MPn-derived methane production by epiphytic bacteria on pelagic Sargassum seaweed from 2017-2019 (Cyanobacteria Hydrocarbons project)

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 via bottle incubations, finding lysis liberated phosphonate-derived methane. The resulting data of methane production relative to various control conditions in the bottle system over time was used as an analog for MPn utilization, in line with previous studies. We noticed the methane production rates were ~linear for the first 2-3 days in each trial, after which production rates began to decrease, likely due to bottle effects. As such, the linear portion of methane production over time was used to calculate the anticipated MPn utilization rate in situ, where confounding variables induced by the experimental design would not impact the microbial activity. These MPn utilization rates are also reported, allowing for more appropriate comparison across trials and use in environmental impact estimates.

The observed activity occurred at concentrations as low as 35 nM MPn and was inhibited by antibiotics, implicating microbial members of the holobiont capable of MPn lysis at realistic environmental concentrations. A survey of macroalgal species inhabiting the Sargasso Sea found a ubiquitous capacity for MPn lysis; such capacity was absent in species inhabiting phosphorus-replete waters of the California Current, pointing to phosphorus limitation as a selective pressure. These results suggest algal holobionts may conditionally acquire phosphorus from phosphonates while simultaneously serving as a source of atmospheric methane. Incubations of macroalgal holobionts were collected from surface waters (Sargassum spp) and shallow reefs (all other 2019) offshore 4km NE of Bermuda, while all 2017 macroalgal species were collected by scientific divers in shallow reefs offshore Santa Barbara, CA.