Diversity and vertical distribution of magnetotactic bacteria along chemical gradients in freshwater microcosms

Abstract

The vertical distribution of magnetotactic bacteria along various physico-chemical gradients in freshwater microcosms was analyzed by a combined approach of viable cell counts, 16S rRNA gene analysis, microsensor profiling and biogeochemical methods. The occurrence of magnetotactic bacteria was restricted to a narrow sediment layer overlapping or closely below the maximum oxygen and nitrate penetration depth. Different species showed different preferences within vertical gradients, but the largest proportion (63-98%) of magnetotactic bacteria was detected within the suboxic zone. In one microcosm the community of magnetotactic bacteria was dominated by one species of a coccoid "Alphaproteobacterium", as detected by denaturing gradient gel electrophoresis in sediment horizons from 1 to 10 mm depth. Maximum numbers of magnetotactic bacteria were up to 1.5 x 10(7) cells/cm3, which corresponded to 1% of the total cell number in the upper sediment layer. The occurrence of magnetotactic bacteria coincided with the availability of significant amounts (6-60 microM) of soluble Fe(II), and in one sample with hydrogen sulfide (up to 40 microM). Although various trends were clearly observed, a strict correlation between the distribution of magnetotactic bacteria and individual geochemical parameters was absent. This is discussed in terms of metabolic adaptation of various strains of magnetotactic bacteria to stratified sediments and diversity of the magnetotactic bacterial communities.

Polarity of bacterial magnetotaxis is controlled by aerotaxis through a common sensory pathway

“…In equilibrium conditions, cells showed a reversal frequency of 0.126 s À 1 or less, which is low compared with data reported for non-MTB [40][41][42][43] . Consistent with the adaptation of MTB for growth in redox gradients in stratified sediments 27 , and also with the dependence of oxygen-sensitive magnetite biomineralization and N 2 fixation on suboxic conditions 44,45 , we found a higher proportion of cells swam faster and reversed less frequently under microoxic than under anoxic or fully oxic conditions. The model of energy taxis suggests cells swimming towards lower or higher oxygen concentrations would experience a change in proton motive force or electron transfer rates 15,46 , resulting in increased motor reversals.…”

“…However, as previous observations indicated that magnetospirilla freshly isolated from environmental samples displayed strong swimming polarity 26,27 , we tested whether polarity could be reinduced in our M. gryphiswaldense WT lab strain 28 , which has been passaged for many generations in the absence of magnetic selection. We therefore performed serial transfers of cells in oxygen gradients (Ox Low -Ox High ), exposing them to either parallel (north polesouth pole) or antiparallel (south pole'north pole) magnetic fields of approximately 10 Â the geomagnetic field strength (0.57 mT) ( ¼ selective conditions, Supplementary Fig.…”

“…Swimming polarity appeared to be an inherent property of cells, and the temporary loss of motility bias after non-selective lab cultivation was not caused by genetic loss, but could be restored by appropriate incubation within oxygen gradients superimposed with magnetic fields mimicking the geomagnetic inclination. As all MTB including magnetospirilla sampled from environmental sites generally display swimming polarity 3,26,27 , it seems likely that in natural redox gradients in which cells persistently experience polarity selection by the geomagnetic field vector polar rather than 'axial' magnetotaxis is the prevalent mode of magnetic navigation common to all MTB. The previously observed 'axial' behaviour of magnetospirilla therefore might represent an artefact, resulting from the loss of selection during lab cultivation.…”

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

“…Magnetosome concentrations in sediments also serve as paleoclimate proxies (11)(12)(13)(14). As they contain 2 to 4% (dry weight) Fe (15,16), MTB could influence iron cycles in nature by acting to sequester Fe in sediments (17). Several studies have addressed the spatiotemporal variation of MTB along vertical profiles (16,18,19), yet much less is known about how their horizontal distribution varies in space and time.…”

“…As they contain 2 to 4% (dry weight) Fe (15,16), MTB could influence iron cycles in nature by acting to sequester Fe in sediments (17). Several studies have addressed the spatiotemporal variation of MTB along vertical profiles (16,18,19), yet much less is known about how their horizontal distribution varies in space and time. Specifically, we question how well MTB populations measured in a given place (a spatial niche) represent a particular environment through time; e.g., do the populations remain fixed in a limited area, move randomly from place to place, or some combination of the two, and over what time scales?…”