Seagrasses Continue to Emit Methane Decades After Death

Found in shallow coastal waters worldwide, seagrass meadows are noteworthy emitters of methane into the atmosphere. Yet only a few studies have quantified their methane contribution and little is known about the metabolic processes and microorganisms involved in it. 

A study published last week (February 14) in PNAS describes the mechanisms by which methane is formed in meadows of Posidonia oceanica, a seagrass species endemic to the Mediterranean Sea. The metagenomes from the sediments surrounding the plants’ roots suggest that microorganisms from different archaeal groups are metabolizing diverse compounds present in the plant tissue to generate methane. According to these findings, these microbial communities produce methane both while the seagrasses are alive and long after they have died. 

Seagrasses, together with mangroves and salt marshes, are considered blue carbon ecosystems—they sequester, store, and bury carbon from the atmosphere, keeping the levels of carbon dioxide from increasing even more. However, they also emit high amounts of methane, a greenhouse gas with a higher warming potential than carbon dioxide. 

Studies like this are important, says Aurora Ricart, a seagrass ecologist at the Bigelow Laboratory for Ocean Sciences in Maine who did not participate in the new study. There is not “much data on seagrass methane emissions. . . Most of the time, we believe [these habitats] are contributing in a positive way to mitigate climate change, specially by sequestering carbon dioxide, and having a clear idea of the overall greenhouse gases fluxes in these systems will really help to understand” their role. The new work, she adds, is quite comprehensive: it reports both the pathway and the microbial community involved in these emissions.

The study is focused on P. oceanica, which can form thick peat layers underground derived from the accumulation of organic debris from roots, rhizomes, and other plant material. The team at the Max Planck Institute for Marine Microbiology in Germany quantified the methane fluxes from these seagrass meadows and found the species to be at the upper end of the emissions range previously described for other seagrasses. 

To further investigate the metabolic pathway involved in these methane emissions, the researchers fed the P. oceanica sediment with different types of carbon-labeled substrates. They observed methanogenesis only when they added methylated compounds, which are typically produced and released by marine plants to deal with osmotic stress. No methane production occurred when they added acetate or hydrogen gas, two common precursors of this process. 

Sina Schorn, marine microbiologist and coauthor of the study, says she and her colleagues were surprised by the fact “that these really common methane production pathways, the classical ones that use very simple organic molecules to form methane . . .  are totally absent in the seagrass ecosystem.” 

Even years after the meadow dies, there is still the capacity of the microbial community to use [compounds still available in the dead plant tissue] and continue to form methane.

—Sina Schorn, Max Planck Institute for Marine Microbiology

They then explored which microbes were living in these habitats by analyzing the metagenome of sediment samples from P. oceanica. Looking at 16S rRNA gene sequences, the team described the composition of both the bacterial and archaeal communities there. Based on the methane fluxes quantified and the “high potential to form methane,” Schorn says she and her colleagues predicted the sediment archaeal community, the only domain harboring methanogenic members, to be “full of methanogens.” But contrary to their expectations, they found less than two percent of taxa considered to be classical methanogens among all the archaeal 16S rRNA sequences. 

This low percentage may point to a “very diverse methanogenic community”—that is, not a specialized community formed only by those well-described methanogens—and this is likely favored by diverse compounds in these plants on which the microbes can feed, Schorn notes. 

This microbial diversity is also supported by the results of the group’s search for the gene coding for methyl-coenzyme M reductase A (mcrA), which is key for methanogenesis. They recovered a few sequences potentially from classical methanogens, but most clustered with the mcrA gene of the uncultured Helarchaeota phylum, a member of the Asgard superphylum that was discovered only a few years ago. 

The function of the mcrA in this group is still debated. Based on its genome, researchers have hypothesized that mcrA in Helarchaeota participates in the oxidation of hydrocarbons such as butane, rather than in methanogenesis. However, given that butane was not identified in these sediments and based on the high methane production rates they report, Schorn says that “it’s definitely worth [it] to reconsider the activity of this enzyme [in Helarchaeota] in the context of the environment where these organisms are found.”  

University of Texas marine microbial ecologist Brett Baker, who was not involved in this study but who discovered Helarchaeota and found the mcrA gene coding in its genome, says these findings are interesting, but at this point, based on the genomes of these microbes, “I’d be really surprised if they were producing methane,” he says. He hypothesizes that even if butane is absent, these microbes may be using other hydrocarbons potentially available in the sediments. Thus, their presence could still be explained without a putative methanogenic activity. 

The Max Planck team also looked at eroded sites no longer covered with living seagrasses, but where the characteristic thick peat layers of P. oceanica can still be found. They assessed whether there was any methanogenic activity in sediments under these eroded seagrass beds, where plants had probably been dead for more than 25 years. They found that methane production in the dead seagrass sediments was occurring at similar levels to that of living seagrasses. According to Schorn, this shows that “even years after the meadow dies, there is still the capacity of the microbial community to use [compounds still available in the dead plant tissue] and continue to form methane.”

Methane production in dead seagrasses may have environmental implications, says Schorn. If seagrass meadows die—a frequent scenario “we face around the world at the moment—they lose this very important ecosystem function of sequestering the carbon from the atmosphere,” yet they continue to produce methane. Thus, according to Schorn, the die-off of these ecosystems could have “bad consequences,” including accelerated global warming.

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