沉积物中非培养趋磁细菌与氧气关系初探

An initial study of the influences of oxygen conditions on wild-type magnetotactic bacteria in sediment

趋磁细菌沿磁场方向排列和游动的现象称之为趋磁性,它们靠感知周围化学环境变化调整其游动方向,即趋化性作用,趋磁细菌游动方式和分布是趋磁性和趋化性共同作用的结果.趋磁细菌主要富集在有氧-无氧交界带附近,趋磁-趋氧(magneto-aerotaxis)模型指出,氧气浓度决定趋磁细菌的游动方向,即对于北半球趋磁细菌,高氧环境下游动方向与磁北方向相同(北向趋磁细菌),在低氧或无氧环境下游动方向与磁北方向相反(南向趋磁细菌).趋磁-趋氧模型目前仍缺少在沉积物环境中的实验检验,沉积物环境中两种非培养趋磁细菌被用来检验其与氧气关系.趋磁细菌垂直分布形成过程中,沉积物上部形成的趋磁细菌初始峰值与氧气梯度有关,但沉积物环境趋于稳定后趋磁细菌分布向深处扩展,说明其他因素的影响.从正常氧气条件到无氧条件的过渡中,趋磁细菌分布整体移至沉积物表层,在稳定无氧条件中趋磁细菌分布向深处扩展,最终与正常氧气条件中相似,说明趋磁细菌对氧气或与氧气有关的环境变化表现出一定的适应性.趋磁细菌在低氧或无氧条件下游动方式与正常氧气条件下相似,趋磁-趋氧模型在两种趋磁细菌上并没有得到充分证实.实验结果说明,趋磁细菌在沉积物中的分布虽与氧气条件有一定联系,但可能并不是影响趋磁细菌游动方式和分布的主要因素.

Magnetotactic bacteria （MTB） orient and migrate along geomagnetic field lines in a process known as magnetotaxis, while their swimming direction is determined by flagella rotation （clockwise or counterclockwise） in response to surrounding chemical conditions, i.e., chemotaxis. The combination of magnetotaxis and chemotaxis （magneto-chemotaxis） is expected to govern MTB behaviors and vertical distribution in chemically stratified sediment. Magneto-aerotaxis is a process in which oxygen concentration determines MTB swimming direction. However, direct validation of magneto-aerotaxis in sediment, where MTB primarily occur, has been limited to date. Such validation is crucial because low alignment degree and complex chemical conditions in sediment are not comparable to those in the aqueous environment. Herein, specifically designed experiments were conducted to investigate the effects of oxygen conditions on two wild-type MTB in sediment, rod-shaped Candidatus Magnetobacterium bavaricum （M. bavaricum） and unspecified cocci collected from Chiemsee, a freshwater lake in southern Germany. In the first experiment, oxygen gradient development and MTB vertical distribution were closely monitored during reformation of new sediment stratification. The MTB population peaked immediately below the sediment-water interface, which was likely because of magneto-aerotaxis. However, there were no observable changes in oxic-anoxic interface （OAI）, and MTB distribution later extended to greater depth, indicating other driving mechanisms. A second experiment was conducted to determine if north-seeking （NS） MTB become south-seeking （SS） when moved into anoxic conditions; however, this conversion was not detected during oxygen concentration reduction. The magneto-aerotaxis of the two examined MTB were not directly validated. Long-term stable anoxic conditions led to MTB distribution downward, which is consistent with the distribution under normal oxygen conditions, indicating MTB adaptation to the environment. These findings indicate that oxygen conditions likely play a role in MTB behaviors and vertical distribution in sediment, but that they do not act as directly influencing or critical factors.