Occurrence of aerobic nitrogen fixing bacteria in wetland and dryland plants

We recently suggested that N2-fixing bacteria other than Azospirillum are present in greater abundance in the roots of wetland rice (2). Using a low concentration of tryptic soy agar, we isolated aerobic heterotrophic bacteria that required low levels of combined nitrogen for nitrogenase activity from wetland rice roots (1). In a liquid medium under batch culture conditions, the bacteria were unable to grow without combined nitrogen under either aerobic or anaerobic conditions. Neither could they grow nor exhibit nitrogenase activity in a semi-solid N-free medium unless the medium was supplemented with a small amount of yeast extract, casamino acids or any amino acid. Among the nitrogenase positive aerobic heterotrophic isolates from the inner tissue of rice roots that accounted for 80% of the isolated heterotrophs, about 90% were found to be Pseudomonas-like, which require a low level of combined nitrogen. We further report here the occurrence of aerobic nitrogen fixing bacteria in association with wetland rice and other plants.