Mexican mangroves have been storing carbon for nearly five millennia

Mangroves are known to thrive in conditions that most other plants cannot tolerate, such as salty coastal waters. Several species have air-conducting, vertical roots which act like snorkels when tides are high, giving them the appearance of trees floating on stilts. 

A new study led by the University of California, Riverside (UCR) and the University of California, San Diego (UCSD) has investigated how marine mangroves off the coast of La Paz in Mexico absorb and release substances such as carbon and nitrogen, through a process called biogeochemical cycling. The experts were amazed to discovered that these mangrove forests have been quietly keeping carbon out of the Earth’s atmosphere for nearly 5,000 years.

“What’s special about these mangrove sites isn’t that they’re the fastest at carbon storage, but that they have kept the carbon for so long,” said study senior co-author Emma Aronson, an environmental microbiologist at UCR. “It is orders of magnitude more carbon storage than most other ecosystems in the region.”

While the scientists expected that carbon would be found in the layer of peat beneath the forest – which consists of a mixture of submerged sediment and partially decayed organic matter, extending up to ten feet below the coastal water line – they did not expect that carbon to be 5,000 years old.

Since biogeochemical cycling is largely driven by microorganisms such as bacteria and fungi, the researchers set out to investigate which of these microbes are thriving in in the peat beneath mangroves. The analysis revealed no fungi living there, most likely due to the fact that little oxygen – which is needed by fungi to break down carbon – can make it to the deepest peat layer. 

While the scientists identified over 1,100 types of bacteria living beneath the mangroves that can function in extreme environments with low or no oxygen, these microbes are not sufficient for breaking down carbon – which could explain the mangroves’ immense potential of safely storing this hazardous gas. 

“The deeper you go into the peat soils, the fewer microorganisms you find. Not much can break down the carbon down there, or the peat itself, for that matter,” said study co-author Mia Maltz, a microbial ecologist at UCR. “Because it persists for so long, it’s not easy to make more of it or replicate the communities of microbes within it.” 

“These sites are protecting carbon that has been there for millennia. Disturbing them would cause a carbon emission that we wouldn’t be able to repair any time soon,” added study lead author Matthew Costa, a coastal ecologist at UCSD.

“If we let these forests keep functioning, they can retain the carbon they’ve sequestered out of our atmosphere, essentially permanently. These mangroves have an important role in mitigating climate change,” he concluded.

The study is published in the journal Marine Ecology Progress Series. 

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By Andrei Ionescu, Earth.com Staff Writer