沉积物中趋磁细菌趋磁性优势的实验研究

AN EXPERIMENTAL STUDY OF MAGNETOTAXIS ADVANTAGE OF MAGNETOTACTIC BACTERIA IN SEDIMENT

趋磁细菌依靠其体内生物合成的磁小体颗粒沿地球磁场定向排列和游动,称为趋磁性。沿磁场的定向排列被认为可以使磁菌更有效地到达最佳生存环境,即趋磁性对磁菌的优势所在。目前对趋磁性优势的研究大多数集中在水环境或培养液环境下,而对于自然沉积物中趋磁性优势的理解还停留在假说阶段。越来越多的研究显示,磁菌在沉积物中的行为方式与水环境中所观测到的现象差别很大,因此,对趋磁性优势的研究也需要在沉积物环境中进行。本文我们通过对比在地磁场、近零磁场和反转磁场条件下两种磁菌:球菌和杆菌(Candidatus Magnetobacterium bavaricum)的时空变化,揭示在沉积物中趋磁性对趋磁细菌的作用。结果显示趋磁性对两种磁菌存在不同的作用,M.bavaricum在近零磁场条件下数量明显下降而在地磁场下又重新恢复,说明趋磁性优势对M.bavaricum的积极作用,然而球菌在近零磁磁场条件下的数量和分布与地磁场条件下相似,可能说明球菌在利用趋化性和行为方式上与M.bavaricum有很大不同。M.bavaricum在反转磁场条件下数量减少,而球菌则接近消失,说明球菌受极向趋磁性的影响比M.bavaricum大。两种磁菌因受沉积物环境和自身趋化性的影响在趋磁性上表现不同,可能M.bavaricum存在不同的趋磁性特点。本文实验结果说明对趋磁性的理解需要立足于复杂的沉积物环境。

Magnetotactic bacteria （MTB） are passively aligned and migrate along magnetic field, which is known as magnetotaxis. Magnetotaxis is believed to provide an advantage for MTB in searching for optimal living conditions. The current knowledge of magnetotaxis and its advantage for MTB is based on observations of MTB in water environment, while how magnetotaxis works in sedimentary environment is currently unclear. Increasing studies reveal that MTB behaviours in sediment are very different from water environment. For example, compared to high degree of alignment with respect to magnetic field, MTB are poorly aligned in sediment. In this study, surface （10-20cm） sediment containing various MTB from Lake Chiemsee, southern Germany, was collected and divided into a few aquaria （30 x 20 x 20cm） in laboratory. To setup microcosms, the sediment in each aquarium was disturbed completely to allow reformation of sediment stratification in the following few months. 3 5cm water was kept above the sediment and water loss due to evaporation was compensated by adding distilled water. The stabilized sediment was dominated by rod-shaped Candidatus Magnetobacterium bavaricum （M. bavaricum） and cocci which were examed in this study. We assess magnetotactic advantage in stable sediment by long-period observations of uncultured MTB populations of cocci and M. bavaricum in an aquarium exposed to the following magnetic field configurations： （1） Geomagnetic field in the laboratory （ca.441~T with 71 ~ downward inclination） , （2） close-to-zero field, and （3）a ca.100~T vertical field whose polarity is switched every 24 hours. Comparison of MTB populations in the geomagnetic field with those observed after long （ca.6 months） exposure to a close-to-zero field provides a first, simple quantification of magnetotaxis advantage in sediment. This comparison is complicated by extremely large spatial and temporal variations of MTB populations, whereby the cell concentration at any given depth can vary by a factor 5 between consecutive samplings within few days, and between profiles taken at fewcm distance. Nevertheless, repeated sampling during normal and close-to-zero field exposure allowed detecting a decrease of MTB concentrations in absence of a magnetic field： this effect was clearer for M. bavaricum, whose mean concentration was 13.1-＋6.1 cells/l^L before cancelling the geomagnetic field, and 5.8-＋ 1.2 cells/jxL during ca.6 months in close-to-zero field, with no clear temporal trend suggesting an extinction. Cell numbers recovered to initial values within ca.1.5 months after the geomagnetic field was reset, and dropped again during a second period of ca. 1.5 month in close-to-zero field. Cocci displayed a larger temporal variability with 11.7-＋4.5 cells/~L during stable periods in the geomagnetic field, and 11.5-＋4.3 cells/~L during close-to-zero field experiments. The absence of a magnetic field did not appear to produce cocci extinction in ca.6 months, nor was a significant change in depth distribution observed. A second experiment in which the polarity of a vertical magnetic field was switched on a daily basis produced a moderate decrease of M. bavaricum concentrations and nearby extinction of cocci. Overall, both types of MTB benefit from the presence of a magnetic field with correct polarity （i.e. pointing downward in the northern hemisphere）, and this benefit appears stronger for M. bavaricum than for cocci. M. bavaricum drops very rapidly after the onset of close-to-zero field conditions, recover to normal concentrations after restoring the geomagnetic field appears gradual. On the other hand, cocci are intolerant to reversed magnetic fields, as expected from the polar magneto-aerotaxis model, confirming the active role of polar magnetotaxis in sediment, while M. bavaricum is only partially affected. The reason for this difference is unclear and might point to a different magnetotactic mechanism for M. bavaricum. The results in this study imply that the understanding of magnetotaxis and its advantages must take the complicated sedimentary environment into account.