Mexican mangroves have been capturing carbon for 5,000 years

Researchers have identified a new reason to protect mangrove forests: they've been quietly keeping carbon out of Earth's atmosphere for the past 5,000 years.

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

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

As these processes are largely driven by microbes, the team also wanted to learn which bacteria and fungi are thriving there.

The team expected that carbon would be found in the layer of peat beneath the forest, but they did not expect that carbon to be 5,000 years old. This result, along with a description of the microbes they identified, is now published in the journal Marine Ecology Progress Series.

"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 Emma Aronson, UCR environmental microbiologist and senior co-author of the study. "It is orders of magnitude more carbon storage than most other ecosystems in the region."

Peat underlying the mangrove trees is a combination of submerged sediment and partially decayed organic matter. In some areas sampled for this study, the peat layer extended roughly 10 feet below the coastal water line.

Little oxygen makes it to the deepest peat layer, which is likely why the team did not find any fungi living in it; normally fungi are found in nearly every environment on Earth. However, oxygen is a requirement for most fungi that specialize in breaking down carbon compounds. The team may explore the absence of fungi further in future mangrove peat studies.

There are more than 1,100 types of bacteria living beneath the mangroves that consume and excrete a variety of chemical elements. Many of them function in extreme environments with low or no oxygen. However, these bacteria are not efficient at breaking down carbon.

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 Mia Maltz, UCR microbial ecologist and study author. "Because it persists for so long, it's not easy to make more of it or replicate the communities of microbes within it."

There are other ecosystems on Earth known to have similarly aged or even older carbon. Arctic or Antarctic permafrost, where the ice hasn't yet thawed allowing a release of gases, are examples. Potentially, other mangrove forests as well. The researchers are now scouting mangrove research sites in Hawaii, Florida and Mexico's Yucatan Peninsula as well.

"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," said Matthew Costa, UC San Diego coastal ecologist and first author on the paper.

Carbon dioxide increases the greenhouse effect that is causing the planet to heat up. Costa believes that one way to keep this issue from worsening is to leave mangroves undisturbed.

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On Jupiter's moon Europa, 'chaos terrains' could be shuttling oxygen to ocean

Salt water within the icy shell of Jupiter's moon Europa could be transporting oxygen into an ice-covered ocean of liquid water where it could potentially help sustain alien life, according to a team of researchers led by The University of Texas at Austin.

This theory has been proposed by others, but the researchers put it to the test by building the world's first physics-based computer simulation of the process, with oxygen hitching a ride on salt water under the moon's "chaos terrains," landscapes made up of cracks, ridges and ice blocks that cover a quarter of the icy world.

The results show that not only is the transport possible, but that the amount of oxygen brought into Europa's ocean could be on a par with the quantity of oxygen in Earth's oceans today.

"Our research puts this process into the realm of the possible," said lead researcher Marc Hesse, a professor at the UT Jackson School of Geosciences Department of Geological Sciences. "It provides a solution to what is considered one of the outstanding problems of the habitability of the Europa subsurface ocean."

The study was recently published in the journal Geophysical Research Letters.

Europa is a top spot to look for alien life because scientists have detected signs of oxygen and water, along with chemicals that could serve as nutrients. However, the moon's ice shell -- which is estimated to be about 15 miles thick -- serves as a barrier between water and oxygen, which is generated by sunlight and charged particles from Jupiter striking the icy surface.

If life as we know it exists in the ocean, there needs to be a way for oxygen to get to it. According to Hesse, the most plausible scenario based on the available evidence is for the oxygen to be carried by salt water, or brine.

Scientists think that chaos terrains form above regions where Europa's ice shell partially melts to form brine, which can mix with oxygen from the surface. The computer model created by the researchers showed what happens to the brine after the formation of the chaos terrain.

The model showed the brine draining in a distinct manner, taking the form of a "porosity wave" that causes pores in the ice to momentarily widen -- allowing the brine to pass through before sealing back up. Hesse compares the process to the classic cartoon gag of a bulge of water making its way down a garden hose.

This mode of transport appears to be an effective way to bring oxygen through the ice, with 86% of the oxygen taken up at the surface riding the wave all the way to the ocean. But the available data allows for a wide range of oxygen levels delivered to Europa's ocean over its history -- with estimates ranging by a factor of 10,000.

According to co-author Steven Vance, a research scientist at NASA's Jet Propulsion Laboratory (JPL) and the supervisor of its Planetary Interiors and Geophysics Group, the highest estimate would make the oxygen levels in Europa's ocean similar to those in Earth's oceans -- which raises hope about the potential for that oxygen to support life in the hidden sea.

"It's enticing to think of some kind of aerobic organisms living just under the ice," he said.

Vance said that NASA's upcoming 2024 Europa Clipper mission may help improve estimates for oxygen and other ingredients for life on the icy moon.

Kevin Hand, a scientist focused on Europa research at NASA JPL who was not part of the study, said that the study presents a compelling explanation for oxygen transport on Europa.

"We know that Europa has useful compounds like oxygen on its surface, but do those make it down into the ocean below, where life can use them?" he said. "In the work by Hesse and his collaborators, the answer seems to be yes."

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