Wang Di, Xu Anming, Elmerich Claudine, Ma Luyan Z
State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, Beijing, China.
University of Chinese Academy of Sciences, Beijing, China.
ISME J. 2017 Jul;11(7):1602-1613. doi: 10.1038/ismej.2017.30. Epub 2017 Mar 24.
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.
被称为生物膜的微生物多细胞群落在农业环境中具有重要意义，但关于固氮细菌形成的生物膜却知之甚少。在此，我们报道了施氏假单胞菌A1501形成生物膜的情况，它是一种自由生活的根际细菌，能够在微需氧和氮限制条件下固氮。施氏假单胞菌A1501倾向于在基本培养基中形成生物膜，尤其是在氮耗尽的条件下。在这种生长条件下，在气液界面形成的生物膜（称为菌膜）和琼脂平板上的菌落生物膜在空气中表现出固氮酶活性。这两种生物膜都含有大的卵形“细胞”，这些“细胞”是多个活细菌嵌入胞外聚合物（EPS）囊中形成的。我们提议将这种大的“细胞”命名为A1501孢囊。我们的结果表明，EPS，尤其是胞外多糖能够使被包裹的细菌在有氧条件下生长时固氮。A1501孢囊的形成会随着碳源或氮源状态的变化而可逆。A1501孢囊的形成依赖于氮限制信号和充足碳源的存在，但与活性固氮酶无关。巴西固氮螺菌形成的菌膜也表现出固氮酶活性，并且含有大的EPS包裹的类似A1501孢囊的“细胞”。我们的数据表明，自由生活的固氮细菌可以将易利用的碳源转化为胞外多糖，以便在自然有氧环境中进行固氮。
