The ocean sucks up heat-trapping carbon dioxide (CO2) building up in our atmosphere with help from tiny plankton.

Like plants on land, plankton convert CO2 into organic carbon via photosynthesis and then can sink into the deep ocean, carrying carbon with them. They decompose when bacteria convert their remains back into CO2.

This "biological pump," if it operated 100 percent efficiently, would mean nearly every atom of carbon drawn into the ocean would be converted to organic carbon, sink into the deep ocean, and remain sequestered from the atmosphere for millennia. But like hail stones that melt before reaching the ground, some carbon never makes it to the deep ocean, allowing CO2 to leak back into the upper ocean and ultimately exchange with the atmosphere.

In a new study published in PNAS, scientists discovered a surprising new short-circuit to the biological pump. They found that sinking particles of stressed and dying phytoplankton release chemicals that have a jolting, steroid-like effect on marine bacteria feeding on the particles. The chemicals juice up the bacteria's metabolism causing them to more rapidly convert organic carbon in the particles back into CO2 before they can sink to the deep ocean.

Snking particles of stressed and dying phytoplankton release chemicals that have a jolting, steroid-like effect on marine bacteria feeding on the particles. Photo by Dawn Moran, Woods Hole Oceanographic Institution

Typically, the detritus of phytoplankton have no special effect on bacteria; they are simply a food source. But the phytoplankton in this study--diatoms--are different. When stressed, some diatoms release bioactive molecules known as polyunsaturated aldehydes (PUAs). The researchers found that these molecules kick the bacteria's metabolism and CO2 respiration rates into hyperdrive -- like skinny weightlifters after a steroid shot.

The bacteria start devouring the falling particles like they are at an all-you-can-eat buffet, and significantly reduce the amount of sinking detritus while releasing CO2.

The researchers went to sea to collect and analyze particle samples from several locations across the North Atlantic, including the Sargasso Sea, the subarctic North Atlantic near Iceland, and the western North Atlantic near Massachusetts. To collect the particles, 6-foot-wide, funnel-shaped sediment traps were submerged 150 meters down (picture huge traffic cones dunked upside down in the ocean) for 24 hours. Once the traps were brought back to surface, the scientists incubated collected particles with PUAs and analyzed changes in bacterial metabolism over a 24-hour period.

The scientists also found much higher concentrations of PUAs within the sinking particles than had been previously been observed in the water column. 

The research was funded by the Gordon and Betty Moore Foundation Grant 3301, with additional support from the National Science Foundation and the Office of Naval Research.