There is a vast amount of methane trapped beneath the seafloor, across the planet. If it were to be released, the results could be potentially catastrophic for all forms of life. This is why finding out how ecosystems respond to methane leaks is important to ocean scientists. Generally, these investigations take place on purpose in controlled environments. But in 2018, a drilling incident caused a methane leak in the South China Sea. But what scientists including those at the University of Bremen, saw next shocked them more. Over the course of the next six years, in situ monitoring of the situation revealed an incredibly rapid response from the ecosystem. Within two years, the bacteria and worms had established a methane-munching ‘worm bed’ as per a study published in National Science Review.
Oceanic burps
Cracks in the sea rocks allow methane to passively leak out into the ocean. But in 2018, gas hydrate exploration in the South China Sea caused a human-made underwater methane leak. They were trying to figure out how much natural gas might exist in marine sediments, so they drilled a 22-centimetre-wide hole in the deep seafloor. Afterwards, they tried to seal the hole with heavy mud. But their attempt failed.The incident wasn’t detected until a year later, but it allowed researchers to study the seabed as the ecosystem quickly underwent a rapid and drastic transformation. A plume of methane gas had shot up from the deep, rising more than a kilometre high into the water column.Before the drilling event occurred, the seabed looked pretty much bare, with little in the way of benthic fauna making a living down there. Immediately following the leak, there was a further sharp decline in the seafloor life as microbial richness dipped by around 50 per cent.
Clean-up time
Between 2018 and 2023, the team, which included microbiologist Emil Ruff of the University of Bremen, tracked the ecosystem’s development. They surveyed the site regularly using echo sounders, underwater cameras, chemical sensors, sediment samples and genetic analysis.Using echosounders, underwater cameras, and chemical sensors, researchers watched in awe as a swarm of ocean life flocked to the hole and contained the leaking gas with “remarkable speed”. “Within just one to two years, an effective methane-consuming ecosystem had already formed,” said microbial ecologist Emil Ruff of the University of Bremen in Germany as per a report by Sciencealert.com. According to past models, a fully functioning methane-munching community on the ocean floor can take between 60 and 100 years to develop. This one was chewing through nearly as much methane in a fraction of the time.Methane-munching bacteria underwater were having a gala time. The anaerobic methanotrophic archaea ANME-2e and ANME-3 had a bonanza, going from scarcely detectable to an expanding population. These microorganisms paved the way for bigger things to come. These single-celled organisms were then followed by an influx of burrowing invertebrates, which created an extensive ‘worm bed’ over the hole. Crustaceans crawled to the scene as well. Within 1 to 2 years, the once barren sediment was covered in an extensive “worm bed”. This was due to the rapid colonization by Spionid and Acrocirrid polychaete worms and Metridinid copepods.“The worms, in turn, benefited from the newly established microorganisms, which likely served as a food source,” explains Ruff. This crew of opportunistic invertebrates are thought to be a good indicator of early-stage seep ecosystems.The blooming of life – big and small – created a kind of animal-microbe biofilter that was breaking down methane about as effectively as methane seeps that had been going on for decades. Reaching 60 to 100 per cent of their methane-degrading capacity, they provided an ecosystem service that formed far quicker than previously thought possible.As tube worms burrowed into the seafloor, they helped mix the sediment, and this probably transported oxygen, nitrate, and sulfate into deeper layers, said Ruff.
After effects
Bubbles of methane were still leaking from the hole in 2023, but as the living filter developed, the plume’s height fell from 1,200 meters (0.74 miles) above the seafloor to roughly 800 meters.However, Ruff points out that the methane filter’s rapid formation came at a cost: a marked change to the original ecosystem. Within a 20-meter (66-foot) radius, the diversity of bacteria and other microorganisms dropped by more than 50% within two years. Even 500 meters (1,640 feet) away, researchers detected elevated methane concentrations, along with a more than 30% drop in microbial diversity.The study, “Rapid de novo assembly of animal-microbe biofilter to mitigate seabed methane leakage,” points to two parallel developments: The seafloor developed a strong capacity for methane breakdown in a short period of time, while the original biological community changed significantly in the process.
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