趋磁细菌在沉积物中的趋磁性和剩磁记录特征

Magnetotactic bacteria (MTB) synthesis intercellular magnetite or/and gregite particles (magnetosomes) in single domain size. MTB orient and migrate along magnetic field lines due to magnetotaxis. The preserved magnetosomes (magnetofossils) in geological sediments are potential carriers of paleomagnetism and paleoenvironment information. Aquatic sediments are the main living habitats for MTB, while many questions on magnetotaxis in sediment and the remnant magnetization influenced by magnetic fields are still not well interpreted. Therefore, this project plans to take intertidal sediment and abundant MTB therein in Fujian, in order to study MTB magnetotaxis in sediment and acquisition of remnant magnetization in various field configurations. Various sediment microcosms containing MTB are placed in magnetic field conditions, including Earth field, close-to-zero field, strong field, horizontal field and reversed field. MTB polarity, cell counts, vertical distributions, and acquisition of remnant magnetization in various field conditions are closely monitored. These experiments are promising to explore the potential magnetic or chemical factors to determine MTB polarity, and the influences of magnetic field conditions on MTB abundance and distribution. Combined with rock magnetic measurements and analysis, the characterization of remnant magnetization is obtained. The results will help to understand how changes of paleomagnetic fields influence MTB, in particular the field intensity declination and field direction changes during field reversals. Accordingly, it will provide insights to study MTB implications in paleomagnetism and paleoenvironments.

趋磁细菌体内合成高纯度的单畴磁铁矿或胶黄铁矿颗粒（磁小体），因趋磁性而沿地磁场排列和游动，地质时期保存下来的磁小体记录着古地磁和古环境的重要信息。然而目前对趋磁细菌在其主要生存环境--沉积物中的趋磁性，及磁场条件对其记录剩磁的影响尚有诸多问题不清楚。鉴于此，本项目拟以福建沿海潮间带沉积物及其中的趋磁细菌为基本研究对象，立足于研究沉积物中的趋磁细菌，观测趋磁细菌在不同磁场条件（地磁场、近零磁场、强磁场、水平磁场和倒转磁场）下的极性变化，研究决定趋磁细菌极性的磁场条件或特定化学条件，以及磁场条件对趋磁细菌数量和分布的影响。结合磁学测量和分析方法，获得趋磁细菌在不同磁场条件下记录剩磁的特征。本研究将有助于理解地质时期趋磁细菌古地磁记录机制，特别是磁极倒转过程中磁场强度降低和方向变化时趋磁细菌的响应过程，为趋磁细菌记录古地磁和古环境重建研究奠定重要基础。

趋磁细菌体内合成磁铁矿或胶黄铁矿颗粒（磁小体），磁小体成链分布使趋磁细菌沿外界磁场定向排列和游动（趋磁性）。因为北半球和南半球分别以趋北磁菌和趋南磁菌为主，普遍认为趋磁细菌极性由磁场方向决定，但越来越多的研究显示趋磁细菌极性受趋化性影响，为了进一步研究磁场和化学环境对不同类型趋磁细菌极性和磁性记录的影响开展本研究。研究发现，（1）大杆菌MB受pH梯度影响可转变其游动方向，小比例（5%-10%）MB通过转变其内在极性以适应不利的磁场或化学环境，对趋磁性演化有重要启示，异于普遍的趋磁-趋氧性，大杆菌这种独特的趋磁-趋化性更好地解释其在沉积物中的分布。（2）通过将趋磁细菌置于向上磁场、近零磁场和水平磁场研究不同类型趋磁细菌对磁场的依赖性。结果显示，不同类型趋磁细菌对磁场的依赖性不同，只有个别趋磁细菌（如弧菌）可在向上磁场中转变极性，杆菌和球菌皆不因磁场而转变极性，之前研究表明近零磁场使杆菌数量降低，但对球菌无影响。这些结果说明前人对趋磁性的认识并不具有普遍适用性，不同类型趋磁细菌对磁场依赖不同，应在沉积物环境中理解趋磁性。（3）前人研究普报道黄土-古土壤中存在趋磁细菌，对其磁性有重要贡献，并响应古气候变化，但未直接验证，本研究利用系统的磁学方法（FORC, 磁学参数，矫顽力组分分解），结合电镜，明确了黄土-古土壤中不存在趋磁细菌或化石磁小体，生物成因磁铁矿对黄土-古土壤的影响需重新评估。