The Ocean's Hidden Fertilizer - Marine Plants Play Major Role in Phosphorus Cycling

Phosphorus is one of the most common substances on Earth. An essential nutrient for every living organism—humans require approximately 700 milligrams per day—we are rarely concerned about consuming enough of it because it is present in most of the foods we eat. Despite its ubiquity and living organisms' utter dependence on it, we know surprisingly little about how it moves, or cycles, through the ocean environment.

Scientists studying the phosphorus cycle in the ocean knew that it was absorbed by plants and animals, and released back to seawater in the form of phosphate as they decay and die. Over the last 10 years, a growing body of research has hinted that phosphorus was being transformed by microbes in the ocean in ways that remained a mystery.

A new study by a research team from the Woods Hole Oceanographic Institution (WHOI) and Columbia University reveals for the first time a marine phosphorus cycle that is much more complex than previously thought. The work also highlights the important but previously hidden role that some microbial communities play in using and breaking down forms of this essential element.

Phosphorus in the oceans cycles between +5 and +3 oxidation states. Most of the oceans' phosphorus is present as oxidized bioavailable phosphate (+5) compounds. Reduced organophosphorus compounds are also present but at much lower concentrations. Through field measurements in the western tropical North Atlantic Ocean and a series of laboratory incubations, Van Mooy et al. measured fast reduction rates of a small but appreciable amount of phosphates by plankton communities, forming phosphites and phosphonates (see the Perspective by Benitez-Nelson). On a global scale, this phosphorus redox cycle adds as much reduced phosphorus to the oceans as all pre-anthropogenic land runoff.