Abstract:[Background] Cyanobacteria are key contributors to biological nitrogen fixation in aquatic and terrestrial ecosystems. [Objective] To increase our knowledge of nitrogen-fixing cyanobacteria in paddy soil and obtain culturable nitrogen-fixing cyanobacterial strains for further studies. [Methods] Three paddy soils with different nitrogen fixation capabilities were selected to isolate nitrogen-fixing cyanobacterial strains with BG11-N medium. The morphological characteristics of newly isolated strains were observed. The nifH gene amplification of genomic DNA was used to clarify their nitrogen fixation potential, acetylene reduction assay and 15N2 tracer method were used to quantitatively determine their nitrogen fixation activity. Preliminary identification of the newly isolated strains were determined through 16S rRNA gene sequence comparison. [Results] Seven autotrophic strains were isolated and purified with BG11-N medium under light conditions. All cells were round or elliptical, uniseriate, unbranched, filamentous and moniliform, and formed mat-like colonies on solid medium. The newly isolated strains grew well in BG11-N medium, were positive for nifH gene amplification of genomic DNA, had high nitrogen fixation activity measured by acetylene reduction assay and 15N2 tracer method, and had capabilities of siderophore production. Through 16S rRNA gene sequence comparison and morphological features, all the seven strains could be preliminarily identified as belonging to Nostocaceae. [Conclusion] The cyanobacterial (Nostocaceae) strains that plays important role in biological nitrogen fixation were isolated from paddy soil. The culturable nitrogen-fixing cyanobacterial strains have high nitrogen fixation and siderophore production ability, which could be used as microbial resources for further studies and have potential research and application value.
