Polar magnetotaxis is sensitive to the field polarity, and becomes disadvantageous in all cases where the field polarity is incorrect with respect to the chemical gradient. Such sit- uations can be encountered in natural sediment in proximity of localized gradients around decomposing organic matter. As expected, cocci are negatively affected by all situations where the field polarity is incorrect, as seen by rapid disappearance. M. bavaricum, on the other hand, possess the surprising ability to adapt to such situations by reversing its mag- netotactic polarity, at least for part of the whole population. This adaptation behaviour appears to be excessively rapid for being attributed to natural selection over several cell generations, as also supported by direct observation under the optical microscope (chap- ter 6).
Chapter 6
Direct observation of
magneto-chemotaxis on individual
magnetotactic bacteria
Abstract
The motion of magnetotactic bacteria (MTB) is the result of the joint action of passive alignment in a magnetic field (magnetotaxis), and the response to chemical stimuli (chemo- taxis). As far as the response to oxygen concentration is concerned, the combination of magnetotaxis and chemotaxis is called magneto-aerotaxis. In this chapter we investigate
the response of individual wild-type cells ofMagnetobacterium bavaricum to a pH gradient
in oxygen-saturated water. The combination of two distinct chemical stimuli (oxygen and pH) creates a complex response of swimming cells observed under the optical microscope
with the hanging drop assay. The normal swimming direction of M. bavaricum in oxygen-
rich water is towards the magnetic N (N-seeking cells, shortly NS). As NS cells swimming in near-neutral water enter a region with different pH, two different behaviours are observed: (1) cells stop abruptly or after some slowdown, or (2) some cells become S-seeking (SS) or engage a rapid back-and-forth (oscillatory) motion along a certain pH front. If the field direction is reversed, formerly oscillatory cells become in most cases consistently NS and in few cases consistently SS. The oscillatory motion can be explained by the combined action of two repellents (oxygen and acidity) on bacteria that perform polar magnetotaxis with a temporal sensing mechanism, in analogy to the magneto-aerotaxis model of Frankel et al. (1997). On the other hand, however,M. bavaricum appears to be able to "switch" the link between chemical sensing and flagella motion, so that some cells become SS in oxygen-rich
114 6. Direct observation of magneto-chemotaxis
water, even when far from any pH-gradient. This ability appears to be an adaptive mecha- nism that enables cells to survive in environments where the chemical gradient is reversed with respect to the usual magnetic field direction, as also observed in sediment (chapter 5).
6.1
Introduction
The ability of magnetotactic bacteria (MTB) to migrate along magnetic field lines is known as magnetotaxis (Blakemore, 1975, 1982; Frankel, 1984). While magnetotaxis determines a preferred axis for the swimming path, the swimming direction (parallel or antiparallel to the field) is given by the sense of flagella rotation (clockwise or counter-clockwise), which is in turn controlled by the chemical environment (chemotaxis). The combination of these two phenomena has been called magneto-aerotaxis by Frankel et al. (1997), in the case of a response to oxygen concentration. Depending on the way of flagella rotation which is affected by the chemical environment, Frankel et al. (1997) distinguished the cases of axial and polar magneto-aerotaxis. Axial magneto-aerotaxis, as observed in magnetic spirilla, is based on the detection of a chemical gradient (i.e. a spatial change of concentration), probably through a temporal sensing mechanism by which cells compare concentrations measured in successive time intervals while moving through the gradient (Berg and Purcell, 1977). Axial magnetotaxis has no preferred swimming direction: if the cell is moving towards more favourable conditions, it keeps swimming along the same direction, while the swimming direction is reversed in case of movement towards less favourable conditions. If cells performing axial magnetotaxis are observed in a homogeneous environment with negligible concentration gradients, such as the oxygen-saturated water drop of a hanging drop assay, the temporal sensing mechanism cannot detect real concentration changes and is affected by random fluctuations. In this case, cells will change their swimming direction at random times, performing a characteristic 1-D oscillatory motion around a mean position. Polar magneto-aerotaxis, on the other hand, appears to be controlled by an upper and a lower oxygen concentration threshold. In the model of Frankel et al. (1997), based on observations on cultured cocci, cells exposed to oxygen concentrations larger than the upper threshold do consistently swim towards one magnetic pole (for example N), while the same cells exposed to concentrations smaller than the lower threshold consistently swim towards the other magnetic pole. The swimming polarity depends on the direction of the magnetic field with respect to the oxygen gradient in which the cells were grown: if the magnetic N points against the oxygen gradient (i.e. the oxygen concentration decreases
6.1 Introduction 115
when moving towards it), cells will swim towards the magnetic N (north seeking, NS) in oxic environments, and towards the magnetic S (south seeking, SS) in anoxic environments. In chemically stratified environments, this type of magnetotaxis will keep MTB within a range of depths comprised between the lower and the upper oxygen concentration thresholds. This range of depths coincides with the oxic-anoxic interface (OAI) of sediment or water
column. In the hanging drop assay, MTB are exposed to oxygen saturation, and are
therefore consistently NS if grown in the Northern hemisphere, and SS if grown in the Southern hemisphere.
The two types of magnetotaxis are easily distinguishable in the hanging drop assay, with an oscillatory behaviour being characteristic of axial magneto-aerotaxis and consis-
tent swimming directions for polar magneto-aerotaxis. According to this criterion, M.
bavaricum performs polar magneto-aerotaxis. However, all attempts to observe SS cells under strictly anoxic conditions, as expected from polar magneto-aerotaxis, gave negative
results for bothM. bavaricum and wild-type cocci (see chapter 3). On the other hand, the
existence of SS migration in sediment has been shown with dedicated experiments (chap- ter 5), proving that magneto-chemotaxis involves both swimming directions despite the impossibility to observe them in the hanging drop assay. SS migration might be induced by other substances instead of oxygen, in which case a lower oxygen concentration threshold does not exist. In the experiments reported by Frankel et al. (1997), such repellents might occur only in the anoxic zone of the MTB culture, forming a concentration gradient that is opposed to that of oxygen. Such opposed gradients (e.g. ammonia and sulphide) are com- mon in stratified environments (Froelich et al., 1979) and might be essential for reversing the swimming direction observed under oxygen saturation. Another unexpected magneto- chemotactic pattern of M. bavaricum observed in sediment is the ability to switch polarity in case of unfavourable combinations between oxygen gradient and vertical magnetic field, so that cells are SS under oxic conditions.
In general, it appears that magnetotaxis is controlled by chemotaxis, as well as responses to physical stimuli such as light (magneto-phototaxis, Chen et al. 2011; Shapiro et al. 2011), and contact with solid obstacles (tactile response, Spormann and Wolfe 1984). Information from specialized receptors must therefore be combined into a single signal that determines the sense of flagella rotation. In order to investigate this mechanism, systematic experiments have been performed with the hanging drop assay in presence of a pH gradient. In this case, MTB cells are exposed to two independent chemical stimuli, which can provide consistent or contradictory signals to the flagella motor, depending on
116 6. Direct observation of magneto-chemotaxis
the position inside the hanging drop.
The pH of typical freshwater and marine MTB habitats is comprised between 5.5 and 9.8 (Jogler et al., 2010; Lefèvre et al., 2011; Mann et al., 1990; Pan et al., 2005a). In general, pH decreases across the OAI, with more acidic conditions in the anoxic zone. For example, Jogler et al. (2010) reported a pH decrease from 8.5 above the OAI of carbonate-
rich sediment, to 7-7.5 at greater depths. The minimum depth for the occurrence of
M. bavaricum usually coincide with a level where dissolved oxygen drops to half of the saturation value, while maximum depths are not correlated with other dissolved ions such as nitrate, ammonia, or pH. Nevertheless, since MTB grow only in defined pH ranges (Blakemore et al., 1979; Martatea and Blakemore, 1981), they might possess a chemotactic
response to H+ or OH− ions. Furthermore, pH might affect flagella rotation, since the
flagella motor is driven by a proton flux (Eisenbach, 2004).