'Quorum sensing' helps control how carbon moves in the sea

It's wondrous how the vast and the infinitesimal combine to make our planet work. Scientists at Woods Hole Oceanographic Institution (WHOI) have found that bacteria in the ocean, gathering in sort of "microbial block parties," communicate and cooperate with each other to have a significant impact on how carbon dioxide is transferred from the atmosphere to the deep sea. In this newly discovered mechanism, bacteria coalesce on tiny particles of carbon-rich detritus sinking in the ocean. They send out chemical signals to discern if other bacteria are in the neighborhood. If enough of their compadres are nearby, the bacteria en masse commence sending out enzymes that break up the particles into more digestible bits (see interactive below).

Tiny marine plants (phytoplankton) die or are eaten by tiny marine animals (zooplankton) which defecate into the water. All this detritus is sticky and agglomerates into heavier particles that sink. This epifluorescence micrograph of a stained gelatinous particle (about 200 microns in size) was harvested from a particle trap set 60 meters deep in Clayoquot Sound, British Columbia, Canada, in 2009. Note individual microbial cells (about 0.5 to 2 microns in length) embedded in gelatinous material together with other plankton "hard parts." (Photo by Tracy Mincer, Woods Hole Oceanographic Institution)

As a result, a substantial amount of carbon does not sink to the depths, which affects both the marine food web and the planet's climate. The re-released carbon can be reused by marine plants, and less carbon dioxide, a heat-trapping greenhouse gas, is drawn out of the air into the ocean. In addition, less carbon is effectively transferred to the bottom of the ocean, from where it cannot easily return to the atmosphere.

The finding represents the first evidence that bacterial communication plays a crucial role in Earth's carbon cycle. It was reported Feb. 22 at the American Geophysical Union's Ocean Sciences meeting in Portland, Ore., by WHOI marine biogeochemists Laura Hmelo, Benjamin Van Mooy, and Tracy Mincer.

Ready, set, secrete enzymes
The carbon cycle works like a planetary conveyor. Carbon is spewed into the atmosphere in the form of carbon dioxide gas-naturally and gradually over geological time from volcanoes and plants, but much more rapidly in recent history from industrial smokestacks and cars. The gas is absorbed by photosynthetic plants, which convert it into bigger organic carbon molecules that they use as food and building blocks for their cells. The oceans harbor multitudes of marine plants; the detritus created when they die or are eaten is sticky and it agglomerates into particles that are heavy and start to sink. These particles, raining into the deep, are sometimes visible and often called "marine snow."

Marine bacteria see food in these particles and begin to colonize them. But the carbon- and nitrogen-rich material is in the form of complex molecules too big to get through bacterial cell membranes. "A protein is too big," Hmelo said. "The bacteria want one amino acid."

The bacteria can manufacture enzymes to break down the molecules, but here's the catch: It costs precious energy to make the enzymes, and a lone bacterium doesn't stand a chance.

"For any one bacterium to engage in this, there's a risk that they wouldn't get a return on their investment," Van Mooy said. "It would have to send out enzymes and hope that they hit something and blow it up so they can get something to eat out of it."

Instead, the bacteria conserve their resources by making other chemicals, called acylated homoserine lactones, or AHLs, which cost much less energy to produce, Hmelo said. They send these AHLs into the environment to see if any other bacteria are around.

If no other AHL-producing bacteria are in the vicinity, the AHLs will diffuse away and quickly degrade, and bacterial "silence" will prevail. But if enough AHL-producing bacteria are nearby, the concentration of AHLs outside the bacteria will eventually rise. That's the chemical signal to each bacterium that it's got a lot of buddies in the area. In this way-a process called quorum sensing-the bacteria sense that they have achieved a quorum, in this case a sufficient density to begin to make and release enzymes. You might think of it as the microbial equivalent of sending out tweets to organize a block party.

Chemical conversations
The scientists investigated this unexplored mechanism by using mesh traps to collect particles of marine snow sinking in Clayoquot Sound off Vancouver, where fjords extend into the ocean. Tides go back and forth over the shallow bottoms of the fjords, stirring up nutrients that spark blooms of marine algae.

Back in the lab at WHOI, the scientists incubated the collected particles and confirmed the presence of AHLs in the marine snow. They also added AHLs to samples and found that the chemical signals stimulated bacteria to make enzymes that broke apart the particles-confirming that bacteria "talk about it" before launching a coordinated assault.

In the natural world, Van Mooy said, only about 10 percent of particles makes it all the way to the ocean depths; the scientists theorize that this quorum-sensing mechanism plays a key role in disaggregating most of the particles.

"So microscopic bacteria buffer the amount of carbon dioxide in the atmosphere through their 'conversations,' " he said. "I think it's amazing that there is an infinite number of these conversations going on in the ocean right now, and they are affecting Earth's carbon cycle."

-Lonny Lippsett

The research was funded by the National Science Foundation.
Source: Oceanus Magazine