Video microscopy reveals that bacteria swim to food
with surprising speed
David L. Shenkenberg
Because phytoplankton use carbon dioxide in photosynthesis, scientists and engineers have thought that an increased abundance of phytoplankton could draw a substantial amount of this greenhouse gas from the atmosphere and thus reduce global warming. They proposed that fertilizing the ocean with iron in regions where it otherwise would be scarce could cause explosive growth of phytoplankton that would then die and sink, and they believed that some of that carbon might ultimately become buried in the sediment of the ocean floor.
Researchers applied video microscopy to quantify the ability of marine bacteria to swim to food, using channels in microfluidic chips to simulate the microenvironment of the bacteria.
This iron-fertilization hypothesis has been tested in the past, and the phytoplankton did grow, but the carbon returned to the atmosphere, in part because bacteria ate it before it ever reached the ocean floor. A million bacteria per milliliter of seawater are waiting for it.
Roman Stocker of MIT in Cambridge, Mass., believes that marine bacteria eat the phytoplankton so efficiently because the bacteria evolved to swim rapidly toward such nutrients, which normally are few and far between in the vast ocean. He and colleagues have shown that the marine bacterium Pseudoalteromonas haloplanktis rapidly exploits ephemeral nutrient sources. They used microfluidic chips to simulate the bacteria’s natural microenvironment and material from phytoplankton cultures as the nutrients.
The investigators used video microscopy to track the bacteria chasing the phytoplankton-derived organic matter in the channels of the microfluidic chip. They set the chip on the stage of a Nikon inverted microscope and used the 10× objective to visualize the bacteria. They repeatedly recorded five- to 40-frame movies with a 1600 × 1200-pixel CCD camera from The Cooke Corporation of Seabrook, N.H, set to operate at 32 fps with a 0.9 × 1.2-mm field of view. They captured images of the nutrients, which were labeled with fluorescein, in epifluorescence mode, followed by images of the bacteria in phase-contrast mode. To measure the speed and other swimming statistics, they used BacTrack software developed by Stocker’s group.
The bacteria rapidly exploited a plume of nutrients in this microfluidic channel. The nutrients were labeled with fluorescein, and the bacteria were imaged in phase-contrast mode. Reprinted with permission of PNAS.
The researchers simulated both the rapid release of nutrients that occurs when a few phytoplankton cells die — a “nutrient pulse”— and the slow trail of nutrients that follows sinking phytoplankton and other marine particles — a “nutrient plume.”
The team clocked the bacteria moving at 35 body lengths per second, faster than the cheetah in terms of speed relative to body length. The fastest 20 percent of the marine bacteria came in contact with 10 times more nutrients than nonmotile cells. On average, the marine bacteria moved toward the nutrients more than 10 times faster than E. coli moved toward its most potent chemoattractants, for twice the nutrient exposure. “This is the first experimental proof that bacteria utilize such nutrient patches extremely rapidly,” Stocker said.
The researchers are examining how turbulence, which is ubiquitous in the ocean, might affect the swimming of bacteria and how marine bacteria interact through chemical signaling.
PNAS, March 18, 2008, pp. 4209-4214.
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