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, we took MTB-bearing sediment from a natural pond, mixed the thoroughly homogenized sediment into two replicate aquaria, and then counted three dominant MTB morphotypes (coccus, spirillum, and rod-shaped MTB cells) at a high spatiotemporal sampling resolution: 36 discrete points in replicate aquaria were sampled every ϳ30 days over 198 days. Population centers of the MTB coccus and MTB spirillum morphotypes moved in continual flux, yet they consistently inhabited separate locations, displaying significant anticorrelation. Rod-shaped MTB were initially concentrated toward the northern end of the aquaria, but at the end of the experiment, they were most densely populated toward the south. The finding that the total number of MTB cells increased over 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 doublings day Ϫ1 and average net growth rates were 0.24, 0.11, and 0.02 doublings day Ϫ1 for MTB cocci, MTB spirilla, and rod-shaped MTB, respectively; minimum growth rates for all three morphotypes were Ϫ0.03 doublings day Ϫ1 . Our results suggest that MTB cocci and MTB spirilla occupy distinctly different niches: their horizontal positioning in sediment is anticorrelated and under constant flux.IMPORTANCE Little is known about the horizontal distribution of magnetotactic bacteria in sediment or how the distribution changes over time. We therefore measured three dominant magnetotactic bacterium morphotypes at 36 places in two replicate aquaria each month for 7 months. We found that the spatial positioning of population centers changed over time and that the two most abundant morphotypes (MTB cocci and MTB spirilla) occupied distinctly different niches in the aquaria. Maximum and average growth and death rates were quantified for each of the three morphotypes based on 72 sites that were measured six times. The findings provided novel insight into the differential behavior of noncultured magnetotactic bacteria.KEYWORDS magnetotactic bacteria, spatiotemporal variation, controlled aquaria A lmost all freshwater and marine environments host a broad spectrum of special kinds of bacteria, called magnetotactic bacteria (MTB) (1), that synthesize membrane-bound, single-domain-sized (35-to 120-nm) magnetite and/or greigite crystals called magnetosomes (2-4). Chains of magnetosomes that contain permanent magnetic moments are fixed within the cell, which enable MTB to swim along magnetic field lines (5, 6). MTB host a wide variety of cell morphotypes, including cocci, vibrios, spirilla, ovoids, rods, and multicellular prokaryotes (7).
