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Science 329 (5989): 342-345

Copyright © 2010 by the American Association for the Advancement of Science

Chemoattraction to Dimethylsulfoniopropionate Throughout the Marine Microbial Food Web

Justin R. Seymour1,2,3,*, Rafel Simó4, Tanvir Ahmed1, and Roman Stocker1

1 Ralph M. Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2 School of Biological Sciences, Flinders University, General Post Office Box 2100, Adelaide, South Australia, 5001, Australia.
3 Plant Functional Biology and Climate Change Cluster (C3), University of Technology, Sydney, Post Office Box 123 Broadway, New South Wales 2007, Australia.
4 Institut de Ciències del Mar (ICM), Consejo Superior de Investigaciones Científicas (CSIC) Passeig Maritim de la Barceloneta 37-49, 08003 Barcelona, Catalonia, Spain.


Figure 1
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Fig. 1. The strength of the attraction to pulses of DMSP and related compounds, as illustrated by the chemotactic index (IC), which is based on cell distributions, and the exposure index (IE), which is based on response speed and substrate diffusion rates (18). Both IC and IE are population-averaged quantities. The color code and top number in each cell describe maximum IC values observed during each experiment. Large positive IC indicates strong chemotactic attraction, whereas IC ≤ 0 corresponds to lack of attraction. Numbers in parentheses correspond to the IE averaged over time (1 to 6 min). Where IC is large but IE is small (e.g., N. designis), organisms respond strongly but not rapidly. ASW denotes the artificial-seawater-only control. Gray boxes are cases where no data were acquired.

 

Figure 2
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Fig. 2. Chemotactic responses to diffusing patches of DMSP and related compounds. Spatiotemporal distributions of (A) the bacterium P. haloplanktis, 500 µM DMSP, (B) the phytoplankter M. pusilla, 20 µM DMSP, and (C) the dinoflagellate Oxyrrhis marina, 20 µM DMSP. Colors denote cell concentration, normalized to a mean of 1. White triangles indicate times at which cell concentrations across the channel width (x) were measured. (D) Sample concentration profiles of organisms across the microchannel width, normalized to a mean of 1 (measurement times are given in the key). Although attraction was observed for many sulfur compounds (solid lines), repulsion also occurred (dashed line). For all cases, the pulse was released at time t = 0 at the center of the microchannel (x = 0) and had an initial width of 300 µm. The full set of spatiotemporal cell distributions is given in fig. S1.

 

Figure 3
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Fig. 3. Swimming behavior of O. marina in response to a 20 µM DMSP pulse. (A and B) Trajectories acquired (A) at t = 15 s and (B) t = 255 s, color-coded as follows: blue, swimming left at >85 µm s–1; red, swimming right at >85 µm s–1; green, trajectories fully within central 300 µm; and gray, all others. Yellow dots indicate starting points, and the gray background is the modeled DMSP concentration field. (C) Chemotactic velocity (i.e., inward speed), U, measured across the channel at t = 15 s. (D) Spatiotemporal distribution of the rate of change of direction relative to background value.

 

Figure 4
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Fig. 4. Chemical micropatch diffusion dynamics. (A) Spatiotemporal variation of the DMSP concentration (µM) following the lysis of a 5-µm radius phytoplankton cell. Distance from the center of the cell is r, time elapsed since lysis is t. (B) Spatial variation of the DMSP concentration with distance from a stressed (18), DMSP-exuding phytoplankton cell, for two cell radii (5 and 10 µm). In (A) and (B), the intracellular DMSP concentration was 100 mM. (C) The initial DMSP concentration in the microfluidic pulses rapidly diffused to considerably lower values. Colors show the computed reduction factor (concentration normalized by the initial concentration in the patch), as a function of time t and distance x across the microchannel.

 


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