphological and physiological characteristics and lipopolysaccharide composition of N2-fixing (C2H2-reducing) root-associated Pseudomonas sp.

A dinitrogen-fixing Pseudomonas sp. was isolated from the roots of the grass Deschampsia caespitosa. The motile organism, which had 4 to 10 polar flagella, was gram negative, obligately aerobic, oxidase positive, arginine dihydrolase positive, and fluorescent. To verify API20B, API20E, and Oxi-Ferm identifications, as well as results from standard microbiological tests and electron microscopic examinations, which all indicated the organism to be a Pseudomonas, we analyzed its lipopolysaccharide. The lipopolysaccharide contained neutral sugars, phosphorus, heptose, hexosamine, 2-keto-3-deoxyoctonate, and fatty acids, which were dodecanoic, 3-hydroxydecanoic, 2-hydroxydodecanoic, and 3-hydroxydodecanoic acids. Both the qualitative and quantitative compositions resembled known data of the genus Pseudomonas. Dinitrogen fixation, determined as C2H2 reduction in semisolid medium, was supported by several carbon sources including malate and glucose. The N2 fixation activity was decreased if the oxygen concentration of the gas phase was lowered to one-tenth of atmospheric concentration. The highest specific nitrogenase activity recorded was 954 nmol C2H4/mg bacterial protein per hour, which is about 30% of that noted for Azospirillum lipoferum used as reference.