Seaweed washing onto sandy shores does more than rot. A new study found that it fuels oxygen-tolerant microorganisms that pump methane into the air, overturning a long-held scientific assumption about coastal ecosystems and their climate role.
Using satellite images and astronaut photographs, NASA has identified that about 31% of the world’s ice-free coastline is sandy. Methanogenic archaea, a form of microorganism, are known to thrive in oxygen-free environments, such as coastal areas, where they break down organic matter to produce methane, a potent greenhouse gas.
A new study, led by researchers from Monash University’s Climate Change Science Hub, examined sandy coastlines across Victoria, Australia, and Avernakø, Denmark, and identified reasons for the unexpectedly high methane levels observed.
“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,” said co-corresponding author Professor Perran Cook, PhD, from Monash’s Climate Hub. “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.”
The researchers combined several approaches to investigate why coastal waters above sandy sediments often have unexpectedly high methane levels. They collected water and sediment samples from beaches in Australia (Port Phillip Bay, Westernport Bay) and Denmark. They measured methane concentrations in surface waters and compared them with radon levels to check if the methane was coming from groundwater. In laboratory experiments, they used slurry incubations (mixing sediments with seawater and seaweed/seagrass) and flow-through reactors (FTRs) to mimic natural water flow through sand. They tested whether methane production stopped when methanogens were chemically inhibited, and whether certain substrates stimulated production.
In addition, the researchers isolated new strains of methanogens (Methanococcoides species) from sediments and sequenced their genomes to confirm their metabolic pathways and oxygen tolerance. Finally, they analyzed DNA from sediments to identify methanogen genes and track community changes when seaweed or seagrass was added.
Methane was consistently found in shallow coastal waters at “supersaturation” concentrations massively higher than what would normally be expected if the water was just in balance with the air. In some cases, nearly 1,900 times more. Methane production was driven by methanogenic microbes that can survive and remain active even after repeated exposure to oxygen – a phenomenon previously thought to be impossible. It had been thought that methanogenic archaea in coastal ecosystems couldn’t survive being exposed to oxygen.
The methanogens relied on methylated compounds released during the breakdown of seaweed and seagrass. Where this kind of debris accumulated, methane production was higher. Unlike methanogens in rice paddies or wetlands (which take weeks to recover), these coastal methanogens resumed methane production within a very short period of time, one to two hours after oxygen returned to anoxic (absence of oxygen) conditions. Flux estimates from these coastal sites showed methane release rates on par with, and sometimes exceeding, those of wetlands and salt marshes, ecosystems already known to be major methane emitters.
“From here, we need to understand this process in more detail,” said lead author and PhD candidate, Ning Hall. “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.”
Obviously, a limitation of the study is that it focused on specific sites in Australia and Denmark; methane release rates vary with geography, sediment type, and local seaweed or seagrass biomass. Additionally, lab experiments are simplifications of complex coastal dynamics. And, while archaeal methanogenesis (methane production) was dominant, other bacterial pathways may also contribute small amounts.
Nonetheless, the research is groundbreaking as it shows that methanogenesis is not limited to strictly oxygen-free environments; sandy coastal zones are now confirmed as important methane emitters. It also challenges “blue carbon” strategies, where seaweed and seagrasses are often promoted as carbon sinks. This study suggests that their breakdown in sandy sediments may release large amounts of methane, offsetting some of their climate change benefits.
“With rising sea temperatures, species invasions and increasing nutrient pollution, we’re seeing more frequent algal blooms and biomass accumulation on beaches,” Cook said. “This could lead to larger and more frequent pulses of methane to the atmosphere, which in turn contributes to rising sea temperatures.”
The research was funded by the Australian Research Council, National Health and Medical Research Council, Australian Government Research Training Program Scholarship, European Research Council, Danish Research Council, and Danish National Research Foundation. It was conducted in collaboration with partners from the University of Southern Denmark, the Monash-led Securing Antarctica’s Environmental Future (SAEF) research center, and the Monash Biomedicine Discovery Institute.
The study was published in the journal Nature Geoscience.
Source: Monash University
