Marine bacteria's mealtime dash is a swimming success

Until 25 years ago, scientists weren’t really aware of the microbial loop, the processing of organic material among the smallest creatures in the ocean: bacteria, phytoplankton, nanozooplankton, viruses, etc. Now they know that the roughly 1 million bacteria per milliliter of ocean play a pivotal role in the microbial loop; by recycling that organic matter, they pass it on to larger animals and prevent it from dropping out of the marine food web.

But quantifying the importance of bacteria in the microbial loop has been difficult, because creating a realistic microenvironment wasn’t possible until recently.

“You can hope to study an organism’s behavior only in the context of its environment. The habitat of a bacterium, on the other hand, is extremely small, on the order of microns to millimeters,” said Stocker. “This has made the study of microbial behavior a formidable technical challenge to date. We have been able to create realistic environmental landscapes for studying marine bacteria in the lab by using microfluidic technology.”

P. haloplanktis is a rapid swimmer, propelling itself by a single rotating flagellum in bursts of speed up to 500 body lengths per second. (The fastest land animal, the cheetah, travels at bursts of speed up to 30 body lengths per second.) During experiments, Stocker and team observed that the bacteria used their rapid motility to very effectively swim toward and follow their food sources. That directed movement in response to a chemical gradient (in this case, nutrients) is known as chemotaxis.

“It will be important to see how widespread the use of rapid chemotaxis is in the ocean,” said Stocker. “We expect this to depend on the environment; in algal blooms, for example, nutrient patches and plumes will be abundant, and speedy bacteria will be favored. Whenever this is the case, nutrients get recycled much more rapidly, making the food web more productive and potentially affecting the rates at which carbon is cycled in the ocean.”