diatoms
diatoms

Scientists have 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 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.

The ocean has been sucking up heat-trapping carbon dioxide (CO2) building up in our atmosphere – with help from tiny plankton. Like plants on land, these plankton convert CO2 into organic carbon via photosynthesis. But unlike land plants, plankton can sink into the deep ocean, carrying carbon with them. Along the way they decompose when bacteria convert their remains back into CO2.

It’s called the “biological pump,” and if it operated 100 percent efficiently, 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 April 27 in the Proceedings of the National Academy of Sciences, scientists at Woods Hole Oceanographic Institution (WHOI) and their colleague from Rutgers University 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.

“We think these compounds are acting as signals to the bacterial community to let them know phytoplankton are dying, lots of ‘free’ food on the way, and to ramp up their metabolisms,” said Bethanie Edwards, lead author of the study and a graduate student in the MIT/WHOI Joint Program in Oceanography. “When the bacteria consume phytoplankton faster, more CO2 is given off in the shallow depths, where it can return to the surface of the ocean and the atmosphere more quickly.”

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.

To collect the particles, the team submerged 6-foot-wide, funnel-shaped sediment traps 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 analysed changes in bacterial metabolism over a 24-hour period.

The sediment traps were placed at several locations across the North Atlantic, including the Sargasso Sea, the subarctic North Atlantic near Iceland, and the western North Atlantic near Massachusetts. The traps were submerged at depths of 150 meters for 24 hours and then the collected particles were analyzed in the lab. (Photo by Suni Shah, Woods Hole Oceanographic Institution)
The sediment traps were placed at several locations across the North Atlantic, including the Sargasso Sea, the subarctic North Atlantic near Iceland, and the western North Atlantic near Massachusetts. The traps were submerged at depths of 150 meters for 24 hours and then the collected particles were analyzed in the lab. (Photo by Suni Shah, Woods Hole Oceanographic Institution)

“Very rarely do you see organisms respond positively to PUAs. In fact, in higher concentrations, they often have a toxic effect, causing a decrease in phytoplankton growth rates and mutations,” Edwards said. “But our results were very surprising. We saw an increase in CO2 production rates, enzyme activity, and bacterial cell growth.”

The scientists also found much higher concentrations of PUAs within the sinking particles than had been previously been observed in the water column. “This suggests that sinking particles are ‘hotspots’ for PUA production,” Edwards said.

“This study shows that when it comes to long-term biological sequestration of atmospheric carbon dioxide in the ocean, not all species of phytoplankton are created equal,” said Don Rice, program director for the National Science Foundation (NSF)’s Chemical Oceanography Program, which partially funded the research.

Further Reading
Bethanie R. Edwards, Kay D. Bidle, and Benjamin A. S. Van Mooy. Dose-dependent regulation of microbial activity on sinking particles by polyunsaturated aldehydes: Implications for the carbon cycle
PNAS 2015 ; published ahead of print April 27, 2015, doi:10.1073/pnas.1422664112

Photo credit: CSIRO [CC BY 3.0]

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