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

Abstract

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.