Magnetotaxis and acquisition of detrital remanent magnetization by magnetotactic bacteria in natural sediment: First experimental results and theory

Abstract

[1] The widespread occurrence of magnetotactic bacteria (MTB) in several types of marine and freshwater sediment, and the role of fossil magnetosomes (magnetofossils) as main remanent magnetization carriers therein, has important paleomagnetic and paleoenvironmental implications. Despite numerous studies on MTB biology and on magnetofossil preservation in geological records, no detailed information is yet available on how magnetotaxis (i.e., the ability to navigate along magnetic field lines) is performed in sedimentary environments, and on how magnetofossils possibly record the Earth magnetic field. We provide for the first time experimental evidence for these processes. MTB living in sediment are poorly aligned with the geomagnetic field, contrary to what is observed in water. This can explain the seemingly excessive magnetic moment of most MTB. The observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients. Experiments with magnetofossil-rich sediment suggest that a natural remanent magnetization (NRM) is acquired by magnetofossils in the so-called benthic mixed layer, where natural MTB populations usually occur. The acquired NRM is proportional to the applied field at least up to $160 mT, and its intensity is compatible with values observed in nature for same sediment types. Therefore, if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes, they can provide useful relative paleointensities. We propose a preliminary model to explain early stages of magnetofossil NRM acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing.

Constant Flux of Spatial Niche Partitioning through High-Resolution Sampling of Magnetotactic Bacteria

“…The finding that the total number of MTB increased over time suggests that the changes in the horizontal distribution do not stem from migration but rather are due to changes in cell division (growth) and death rates, which then trended toward relatively equal numbers at the latter stages of the experiment in both aquaria. This is consistent with the observations of Mao et al (21), who found that MTB move via a slightly biased random walk in the external magnetic field in the sediment. While the swimming velocity of MTB cocci in water can reach 112 m/s (25), their swimming velocity in sediment is unknown.…”

“…After aquarium A was rotated 180°on day 44, MTB rods did not flourish in the "new" northern part of the aquarium or die off toward the south. Because the sediment was mixed into the aquaria in the magnetic field, and because MTB swimming velocities are higher in water than in sediment (21), the fact that MTB rods were concentrated in the north could indicate that pouring the sediment into the aquaria in the ambient field might have biased the initial results. On the other hand, the finding that MTB rods died off in aquarium A and not in aquarium B, whereas MTB cocci and MTB spirilla were less affected by the rotation, suggests that the change in the magnetic environment influenced the MTB rod community.…”

“…We collected sediment from a natural pond situated in a forest near Landshut, Germany, 80 km northeast of Munich (48°35=14.98ЉN, 12°04=43.71ЉE) that hosts abundant and well-characterized MTB (21,36). Grain size analyses of five sediment samples showed log-normal distributions with maxima at 15 to 23 m, whose dominant magnetic contribution comes from noninteracting single-domain particles, probably in the form of intact magnetosome chains (37).…”

Magnetic properties of uncultivated magnetotactic bacteria and their contribution to a stratified estuary iron cycle

“…This result is significant for three reasons. First, prior associations of BS-BM and BH-BI with magnetofossils remains speculative because numerical inversion of magnetization curves through which they were originally conceived 19 and continuously presumed 16,[48][49][50][51] has non-unique solutions. In addition, the coercivity distributions (characterized by skewness, median coercivity and dispersion factor) of diverse MTB types found in nature are poorly constrained.…”

“…Third, the skewed coercivity distributions that characterize uncultivated MTB point to the importance of modelling them as such, rather than symmetric distributions, so as to avoid the addition of coercivity distribution components that are not physically meaningful when quantifying the amount of magnetofossils. Both approaches are widely used 19,[50][51][52][53] .…”