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Spatial and temporal distributions of magnetotactic bacteria in control aquaria and a freshwater pond
Spatial and temporal distributions of magnetotactic bacteria in control aquaria and a freshwater pond
Magnetotactic bacteria (MTB) swim along magnetic field lines in water. They are found in aquatic habitats throughout the world, yet knowledge of their spatial and temporal distribution remains limited. To help remedy this, MTB-bearing sediment was taken from a natural pond, mixed into two replicate aquaria, and then the three dominant MTB morphotypes (cocci, spirilla and rod-shapes) were counted at high spatiotemporal sampling resolution: 36 discrete points in replicate aquaria were sampled every ~30 days over 198 days. Population centers of cocci and spirilla morphotypes moved in continual flux, yet they consistently inhabited separate locations displaying significant anti-correlation. Rod-shaped MTB were initially concentrated toward the northern end of the aquaria, but at the end of the experiment were most densely populated toward the south. That the total number of MTB cells increased through time during the experiment argues that population reorganization arose from relative changes in cell division and death and not from migration. The maximum net growth rates were 10, 3 and 1 day-1 and average net growth rates were 0.24, 0.11 and 0.02 day-1 for cocci, spirilla and rod-shaped MTB respectively; minimum growth rates for all three morphotypes were -0.03 day-1. Our results suggest that cocci and spirilla occupy distinctly different niches--their horizontal positioning in sediment are anti-correlated and under constant flux. Magnetotactic bacteria (MTB) synthesize ferrimagnetic crystals that contribute to the remanent magnetization in sediments, yet knowledge of how MTB populations vary in natural environments over time remained limited. One chapter in this thesis documents the abundances of three MTB morphotypes from nine sites collected and measured every month over a two-year period from a pond near Munich, Germany. Morphotype populations underwent coherent temporal trends among the nine sites. Spirilla populations attained maxima in the summer when temperatures were highest and oxygen concentrations were lowest. Spirilla and cocci exhibited relative antipathy in 2015 when both morphotypes reacted in distinctly opposite ways to oxygen levels. In 2016, they were positively correlated with each other but displayed no coherency with oxygen. Magnetic properties of the sediments varied with water depth: deeper sites, which were also lower in organic carbon, nitrogen and oxygen concentrations than shallower sites, had higher saturation magnetizations and were richer in single domain particles. Although absolute MTB concentrations differed in space, all three measured morphotypes underwent similar relative changes in the time domain at the nine sites; hence, long-term trends of MTB populations in natural ecosystems are likely indicative of the ecosystem as a whole. This is in distinct contrast to artificial habitats in the laboratory that have not reached steady-state conditions. Redeposition experiments were performed over a range of magnetic field intensities and inclinations using the magnetite-bearing, natural sediments from the pond near Munich. Remanent magnetization systematically increased with applied magnetic field, yet relative paleointensities from 25 repeat redeposition experiments performed at identical field conditions vary by a factor of two. The question arises as to what accounts for the variability and what can we do to diminish/remove the variability to improve the precision of relative paleointensity estimates? Recalling that the saturation magnetization (Ms) lies in the long axis direction of a magnetite grain, then the maximum axis of the magnetic anisotropy ellipsoid should parallel the paleomagnetic direction if the magnetization reflects a true depositional remanence acquired via torque. As higher external fields increase the remanence in sediments by tightening the alignment of the grains, the magnetic anisotropy ellipsoid should become increasingly prolate proportional to field strength. This effect is indeed demonstrated in new experiments using anisotropy of anhysteretic remanent magnetization (AARM): nearly all AARM indices vary in proportion with magnetic field strength. Conversely, anisotropy of magnetic susceptibility does not, as it is overwhelmed by the paramagnetic fraction that shows a sedimentary fabric. AARM thus holds promise to improve relative paleointensity estimates from sediments.
magnetotactic bacteria, spatiotemporal variation, control aquaria, freshwater pond, redeposition, remanence anisotropy
He, Kuang
2018
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
He, Kuang (2018): Spatial and temporal distributions of magnetotactic bacteria in control aquaria and a freshwater pond. Dissertation, LMU München: Faculty of Geosciences
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Abstract

Magnetotactic bacteria (MTB) swim along magnetic field lines in water. They are found in aquatic habitats throughout the world, yet knowledge of their spatial and temporal distribution remains limited. To help remedy this, MTB-bearing sediment was taken from a natural pond, mixed into two replicate aquaria, and then the three dominant MTB morphotypes (cocci, spirilla and rod-shapes) were counted at high spatiotemporal sampling resolution: 36 discrete points in replicate aquaria were sampled every ~30 days over 198 days. Population centers of cocci and spirilla morphotypes moved in continual flux, yet they consistently inhabited separate locations displaying significant anti-correlation. Rod-shaped MTB were initially concentrated toward the northern end of the aquaria, but at the end of the experiment were most densely populated toward the south. That the total number of MTB cells increased through time during the experiment argues that population reorganization arose from relative changes in cell division and death and not from migration. The maximum net growth rates were 10, 3 and 1 day-1 and average net growth rates were 0.24, 0.11 and 0.02 day-1 for cocci, spirilla and rod-shaped MTB respectively; minimum growth rates for all three morphotypes were -0.03 day-1. Our results suggest that cocci and spirilla occupy distinctly different niches--their horizontal positioning in sediment are anti-correlated and under constant flux. Magnetotactic bacteria (MTB) synthesize ferrimagnetic crystals that contribute to the remanent magnetization in sediments, yet knowledge of how MTB populations vary in natural environments over time remained limited. One chapter in this thesis documents the abundances of three MTB morphotypes from nine sites collected and measured every month over a two-year period from a pond near Munich, Germany. Morphotype populations underwent coherent temporal trends among the nine sites. Spirilla populations attained maxima in the summer when temperatures were highest and oxygen concentrations were lowest. Spirilla and cocci exhibited relative antipathy in 2015 when both morphotypes reacted in distinctly opposite ways to oxygen levels. In 2016, they were positively correlated with each other but displayed no coherency with oxygen. Magnetic properties of the sediments varied with water depth: deeper sites, which were also lower in organic carbon, nitrogen and oxygen concentrations than shallower sites, had higher saturation magnetizations and were richer in single domain particles. Although absolute MTB concentrations differed in space, all three measured morphotypes underwent similar relative changes in the time domain at the nine sites; hence, long-term trends of MTB populations in natural ecosystems are likely indicative of the ecosystem as a whole. This is in distinct contrast to artificial habitats in the laboratory that have not reached steady-state conditions. Redeposition experiments were performed over a range of magnetic field intensities and inclinations using the magnetite-bearing, natural sediments from the pond near Munich. Remanent magnetization systematically increased with applied magnetic field, yet relative paleointensities from 25 repeat redeposition experiments performed at identical field conditions vary by a factor of two. The question arises as to what accounts for the variability and what can we do to diminish/remove the variability to improve the precision of relative paleointensity estimates? Recalling that the saturation magnetization (Ms) lies in the long axis direction of a magnetite grain, then the maximum axis of the magnetic anisotropy ellipsoid should parallel the paleomagnetic direction if the magnetization reflects a true depositional remanence acquired via torque. As higher external fields increase the remanence in sediments by tightening the alignment of the grains, the magnetic anisotropy ellipsoid should become increasingly prolate proportional to field strength. This effect is indeed demonstrated in new experiments using anisotropy of anhysteretic remanent magnetization (AARM): nearly all AARM indices vary in proportion with magnetic field strength. Conversely, anisotropy of magnetic susceptibility does not, as it is overwhelmed by the paramagnetic fraction that shows a sedimentary fabric. AARM thus holds promise to improve relative paleointensity estimates from sediments.