Mysterious form of phosphorus explained

Facing famine, some crafty marine microbes may set up an exclusive black market for their rations of phosphorus.

By converting the essential element of life into an unconventional chemical state, certain marine microbes could create a thriving exchange for phosphorus just among themselves, researchers report in the May 15 Science. The finding helps solve a long-standing mystery of why marine microbes create the unusual form of phosphorus, called phosphonate. It may also upend scientists’ theories of how phosphorus-containing compounds circulate among organisms and ecosystems, and the impact of that cycle.

“So much hinges on our understanding of phosphorus cycling,” says chemical oceanographer Benjamin Van Mooy of the Woods Hole Oceanographic Institution in Massachusetts. For example, some scientists use movements of phosphorus in the ocean to estimate how much carbon dioxide gets absorbed by the sea, acidifying waters.

So far, no such estimation takes phosphonates into account, says microbial oceanographer David Karl of the University of Hawaii in Honolulu.

Instead, scientists have focused on phosphate, a common form of phosphorus found in all genetic material and in ATP, cells’ energy currency. In phosphate, the phosphorus atom has an oxidation state — a theoretical measure of how many electrons it shares — of +5. In an energy-expensive conversion, some microbes can transform phosphate into phosphonate molecules, which have a phosphorus atom with an oxidation state of +3.

In waters near the Caribbean islands, where phosphorus is scarce, Van Mooy and colleagues found that cyanobacteria in the genus Trichodesmium, along with unidentified microbial accomplices, readily convert phosphate to phosphonates. The microbes then release the phosphonates into the water, potentially sharing it among themselves like a “shadow currency,” Van Mooy says.

He and colleagues estimate that the amount of phosphorus converted into phosphonates by the microbes could account for a significant fraction of the element’s circulation — even more than the amount of phosphorus that naturally cycles into the oceans from land.

“That’s huge,” says marine microbiologist Edward DeLong, also of the University of Hawaii. “It’s going to change the game in terms of how people think about phosphorus cycling in the ocean.”