Microbes are frequently thought of as the lowest of the low, sitting at the bottom of the food chain, decomposing other organisms’ scraps.

But a new study suggests that microbes act as top-level competitors. In this month’s issue of the journal Ecology, Georgia Institute of Technology biologist Mark Hay and his colleagues found that microbes make rotting food stink so they can compete with bigger predators that would otherwise scavenge the food source.

“Microbes don’t simply sit passively around, waiting for upper levels of the food chain to deliver them the leftovers,” Hay said.  Instead, they meddle directly with the top of the food chain, making certain foods off-limits to upper-level consumers.

By conducting experiments in Georgia’s coastal salt marshes, Hay and his team determined exactly how microbial infection deterred stone crabs from consuming rotting fish. The scientists baited crab traps with fresh fish and rotting fish and then recorded which traps the crabs preferred. They found that crabs preferred fresh fish to rotting fish, but that traps baited with aged, antibiotic-treated fish were just as attractive to the crabs as were fresh fish.

The research team also performed a series of control experiments, all of which suggested that it is the microbe-produced chemicals in rotting fish—rather than the age, the texture or the nutrient level—that makes it unpalatable to the crabs.

“The chemicals that smelled bad to us tasted bad to the crabs, so it was the taste of the chemicals produced by the microbes that made the fish repugnant,” said Deron Burkepile, an ecologist at Yale University and coauthor of the study.

While fresh fish are inhabited by mostly aerobic, or oxygen-consuming, bacteria, rotting fish are populated by anaerobic bacteria, which does not require oxygen. It is this anaerobic bacteria that produces the noxious chemicals that create a nasty stench and flavor and could sicken animals.

“[Microbes] fight animals and other microbes for food, and, since they don’t have teeth and claws, they use chemical warfare to do it,” Burkepile said. “These interactions happen in our refrigerators every day. When we throw away the fuzzy strawberries or the moldy steak, it’s because the microbes have won and successfully staked a claim to their food.”

Hay’s study suggests that microbes should no longer be associated only with food-borne disease. 

“It’s interesting to consider that rotting food is repulsive for a reason,” said Kevin Lafferty, a marine ecologist with the U.S. Geological Survey and the University of California, Santa Barbara.

Scientists should give microbes more serious consideration when making decisions about conservation, Hay said, since microbes play an important role in the marine food web.

“Managers commonly consider issues of predation and competition among higher organisms in making management decisions,” Hay said. “They rarely view microbes as ‘players’—they can be, often are, and need to be factored in at appropriate levels.”

Originally published November 21, 2006

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