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Bit Player Turns Out to Be the Star

The trace element molybdenum, and not that poseur phosphorous, governs the profligacy of your average tropical rainforest.

Molybdenum, an element that always sounds like a character populating a Henry Fielding novel, turns out to be the protagonist in one saga arising from tropical rainforests. New research from Princeton suggests that this trace element supports the nitrogen infrastructure that allows the rainforest to grow like, well, a jungle.

And as in an old novel, molybdenum’s role hinged on a case of mistaken identity, as scientists had long thought phosphorous, that diabolical (and more common) 13th element, lay behind the growth.

"We were surprised," a release from Princeton quotes Lars Hedin, a professor of ecology and evolutionary biology at the Princeton Environmental Institute who led the research, as saying. "It's not what we were expecting."

The research conducted on Panama’s Gigante Peninsula matters to the average non-jungle-dwelling Miller-McCune reader in part because the world’s tropical rainforests are seen as giant carbon dioxide sponges for a rapidly warming planet. If their best growth requires the presence of molybdenum, which is 10,000 times less abundant in the jungle than phosphorous, that in turn limits how much carbon dioxide can be drawn out of the atmosphere. (Complete the rest of the syllogism on your own dime.)

"Just like trace amounts of vitamins are essential for human health, this exceedingly rare trace metal is indispensable for the vital function of tropical rainforests in the larger Earth system," Hedin said in the release. To be specific (and to steal generously from the release), the element controls the biological conversion — via the enzyme nitrogenase — of atmospheric nitrogen into the soil fertilizer nitrogen plants need to thrive.

"Nitrogenase without molybdenum is like a car engine without spark plugs," the release quotes Alexander Barron, the lead author on the paper and a graduate student in Hedin's lab, as saying. Barron is now working on climate change policy at the national and even international level.

One school of thought had believed the world’s jungles were already maxed out in terms of nitrogen, in part because of the glut of phosphorous. Meta-study work by University of California, Irvine scientists announced earlier this year showed that nitrogen runoff from human sources could increase growth in the tropics substantially — a clue in retrospect that some governor was acting on tropical nitrogen.