Methylphosphonate Is Utilized by Commensal Microbiota of Macroalgae in the Oligotrophic Sargasso Sea

Abstract

The essential nutrient phosphorus is biologically scarce in the Sargasso Sea, yet the pelagic macroalgae Sargassum, for which this area of the North Atlantic Ocean is named, thrives. We tested the hypothesis that Sargassum holobionts utilize methylphosphonate (MPn) as an alternative source of phosphorus, finding lysis liberated phosphonate-derived methane. The observed activity occurred at concentrations as low as 35 nM MPn and was inhibited by antibiotics, implicating bacterial epibionts of the holobiont capable of MPn lysis at realistic environmental concentrations. Dark incubations resulted in diminished methane production, consistent with commensalism between microbe and host. A survey of macroalgal species inhabiting the Sargasso Sea found ubiquitous capacity for MPn lysis; such capacity was absent in species inhabiting phosphorus-replete waters of the California Current, pointing to phosphorous limitation as a selective pressure. These results suggest that bacterial epibionts of algal communities acquire phosphorus from phosphonates while simultaneously serving as a source of atmospheric methane.