Nitrogen (N2) fixation activity under different oxygen concentration of methanotrophic bacteria isolated from rice fields and their molecular identification

Abstract

Rice fields are a significantly sources of atmospheric methane. Methanotrophic bacteria are unique in their ability to utilize methane as a sole carbon source and their ability to fix N2. This research successfully characterized N2 fixation activity under different oxygen concentrations of methanotrophic bacteria isolated from rice fields. From 19 tested isolates, four isolates performed activity to fix N2. They could fix N2 on different concentration of air saturation (10 % up to 100%). The growth of methanotrophs is not directly corelated with the N2 fixation activity, and their N2 fixation activities are affected by O2 concentrations. The BGM 3 and BGM 9 isolates had very good N2 fixation activity. Their activities were increased by increasing air saturation up to 50% (approximately 10% O2), but then decrease by increasing air saturation from 50% (approximately 10% O2) to 100% (approximately 20% O2). However, the highest N2 fixation activity was performed by the BGM 9 isolate at 30% air saturation (approximately 6% O2), and the isolate was identified as Methylococcus capsulatus. This information can support application of the isolates to achieve sustainable and environmentally friendly agricultural system.