The Salp Is Nature’s Near-Perfect Little Engine
Role extends to removing carbon dioxide from upper ocean and atmosphere
What if trains, planes and automobiles all were powered simply by the air through which they move?Â What if their exhaust and by-products helped the environment?
Such an energy-efficient, self-propelling mechanism already exists in nature.
The salp, a small, barrel-shaped organism that resembles a streamlined jellyfish, gets everything it needs from ocean waters to feed and propel itself.
Scientists believe its waste material may help remove carbon dioxide (CO2) from the upper ocean and the atmosphere.
Now researchers at the Woods Hole Oceanographic Institution (WHOI) and MIT have found that the half-inch to 5-inch-long creatures are even more efficient than had been believed.
"This innovative research is providing an understanding of how a key organism in marine food webs affects important biogeochemical processes," said David Garrison, director of the National Science Foundation (NSF)’s biological oceanography program, which funded the research.
Reporting this week in the journal Proceedings of the National Academy of Sciences (PNAS), the scientists have found that mid-ocean-dwelling salps are capable of capturing and eating extremely small organisms as well as larger ones, rendering them even hardier–and perhaps more plentiful–than had been believed.
"We had long thought that salps were about the most efficient filter-feeders in the ocean," said Larry Madin, WHOI Director of Research and one of the paper’s authors.
"But these results extend their impact down to the smallest available size fraction, showing they consume particles spanning four orders of magnitude in size. This is like eating everything from a mouse to a horse."
Salps capture food particles, mostly phytoplankton, with an internal mucus filter net.Â Until now, it was thought that included only particles larger than the 1.5-micron-wide holes in the mesh; smaller particles would slip through.
But a mathematical model suggested salps somehow might be capturing food particles smaller than that, said Kelly Sutherland, who co-authored the PNAS paper after her PhD research at MIT and WHOI.
In the laboratory at WHOI, Sutherland and her colleagues offered salps food particles of three sizes: smaller, around the same size as, and larger than the mesh openings.
"We found that more small particles were captured than expected," said Sutherland, now a post-doctoral researcher at Caltech. "When exposed to ocean-like particle concentrations, 80 percent of the particles that were captured were the smallest particles offered in the experiment."
The finding helps explain how salps–which can exist either singly or in "chains" that may contain a hundred or more–are able to survive in the open ocean where the supply of larger food particles is low.
"Their ability to filter the smallest particles may allow them to survive where other grazers can’t," said Madin.
Perhaps most significantly, the result enhances the importance of the salps’ role in carbon cycling. As they eat small, as well as large, particles, "they consume the entire ‘microbial loop’ and pack it into large, dense fecal pellets," Madin says.
The larger and denser the carbon-containing pellets, the sooner they sink to the ocean bottom. "This removes carbon from the surface waters," said Sutherland, "and brings it to a depth where you won’t see it again for years to centuries."
And the more carbon that sinks to the bottom, the more space there is for the upper ocean to accumulate carbon, hence limiting the amount that rises into the atmosphere as CO2, said paper co-author Roman Stocker of MIT.
"The most important aspect of this work is the very effective shortcut that salps introduce in the process of particle aggregation," Stocker said. "Typically, aggregation of particles proceeds slowly, by steps, from tiny particles coagulating into slightly larger ones."
"Now, the efficient foraging of salps on particles as small as a fraction of a micrometer introduces a substantial shortcut in this process, since digestion and excretion package these tiny particles into much larger particles, which thus sink a lot faster."
This process starts with the mesh made of fine mucus fibers inside the salp’s hollow body.
Salps, which can live for weeks or months, swim and eat in rhythmic pulses, each of which draws seawater in through an opening at the front end of the animal. The mesh captures the food particles, then rolls into a strand and goes into the gut, where it is digested.
"It was assumed that very small cells or particles were eaten mainly by other microscopic consumers, like protozoans, or by a few specialized metazoan grazers like appendicularians," said Madin.
"This research indicates that salps can eat much smaller organisms, like bacteria and the smallest phytoplankton, organisms that are numerous and widely distributed in the ocean."
The work, also funded by the WHOI Ocean Life Institute, "implies that salps are more efficient vacuum cleaners than we thought," said Stocker.
"Their amazing performance relies on a feat of bioengineering–the production of a nanometer-scale mucus net–the biomechanics of which remain a mystery."
Image 1: Researchers Larry Madin and Kelly Rakow Sutherland get up close with a salp in the lab. Credit: Tom Kleindinst, Woods Hole Oceanographic Institution
Image 2: Salps float through the ocean’s sunlit surface waters. Credit: Kelly Sutherland and Larry Madin, WHOI
Image 3: Epifluorescent microscope image of salp filtering mesh: the best food-trapper in the sea. Credit: Kelly Sutherland and Larry Madin, WHOI
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