Abstract
The multicellular communities of microorganisms known as biofilms are of high significance in agricultural setting, yet it is largely unknown about the biofilm formed by nitrogen-fixing bacteria. Here we report the biofilm formation by Pseudomonas stutzeri A1501, a free-living rhizospheric bacterium, capable of fixing nitrogen under microaerobic and nitrogen-limiting conditions. P. stutzeri A1501 tended to form biofilm in minimal media, especially under nitrogen depletion condition. Under such growth condition, the biofilms formed at the air–liquid interface (termed as pellicles) and the colony biofilms on agar plates exhibited nitrogenase activity in air. The two kinds of biofilms both contained large ovoid shape ‘cells’ that were multiple living bacteria embedded in a sac of extracellular polymeric substances (EPSs). We proposed to name such large ‘cells’ as A1501 cyst. Our results suggest that the EPS, especially exopolysaccharides enabled the encased bacteria to fix nitrogen while grown under aerobic condition. The formation of A1501 cysts was reversible in response to the changes of carbon or nitrogen source status. A1501 cyst formation depended on nitrogen-limiting signaling and the presence of sufficient carbon sources, yet was independent of an active nitrogenase. The pellicles formed by Azospirillum brasilense, another free-living nitrogen-fixing rhizobacterium, which also exhibited nitrogenase activity and contained the large EPS-encapsuled A1501 cyst-like ‘cells’. Our data imply that free-living nitrogen-fixing bacteria could convert the easy-used carbon sources to exopolysaccharides in order to enable nitrogen fixation in a natural aerobic environment.
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Acknowledgements
We thank Dr Min Lin from the Biotechnology Research Institute, Chinese Academy of Agricultural Sciences, for providing P. stutzeri strain. Mr Chunli Li and Ms Jingnan Liang from the Institute of Microbiology, Chinese Academy of Sciences for technical support on SEM and TEM, respectively. This work was supported by the National Basic Research Program of China (973 Program 2014CB846002 to LM and 2015CB150600), and the National Natural Science Foundation of China (31570126 to LM and 31200044 to DW). This work was conducted with the support of Institut Pasteur, Paris, France.
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Wang, D., Xu, A., Elmerich, C. et al. Biofilm formation enables free-living nitrogen-fixing rhizobacteria to fix nitrogen under aerobic conditions. ISME J 11, 1602–1613 (2017). https://doi.org/10.1038/ismej.2017.30
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DOI: https://doi.org/10.1038/ismej.2017.30
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