Seagrasses continue to release methane after they die

Seagrasses cover shallow coastal regions of temperate and tropical seas worldwide, and form the basis of a crucial ecosystem that is home to many animals, including some endangered species of fish, sea turtles, or seahorses. Moreover, they protect coastal areas from erosion and sequester millions of tons of carbon dioxide from the atmosphere each year. 

Unfortunately, they also emit greenhouse gases, particularly methane, which is even more damaging for the climate than carbon dioxide. A new study led by the Max Plank Institute for Marine Biology in Germany has recently found that seagrasses emit this gas even decades after they died off.

“In seagrass sediments, methane is formed solely from one class of organic compounds,” said study lead author Sina Schorn, a microbiologist at the Max Plank Institute. “These so-called methylated compounds are produced by the seagrass plant itself. Specialized microorganisms, the methanogenic archaea, then convert these compounds into methane.”

The release of methane into the water column is very fast, since the plant tissue acts as a straw, helping the gas quickly escape from the seabed into the water. Furthermore, because the seagrasses only grow in shallow water, pelagic microorganisms don’t have the opportunity to consume the methane before ending up in the atmosphere. 

As part of their study, the researchers also sampled a dead seagrass meadow. To their surprise, the rates of methane production in this meadow were similar to those in intact meadows. “We believe that the reason behind this continuing methane production is that methylated compounds persist in the plant tissue for a very long time,” even up to two decades after the plants die, explained study senior author Jana Milucka, head of the Greenhouse Gases Research Group at the Max Planck Institute.

“Currently, we are seeing a die-off of seagrass meadows worldwide which has a devastating effect on the coastal ecosystems. Our results caution that whereas upon the death of the plant carbon dioxide from the atmosphere will no longer be sequestered and stored in the sediment as ‘blue carbon’, methane may still continue to be released.”  

These results suggest that methane emissions from dead seagrass partially offset the blue carbon effect of living meadows, thus highlighting the ecologic importance of these plants and the urgent need to protect and conserve them. 

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Andrei Ionescu, Earth.com Staff Writer