Study Reveals Microbes’ Hidden Role in Fertilizing Oceans

May 14, 2015
In order to understand how phosphorus moves through the oceans, researchers did shipboard experiments with Trichodesmium, a type of bacteria that forms visible colonies. The test tube is about the diameter of a U.S. quarter. (Carly Buchwald, WHOI)
Phosphorus is an essential nutrient for every living organism, well known for its role in fueling everything from the human body to farm fields. But up to now, surprisingly little has been known about how the element cycles through the oceans. A new study has broken through some of this mystery, by showing the hidden role that the oceans’ tiniest creatures play. The study appears this week in the leading journal Science.
Like phosphorus-based fertilizers that boost plant growth on land, phosphorus in the oceans fertilizes microbes and tiny marine plants called phytoplankton, which compose the base of the food chain. But it has been unclear exactly how these organisms both use and produce the different forms of phosphorus found in the oceans—especially little-understood versions called phosphonates. Scientists knew that microbes take up and then release phosphorus back to seawater in different forms, as these creatures die and decay.  But they were not sure where phosphonates or other forms of phosphorus come from.
“Phosphonates have always been a huge mystery,” said lead author Benjamin Van Mooy, a biochemist at Woods Hole Oceanographic Institution. “No one’s been able to figure out exactly what they are, and more importantly, if they’re made and consumed quickly by microbes, or if they’re just laying around in the ocean.”
The scientists were able to show that phosphonates are being manufactured by certain microbes.
“Although evidence of the cycling of phosphonates has been mounting for nearly a decade, these results show for the first time that microbes are producing phosphonates in the ocean, and that it is happening very quickly,” said coauthor Sonya Dyhrman, a marine microbiologist at Columbia University’s Lamont-Doherty Earth Observatory.
To find out more, the researchers performed a series of experiments at sea in summer 2014. They took samples of microbes from seawater obtained at stops during a cruise from Bermuda to Barbados. Then they added a better-studied form of phosphorus, called phosphate, impregnated with radioactivity. Using a technique called ion chromatography, they used the radioactivity as a tracer to see what, if anything, the microbes did with the phosphate.
What they saw was that about 5 percent of the phosphate in shallow water samples were taken up by the microbes and changed to phosphonate. In deeper samples, from depths of 40 and 150 meters, about 15 to 20 percent of the phosphate was reduced to phosphonate.
“In near real time, we could tell that the phosphate we added was being transformed to phosphonate, and everyone on the ship got to share in this moment and the process of discovery,” said Dyhrman. “Such work will help us further resolve the complexities of how this critical element is cycled in the ocean,” she said.
The researchers say that a better understanding of marine phosphorus cycling is important not only for its central role in the food chain, but in understanding the ability of ocean creatures to absorb atmospheric carbon dioxide as they grow. Phosphonates are probably only a small part of the picture; the scientists say they still need to identify the other forms of phosphorus  being produced and metabolized by marine microbes, and what physiological roles they serve.
“There’s still a lot we don’t know about the sea,” said Don Rice, program director in the National Science Foundation’s Division of Ocean Sciences, which funded the research.
The work was also supported by grants from the Simons Foundation.



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