Seaweed on sandy coastlines contributes to methane emissions

New research led by Monash University has revealed that sandy coastlines, which make up half the world’s continental margins, are a previously overlooked source of methane. Principal investigator Professor Perran Cook said the research challenges the accepted role of coastal vegetation as carbon storage.

The research identified two new strains of methanogens, or methane-producing microbes, at field sites across Port Phillip Bay and Westernport Bay in Victoria, as well as in Denmark. These microbes metabolise compounds released from decaying seaweed and seagrass, producing methane as a byproduct.

It was previously understood these microbes couldn’t survive when exposed to oxygen in coastal ecosystems. But the new research, published in the journal Nature Geoscience, proves they are capable of rapid recovery and methane production following oxygen exposure.

“This new finding not only challenges a fundamental assumption in marine science, but calls into question what we thought we knew about the role of sandy coastline ecosystems in greenhouse gas production,” Cook said.

“Our work contributes to the growing body of evidence that shows methane emissions from decaying biomass like seaweed may offset much of the carbon dioxide removal attributed to coastal ecosystems.

“Understanding how much naturally occurring methane emissions are coming from coastal areas is also important for the climate models we rely on to understand the impacts of climate change and determine climate action.”

Cook said several outbreaks of algal blooms, such as the recent occurrence that has plagued parts of coastal South Australia this year, may also be enhancing methane emissions in instances where they are washed up on the beach.

“With rising sea temperatures, species invasions and increasing nutrient pollution, we’re seeing more frequent algal blooms and biomass accumulation on beaches,” he said.

“This could lead to larger and more frequent pulses of methane to the atmosphere, which in turn contributes to rising sea temperatures.”

First author and Monash PhD candidate Ning Hall said the team will continue its work to understand the implications of methane production in coastal ecosystems, examining how widespread it is and the chemical compounds produced.

“From here, we need to understand this process in more detail,” Hall said.

“Our research will look at how different species of seaweeds and ocean conditions affect these microbes.

“This will then allow us to reassess and better predict how much methane is being produced in the coastal zone.”

Image caption: Field work was conducted at St Kilda as part of the new study. Image credit: Ning Hall, Monash University.