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American researchers get to the bottom of marine molecule mystery

Researchers at Woods Hole Oceanographic Institution (WHOI) and the University of South Carolina has managed to solve a conundrum that’s been puzzling marine scientists for roughly a decade – where does all the oceanic phosphonate come from?

Roughly a decade ago, phosphonate – a rare form of organic phosphorus – was discovered in marine organic matter. Not only were researchers baffled to find this rare form of phosphorus in the ocean; they were also flummoxed by the high concentrations in which it was found throughout the sea. No one could explain where it came from and why it could be found in such abundance.

That is, no one could explain it until now.

In 2006, biologist Sonya Dyhrman and her WHOI team commenced a field and laboratory study on a group of phytoplankton called Trichodesmium. Trichodesmium is a microscopic marine microbe found in ample amounts throughout warm tropical and subtropical waters where nutrients are scarce. The WHOI team were able to show that Trichodesmium uses phosphonate to support carbon and nitrogen fixation, and that a special set of genes have given them this capacity. This triggered Dyhrman’s curiosity – where did Trichodesmium get its phosphonate from in the first place?

To solve the mystery, Dyhrman partnered up with Claudia Benitez-Nelson, a marine geochemist with the University of South Carolina, and started analyzing various phytoplanktons using nuclear magnetic resonance.

We’ve been fascinated by these phosphonate compounds for a while,” said Benitez-Nelson. “Sonya and I decided that something had to be producing them, and we had to start looking at all these organisms to figure out who it was.”

After culturing several different kinds of phytoplankton and analyzing them using nuclear magnetic resonance (NMR) spectroscopy, we found high concentrations of phosphonate in cultures of a specific Trichodesmium species – in fact an average of 10 percent of the cellular phosphorus is in the form of phosphonate“, explained Dyhrman. “Ten percent may not sound like much, but this is the most phosphonate ever detected in a marine microbe.”

When we first saw the phosphonate peak in the Trichodesmium culture, we were stunned, after a 10-year mystery it seemed ironic for Trichodesmium to both consume and produce this compound“, said Benitez-Nelson. “We ran it again. We grew them under different nutrient conditions and, sure enough, the results were the same.”

Since nitrogen is scarce in the open ocean, nitrogen fixing organisms like Trichodesmium are imperative to the marine food web. Trichodesmium phytoplankton will not only bring carbon into the food chain by absorbing it from the atmosphere like other phytoplankton; they will also provide the food chain with essential nitrogen due to their ability to absorb nitrogen gas from the air and transform it into a compound that other organisms can use.

Not only does this solve a mystery about where these forms of phosphorus are coming from, but the fact that it is Trichodesmium has ramifications for how the phosphorus cycle is linked to the cycling of carbon and nitrogen and how those cycles will function in the future ocean,” said Dyhrman.

The Dyhrman and Benitez-Nelson study was recently published in the journal Nature Geoscience.

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