5,000 years ago, Mexican mangroves were capturing carbon

Researchers have identified a new reason to protect mangroves: They have quietly kept carbon from Earth’s atmosphere for the past 5,000 years.

Mangroves grow in conditions that most plants cannot tolerate, such as in brackish coastal waters. Some species have vertical air-conducting roots that act like plunges when the tide is high, giving the appearance of trees perched on stilts.

A research team led by the University of California, Riverside and the University of California, San Diego, set out to understand how marine mangroves off the coast of La Paz, Mexico, absorb and release elements such as nitrogen and carbon, processes called biogeochemical cycling.

Since these processes are largely driven by microbes, the team also wanted to see what bacteria and fungi thrive there.

The team expected to find carbon in the peat layer below the forest, but they didn’t expect the carbon to be 5,000 years old. This finding, along with a description of the microbes they identified, has now been published in the journal Marine Ecology Advance Series.

“What sets these mangrove sites apart is not that they are the fastest to store carbon, but that they have retained carbon for a long time,” said Emma Aronson, an environmental microbiologist at UCR and co-author of the study. “It’s storing carbon in larger volumes than most other ecosystems in the area.”

The peat under mangroves is a mixture of flooded sediments and partially decomposed organic matter. In some of the areas sampled for this study, the peat layer extended about 10 feet below the coastal water line.

Little oxygen reaches the deepest layer of the peat, which is likely why the team couldn’t find any fungi living in it; Fungi are commonly found in nearly every environment on Earth. However, oxygen is a requirement for most fungi that are specialized in breaking down carbon compounds. The team may explore the absence of the fungus further in future mangrove peat studies.

There are more than 1,100 species of bacteria living under the mangroves that consume and secrete a variety of chemical elements. Many of them work in harsh environments with little or no oxygen. However, these bacteria are not efficient at cracking carbon.

The deeper you go into peat soil, the fewer microorganisms you will find. Not much can break down the carbon there, or the peat itself, said Mia Maltz, a microbial ecologist and author of the study at the University of California. “Because they last so long, it’s not easy to produce more of them or replicate the microbial communities within them.”

There are other ecosystems on Earth that are known to contain carbon similar to aging or even older. An example is the ice in the Arctic or Antarctic, where the ice has not yet melted allowing gases to be released. Other mangrove forests are likely as well. Researchers are now exploring mangrove research sites in Hawaii, Florida and Mexico’s Yucatan Peninsula as well.

“These sites protect carbon that has been around for thousands of years. Matthew Costa, a coastal ecologist at the University of California, San Diego and first author on the research paper, said:

Carbon dioxide increases the greenhouse effect that is causing the planet to warm. Costa believes that one way to prevent this problem from getting worse is to leave the mangroves undisturbed.

“If we let these forests continue to function, they could retain the carbon they sequestered from our atmosphere, essentially permanently,” Costa said. “Mangroves play an important role in mitigating climate change.”