Gieseke Armin, Tarre Sheldon, Green Michal, de Beer Dirk
Microsensor Group, Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany.
Appl Environ Microbiol. 2006 Jun;72(6):4283-92. doi: 10.1128/AEM.00241-06.
The sensitivity of nitrifying bacteria to acidic conditions is a well-known phenomenon and generally attributed to the lack and/or toxicity of substrates (NH3 and HNO2) with decreasing pHs. In contrast, we observed strong nitrification at a pH around 4 in biofilms grown on chalk particles and investigated the following hypotheses: the presence of less acidic microenvironments and/or the existence of acid-tolerant nitrifiers. Microelectrode measurements (in situ and under various experimental conditions) showed no evidence of a neutral microenvironment, either within the highly active biofilm colonizing the chalk surface or within a control biofilm grown on a nonbuffering (i.e., sintered glass) surface under acidic pH. A 16S rRNA approach (clone libraries and fluorescence in situ hybridizations) did not reveal uncommon nitrifying (potentially acid-tolerant) strains. Instead, we found a strongly acidic microenvironment, evidence for a clear adaptation to the low pH in situ, and the presence of nitrifying populations related to subgroups with low Km s for ammonia (Nitrosopira spp., Nitrosomonas oligotropha, and Nitrospira spp.). Acid-consuming (chalk dissolution) and acid-producing (ammonia oxidation) processes are equilibrated on a low-pH steady state that is controlled by mass transfer limitation through the biofilm. Strong affinity to ammonia and possibly the expression of additional functions, e.g., ammonium transporters, are adaptations that allow nitrifiers to cope with acidic conditions in biofilms and other habitats.
硝化细菌对酸性条件的敏感性是一个众所周知的现象,通常归因于随着pH值降低底物(NH3和HNO2)的缺乏和/或毒性。相比之下,我们观察到在白垩颗粒上生长的生物膜在pH约为4时具有强烈的硝化作用,并研究了以下假设:存在酸性较弱的微环境和/或存在耐酸硝化菌。微电极测量(原位和在各种实验条件下)表明,在定殖于白垩表面的高活性生物膜内或在酸性pH下在非缓冲(即烧结玻璃)表面上生长的对照生物膜内,均没有中性微环境的迹象。16S rRNA方法(克隆文库和荧光原位杂交)未揭示不常见的硝化(潜在耐酸)菌株。相反,我们发现了一个强酸性微环境,有证据表明其在原位对低pH有明显适应性,并且存在与对氨具有低Km值的亚群相关的硝化菌群(亚硝化螺菌属、寡营养亚硝化单胞菌和硝化螺菌属)。耗酸(白垩溶解)和产酸(氨氧化)过程在一个低pH稳态下达到平衡,该稳态由通过生物膜的传质限制控制。对氨的强亲和力以及可能的其他功能(例如铵转运蛋白)的表达是使硝化菌能够应对生物膜和其他生境中酸性条件的适应性特征。